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Physiological parameters governing the action of pancreatic lipase

Published online by Cambridge University Press:  01 March 2010

Iain A. Brownlee*
Affiliation:
Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle upon TyneNE2 4HH, UK
Deborah J. Forster
Affiliation:
Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle upon TyneNE2 4HH, UK
Matthew D. Wilcox
Affiliation:
Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle upon TyneNE2 4HH, UK
Peter W. Dettmar
Affiliation:
Technostics Ltd, The Deep Business Centre, HullHU1 4BG, UK
Chris J. Seal
Affiliation:
Human Nutrition Research Centre, School of Agriculture, Food & Rural Development, Agriculture Building, Newcastle University, Newcastle upon TyneNE1 7RU, UK
Jeff P. Pearson
Affiliation:
Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle upon TyneNE2 4HH, UK
*
*Corresponding author: Dr Iain A. Brownlee, fax +44 191 222 7424, email i.a.brownlee@ncl.ac.uk
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Abstract

The most widely used pharmacological therapies for obesity and weight management are based on inhibition of gastrointestinal lipases, resulting in a reduced energy yield of ingested foods by reducing dietary lipid absorption. Colipase-dependent pancreatic lipase is believed to be the major gastrointestinal enzyme involved in catalysis of lipid ester bonds. There is scant literature on the action of pancreatic lipase under the range of physiological conditions that occur within the human small intestine, and the literature that does exist is often contradictory. Due to the importance of pancreatic lipase activity to nutrition and weight management, the present review aims to assess the current body of knowledge with regards to the physiology behind the action of this unique gastrointestinal enzyme system. Existing data would suggest that pancreatic lipase activity is affected by intestinal pH, the presence of colipase and bile salts, but not by the physiological range of Ca ion concentration (as is commonly assumed). The control of secretion of pancreatic lipase and its associated factors appears to be driven by gastrointestinal luminal content, particularly the presence of acid or digested proteins and fats in the duodenal lumen. Secretion of colipase, bile acids and pancreatic lipase is driven by cholecystokinin and secretin release.

Type
Review Article
Copyright
Copyright © The Authors 2010

Introduction

A myriad of factors of dietary, endogenous and bacterial origin occur in the lumen of the small intestine, and therefore have the potential to affect the digestive processes there within. Pancreatic lipase (also referred to as TAG acyl hydrolase(Reference Embleton and Pouton1) and originally named steapsin(Reference Palmer2); EC 3.1.1.3) is one of a range of enzymes secreted by the pancreatic acinar cells that is involved in digesting dietary lipids for subsequent absorption in the small intestine.

cDNA for human pancreatic lipase was first isolated in the 1980s(Reference Lowe, Rosenblum and Strauss3). The primary and tertiary structures of human pancreatic lipase were described soon after(Reference Winkler, D'Arcy and Hunziker4). Three subgroups of human pancreatic lipases have been identified which share about 70 % sequence identity(Reference Van Tilbeurgh, Bezzine and Cambillau5). Pancreatic lipase-related proteins 1 and 2 have similar structure and action to pancreatic lipase, although it is possible they may have different substrate specificity. These enzymes are also produced by the pancreas and secreted into the duodenum, and are characterised in detail elsewhere(Reference Lowe6Reference Berton, Sebban-Kreuzer and Crenon13). The term ‘pancreatic lipase’ in the present review refers to ‘classical’ or ‘colipase-dependent’ pancreatic lipase from this point on.

Human pancreatic lipase is a 50 kDa peptide chain consisting of two structural domains: the catalytically active, 336-amino acid long N-terminal domain, and the shorter (113 residues) C-terminal domain(Reference Miled, Canaan and Dupuis14), which binds to the cofactor colipase (see below). The lipase family of enzymes acts at the interface of lipid and aqueous mixtures. This interfacial binding is unique in digestive enzymes. The majority of lipases are water soluble and catalyse cleavage of water-insoluble TAG(Reference Beisson, Tiss and Riviere15). Pancreatic lipase differs from other mammalian digestive hydrolase systems in that it is not a single domain enzyme. Its catalytic activity is increased in the presence of the cofactor colipase. As pancreatic lipase acts at the interface of lipid and water, its action is also dependent on the presence of bile acids or other moieties capable of emulsifying lipids(Reference Embleton and Pouton1). As a result of these factors, the classic lock-and-key model does not lend itself well to understanding lipase catalytic behaviour and activity.

Plasma levels of pancreatic lipase are routinely measured in the diagnosis of pancreatic dysfunction(Reference Yadav, Agarwal and Pitchumoni16). In addition, pancreatic enzyme mixtures are often given therapeutically to patients with pancreatic exocrine deficiency, including those with chronic pancreatitis and cystic fibrosis(Reference Ferrone, Raimondo and Scolapio17). In measuring the levels of pancreatic lipase activity, a number of tests that use synthetic substrates have been developed. To reduce interference of other lipolytic factors in plasma, these lipase activity assays are carried out at high (often unphysiological) pH. While such models are clearly useful diagnostic tools, it must be noted that these types of assays do not adequately predict the action of pancreatic lipases under gastrointestinal conditions.

Tetrahydrolipstatin (commonly referred to as orlistat) is a hydrogenated derivative of a naturally occurring compound originally isolated from Streptomyces toxytricini (Reference Hadvary, Sidler and Meister18). It was developed as a pharmacological agent to inhibit dietary lipid absorption during the 1980s and was approved for drug use in 1998(Reference Padwal and Majumdar19). Orlistat inhibits a range of human lipases (including gastric and pancreatic) through covalent binding to the active site serine residue that is essential for enzymic action(Reference Hadvary, Sidler and Meister18).

Previous clinical data suggest that orlistat reduces fat absorption by about 30 % in human subjects. Double-blind, placebo-controlled evidence also demonstrates the efficacy of orlistat in reducing body weight in obese individuals(Reference Sjostrom, Rissanen and Andersen20Reference Finer, James and Kopelman22), as well as further benefitting other anthropometric and plasma biomarkers of CVD risk in this population group(Reference Davidson, Hauptman and DiGirolamo23, Reference Rossner, Sjostrom and Noack24). The clinical efficacy of orlistat is tempered by common gastrointestinal side effects, including steatorrhoea and faecal incontinence that result from excess dietary lipids entering the colon. While poor compliance for orlistat usage has been reported as a result of these unwanted side effects(Reference Elfhag, Finer and Rossner25), inclusion of extra viscous, non-fermentable fibre in the diet appears to greatly reduce their occurrence(Reference Cavaliere, Floriano and Medeiros-Neto26).

The above evidence for orlistat would suggest that inhibition of pancreatic lipase is an effective strategy for benefitting weight management (and therefore obesity therapy). Through the study of the physiological and biochemical processes involved in intestinal lipid hydrolysis, it may be possible to develop novel pharmaceutical and/or food products that effectively inhibit pancreatic lipase but do not cause unwanted gastrointestinal side effects.

The rest of the present review will summarise evidence on how pancreatic lipase functions, including details on how physiological factors affect its activity and secretion.

Pancreatic lipase

Secretion

Pancreatic acinar cells are highly polarised, with a small apical membrane domain, and a much larger basolateral domain(Reference Wäsle and Edwardson27). They synthesise, store and secrete a variety of enzymes into the duodenal lumen on demand. These enzymes, including pancreatic lipases, are stored in zymogen granules within the apical pole of the cells. A wide range of secretagogues, including acetylcholine and cholecystokinin (CCK), stimulate basolateral membrane receptors in the acinar cells. This generates an intracellular response driven by Ca++, diacylglycerol and cAMP(Reference Williams28). These secondary messengers stimulate fusion of the zymogen granules with the apical membrane, leading to digestive enzyme secretion from the cells. These processes are discussed in greater detail elsewhere(Reference Wäsle and Edwardson27, Reference Thevenod29). While the intracellular pathways involved in pancreatic enzyme release (including protein sorting and zymogen granule binding) are not fully understood(Reference Wäsle and Edwardson27, Reference Williams28), a number of pathways have been suggested for control of the luminal factors relevant to lipolytic activity.

While it is currently unsure whether the release of separate pancreatic enzymes is independent of each other or not(Reference Maouyo and Morisset30), CCK appears to be the main hormonal drive for increased zymogen secretion and, therefore, lipase secretion. CCK is released from I cells and enteric nerves in the duodenal and jejunal mucosa in response to the presence of fats and amino acids in the intestinal lumen(Reference Williams28). Released CCK is believed to affect intestinal stimulation through contact with CCKA receptors found within the pancreas and the intestinal crypt epithelia(Reference Crawley31, Reference Wank32).

Further stimulation for release of pancreatic enzymes comes from parasympathetic acetylcholine release from pre- and postganglionic neurones. This acetylcholine release stimulates muscarinic M3 receptors, which in turn triggers the intracellular cascades driving zymogen granule exocytosis. Cholinergic stimuli will occur within cephalic, gastric and intestinal phases of digestion(Reference Power and Schulkin33).

Pancreatic lipase appears to lose catalytic activity below pH 5. This observation would suggest a potential exacerbation of low lipase activity in disorders causing pancreatic exocrine insufficiency, as not only will lower amounts of lipase be secreted into the duodenal lumen, but it may also be catalytically inactive due to low intestinal pH, caused by a lack of pancreatic bicarbonate secretion.

Previous observations in fasting (healthy) human subjects have noted that peak levels of pancreatic enzyme secretion (including pancreatic lipase) appear to occur at the commencement of phase III of the duodenal migrating motor complex (characterised by regular, high-amplitude phasic contractions). Additional peak levels of secretion also appear to occur at non-specific time points within the interdigestive (fasting) stages of motility(Reference Keller, Groger and Cherian34, Reference Domínguez-Muñoz, Bregulla and Nelson35).

During the fed state, intestinal motor responses to the presence of nutrients in the duodenum appear to occur over the same period of time as pancreatic secretory responses(Reference Keller, Runzi and Goebell36). This is to be expected, as both responses are elicited by similar hormonal drives.

Secretin which is released from duodenal S cells in response to the presence of luminal contents and parasympathetic stimulation may also work in synergy with CCK in up-regulating the release of pancreatic enzymes(Reference Chey, Lee and Chang37). Release of pancreatic polypeptide and somatostatin from the pancreas results in local inhibition of acinar cell exocytosis(Reference Konturek, Konturek and Domschke38). Cholinergic stimulation of pancreatic enzyme exocytosis is also inhibited by elevated circulating concentrations of pancreatic polypeptide(Reference Adrian, Besterman and Mallinson39), glucagon-like peptide-1(Reference Wettergren, Schjoldager and Mortensen40, Reference Wettergren, Schjoldager and Mortensen41) and peptide YY (released from the ileum) and ghrelin and leptin(Reference Konturek, Zabielski and Konturek42, Reference Symersky, Biemond and Frolich43).

Catalytic action

The pancreatic lipase complex catalyses hydrolysis of the ester bonds that attach fatty acids to the glycerol backbone in di- and triacylglycerols. Glycerol contains an alcohol –OH group at each of its three carbons to which fatty acids can be linked by ester bonds(Reference Perona, Ruiz-Guttierez and Nollet44). Pancreatic lipase has no activity towards the central sn-2 ester bonds, but is specific to the cleavage of the outer sn-1 and sn-3 esters. However, sn-2 ester bonds slowly undergo a non-enzymic isomerisation to 1-monoacylglycerols under the alkaline conditions of the small intestine, subsequently making them available for hydrolysis as well, potentially allowing full TAG hydrolysis(Reference Embleton and Pouton1).

Pancreatic lipase retains the same serine 152–histidine 263–aspartate 176 triad that is conserved within other members of the lipase family(Reference Winkler, D'Arcy and Hunziker4, Reference Colin, Deprez-Beauclair and Allouche45). Serine–histidine–aspartate triads also drive catalytic activity in serine proteases(Reference Miled, Canaan and Dupuis14). Four main steps, involving changes in conformation or charge, occur to the pancreatic lipase molecule(Reference Miled, Canaan and Dupuis14):

  1. (1) Initial closed conformation: a loop structure within the N-terminal (residues 237–261), often referred to as the ‘lid’ domain, initially covers the active site. Two other domains maintain the lid in this closed confirmation by van der Waal's forces; β5 (75–84) and β9 (203–223).

  2. (2) Transition to open confirmation: the β5 domain moves away from the lid domain, causing the lid domain to uncover the active site (the open conformation).

  3. (3) Formation of an oxyanion hole: the movement of the β5 domain also creates an electrophilic region around the triad serine residue. This oxyanion hole helps stabilise the intermediate catalytic product formed during the reaction. Aromatic side chains from residues tyrosine 114, phenylalanine 215 and phenylalanine 77 endow the oxyanion hole with hydrophobicity, as does the presence of proline 180, isoleucine 209 and leucine 202.

  4. (4) Binding to substrate and catalysis: two acyl-binding sites in the β9 domain allow the carbonyl carbon of the primary ester bond access to the triad serine 152 residue. The sn-1 (or because of molecular symmetry, the sn-3) acyl chain is held within the oxyanion hole. The sn-2 chain lies within a second hydrophobic groove formed by side chains of lid domain residues (251 to 259) and by isoleucine 78 in the β5 domain(Reference Van Tilbeurgh, Egloff and Martinez46). This results in hydrolysis of the acylglycerol substrate. Two residues (leucine 213 and phenylalanine 215) on the β9 domain contact with alkyl chains are implicated in increasing hydrolysis product stability(Reference Miled, Canaan and Dupuis14).

Interestingly, pancreatic lipase still shows appreciable in vitro activity to hydrolyse dietary TAG in the absence of colipase, but cannot function without the presence of bile salts, as shown in Fig. 1.

Fig. 1 Effect of colipase presence and bile salt concentration on porcine pancreatic lipase activity. (a) Lipase activity over a pH range in the presence (■; 23·8 μg/ml) and absence (□) of colipase. (b) Lipase activity over a range of bile salt (sodium taurodeoxycholate; NaTDC) concentrations at pH 7. Values are means, with standard errors represented by vertical bars. Olive oil micelles were used as a substrate using procedures modified from Vogel & Zieve(Reference Vogel and Zieve97).

Effect of pH on pancreatic lipase activity

As they must maintain activity under a wide range of physiological conditions, gastrointestinal enzymes show a much greater resistance to irreversible denaturation. In particular, pancreatic lipase has to be stable under a wide range of pH within the small intestine. The enzyme is secreted as part of the pancreatic juice at about pH 8. The approximate pH within the small intestine is suggested to be from about 6·5 in the duodenum to over 7 in the distal ileum. However, previous assessments agree that there is a generally higher pH more distally in the small intestine, and a much wider pH range occurs in both the healthy and diseased state (see Table 1).

Table 1 Reported pH range in the human small intestine

* Range includes highest and lowest mean duodenal pH values for participants with varying duodenal chymotrypsin activity.

Range includes highest and lowest mean segmental values given.

Only mean proximal and distal values cited.

§ Range includes highest and lowest mean pH values over a 240 min time-course in duodenum and jejunum.

Control of small-intestinal pH

Control of acid output of the stomach (i.e. pH of the digesta entering the small intestine) is reviewed in detail elsewhere(Reference Dockray, Varro and Dimaline47, Reference Hersey and Sachs48). However, as pancreatic lipase activity, and that of other hydrolytic enzymes in the small intestine, favours a more neutral pH, processes must be in place to quickly and effectively raise intestinal pH. The pH of the small-intestinal lumen can be raised by secretion of bicarbonate ions at three separate sites: pancreatic and hepatic ducts and small-intestinal epithelial cells (particularly those located in intestinal crypts). The model of bicarbonate secretion is analogous in these three sites at a cellular level (see Fig. 2), although it must be noted that the small-intestinal epithelia also allows paracellular migration of bicarbonate, due to the intercellular junctions being considerably leakier(Reference Allen, Flemstrom and Garner49) than hepatic and pancreatic duct junctions. Stimulated output of bicarbonate from the luminal epithelium has been suggested to vary along the length of the small intestine, with the highest in outputs in the proximal duodenum equating to approximately 200 μmol/cm per h(Reference Isenberg, Hogan and Koss50). This suggests (assuming small-intestinal length of 5 m) an intestinal output of bicarbonate of up to 100 mmol/h. Secretin stimulation of healthy human participants did not appear to affect hepatobiliary bicarbonate concentration (30 mmol/l pre-stimulation v. 35 mmol/l post-stimulation), but secretion volume rose from 20 ml/h to nearly 80 ml/h(Reference Nyberg51), suggesting a total stimulated bicarbonate output of < 3 mmol/h. Similar studies on pancreatic juice suggest a stimulated volume output of 200 ml/h(Reference Ochi, Harada and Mizushima52) with bicarbonate output reaching 90 mmol/l(Reference Stevens, Conwell and Zuccaro53) (total stimulated output of < 20 mmol/h).

Fig. 2 Putative cellular mechanisms involved with bicarbonate secretion from pancreatic duct, hepatic duct and small-intestinal epithelial cells. Intracellular accumulation of bicarbonate occurs through conversion of CO2 (which passively diffuses into the cell from the blood) by carbonic anhydrase and the action of the basolateral Na–HCO3 co-transporter (NBC). In the unstimulated cell, bicarbonate is removed from the cell by anion exchangers (AE) at the apical and basolateral membranes. Within the stimulated cell, the basolateral AE action is halted. The conductance of the apical chloride leak channel (CFTR) is raised, which results in higher localised chloride concentrations apically, thus driving increased bicarbonate release through the apical AE. During the latter stages of stimulation of bicarbonate release (i.e. when the apical and luminal concentration of bicarbonate is high), it is believed that the apical AE becomes inhibited. Bicarbonate efflux then occurs through the CFTR. Passive diffusion of bicarbonate from the blood to the lumen (left of figure) can only occur in the leaky epithelia of the intestine. Adapted from details in Allen & Flemström(Reference Allen and Flemström54), Kanno et al. (Reference Kanno, LeSage and Glaser98) and Steward et al. (Reference Steward, Ishiguro and Case99). Other membrane transporters indirectly involved in driving these processes, such as the basolateral Na+:K+:2Cl−  ATPase and K leak channels, are not included for clarity. NHE, Na+–H+ exchanger.

Secretin is the principal hormonal stimulant driving bicarbonate and fluid secretion into the small intestine(Reference Konturek, Zabielski and Konturek42, Reference Allen and Flemström54, Reference Saetre, Andersen and Houe55). Secretin acts through attachment to G-protein-coupled receptors that also show affinity for vasoactive intestinal peptide(Reference Ulrich, Holtmann and Miller56). Unlike pancreatic enzyme secretions, pancreatic release of bicarbonate appears to be unaffected by pancreatic polypeptide(Reference Adrian, Besterman and Mallinson39).

Bile acids

Bile acids are secreted by the liver, and stored in the gallbladder in man. From here, they are ejected into the duodenum via the biliary duct by contraction of the surrounding smooth muscle. Bile acids are synthesised from cholesterol within hepatocytes. About 95 % of bile acids within secreted bile are actually from the recirculating pool of bile that occurs from liver to duodenum to distal ileum/large bowel to liver(Reference Muller and Jansen57). Recycled bile acids are reabsorbed at the hepatocyte serosal (or sinusoidal) membrane by a range of specific Na-dependent and Na-independent transporters.

The majority of bile acids secreted (both recycled and new) in man are conjugated within the hepatocytes to glycine and taurine amino acid residues. This process is catalysed by bile acyl-CoA:amino acid N-acyltransferase, which is localised in peroxisomes and the cytosol(Reference Ferdinandusse and Houten58). Conjugated bile acids are considerably more soluble in the intestinal milieu than unconjugated bile acids(Reference Kullak-Ublick, Stieger and Meier59), thus endowing bile salts with greater solubility to benefit their role in making dietary lipids more available to pancreatic lipase. Bile acids and salts act as biological detergents to emulsify dietary lipids, thereby greatly increasing the surface area of the lipid–aqueous interface for pancreatic lipase action. Critical micellar concentrations of different types of bile acids tend to occur within a low millimolar range, but bile acids occur well in excess of this concentration in the small intestine(Reference Borgstrom and Erlanson60, Reference Tiss, Ransac and Lengsfeld61).

Following conjugation, bile salts are then secreted into hepatic canaliculi – secretory lumens formed between two or more adjacent hepatocytes and their tight junction complexes. Transport of conjugated bile across the canalicular membrane occurs through ATP-driven bile salt export pumps(Reference Kullak-Ublick, Stieger and Meier59). The canaliculi empty into larger bile ductules that eventually drain into the common bile duct(Reference Coleman62). In man, and many other mammals, the bile drains into the gallbladder for subsequent release. The rat does not have a gallbladder, and so is considered a poor model of human bile circulation, and hence a poor model of fatty acid and cholesterol metabolism.

CCK appears to be the major effector of bile acid secretion, acting to mediate bile release through sphincter of Oddi relaxation and contraction of the smooth muscle surrounding the gallbladder(Reference Schmidt, Creutzfeldt and Schleser63). Other stimuli for gallbladder contraction/sphincter of Oddi relaxation appear to be motilin and vagal efferent impulses, whereas pancreatic polypeptide and somatostatin release acts to inhibit gallbladder output(Reference Niebergall-Roth, Teyssen and Singer64).

Previous reports suggests that bile acid action results in an emulsion of stable lipid particles of less than 0·5 μm in diameter(Reference Embleton and Pouton1). In vitro, higher ratios of unconjugated bile acids appear to inhibit lipase activity(Reference Knarreborg, Jensen and Engberg65), while conjugated bile acids stimulate lipolysis below critical micellar concentrations, but appear to be strongly inhibitory at higher concentrations(Reference Borgstrom and Erlanson60).

A number of reports have suggested that bile acids actually inhibit lipase activity as a result of the observed effect of reducing interfacial adhesion (for example, Gargouri et al. (Reference Gargouri, Julien and Bois66); Patton & Carey(Reference Patton and Carey67)). In the physiological situation, however, the detergent action of bile will greatly increase the surface area within the small-intestinal lumen, thereby increasing total accessibility(Reference Van Tilbeurgh, Bezzine and Cambillau5).

Colipase

Human procolipase is a ninety-five-amino acid protein secreted by the pancreatic acinar cells. It is rapidly converted to colipase by tryptic and proteolytic action(Reference Kerfelec, Allouche and Colin68), causing cleavage at the Arg 5–Gly 6 bond(Reference Cordle and Lowe69). Within this conversion, a five-amino acid fragment is lost. This pentapeptide, also referred to as enterostatin(Reference Okada, York and Bray70), has been suggested to be important in appetite control in animal studies(Reference D'Agostino, Cordle and Kullman71).

Due to shared pathways of secretion, a similar range of neurohumoral mediators is likely to affect up- and down-regulation of procolipase release as that of pancreatic lipase (see above).

Colipase is an 11 000 Da protein that is catalytically inactive on its own, and binds to pancreatic lipases in a 1:1 ratio(Reference Dahim and Brockman72). Structurally, colipase is a small amphipathic protein, stabilised against acid denaturation by five disulfide bridges(Reference Sugar, Mizuno and Momsen73). As it can interact with both aqueous and non-aqueous compounds, it is believed to be important in helping ‘anchor’ pancreatic lipases to lipid droplets and micelles. The presence of NEFA in lipid micelles has been suggested to favour colipase interfacial binding(Reference Dahim and Brockman72, Reference Borgstrom74).

Murine knock-out studies have shown that procolipase-deficient mice had a 60 % lower postnatal survival rate (presumably due to a decrease in the available energy during weaning) and also had reduced body weight (by 30 %) compared with wild types during and after weaning. In addition, when knock-out mice were fed a high-fat diet, they exhibited fat malabsorption through steatorrhoea(Reference D'Agostino, Cordle and Kullman71).

Crystallographic analysis suggests that colipase binds to the non-catalytic, C-terminal domain of pancreatic lipase(Reference Van Tilbeurgh, Bezzine and Cambillau5). This interaction between colipase and the C-terminal is through the amino acids in two colipase hairpin loops. The weak association between the two proteins is increased in the presence of a lipid interface(Reference Van Tilbeurgh, Bezzine and Cambillau5). The binding of colipase to pancreatic lipases is also believed to play a role in stabilising the open-lid conformation(Reference Dahim and Brockman72). Mutations at the Glu 15 residue of the colipase protein, which occurs within the area that binds to the lipase lid, greatly reduced the potential of colipase to maintain the open-lid confirmation(Reference Van Tilbeurgh, Bezzine and Cambillau5).

The main area of the colipase that interacts with micelles is believed to be the 70–85 loop(Reference Van Tilbeurgh, Bezzine and Cambillau5). Further residues have been suggested to be important in this interaction through a range of studies. Computational modelling of colipase binding with lipid droplets and bile salt micelles hypothesises that hydrophobic finger areas of the colipase molecule are the interfacial binding site(Reference Kerfelec, Allouche and Colin68). Interfacial interactions of colipase have been shown by in vitro study to be dependent on the lipid substrate involved, as well as the ionic strength and pH of the aqueous phase(Reference Kerfelec, Allouche and Colin68). Previous studies have demonstrated that colipase–lipase binding occurs in such a way that the hydrophobic regions of the colipase molecule are orientated towards the lipid–aqueous interface(Reference Van Tilbeurgh, Bezzine and Cambillau5, Reference Cordle and Lowe75). Site-directed mutagenesis towards this area revealed that Tyr 55 and 59 residues appeared to be important in the ability of colipase to act as a cofactor(Reference Cordle and Lowe75) in pancreatic lipase produced from human cDNA. Further suggestion that this hydrophobic region of colipase was integral in interface binding came from NMR analysis of conserved areas of porcine and equine colipases. Within these studies, two aromatic residues (Tyr 55 and either Trp or Phe residues at the 52 position) were also associated with interaction with bile salt micelles(Reference Dominguez, Sebban-Kreuzer and Bornet76).

Calcium ions

Unlike other factors involved in intestinal lipolysis, Ca++ concentration within the small-intestinal lumen is mainly as a result of dietary intake. Ca is believed to be involved in the catalytic activity of the pancreatic lipase–coenzyme complex. Previous in vitro lipolysis studies have demonstrated an increase in purified human pancreatic lipase activity of about 8 % in the presence of 0·5 mm-Ca compared with a Ca-free solution. No further increase in activity was seen with higher concentrations of Ca up to 4 mm(Reference Lessinger, Férard and Mignot77). Early work in this area using radioisotope studies suggested that Ca has no binding affinity for either colipase or lipase. However, low concentrations (plateau of maximal activity at 40 μm-Ca++) still appeared to increase lipolysis Vmax (maximum velocity of reaction) and decrease the apparent K m for the olive oil emulsion substrate(Reference Kimura, Futami and Tarui78). It has been postulated that the presence of Ca++ reduces the lag phase of pancreatic lipase-mediated lipolysis(Reference Alvarez and Stella79). The presence of Ca ions was shown to reduce the surface charge of micelle droplets, but this did not correlate with lag phase duration(Reference Wickham, Garrood and Leney80). From the above evidence, it appears that Ca++ is not essential for intestinal lipolysis. It is unlikely that luminal concentrations (largely governed by dietary intake) will fall below micromolar levels, and even if they do, this only has minor consequence to pancreatic lipase activity.

Luminal control of intestinal lipolytic activity

Enteroendocrine system

While control of intestinal lipolysis is mediated by endocrine, neural and paracrine factors basolaterally, the initial stimulus that affects the release of these factors comes from the composition of the digesta within the gut lumen. Conditions that drive the release of these agonists and antagonists of pancreatic exocrine secretion and other processes involved in intestinal lipolysis are mediated through sampling (chemo-sensing) of the luminal contents by enteroendocrine cells(Reference Dockray81, Reference Sternini, Anselmi and Rozengurt82). Mechanical and luminal stimuli cause release of enteroendocrine intracellular Ca++ stores, resulting in the release of humoral mediators of digestive control. Within the small intestine, specific luminal factors are known to drive lipolytic activity.

Factors such as the presence of an acidic bolus and digested proteins and fats have been demonstrated to cause the release of CCK or secretin from the small-intestinal mucosa in human studies(Reference Liddle83). In particular, strong CCK releases have been noted in response to the presence of luminal fatty acid with a chain length of > eleven carbons(Reference McLaughlin, Luca and Jones84) or the amino acids tryptophan and phenylalanine(Reference Liddle83). Secretin release appears to be mainly driven by intestinal luminal acidity(Reference Holst, Lauritzen and Jensen85, Reference Nishiwaki, Satake and Kitamura86), particularly below pH 4·5.

Luminal down-regulation of lipolytic secretions comes from intestinal tryptic and proteolytic activity(Reference Owyang, Louie and Tatum87, Reference Ihse, Lilja and Lundquist88). This feedback regulation was shown to only affect CCK-mediated pathways, not cholinergic pathways, in human studies(Reference Owyang, May and Louie89). Tryptic activity has also been shown to reduce pancreatic lipase activity in vitro, due to proteolysis of the active enzyme(Reference Layer, Jansen and Cherian90).

Dietary inhibitors of lipolytic activity

More recently, a number of other naturally derived compounds have been noted to have an impact on pancreatic and gastric lipase activity in vitro or in animal studies, including those from seaweed(Reference Ben Rebah, Smaoui and Frikha91, Reference Bitou, Ninomiya and Tsujita92), green tea(Reference Koo and Noh93), berries(Reference McDougall, Kulkarni and Stewart94) and wheat flour(Reference Tani, Ohishi and Watanabe95). A recent review has also highlighted a number of other plant products that inhibit pancreatic lipase action, including saponins and polyphenols(Reference Birari and Bhutani96). It must be noted that such compounds are likely to have similar unwanted gastrointestinal side effects to orlistat unless consumed at lower amounts or in the presence of bulking agents(Reference Cavaliere, Floriano and Medeiros-Neto26).

Summary

Intestinal lipolysis is perhaps the most complex endogenous enzyme-driven process that occurs within the human gut. Three major factors (pancreatic lipase, colipase and bile acids) are released in response to various luminal conditions that drive the rate of dietary lipid digestion. The formation of lipid micelles and their potential to bind to lipase–colipase complexes are important rate-determining steps in this catalytic process.

The present review highlights the cascade of luminal, local and neurohumoral factors associated with the rate of lipid hydrolysis in the small intestine. Novel treatments that aim to target lipolytic pathways to treat obesity should aim to do so in a way that does not produce unwanted gastrointestinal side effects in order to increase patient compliance.

Acknowledgements

I. A. B.'s salary was paid through a Biotechnology and Biological Sciences Research Council (BBSRC) Diet and Health Research Industry Club (DRINC) project grant (no. BB/G00563X/1 held by J. P. P. and C. J. S.), while M. D. W. was sponsored by a BBSRC Collaborative Awards in Science and Engineering (CASE) studentship (no. BBS/S/M/2006/13 035 held by J. P. P.) during the preparation of this paper.

All authors contributed equally to the preparation of this paper.

The authors report no conflicts of interest arising from the publication of this review.

References

1Embleton, JK & Pouton, CW (1997) Structure and function of gastro-intestinal lipases. Adv Drug Deliv Rev 25, 1532.CrossRefGoogle Scholar
2Palmer, LS (1922) The influence of various antiseptics on the activity of lipase. J Am Chem Soc 44, 15271538.CrossRefGoogle Scholar
3Lowe, ME, Rosenblum, JL & Strauss, AW (1989) Cloning and characterization of human pancreatic lipase cDNA. J Biol Chem 264, 2004220048.CrossRefGoogle ScholarPubMed
4Winkler, FK, D'Arcy, A & Hunziker, W (1990) Structure of human pancreatic lipase. Nature 343, 771774.CrossRefGoogle ScholarPubMed
5Van Tilbeurgh, H, Bezzine, S, Cambillau, C, et al. . (1999) Colipase: structure and interaction with pancreatic lipase. Biochim Biophys Acta 1441, 173184.CrossRefGoogle ScholarPubMed
6Lowe, ME (2000) Properties and function of pancreatic lipase related protein 2. Biochimie 82, 9971004.CrossRefGoogle ScholarPubMed
7Giller, T, Buchwald, P, Blum-Kaelin, D, et al. . (1992) Two novel human pancreatic lipase related proteins, hPLRP1 and hPLRP2. Differences in colipase dependence and in lipase activity. J Biol Chem 267, 1650916516.CrossRefGoogle ScholarPubMed
8Eydoux, C, Spinelli, S, Davis, TL, et al. . (2008) Structure of human pancreatic lipase-related protein 2 with the lid in an open conformation. Biochemistry 47, 95539564.CrossRefGoogle Scholar
9De Caro, J, Sias, B, Grandval, P, et al. . (2004) Characterization of pancreatic lipase-related protein 2 isolated from human pancreatic juice. Biochim Biophys Acta 1701, 8999.CrossRefGoogle ScholarPubMed
10De Caro, J, Carriere, F, Barboni, P, et al. . (1998) Pancreatic lipase-related protein 1 (PLRP1) is present in the pancreatic juice of several species. Biochim Biophys Acta 387, 331341.CrossRefGoogle Scholar
11Crenon, I, Jayne, S, Kerfelec, B, et al. . (1998) Pancreatic lipase-related protein type 1: a double mutation restores a significant lipase activity. Biochem Biophys Res Comm 246, 513517.CrossRefGoogle ScholarPubMed
12Carriere, F, Withers-Martinez, C, Van Tilbeurgh, H, et al. . (1998) Structural basis for the substrate selectivity of pancreatic lipases and some related proteins. Biochim Biophys Acta 1376, 417432.CrossRefGoogle ScholarPubMed
13Berton, A, Sebban-Kreuzer, C & Crenon, I (2007) Role of the structural domains in the functional properties of pancreatic lipase-related protein 2. FEBS J 274, 60116023.CrossRefGoogle ScholarPubMed
14Miled, N, Canaan, S, Dupuis, L, et al. . (2000) Digestive lipases: from three-dimensional structure to physiology. Biochimie 82, 973986.CrossRefGoogle ScholarPubMed
15Beisson, F, Tiss, A, Riviere, C, et al. . (2000) Methods for lipase detection and assay: a critical review. Eur J Lipid Sci Technol 102, 133153.3.0.CO;2-X>CrossRefGoogle Scholar
16Yadav, D, Agarwal, N & Pitchumoni, CS (2002) A critical evaluation of laboratory tests in acute pancreatitis. Am J Gastroenterol 97, 13091318.CrossRefGoogle ScholarPubMed
17Ferrone, M, Raimondo, M & Scolapio, JS (2007) Pancreatic enzyme pharmacotherapy. Pharmacotherapy 27, 910920.CrossRefGoogle ScholarPubMed
18Hadvary, P, Sidler, W, Meister, W, et al. . (1991) The lipase inhibitor tetrahydrolipstatin binds covalently to the putative active site serine of pancreatic lipase. J Biol Chem 266, 20212027.CrossRefGoogle Scholar
19Padwal, RS & Majumdar, SR (2007) Drug treatments for obesity: orlistat, sibutramine, and rimonabant. Lancet 369, 7177.CrossRefGoogle ScholarPubMed
20Sjostrom, L, Rissanen, A, Andersen, T, et al. . (1998) Randomised placebo-controlled trial of orlistat for weight loss and prevention of weight regain in obese patients. Lancet 352, 167172.CrossRefGoogle ScholarPubMed
21Torgerson, JS, Hauptman, J, Boldrin, MN, et al. . (2004) XENical in the Prevention of Diabetes in Obese Subjects (XENDOS) Study: a randomized study of orlistat as an adjunct to lifestyle changes for the prevention of type 2 diabetes in obese patients. Diabetes Care 27, 155161.CrossRefGoogle ScholarPubMed
22Finer, N, James, WPT, Kopelman, PG, et al. . (2000) One-year treatment of obesity: a randomized, double-blind, placebo-controlled, multicentre study of orlistat, a gastrointestinal lipase inhibitor. Int J Obes 24, 306313.CrossRefGoogle ScholarPubMed
23Davidson, MH, Hauptman, J, DiGirolamo, M, et al. . (1999) Weight control and risk factor reduction in obese subjects treated for 2 years with orlistat: a randomized controlled trial. JAMA 281, 235242.CrossRefGoogle ScholarPubMed
24Rossner, S, Sjostrom, L, Noack, R, et al. . (2000) Weight loss, weight maintenance, and improved cardiovascular risk factors after 2 years treatment with orlistat for obesity. Obes Res 8, 4961.CrossRefGoogle ScholarPubMed
25Elfhag, K, Finer, N & Rossner, S (2008) Who will lose weight on sibutramine and orlistat? Psychological correlates for treatment success. Diabetes Obes Metab 10, 498505.CrossRefGoogle ScholarPubMed
26Cavaliere, H, Floriano, I & Medeiros-Neto, G (2001) Gastrointestinal side effects of orlistat may be prevented by concomitant prescription of natural fibers (psyllium mucilloid). Int J Obes 25, 10951099.CrossRefGoogle ScholarPubMed
27Wäsle, B & Edwardson, JM (2002) The regulation of exocytosis in the pancreatic acinar cell. Cell Signal 14, 191197.CrossRefGoogle ScholarPubMed
28Williams, JA (2001) Intracellular signaling mechanisms activated by cholecystokinin-regulating synthesis and secretion of digestive enzymes in pancreatic acinar cells. Annu Rev Physiol 63, 7797.CrossRefGoogle ScholarPubMed
29Thevenod, F (2002) Ion channels in secretory granules of the pancreas and their role in exocytosis and release of secretory proteins. Am J Physiol Cell Physiol 283, C651C672.CrossRefGoogle ScholarPubMed
30Maouyo, D & Morisset, J (1995) Amazing pancreas: specific regulation of pancreatic secretion of individual digestive enzymes in rats. Am J Physiol Endocrinol Metab 268, E349E359.CrossRefGoogle ScholarPubMed
31Crawley, JN (1994) Biological actions of cholecystokinin. Peptides 15, 731755.CrossRefGoogle ScholarPubMed
32Wank, SA (1995) Cholecystokinin receptors. Am J Physiol Gastrointest Liver Physiol 269, G628G646.CrossRefGoogle ScholarPubMed
33Power, ML & Schulkin, J (2008) Anticipatory physiological regulation in feeding biology: cephalic phase responses. Appetite 50, 194206.CrossRefGoogle ScholarPubMed
34Keller, J, Groger, G, Cherian, L, et al. . (2001) Circadian coupling between pancreatic secretion and intestinal motility in humans. Am J Physiol Gastrointest Liver Physiol 280, G273G278.CrossRefGoogle ScholarPubMed
35Domínguez-Muñoz, JE, Bregulla, M, Nelson, DK, et al. . (1998) Independent cycles of exocrine pancreatic secretion, hormones and gastroduodenal motility in healthy fasting humans: reassessment of a complex partnership. Neurogastroenterol Motil 10, 2734.CrossRefGoogle ScholarPubMed
36Keller, J, Runzi, M, Goebell, H, et al. . (1997) Duodenal and ileal nutrient deliveries regulate human intestinal motor and pancreatic responses to a meal. Am J Physiol Gastrointest Liver Physiol 272, G632G637.CrossRefGoogle ScholarPubMed
37Chey, WY, Lee, KY, Chang, TM, et al. . (1984) Potentiating effect of secretin on cholecystokinin-stimulated pancreatic secretion in dogs. Am J Physiol 246, G248G252.Google ScholarPubMed
38Konturek, SJ, Konturek, JW & Domschke, S (1986) Effects of secretin antibody on gastric acid inhibition and pancreatic bicarbonate stimulation by acidified liver extract meal in dogs. Hepatogastroenterology 33, 170175.Google ScholarPubMed
39Adrian, TE, Besterman, HS & Mallinson, CN (1979) Inhibition of secretin stimulated pancreatic secretion by pancreatic polypeptide. Gut 20, 3740.CrossRefGoogle ScholarPubMed
40Wettergren, A, Schjoldager, B, Mortensen, PE, et al. . (1993) Effect of GLP-1 on gastric motility and gastric and pancreatic secretion in man. Digestion 54, 384385.Google Scholar
41Wettergren, A, Schjoldager, B, Mortensen, PE, et al. . (1993) Truncated GLP-1 (proglucagon 78-107-amide) inhibits gastric and pancreatic functions in man. Dig Dis Sci 38, 665673.CrossRefGoogle ScholarPubMed
42Konturek, SJ, Zabielski, R, Konturek, JW, et al. . (2003) Neuroendocrinology of the pancreas; role of brain–gut axis in pancreatic secretion. Eur J Pharmacol 481, 114.CrossRefGoogle ScholarPubMed
43Symersky, T, Biemond, I, Frolich, M, et al. . (2005) Effect of peptide YY on pancreatico-biliary secretion in humans. Scand J Gastroenterol 40, 944949.CrossRefGoogle ScholarPubMed
44Perona, JS & Ruiz-Guttierez, V (2004) Analysis of neutral lipids: triacylglycerols. In Handbook of Food Analysis: Physical Characterization and Nutrient Analysis, pp. 275313 [Nollet, LML, editor]. London: CRC Press.Google Scholar
45Colin, DY, Deprez-Beauclair, P, Allouche, M, et al. . (2008) Exploring the active site cavity of human pancreatic lipase. Biochem Biophys Res Commun 370, 394398.CrossRefGoogle ScholarPubMed
46Van Tilbeurgh, H, Egloff, MP, Martinez, C, et al. . (1993) Interfacial activation of the lipase-procolipase complex by mixed micelles revealed by X-ray crystallography. Nature 362, 814820.CrossRefGoogle ScholarPubMed
47Dockray, GJ, Varro, A, Dimaline, R, et al. . (2001) The gastrins: their production and biological activities. Annu Rev Physiol 63, 119139.CrossRefGoogle ScholarPubMed
48Hersey, SJ & Sachs, G (1995) Gastric acid secretion. Physiol Rev 75, 155189.CrossRefGoogle ScholarPubMed
49Allen, A, Flemstrom, G, Garner, A, et al. . (1993) Gastroduodenal mucosal protection. Physiol Rev 73, 823857.CrossRefGoogle ScholarPubMed
50Isenberg, JI, Hogan, DL, Koss, MA, et al. . (1986) Human duodenal mucosal bicarbonate secretion. Evidence for basal secretion and stimulation by hydrochloric acid and a synthetic prostaglandin E1 analogue. Gastroenterology 91, 370378.CrossRefGoogle Scholar
51Nyberg, B (1994) Effects of intravenous infusion of secretin on bile secretion in humans. Eur J Gastroenterol Hepatol 6, 931936.CrossRefGoogle Scholar
52Ochi, K, Harada, H, Mizushima, T, et al. . (1997) Intraductal secretin test is as useful as duodenal secretin test in assessing exocrine pancreatic function. Dig Dis Sci 42, 492496.CrossRefGoogle ScholarPubMed
53Stevens, T, Conwell, DL, Zuccaro, G, et al. . (2004) Electrolyte composition of endoscopically collected duodenal drainage fluid after synthetic porcine secretin stimulation in healthy subjects. Gastrointest Endosc 60, 351355.CrossRefGoogle ScholarPubMed
54Allen, A & Flemström, G (2005) Gastroduodenal mucus bicarbonate barrier: protection against acid and pepsin. Am J Physiol Cell Physiol 288, C1C19.CrossRefGoogle ScholarPubMed
55Saetre, SS, Andersen, NJ, Houe, T, et al. . (1998) Regulation of porcine biliary secretion by secretin. Acta Physiol Scand 163, 113119.CrossRefGoogle ScholarPubMed
56Ulrich, CD II, Holtmann, M & Miller, LJ (1998) Secretin and vasoactive intestinal peptide receptors: members of a unique family of G protein-coupled receptors. Gastroenterology 114, 382397.CrossRefGoogle ScholarPubMed
57Muller, M & Jansen, PLM (1997) Molecular aspects of hepatobiliary transport. Am J Physiol Gastrointest Liver Physiol 272, G1285G1303.CrossRefGoogle ScholarPubMed
58Ferdinandusse, S & Houten, SM (2006) Peroxisomes and bile acid biosynthesis. Biochim Biophys Acta 1763, 14271440.CrossRefGoogle ScholarPubMed
59Kullak-Ublick, GA, Stieger, B & Meier, PJ (2004) Enterohepatic bile salt transporters in normal physiology and liver disease. Gastroenterology 126, 322342.CrossRefGoogle ScholarPubMed
60Borgstrom, B & Erlanson, C (1973) Pancreatic lipase and co lipase. Interactions and effects of bile salts and other detergents. Eur J Biochem 37, 6068.CrossRefGoogle ScholarPubMed
61Tiss, A, Ransac, S, Lengsfeld, H, et al. . (2001) Surface behaviour of bile salts and tetrahydrolipstatin at air/water and oil/water interfaces. Chem Phys Lipids 111, 7385.CrossRefGoogle ScholarPubMed
62Coleman, R (1987) Biochemistry of bile secretion. Biochem J 244, 249261.CrossRefGoogle ScholarPubMed
63Schmidt, WE, Creutzfeldt, W, Schleser, A, et al. . (1991) Role of CCK in regulation of pancreaticobiliary functions and GI motility in humans: effects of loxiglumide. Am J Physiol Gastrointest Liver Physiol 260, G197G206.CrossRefGoogle ScholarPubMed
64Niebergall-Roth, E, Teyssen, S & Singer, MV (1997) Neurohormonal control of gallbladder motility. Scand J Gastroenterol 32, 737750.CrossRefGoogle ScholarPubMed
65Knarreborg, A, Jensen, SK & Engberg, RM (2003) Pancreatic lipase activity as influenced by unconjugated bile acids and pH, measured in vitro and in vivo. J Nutr Biochem 14, 259265.CrossRefGoogle ScholarPubMed
66Gargouri, Y, Julien, R, Bois, AG, et al. . (1983) Studies on the detergent inhibition of pancreatic lipase activity. J Lipid Res 24, 13361342.CrossRefGoogle ScholarPubMed
67Patton, JS & Carey, MC (1981) Inhibition of human pancreatic lipase–colipase activity by mixed bile salt–phospholipid micelles. Am J Physiol Gastrointest Liver Physiol 4, 328336.CrossRefGoogle Scholar
68Kerfelec, B, Allouche, M, Colin, D, et al. . (2008) Computational study of colipase interaction with lipid droplets and bile salt micelles. Proteins 73, 828838.CrossRefGoogle ScholarPubMed
69Cordle, RA & Lowe, ME (1998) Purification and characterization of human procolipase expressed in yeast cells. Protein Expr Purif 13, 3035.CrossRefGoogle ScholarPubMed
70Okada, S, York, DA, Bray, GA, et al. . (1991) Enterostatin (Val-Pro-Asp-Pro-Arg), the activation peptide of procolipase, selectively reduces fat intake. Physiol Behav 49, 11851189.CrossRefGoogle ScholarPubMed
71D'Agostino, D, Cordle, RA, Kullman, J, et al. . (2002) Decreased postnatal survival and altered body weight regulation in procolipase-deficient mice. J Biol Chem 277, 71707177.CrossRefGoogle ScholarPubMed
72Dahim, M & Brockman, H (1998) How colipase–fatty acid interactions mediate adsorption of pancreatic lipase to interfaces. Biochemistry 37, 83698377.CrossRefGoogle ScholarPubMed
73Sugar, IP, Mizuno, NK, Momsen, MM, et al. . (2001) Lipid lateral organization in fluid interfaces controls the rate of colipase association. Biophys J 81, 33873397.CrossRefGoogle ScholarPubMed
74Borgstrom, B (1980) Importance of phospholipids, pancreatic phospholipase A2, and fatty acid for the digestion of dietary fat. In vitro experiments with the porcine enzymes. Gastroenterology 78, 954962.CrossRefGoogle ScholarPubMed
75Cordle, RA & Lowe, ME (1998) The hydrophobic surface of colipase influences lipase activity at an oil–water interface. J Lipid Res 39, 17591767.CrossRefGoogle ScholarPubMed
76Dominguez, C, Sebban-Kreuzer, C, Bornet, O, et al. . (2000) Interactions of bile salt micelles and colipase studied through intermolecular nOes. FEBS Lett 482, 109112.CrossRefGoogle ScholarPubMed
77Lessinger, JM, Férard, G, Mignot, V, et al. . (1996) Catalytic properties and stability of lipase purified from human pancreatic juice. Clin Chim Acta 251, 119129.CrossRefGoogle ScholarPubMed
78Kimura, H, Futami, Y, Tarui, S, et al. . (1982) Activation of human pancreatic lipase activity by calcium and bile salts. J Biochem 92, 243251.CrossRefGoogle ScholarPubMed
79Alvarez, FJ & Stella, VJ (1989) The role of calcium ions and bile salts on the pancreatic lipase-catalyzed hydrolysis of triglyceride emulsions stabilized with lecithin. Pharm Res 6, 449457.CrossRefGoogle ScholarPubMed
80Wickham, M, Garrood, M, Leney, J, et al. . (1998) Modification of a phospholipid stabilized emulsion interface by bile salt: effect on pancreatic lipase activity. J Lipid Res 39, 623632.CrossRefGoogle ScholarPubMed
81Dockray, GJ (2003) Luminal sensing in the gut: an overview. J Physiol Pharmacol 54, Suppl. 4, 917.Google ScholarPubMed
82Sternini, C, Anselmi, L & Rozengurt, E (2008) Enteroendocrine cells: a site of ‘taste’ in gastrointestinal chemosensing. Curr Opin Endocrinol Diabetes Obes 15, 7378.CrossRefGoogle ScholarPubMed
83Liddle, RA (2000) Regulation of cholecystokinin secretion in humans. J Gastroenterol 35, 181187.CrossRefGoogle ScholarPubMed
84McLaughlin, J, Luca, MG, Jones, MN, et al. . (1999) Fatty acid chain length determines cholecystokinin secretion and effect on human gastric motility. Gastroenterology 116, 4653.CrossRefGoogle ScholarPubMed
85Holst, JJ, Lauritzen, K & Jensen, SL (1981) Secretin release from the isolated, vascularly perfused pig duodenum. J Physiol 318, 327337.CrossRefGoogle ScholarPubMed
86Nishiwaki, H, Satake, K & Kitamura, T (1983) Postprandial plasma secretin response in patients following gastrectomy. Surg Gynecol Obstet 156, 6972.Google ScholarPubMed
87Owyang, C, Louie, DS & Tatum, D (1986) Feedback regulation of pancreatic enzyme secretion. Suppression of cholecystokinin release by trypsin. J Clin Invest 77, 20422047.CrossRefGoogle ScholarPubMed
88Ihse, I, Lilja, P & Lundquist, I (1977) Feedback regulation of pancreatic enzyme secretion by intestinal trypsin in man. Digestion 15, 303308.CrossRefGoogle ScholarPubMed
89Owyang, C, May, D & Louie, DS (1986) Trypsin suppression of pancreatic enzyme secretion. Differential effect on cholecystokinin release and the enteropancreatic reflex. Gastroenterology 91, 637643.CrossRefGoogle ScholarPubMed
90Layer, P, Jansen, JBMJ, Cherian, L, et al. . (1990) Feedback regulation of human pancreatic secretion. Effects of protease inhibition on duodenal delivery and small intestinal transit of pancreatic enzymes. Gastroenterology 98, 13111319.CrossRefGoogle ScholarPubMed
91Ben Rebah, F, Smaoui, S, Frikha, F, et al. . (2008) Inhibitory effects of Tunisian marine algal extracts on digestive lipases. Appl Biochem Biotechnol 151, 7179.CrossRefGoogle ScholarPubMed
92Bitou, N, Ninomiya, M, Tsujita, T, et al. . (1999) Screening of lipase inhibitors from marine algae. Lipids 34, 441445.CrossRefGoogle ScholarPubMed
93Koo, SI & Noh, SK (2007) Green tea as inhibitor of the intestinal absorption of lipids: potential mechanism for its lipid-lowering effect. J Nutr Biochem 18, 179183.CrossRefGoogle ScholarPubMed
94McDougall, GJ, Kulkarni, NN & Stewart, D (2009) Berry polyphenols inhibit pancreatic lipase activity in vitro. Food Chem 115, 193199.CrossRefGoogle Scholar
95Tani, H, Ohishi, H & Watanabe, K (1995) When flour lipase inhibitor decreases serum lipid levels in male rats. J Nutr Sci Vitaminol 41, 699706.CrossRefGoogle ScholarPubMed
96Birari, RB & Bhutani, KK (2007) Pancreatic lipase inhibitors from natural sources: unexplored potential. Drug Discov Today 12, 879889.CrossRefGoogle ScholarPubMed
97Vogel, WC & Zieve, L (1963) A rapid and sensitive turbidimetric method for serum lipase based upon differences between the lipases of normal and pancreatitis serum. Clin Chem 9, 168181.CrossRefGoogle ScholarPubMed
98Kanno, N, LeSage, G, Glaser, S, et al. . (2001) Regulation of cholangiocyte bicarbonate secretion. Am J Physiol Gastrointest Liver Physiol 281, G612G625.CrossRefGoogle ScholarPubMed
99Steward, MC, Ishiguro, H & Case, RM (2005) Mechanisms of bicarbonate secretion in the pancreatic duct. Ann Rev Physiol 67, 377409.CrossRefGoogle ScholarPubMed
100Hernell, O, Staggers, JE & Carey, MC (1990) Physical-chemical behavior of dietary and biliary lipids during intestinal digestion and absorption. 2. Phase analysis and aggregation states of luminal lipids during duodenal fat digestion in healthy adult human beings. Biochemistry 29, 20412056.CrossRefGoogle ScholarPubMed
101Kay, GH, Tetlow, VA & Braganza, JM (1983) Relationship between PABA and Lundh tests: lack of influence of duodenal pH in vivo. Clin Chim Acta 128, 115124.CrossRefGoogle ScholarPubMed
102Watson, BW, Meldrum, SJ, Riddle, HC, et al. . (1972) pH profile of gut as measured by radiotelemetry capsule. BMJ 2, 104106.CrossRefGoogle Scholar
103Evans, DF, Pye, G, Bramley, R, et al. . (1988) Measurement of gastrointestinal pH profiles in normal ambulant human subjects. Gut 29, 10351041.CrossRefGoogle ScholarPubMed
104Zentler-Munro, PL, Fitzpatrick, WJF, Batten, JC, et al. . (1984) Effect of intrajejunal acidity on aqueous phase bile acid and lipid concentrations in pancreatic steatorrhoea due to cystic fibrosis. Gut 25, 500507.CrossRefGoogle ScholarPubMed
105Fallingborg, J, Christensen, LA, Ingeman-Nielsen, M, et al. . (1989) pH-profile and regional transit times of the normal gut measured by a radiotelemetry device. Aliment Pharmacol Ther 3, 605613.CrossRefGoogle ScholarPubMed
106Sasaki, Y, Hada, R, Nakajima, H, et al. . (1997) Improved localizing method of radiopill in measurement of entire gastrointestinal pH profiles: colonic luminal pH in normal subjects and patients with Crohn's disease. Am J Gastroenterol 92, 114118.Google ScholarPubMed
107Fallingborg, J, Pedersen, P & Jacobsen, BA (1998) Small intestinal transit time and intraluminal pH in ileocecal resected patients with Crohn's disease. Dig Dis Sci 43, 702705.CrossRefGoogle ScholarPubMed
108Press, AG, Hauptmann, IA, Hauptmann, L, et al. . (1998) Gastrointestinal pH profiles in patients with inflammatory bowel disease. Aliment Pharmacol Ther 12, 673678.CrossRefGoogle ScholarPubMed
109Brouwers, J, Ingels, F, Tack, J, et al. . (2005) Determination of intraluminal theophylline concentrations after oral intake of an immediate- and a slow-release dosage form. J Pharm Pharmacol 57, 987995.CrossRefGoogle Scholar
110Perez De La Cruz Moreno, M, Oth, M, Deferme, S, et al. . (2006) Characterization of fasted-state human intestinal fluids collected from duodenum and jejunum. J Pharm Pharmacol 58, 10791089.CrossRefGoogle ScholarPubMed
111Ibekwe, VC, Fadda, HM, McConnell, EL, et al. . (2008) Interplay between intestinal pH, transit time and feed status on the in vivo performance of pH responsive ileo-colonic release systems. Pharm Res 25, 18281835.CrossRefGoogle ScholarPubMed
Figure 0

Fig. 1 Effect of colipase presence and bile salt concentration on porcine pancreatic lipase activity. (a) Lipase activity over a pH range in the presence (■; 23·8 μg/ml) and absence (□) of colipase. (b) Lipase activity over a range of bile salt (sodium taurodeoxycholate; NaTDC) concentrations at pH 7. Values are means, with standard errors represented by vertical bars. Olive oil micelles were used as a substrate using procedures modified from Vogel & Zieve(97).

Figure 1

Table 1 Reported pH range in the human small intestine

Figure 2

Fig. 2 Putative cellular mechanisms involved with bicarbonate secretion from pancreatic duct, hepatic duct and small-intestinal epithelial cells. Intracellular accumulation of bicarbonate occurs through conversion of CO2 (which passively diffuses into the cell from the blood) by carbonic anhydrase and the action of the basolateral Na–HCO3 co-transporter (NBC). In the unstimulated cell, bicarbonate is removed from the cell by anion exchangers (AE) at the apical and basolateral membranes. Within the stimulated cell, the basolateral AE action is halted. The conductance of the apical chloride leak channel (CFTR) is raised, which results in higher localised chloride concentrations apically, thus driving increased bicarbonate release through the apical AE. During the latter stages of stimulation of bicarbonate release (i.e. when the apical and luminal concentration of bicarbonate is high), it is believed that the apical AE becomes inhibited. Bicarbonate efflux then occurs through the CFTR. Passive diffusion of bicarbonate from the blood to the lumen (left of figure) can only occur in the leaky epithelia of the intestine. Adapted from details in Allen & Flemström(54), Kanno et al.(98) and Steward et al.(99). Other membrane transporters indirectly involved in driving these processes, such as the basolateral Na+:K+:2Cl−  ATPase and K leak channels, are not included for clarity. NHE, Na+–H+ exchanger.