Importance of the N-terminal sequence in porcine pancreatic colipase

Pancreatic lipase and its related proteins: where are we now?

Obesity is a disease of epidemic proportions, with a worrisome upward trend. The high consumption of lipids, a major energy source, leads to obesity because of their high calorific value. Pancreatic lipase (PTL), produced by pancreatic acinar cells, hydrolyzes 50-70% of triacylglycerol (TAG) from food. PTL-related protein 1 (PLRP1) and 2 (PLRP2) are also produced by these cells. In vertebrates, PLRP1 has relatively less lipolytic activity, whereas PLRP2 has an essential role in lipid digestion, especially in infants. In this review, we summarize the structure and function of PTL, PLRP1, and PLRP2, and the metabolic fate of PTL inhibitors. We also discuss the current status of clinical trials on orlistat and its combinations for obesity treatment.

Biochemical and structural comparative study between bird and mammal pancreatic colipases

Three colipases were purified from pancreas of two birds (ostrich and turkey) and one mammal (dromedary). After acidic and/or heat treatment and precipitation by sulfate ammonium and then ethanol, cofactors were purified by Sephadex G-50 gel filtration followed by ion-exchange chromatography first on Mono S and then on Mono Q. One molecular form was obtained from each species with a molecular mass of approximately 10 kDa. Cofactors were not glycosylated. The N-terminal sequences of the three purified cofactors showed high sequence homology. A 90 amino acid sequence of the ostrich cofactor was established based on peptide sequences from four different digests of the denaturated protein using trypsin, chymotrypsin, thermolysin, or staphylococcal protease. This sequence exhibited a high degree of homology with chicken and mammal cofactors. Bile salt-inhibited pancreatic lipases from five species were activated to variable extents by colipases from bird and mammal origins. The bird pancreatic lipase-colipase system appears to be functionally similar to homologous lipolytic systems from higher mammals. Our comparative study showed that mammal colipase presents a lower activation level toward bird lipases than the bird counterpart. Three-dimensional modeling of ostrich colipase suggested a structural explanation of this fact.

Purification and characterization of human procolipase expressed in yeast cells

We report the successful, efficient, and large-scale expression of recombinant human procolipase in yeast. Using the full-length cDNA of human procolipase, constructs were made using either the native human procolipase signal peptide sequence or the signal peptide sequence of yeast. These constructs were used to transform yeast cells, and expression was followed. Only minimal expression was seen with the procolipase using the native human signal peptide. Robust secretion of the procolipase occurred when the yeast signal peptide was exchanged for the native signal peptide. Expression yielded more than 30 mg/liter. The recombinant protein was purified from the medium by immunoaffinity chromatography. The highly purified procolipase was free of proteolytic degradation and displayed activity and binding characteristics that were indistinguishable from those of tissue-purified human pancreatic colipase. Expression in yeast cells provides a useful tool for expressing intact, unprocessed recombinant wild-type and mutated procolipase.

Structure and function of pancreatic lipase and colipase

Dietary fats are essential for life and good health. Efficient absorption of dietary fats is dependent on the action of pancreatic triglyceride lipase. In the last few years, large advances have been made in describing the structure and lipolytic mechanism of human pancreatic triglyceride lipase and of colipase, another pancreatic protein that interacts with pancreatic triglyceride lipase and that is required for lipase activity in the duodenum. This review discusses the advances made in protein structure and in understanding the relationships of structure to function of pancreatic triglyceride lipase and colipase.

Lipid binding and activating properties of porcine pancreatic colipase split at the Ile79-Thr80 bond

Porcine colipase, the protein cofactor of pancreatic lipase, was isolated from pancreas freshly collected on animals and from a side fraction from the production of insulin (Novo Nordisk A/S). Samples of purified colipase were analyzed for homogeneity by polyacrylamide gel electrophoresis, reverse-phase high-performance liquid chromatography (RPLC), quantitative N-terminal sequence determination and mass spectrometry. The activating properties of colipase preparations were assayed against tributyrin, triolein or the commercial Intralipid emulsion, in presence of bile salt. Two fractions of colipase with the same specific activity were purified from fresh pancreas. The major fraction (85%) contained one single protein corresponding to fragment 1-93 of the 95-residue form of colipase (procolipase) previously characterized in porcine pancreatic juice. The other fraction (15%) corresponded to fragment 1-91 of procolipase. Also, two fractions of colipase were purified from the side fraction supplied by Novo. These fractions consisted of the 95-residue proform of colipase and of fragment 1-93, respectively, both specifically cleaved at the Ile79-Thr80 peptide bond with partial removal of isoleucine at position 79 and serine at position 78. Procolipase split at the 79-80 bond retained full activity on tributyrin and triolein and on the Intralipid emulsion but the kinetics of hydrolysis of triacylglycerol substrates showed much longer lag periods than those observed with native procolipase. Also, all forms of procolipase split at the 79-80 bond showed one peak in RPLC but their retention time was markedly decreased as compared to that of native procolipase which indicated a weaker hydrophobic binding capacity. The value of the retention time was of the same order of magnitude as that of inactive reduced procolipase. Treatment of native procolipase by pancreatic endopeptidases showed that elastase is likely responsible for specific cleavage at the 79-80 bond of procolipase purified from the Novo extract. Limited proteolysis by trypsin of the proforms of colipase split at the 79-80 bond reduced the lag period. Results presented in this communication provide the first direct evidence showing that the finger-shaped peptide segment between half-cystine residues at positions 69 and 87 is involved in colipase-lipid interaction as previously hypothesized from the three-dimensional structure of the protein.

Crystallographic study of the structure of colipase and of the interaction with pancreatic lipase

Colipase (Mr 10 kDa) confers catalytic activity to pancreatic lipase under physiological conditions (high bile salt concentrations). Previously determined 3-A-resolution X-ray structures of lipase-colipase complexes have shown that, in the absence of substrate, colipase binds to the noncatalytic C-terminal domain of pancreatic lipase (van Tilbeurgh H, Sarda L, Verger R, Cambillau C, 1992, Nature 359:159-162; van Tilbeurgh et al., 1993a, Nature 362:814-820). Upon lipid binding, conformational changes at the active site of pancreatic lipase bring a surface loop (the lid) in contact with colipase, creating a second binding site for this cofactor. Covalent inhibition of the pancreatic lipase by a phosphonate inhibitor yields better diffracting crystals of the lipase-colipase complex. From the 2.4-A-resolution structure of this complex, we give an accurate description of the colipase. It confirms the previous proposed disulfide connections (van Tilbeurgh H, Sarda L, Verger R, Cambillau C, 1992, Nature 359:159-162; van Tilbeurgh et al., 1993a, Nature 362:814-820) that were in disagreement with the biochemical assignment (Chaillan C, Kerfelec B, Foglizzo E, Chapus C, 1992, Biochem Biophys Res Commun 184:206-211). Colipase lacks well-defined secondary structure elements. This small protein seems to be stabilized mainly by an extended network of five disulfide bridges that runs throughout the flatly shaped molecule, reticulating its four finger-like loops. The colipase surface can be divided into a rather hydrophilic part, interacting with lipase, and a more hydrophobic part, formed by the tips of the fingers. The interaction between colipase and the C-terminal domain of lipase is stabilized by eight hydrogen bonds and about 80 van der Waals contacts. Upon opening of the lid, three more hydrogen bonds and about 28 van der Waals contacts are added, explaining the higher apparent affinity in the presence of a lipid/water interface. The tips of the fingers are very mobile and constitute the lipid interaction surface. Two detergent molecules that interact with colipase were observed in the crystal, covering part of the hydrophobic surface.

Pancreatic triglyceride lipase and colipase: insights into dietary fat digestion

Dietary fats have an impact on health and disease. A pancreatic exocrine protein, pancreatic triglyceride lipase, is essential for the efficient digestion of dietary fats. This enzyme requires another pancreatic exocrine protein, colipase, for full activity in the gut lumen. In addition to its importance in fat digestion, pancreatic triglyceride lipase has potential applications in medical therapy, medical diagnostics, and industry. This potential stimulated interest in lipases; radiograph during the last few years, studies applying the technologies of molecular biology and radiograph crystallography greatly increased our knowledge about pancreatic triglyceride lipase and colipase protein structure, enzyme mechanism, and gene structure. This review focuses on these recent advances and discusses models for the kinetic properties of pancreatic triglyceride lipase and for the interaction of pancreatic triglyceride lipase with colipase.

Interfacial activation of the lipase-procolipase complex by mixed micelles revealed by X-ray crystallography

The three-dimensional structure of the lipase-procolipase complex, co-crystallized with mixed micelles of phosphatidylcholine and bile salt, has been determined at 3 A resolution by X-ray crystallography. The lid, a surface helix covering the catalytic triad of lipase, adopts a totally different conformation which allows phospholipid to bind to the enzyme's active site. The open lid is an essential component of the active site and interacts with procolipase. Together they form the lipid-water interface binding site. This reorganization of the lid structure provokes a second drastic conformational change in an active site loop, which in its turn creates the oxyanion hole (induced fit).

Conformational prediction studies on pancreatic colipase

Comparison of the primary structures of pancreatic colipases from man, pig, horse and rat shows a high degree of homology between proteins. Fifty-two out of the 95 residues of the polypeptide are identical. All colipases contain 10 half-cystines which are located at invariant positions. The secondary structure of colipases has been predicted from the sequence using the statistical method of Chou and Fasman and the method of Gibrat, Garnier and Robson based on information theory. Predictions indicate that colipases have a low content of alpha-helix and beta-strand structure. The two segments at positions 7-10 and 56-59, assumed to be part of the lipid binding domain, have predicted beta-sheet conformation and should be in close spatial vicinity to each other in the proteins. Four beta-turns are predicted in all colipases at positions 3-6, 46-49, 61-64, and 81-84. They might contribute, with the five disulfide bridges, to a tight packing of the protein molecule. Surface residues and major sequential antigenic determinants of mammalian colipases have been predicted using methods based either on hydrophilicity/hydropathy scales or amino acid mutability. From these studies, it appears that colipases exhibit large conformational homologies. In the absence of data on the tertiary structure of colipase, predictive methods, together with physico-chemical and immunological studies, provide valuable information on the conformation of the protein in relation to the topology of residues involved in the functional and antigenic sites.

Production and characterization of four monoclonal antibodies against porcine pancreatic colipase

Four monoclonal antibodies directed against porcine colipase have been generated by hybridization of myeloma cells with spleen cells of BALB/c immunized mice. Antibodies were screened by binding to immobilized colipase in a solid-phase assay. Monoclonal antibodies were purified by affinity chromatography on colipase coupled to Sepharose. All monoclonal antibodies are of the IgG1 class with high affinity for the antigen. The dissociation constant of the complex formed in solution between porcine colipase and antibody varied from 1.1 X 10(-10) M to 1.8 X 10(-8) M. Epitope specificity was studied for each antibody and in pairs with an enzyme-linked immunosorbent assay (ELISA). Results indicate that the four monoclonal antibodies react with at least three different antigenic regions of colipase. Finally, three monoclonal antibodies were found to be potent inhibitors of colipase activity. Antiporcine monoclonal antibodies appear to be suitable probes for studying the lipid affinity site of the protein cofactor of pancreatic lipase.

Inhibition of pancreatic colipase by antibodies and Fab fragments. Selective effects of two fractions of antibodies on the functional sites of the cofactor

Rabbit antiserum was raised against porcine pancreatic colipase and Fab fragments were prepared by papain digestion of purified antibodies followed by purification on protein A-Sepharose. Fab fragments showed inactivation toward porcine colipase activity similar to that of antiserum and purified antibodies. From inactivation studies carried out by incubating porcine colipase and lipase with Fab fragments in the absence of lipid or in the presence of triolein and sodium deoxycholate, it could be concluded that polyclonal antiporcine colipase antibodies contain fractions that bind specifically to epitopes at or near the functional regions of the porcine cofactor. Studies with an enzyme-linked immunosorbent assay showed that cross-reactivity of horse or chicken colipase with antiporcine colipase antiserum was lower than that of the human or porcine protein. Results of immunoactivation kinetic studies performed with the same proteins, fully confirmed these observations. Partial cross-reactivity between porcine and chicken colipases allowed us to fractionate antibodies by immunoaffinity chromatography on immobilized chicken colipase. Fraction I contains antibodies absorbed on porcine colipase not accessible when the cofactor is bound to lipid. Antibodies of fraction II, nonadsorbed on chicken colipase, inactivate porcine colipase preincubated with triolein/deoxycholate. Lipase had a protective effect against inactivation. Antibodies of fraction II bind likely to epitopes close to the specific region of colipase interacting with lipase. Our conclusions are in good agreement with analysis of the sequence of porcine, equine and human colipases by calculating local hydrophilicity indices.

The primary sequence of human pancreatic colipase

The amino acid sequence of an activated colipase purified from human pancreas was determined. The protein consists of a single polypeptide chain of 86 amino acids (human colipase86) and has a molecular weight of 9289. The sequence was determined by automated Edman degradation of the reduced and S-carboxymethylated protein and of two CNBr peptides. Sequence determination of porcine procolipase II was also performed, which showed that in the original sequence determination apparently two residues were missed. These residues were determined to be a leucine at position 37 and a serine in position 50. For comparison with porcine and equine procolipases, the residues composing human colipase are numbered from 6 to 91. No human procolipase has been isolated so far. The colipases from man, pig, horse and chicken show a high degree of homology: human colipase differs from the other proteins by substitutions of 19 (porcine), 24 (equine A) and 21 (equine B) residues, respectively.

Isolation and characterization of colipase from porcine and human pancreatic juice by immunoaffinity chromatography

Pure colipase was prepared by immunoaffinity chromatography from porcine and human pancreatic juice. A single form of the porcine colipase was obtained, having the structural and biological properties of previously characterized porcine procolipase A. Two forms of activated colipase (N-terminal Gly) were isolated from human pancreatic juice by the same procedure. The existence of two forms of activated colipase might arise from rapid activation of a precursor form of human colipase during collection of the pancreatic juice.

Studies on the immunological cross-reactivity of various pancreatic colipases. Isolation by immunoaffinity chromatography of a single form of procolipase from porcine pancreas

Antibodies against porcine procolipase B were produced in rabbits. The antiserum was used to immunoinactivate various forms of native and trypsin-treated porcine colipase. Our results indicate that all forms of the porcine cofactor bind to anti-porcine procolipase B antibodies. Human colipase showed lower affinity for the antibodies than porcine colipase. No cross-reactivity was observed between pig and horse, cow, dog or chicken colipases. Immunological studies on porcine colipase, carried out in the presence of lipid, provided evidence that antibodies bind to colipase at or near the lipase binding site. The binding of antibodies to colipase is not affected by the adsorption of the cofactor at a lipid interface. Using a predictive method for identification of the antigenic determinants, it was found that, in pig colipase, regions at positions 42-48 and 70-74 might represent antigenic sites. In the horse protein, the peptide segment 42-48 was also recognized as a possible antigenic site. An immunoadsorbent gel column was prepared for a one-step isolation of porcine colipase. In contrast to purification methods described so far, immunoaffinity chromatography yielded only one form of the porcine cofactor when starting from a pancreatic extract. This protein preparation has structural, biochemical and immunochemical properties similar to that of porcine procolipase A previously isolated from pancreas in the presence of detergent.