Pancreatic colipase: crystallographic and biochemical aspects

Lipases and esterases: a review of their sequences, structure and evolution

This chapter aims to provide a brief review on the enzyme family of lipases and esterases. The sequences, 3D structures and pH dependent electrostatic signatures are presented and analyzed. Since the family comprises more than 100 sequences, we have tried to focus on the most interesting features from our perspective, which translates into finding similarities and differences between members of this family, in particular in and around the active sites, and to identify residues that are partially or totally conserved. Such residues we believe are either important for maintaining the structural scaf-fold of the protein or to maintain activity or specificity. The structure function relationship for these proteins is therefore of central interest. Can we uniquely identify a protein from this large family of sequences--and if so, what is the identifier? The protein family displays some highly complex features: many of the proteins are interfacially activated, i.e. they need to be in physical contact with the aggregated substrate. Access to the active site is blocked with either a loop fragment or an alpha-helical fragment in the absence of interfacial contact. Although the number of known, relevant protein 3D structures is growing steadily, we are nevertheless faced with a virtual explosion in the number of known or deduced amino acid sequences. It is therefore unrealistic to expect that all protein sequences within the foreseeable future will have their 3D structure determined by X-ray diffractional analysis or through other methods. When feasible the gene and/or the amino acid sequences will be analyzed from an evolutionary perspective. As the 3D folds are often remarkably similar, both among the triglyceride lipases as well as among the esterases, the functional diversities (e.g. specificity) must originate in differences in surface residue utilization, in particular of charged residues. The pH variations in the isopotential surfaces of some of the most interesting lipases are presented and a qualitative interpretation proposed. Finally we illustrate that NMR has potential for becoming an important tool in the study of lipases, esterases and their kinetics.

Molecular cloning and expression of two horse pancreatic cDNA encoding colipase A and B

Pancreatic colipase plays an essential role in the intestinal fat digestion by anchoring lipase on lipid/water interfaces in the presence of bile salts. In contrast to other species, two molecular forms of colipase, A and B, have been found in horse. The two corresponding cDNAs were isolated from a horse pancreatic library and their nucleotide sequences were determined. Moreover, for the first time, active colipase has been obtained after transfection of COS cells by either colipase A or B cDNA.

Separation and characterization of the precursor and activated forms of porcine and human pancreatic colipase by reversed-phase liquid chromatography

Reversed-phase liquid chromatography was used as an alternative method for the characterization of the precursor and activated forms of porcine and human pancreatic colipase. Using a Beckman Ultrasphere column with an increasing acetonitrile gradient in 0.1% trifluoroacetic acid, it was possible to obtain well-resolved separation of the precursor form of colipase (procolipase) from its trypsin-activated derivative. This protocol was used (1) to study the activation of porcine procolipase by trypsin or thrombin in vitro, (2) to assess the homogeneity of porcine colipase preparations used in tridimensional structure studies and in combination with immunoaffinity chromatography, (3) to identify the form of colipase present in samples of human pancreatic juice.

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.

Antigen specificity of anticolipase monoclonal antibodies: characterization of colipase-Fab complexes using gel filtration high-performance liquid chromatography

The epitope specificity of eight mouse monoclonal antiporcine procolipase antibodies (MAbs) was characterized on the basis of their competitive binding with antigen. Binary and ternary Fab-colipase complexes formed between antibody and porcine procolipase or its trypsin activated derivative were identified using gel filtration HPLC. The eight MAbs were divided in two groups that recognized overlapping epitopes located in distinct antigenic regions on procolipase. The gel filtration HPLC technique allowed to characterize two MAbs which did not react with solid-phase coupled antigen. Three MAbs formed Fab-antigen complexes with procolipase and not with activated colipase which suggests that epitopes recognized by these MAbs involve residues of the N-terminal pentapeptide of procolipase.

Assignment of the human pancreatic colipase gene to chromosome 6p21.1 to pter

Pancreatic colipase is a 12-kDa polypeptide cofactor for pancreatic lipase (EC 3.1.1.3), an enzyme essential for the absorption of dietary long-chain triglyceride fatty acids. Colipase is thought to anchor lipase noncovalently to the surface of lipid micelles, counteracting the destabilizing influence of intestinal bile salts. Using primers derived from the known amino acid sequence, we have used the polymerase chain reaction to produce a cDNA clone corresponding to the complete coding region of the human procolipase mRNA. Southern blot analysis of genomic DNA from a panel of mouse-human somatic cell hybrids indicated that the colipase gene (CLPS) resides on human chromosome 6. Further analysis of somatic cell hybrids carrying chromosome 6 translocations permitted regional localization of CLPS to the 6p21.1-pter region.

Rat pancreatic colipase mRNA: nucleotide sequence of a cDNA clone and nutritional regulation by a lipidic diet

A cDNA clone encoding rat pancreatic colipase was isolated using as a probe a synthetic deoxyoligonucleotide corresponding to a highly conserved amino acid sequence region in colipases from other species. The cloned messenger codes for a protein of 95 amino acids plus a signal peptide of 17 amino acids. The structure of the full-length cDNA was also determined and the corresponding amino acid sequence showed a high degree of homology with those of other known colipases. Quantification of the homologous mRNA in the pancreas of animals fed a high-lipid diet was consistent with a specific though moderate induction of colipase messenger by the nutritional manipulation.

Inhibitory properties and antigenic specificity of monoclonal antibodies to pancreatic colipase

To understand the mechanism by which colipase acts as a protein cofactor for anchoring pancreatic lipase at triacylglycerol/water interface, we have used an immunochemical approach. Ten monoclonal antibodies (Mabs) against porcine pancreatic procolipase were produced. Purified immunoglobulins and Fab fragments were studied for their capacity to inhibit colipase-dependent lipase activity. These studies were carried out by using procolipase, the secretory form of the cofactor, and its trypsin-treated form obtained by removal of the amino terminal pentapeptide by trypsin. Reactivities of Mabs with both forms of the cofactor were also studied by immunoenzymatic methods. Mabs 6.1, 49.20. 75.8, 270.13 and 419.1 were found to inhibit lipolysis by preventing the binding of procolipase or trypsin-treated colipase to the lipid substrate. Mab 72.11 inhibited procolipase binding but had no effect on trypsin-treated colipase. Mab 72.11 reacted with procolipase in ELISA but showed no reactivity with trypsin-treated colipase. Finally, preincubation of Mab 72.11 with porcine procolipase prevented specific cleavage at the Arg5-Gly6 bond by trypsin. It could be concluded, that the five first residues of procolipase are structural elements of the antigenic determinant recognized by Mab 72.11. Results of ELISA additivity tests (cotitrations) further indicated that epitopes for Mabs 6.1, 72.11, 270.13 and 419.1 and for Mabs 49.20 and 75.8 are located in two distinct antigenic regions of the procolipase molecule. It appears then that the lipid binding domain of the pancreatic lipase protein cofactor comprises two regions. The first region corresponds to the amino terminal fragment of the protein. The second region is likely identical with the peptide segment at position 51-59 as previously hypothesized from NMR and spectrophotometric studies. Studies carried out on procolipase chemically modified at tyrosine residues provided evidence that epitopes for Mabs 49.20 and 75.8 are in or close to the region which contains tyrosines at positions 55 and 59, and that the two peptide regions essential for interfacial binding are spatially adjacent in the procolipase and the trypsin-treated form of the cofactor. General conclusions are in accordance with the location of antigenic regions of procolipase determined by predictive methods.

Minireview on pancreatic lipase and colipase

By hydrolyzing the dietary triacylglycerols, pancreatic lipase causes catalysis in heterogeneous medium. In vivo, lipase action cannot take place without colipase due to the presence of bile salts. The cofactor enables lipase anchoring to the water-lipid interface. The lipase-colipase system furnishes an excellent example of specific interactions (protein-protein and protein-lipid). The studies of lipase catalytic properties brought to light the importance of certain parameters related to the 'quality of the interface'. The structure-function relationship analyses revealed a certain number of functional amino acid residues in lipase and colipase involved either in the catalytic site of the enzyme or in the recognition sites (lipase-colipase and protein-interface). Comparisons of the sequences of lipases derived from different sources display interesting similarities in certain cases.

A comparison of dogfish and porcine pancreatic lipases

1. A 16-fold purification of lipase activity from the pancreas of the dogfish (Squalus acanthias) was obtained by preparative isoelectric focussing. 2. The dogfish enzyme has a higher pH optimum (8.5) and a broader spectrum of activity above and below its optimum than a commercial porcine lipase preparation (optimum, 7.5). 3. The temperature optimum of dogfish lipase was 35 degrees C and it displayed greater stability than the porcine lipase preparation towards higher temperatures in the absence of substrate. 4. Dogfish lipase was active against a broad range of triacylglycerols and a wax ester. It had greater relative activity against the latter than the porcine lipase but hydrolyzed low mol. wt fatty acids less readily. In general, the effect of temperature was less on the dogfish enzyme than the porcine one.

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.

Further results on lipase-colipase interactions studied by affinity chromatography

Affinity chromatography of lipase on a colipase-coupled gel was studied in the present paper. The elution volume of the associable lipase increased when the loaded amount decreased. A KD value of 1.9 X 10(-6) M at pH 6.2 was thus deduced. A minimum value of 1.5 X 10(-6) M was obtained at pH 5.1-5.3. Mixed micelles associated with coupled colipase, but no modifications of lipase-colipase interactions took place when mixed micelles were added to the elution buffer. DMMA-modified coupled colipase failed to interact with lipase, owing to the specific orientation of the modified cofactor in the gel.

Purification and characterization of pancreatic colipase from the dogfish (Squalus acanthius)

Pure colipase from dogfish (Squalus acanthius) was obtained from an extract of pancreatic gland. It has a high isoelectric point (10.2) and the molecular weight was calculated to be 9108-9383. The N-terminal sequence was shown to be Gly-Leu-Phe-Leu-Asn-Leu-Ser-Ala-Gly-Glu-Leu-Cys-Val-Gly-Ser-Phe-Gln -Cys-Lys-Ser-Ser-Cys-Cys-Gln-Arg-Glu-Thr-Gly-Leu-Ser-Leu-Ala -Arg-Cys-Ala-. This sequence shows great homology with colipases from man, horse, pig and hen. There were indications of the existence of a proform of dogfish colipase. The propeptide was found to be Ala-Pro-Glu-Arg.

Evidence for the existence of procolipase in chicken pancreas and pancreatic juice

Purified antibodies raised against chicken colipase were coupled to Sepharose 4B and colipase was isolated in a single step by immunoaffinity chromatography from an extract of chicken pancreas prepared under conditions where trypsin activation is avoided. The purified protein has a single amino terminal residue of alanine and its biochemical properties are similar to those of the precursor form of colipase (procolipase) previously isolated from porcine and equine pancreas or pancreatic juice. Further evidence for the existence of procolipase was obtained from kinetic studies of the hydrolysis of the Intralipid emulsion by untreated and trypsin-treated chicken pancreatic juice.

One-step purification of procolipase from human pancreatic juice by immobilized antibodies against human colipase86

Purified antibodies to human colipase86 were coupled to CNBr-activated Sepharose 4B. The immunoadsorption column thus obtained was used to purify procolipase from human pancreatic juice in one step by immunoaffinity chromatography. A single form of procolipase was obtained, having similar biological properties as previously characterized procolipases from horse and pig. The sequence of the N-terminal propeptide was determined to be Ala-Pro-Gly-Pro-Arg. In bovine, equine and porcine procolipases the corresponding N-terminal sequence is Val-Pro-Asp-Pro-Arg.

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.

Binding between immobilized anti-colipase purified antibodies and colipase. Radioimmunoassay of colipase from pig plasma and pancreatic juice

Procedures for purification of porcine colipase II (Gly6-Gly89) and for obtaining purified anti-colipase antibodies are described. The interactions between antibodies immobilized on an Ultrogel AcA 22 column and colipase were investigated and colipase radioimmunoassay carried out. The immobilized antibody-colipase binding was preserved in the presence of mixed micelles, lipase, or both when added to the elution mixture. Bound colipase maintained its capability of interacting with mixed micelles, but not with lipase in either the presence or the absence of mixed micelles. It could be inferred that the antigenic site(s) is independent of the interfacial recognition site and close to the site of lipase recognition. Results are reported suggesting that one or both colipase histidyl residue-containing sequences are involved as antigenic determinant(s). Immunoreactive colipase, bound to a macromolecular protein complex, was found in the plasma of pig. This finding could be explained by an endocrine 'leakage' of colipase from the exocrine pancreatic cell rather than by passage through the intestinal mucosa.