Conformation of colipase. Prediction of the secondary structure, circular dichroism and 360 MHz proton NMR studies of porcine colipase A

Identification of amino acids in human colipase that mediate adsorption to lipid emulsions and mixed micelles

The adsorption of colipase is essential for pancreatic triglyceride lipase activity and efficient dietary fat digestion. Yet, little is known about which specific amino acids in the hydrophobic surface of colipase influence adsorption. In this study, we systematically substituted alanine or tryptophan at residues implicated in adsorption of colipase to an interface. We expressed, purified recombinant colipase mutants and characterized the ability of each alanine mutant to restore activity to lipase in the presence of bile salts. The functions of L16A, Y55A, I79A and F84A colipase were most impaired with activities ranging from 20 to 60% of wild-type colipase. We next characterized the fluorescence properties of the tryptophan mutants in the absence and presence of bile-salt-oleic acid mixed micelles. We performed steady-state emission spectra to determine peak shift and I330/I350 ratio and acrylamide quenching curves to characterize the environment of the residues. The analysis supports a model of adsorption that includes residues Leu 34 and Leu 36 on the 2nd loop, Tyr 55 and Tyr 59 on the 3rd loop and Ile 75 and Ile 79 on the 4th loop. The analysis confirms that Phe 84 is not part of the adsorption surface and likely stabilizes the conformation of colipase. Contrary to the predictions of computer modeling, the results provide strong support for an essential role of Tyr 55 in colipase adsorption to mixed micelles. The results indicate that the adsorption of colipase to mixed micelles is mediated by specific residues residing in a defined surface of colipase.

Colipase stabilizes the lid domain of pancreatic triglyceride lipase

Pancreatic lipase is characterized by increased activity against water-insoluble substrates and by dependence on another protein, colipase, for binding to the substrate interface. In most models of pancreatic lipase activity, colipase functions to anchor lipase on the substrate interface. Recent studies of the x-ray crystal structure of the complex between colipase and lipase suggest another function for colipase in maintaining the active conformation of lipase. We tested this hypothesis by introducing mutations into colipase at position 15, a residue that contacts the lid domain lipase in the open conformation. Multiple mutant colipases were expressed and shown to have decreased activity. To further investigate the function of the interaction between Glu15 of colipase and lipase, we examined one mutant, E15R, in detail. This mutant had 175-fold less activity compared with wild-type colipase. Although E15R had decreased activity, it was as effective as wild-type lipase in anchoring lipase to mixed emulsions of bile salt and tributyrin. The importance of the interaction with the lid domain was tested by determining the activity of E15R with lid deletion mutants of lipase. E15R was as active as wild-type colipase with these mutant lipases. These results indicate that Glu15 is critical for activity of the colipase-lipase complex at an interface and that colipase has a function in lipolysis in addition to anchoring lipase to an interface. We propose that this function is to stabilize the lid domain of lipase in the open conformation, thereby facilitating lipolysis.

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.

Construction and expression of synthetic wild-type and mutant genes encoding porcine pancreatic colipase: tryptophan fluorescence studies

Based on the known (95-residue) amino acid (aa) sequence of porcine pancreatic colipase (CLP), a cofactor of pancreatic lipase, a 297 bp gene was designed and assembled from eight synthetic, overlapping DNA fragments. Optimized for expression in bacteria, the CLP-encoding gene (CLP) was inserted into the lacZ gene fragment contained in the small expression vector, pUC8, and cloned in Escherichia coli JM109. Expression of this construct yielded a protein approx. 11 kDa in size, equivalent to CLP, with an Mr of 10,336, plus ten additional amino acids at the N-terminus. The recombinant CLP (reCLP) was solubilized from bacterial inclusion bodies and then purified and refolded. A mutant CLP gene, changing Tyr-55 to Trp, was then constructed by site-directed mutagenesis. Since porcine CLP contains no Trp, this strategy provided a protein with an internal fluorescent probe for biophysical studies. The presence of Trp in the mutant protein was confirmed using fluorescence spectroscopy. Both wild-type (wt) and mutant reCLP reacted on Western blots with an affinity-purified rabbit anti-CLP antibody, raised against native CLP. The Tyr-55 to Trp exchange did not affect the activity of reCLP. Fluorescence studies of the interaction between reCLP and the bile salt, taurodeoxycholate (TDOC), showed that Trp-55 in the hydrophobic binding site of mutant reCLP inserted into the interior of the bile salt micelle.

Synthesis and characterization of the dansyltyrosine derivatives of porcine pancreatic colipase

Steady-state and time-resolved fluorescence techniques were used to study dansyltyrosine derivatives of porcine pancreatic colipase. Nitration, reduction, acylation, and dansylation reactions were utilized to synthesize two fluorescently labeled colipases: (o-aminodansyltyrosine 55 porcine colipase) (DNStyr55PC) and o-aminodansyltyrosine 59 porcine colipase (DNStyr59PC). DNStyr55PC was 200% active, while the DNStyr59 derivative maintained 80% activity in a pH stat assay. Emission spectra, lifetime analysis, acrylamide quenching, polarization, and anisotropy decay studies indicated that Tyr55 was located on the solvent-exposed surface of the protein, where the fluorophore experienced free rotation. Identical experiments done on DNStyr59PC indicated that Tyr59 was in a partially buried environment and the motion of the dansyl tyrosine group was hindered. The double-exponential decay of the fluorescence emission of N-acetyl-o-aminodansyltyrosine ethyl ester (DNStyr) and the DNStyr derivatives of colipase was investigated with pH, temperature, solvent, and emission-resolved-lifetime experiments. The existence of excited-state processes was eliminated in both pH and emission-resolved-lifetime experiments, whereas temperature studies indicated either a rotational isomer or a differential solvent quenching mechanism for multiple decay kinetics. These experiments also showed that DNStyr was a sensitive probe of solvent polarity and viscosity, but not of pH.

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.

The role of aromatic side chain residues in micelle binding by pancreatic colipase. Fluorescence studies of the porcine and equine proteins

Fluorescence techniques have been employed to study the interaction of porcine and equine colipase with pure taurodeoxycholate and mixed micelles. Nitrotyrosine-55 of porcine colipase is obtained by modification with tetranitromethane (low excess, in the presence of taurodeoxycholate) of the protein followed by gel filtration and ion-exchange chromatography. Verification of the residue modified was obtained by h.p.l.c. peptide purification and sequence analysis. Reduction and quantitative reaction with dansyl chloride yields a fluorescent derivative that is twice as active in conjunction with lipase as is native colipase and that exhibits a strong emission band at 550 nm. Addition of micellar concentrations of taurodeoxycholate causes a 4.3-fold increase in the emission maximum as well as a 70 nm blue shift to 480 nm. Inclusion of oleic acid to form a mixed micelle reduces these spectral effects. Scatchard analysis of the data yield a Kd of 6.8 X 10(-4) M and a single colipase-binding site for taurodeoxycholate micelles. The data, by analogy to a phospholipase system, are consistent with a direct insertion of dansyl-NH-tyrosine-55 into the micelle. The presence of a single tryptophan residue (Trp-52) in equine colipase provides an intrinsic fluorescent probe for studying protein-micelle interaction. The emission maximum of horse colipase at 345 nm indicates a solvent-accessible tryptophan residue which becomes less so on binding of micelles. A blue shift of 8 nm and a 2-fold increase in amplitude is indicative of a more hydrophobic environment for tryptophan induced by taurodeoxycholate micelles. There is also a decrease in KSV for acrylamide quenching in the presence of micelles, which further supports a loss of solvent accessibility. The most dramatic pH effects are observed with KI quenching, and may indicate the presence of negative charges near Trp-52.

Spectrofluorimetric study of the bile salt micelle binding site of pig and horse colipases

Pig and horse colipases contain three tyrosine residues. In addition, horse colipase possesses a tryptophan residue. Some of the tyrosine residues are involved in the association of colipase and a bile salt micelle. The present report demonstrates that the aromatic residues responsible for colipase fluorescence are in an aqueous environment. In the presence of bile salt micelles, changes in colipase fluorescence properties indicate that the intrinsic fluorophores are located in a more hydrophobic environment upon colipase-micelle complex formation. In addition, the fluorescence of an NBD group fixed on lysine 60, which is very close to the aromatic region in the pig colipase, is also altered in the presence of micelles. These results show that the micelle binding site is not limited to the tyrosine residues but may be broadened to adjacent residues such as lysine 60 and also tryptophan 52 in horse colipase.

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.

Nitration of the tyrosine residues of porcine pancreatic colipase with tetranitromethane, and properties of the nitrated derivatives

The nitration of the long form (N-terminal valine) of porcine pancreatic colipase with tetranitromethane was investigated under a variety of conditions. Fractionation of the nitrated monomers on DE-cellulose led to well-defined derivatives containing one, two and three nitrotyrosines per mol. Automated Edman degradation of the nitrated peptides, especially that of the staphylococcal proteinase peptide (49-64) showed that Tyr-54 was nitrated very fast under all conditions. This residue was the only one to be nitrated in water. Partial nitration of Tyr-59 was induced by bile salt micelles, while both Tyr-59 and Tyr-58 reacted extensively in the presence of lysophosphatidylcholine micelles (in which tetranitromethane is concentrated 150-fold compared to water) or of a liquid tetranitromethane-water interface. The strong negative Cotton effect at 410 nm which has already been observed using unfractionated preparations of nitrated colipase (Behnke W.D. (1982) Biochim. Biophys. Acta 708, 118-123) is linked with the nitration of Tyr-59 and it is markedly reduced by taurodeoxycholate micelles, suggesting a conformational change induced by the micelles in the tyrosine region. Moreover, the pKa of the nitrotyrosine residues in nitrated colipase is the same as that of free nitrotyrosine (pKa = 6.8) and it is shifted to 7.6 in the presence of taurodeoxycholate micelles. Micelles protected colipase against polymerization during nitration. These data suggest that Tyr-58 and Tyr-59 are part of the interface recognition site of colipase. The participation of Tyr-55 in binding is not excluded. The upwards nitrotyrosine pKa shift in the colipase micelle complex may explain why nitrated colipase can reactivate lipase in a triacylglycerol-taurodeoxycholate system at pH 7.5.

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.

The reaction of porcine colipase a with tetranitromethane. Generation of extrinsic cotton effects in the visible region

Porcine pancreatic colipase was reacted with a large excess of tetranitromethane in a two-phase system. Modification of tyrosine to produce nitrotyrosine generates extrinsic Cotton effects near 410 nm in the visible region which are not affected by submicellar concentrations of taurodeoxycholate. On the other hand, supramicellar concentrations specifically altered the spectrum near 350 nm. The total reaction products included both inter- and intramolecular cross-links and the initial mixture can be separated into monomers and polymers (dimers). Despite the complicated reactions that have taken place, the monomer has maintained full activity in a tributyrin-taurodeoxycholate assay system and the evidence supports a more indirect role for tyrosine in colipase interface reactions.

Porcine pancreatic lipase. Completion of the primary structure

The complete primary structure of a lipase (triacylglycerol hydrolase; EC 3.1.1.3) is presented for the first time. The porcine pancreatic enzyme which was investigated is composed of a single chain of 449 amino acids. Upon fragmentation by CNBr, five peptides were obtained. The sequence of four of them (CN I-CN IV) has already been published. The present report deals with the arrangement of the 142 amino acids of the C-terminal peptide CN V, thus completing the analysis of the whole molecule. Special problems resulting from incomplete cleavage of some peptide bonds in CN V and aggregation of large peptides were overcome using Sephadex filtration of succinylated derivatives in 50% acetic acid, automated sequence analysis of peptide mixtures and subdigestion of material which could not be directly resolved. No obvious homology was found when the sequence of porcine lipase was compared with other protein, including pancreatic phospholipase A2 and colipase from the same species. However, a few similarities which might be significant were detected between the environment and relative position of certain half cystines in lipase and colipase, as well as between two tyrosine-rich regions existing in both proteins.

Limited trypsinolysis of porcine and equine colipases. Spectroscopic and kinetic studies

Porcine and equine colipases have been submitted to mild tryptic digestion. Proteolysis occurs at the Arg5-Gly6 bond with the loss of the N-terminal pentapeptide. Studies of native and trypsin-treated colipases by circular dichroism and laser chemically induced dynamic nuclear polarization indicate that proteolysis induces conformational changes in the region of the tyrosine cluster. Experiments in the presence of phospholipid provide further evidence showing that these residues are in or close to the region of the protein interacting with aggregated lipids. Kinetic studies of the reaction of bile salt-inhibited lipase with emulsified triolein in the absence and in the presence of lecithin show that tryptic hydrolysis of the protein cofactor increases its affinity for the enzyme in the presence of lipid substrate. In both cases, it was found that the apparent dissociation constant of the lipase-colipase complex is decreased by one order of magnitude. Our results confirm that the biological activity of the lipase cofactor is enhanced by specific tryptic cleavage in the amino terminal region of the polypeptide and support the suggestion by Borgström et al. (Borgström, B., Wieloch, T., Erlanson-Albertsson (1981) FEBS. Lett. 108, 407-410) that the secreted form of colipase is a precursor.

Importance of the N-terminal sequence in porcine pancreatic colipase

Colipase exists in pancreatic juice in a pro-form which is activated by limited trypsin hydrolysis. During this activation, the N-terminal pentapeptide 1Val-Pro-Asp-Pro-5Arg is cleaved. The new N-terminal sequence formed, 6Gly-Ile-Ile-Ile-10Asn, contains three isoleucine residues. The importance of these for stimulating lipase activity has been investigated by successive Edman degradation of epsilon-acetimidolysine residues followed by limited trypsin hydrolysis. The epsilon-amidinated colipase obtained was fully active both with a phospholipid-covered triacylglycerol (Intralipid) and tributyrin as substrate. After removal of the three isoleucine residues, the activity of colipase was lost with Intralipid but not with tributyrin as substrate. The shortened colipases regained their Intralipid activity upon addition of long-chain fatty acids. The binding of colipase to lipase was not affected by removal of the isoleucine residues.

Structure and conformation of human pancreatic carboxyl-ester hydrolase

Human pancreatic carboxyl-ester hydrolase is a glycoprotein with a molecular weight of 100 000 and a high content in carbohydrate, 20%. Sedimentation studies indicate that the molecule resembles an ellipsoid. The results of hydrodynamic and vacuum-ultraviolet circular dichroism investigation allow one to propose a model of the carboxyl-ester hydrolase three-dimensional structure. The enzyme belongs to the "all beta" class of proteins; about 54-60% of its residues are in beta-sheets and in beta-turns, most probably forming the surface of an ellipsoid. A similarity with prealbumin structure is proposed and a comparison with structure of other pancreatic enzymes is presented.

Amino acid sequence of horse colipase B

The complete sequence of the 96 residues composing horse colipase B has been determined by automated analysis of the intact protein, of two CNBr peptides and two tryptic peptides arising, respectively, from the citraconylated chain and from the unreduced protein. The single histidine of the protein is located at position 29 as in horse colipase A. His86, present in the C-terminal region of the pig cofactor and supposed to play a role in the folding molecule, is not conserved in horse B. Large pieces of the pig and horse B chains were found to be identical or very similar, especially the N-terminal sequence and the central segment Ala49-Cys65 including the three tyrosines of the molecule. The four lysines and the ten half cystines are also conserved.

Stabilization of the C-terminal part of pig and horse colipase by carboxypeptidase and trypsin inhibitors

Pig and horse colipases have been purified by a common procedure using trypsin and carboxypeptidase inhibitors as stabilizers. Two forms of pig colipase were identified: a predominant A1 form with about 103-105 residues, and a minor slightly degraded A2 form in which the last two C-terminal residues, Asp and Ser, were lacking. This type of degradation is considerably slowed down by carboxypeptidase inhibitors. A total of four forms of the horse cofactor were characterized: two (A1 and B1) were probably isocolipases which differed by only a few substitutions. Both contained the same number of residues (about 96), an N-terminal valine and an Arg-Ser-Glu-(Glx)1,2-ArgC-terminal sequence. A2 and B2 were slightly degraded forms probably resulting from tryptic cleavage of the Arg-Ser bond in the above sequence. The presence of methionine in the horse cofactor allowed fragmentation by cyanogen bromide. The C-terminal fragment was composed of 16 or 17 residues and contained no histidine. The single histidine of horse B1 was found in the intermediary fragment between Met-18 and Met-(n-17). These data show that the C-terminal parts of both pig and horse colipases are still more exposed to proteolytic degradations than the N-terminal parts. Preliminary attempts to crystallize B1 were carried out.

Horse pancreatic lipase. Interaction with colipase from various species

Horse pancreatic lipase has been purified from tissue homogenates. Molecular and catalytic properties of horse lipase are comparable to those of the pancreatic lipases previously isolated. Kinetic studies of the inhibition of horse lipase activity by bile salts and of reactivation by pure colipase from three species (horse, ox and pig) allowed to calculate the apparent dissociation constant (Kd) of the lipase-colipase complex in the presence of the substrate (triolein). Identical values of Kd were found in all three cases (Kd = 1.1 10(-9) M). These values are lower by several orders of magnitude than that published for the binding between lipase and colipase in the absence of substrate. Qualitative experiments show that the activation of horse lipase can be accomplished by rat, dog and chicken colipase as well. The interaction between lipase and colipase is enhanced when the complex is adsorbed at the lipid-water interface. This specific protein-protein interaction is preserved in heterologous mixtures using colipases from other animal species.

Isolation and partial structural characterization of chicken pancreatic colipase

Colipase has been isolated from acidic extracts of chicken pancreatic tissue homogenized with Triton X-100. The cofactor fully activates bile salt inhibited mammalian lipases. The amino terminal sequence of the avian protein has been determined up to position 39 and compared to the homologous region of the mammalian colipases (pig, horse, man) previously studied. From this comparison, it appears that a high degree of homology exists between the proteins.

Enhanced activity of brain lipase in the presence of adrenocorticotrophic hormone. Comparison with a colipase effect

The characteristics of the pH-dependent stimulatory influence of adrenocorticotrophic hormone (ACTH) on lipolysis (Arnaud, J., Nobili, O. and Boyer, J. (1980) Biochim, Biophys. Acta 617, 524-528) were investigated further. ACTH enhanced 7-30 times the rates of hydrolysis of emulsified trioleoylglycerol by a rat brain lipase, when added to the medium both before and after the enzyme. When lipase activity was inhibited by sodium taurocholate, ACTH fully reversed the inhibition at bile salt concentration up to 2 mM. The reactivation process followed a sharp S-shaped pattern, leveling off at about 10(-4) M ACTH. With and without bile salt, the stimulatory effect of ACTH culminated at pH 5.75, and was dependent on the presence of trace amount of a water-insoluble solvent in the substrate emulsion. Taken together, the results suggest that ACTH acts at the lipid-water interface in facilitating the enzyme-substrate interaction. The relevance of the hormonal influence to a colipase-like effect is discussed.