Proton N.M.R. study of the conformational dynamics of porcine pancreatic colipase. Titration of aromatic residues

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.

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.

Solution structure of porcine pancreatic procolipase as determined from 1H homonuclear two-dimensional and three-dimensional NMR

Procolipase is the precursor of colipase, which acts as protein cofactor for the activity of pancreatic lipase. The solution structure of procolipase has been determined by 1H NMR using two- and three-dimensional measurements. The secondary structure determination identified two separate three-stranded beta-sheet regions with concomitant hydrogen bond patterns. The tertiary structure of the protein was determined using 863 non-trivial proton--proton distance constraints, 14 hydrogen bond distance constraints and 55 phi and 25 X1 dihedral constraints. The structure that was obtained from distance geometry and energy refinement contains three highly disordered loops as well as a disordered N- and C-terminal region. The remaining part of the structure is well defined with a root-mean-square deviation (rmsd) relative to the average of 0.09 +/- 0.02 nm for backbone atoms (residues 11-30, 37-50, 57-69, 83-89). The protein comprises two identical domains, each containing a three-strand beta-sheet and two disulfide bonds: a 15-residue region in each domain superimposes with 0.07 nm rmsd, measured on backbone atoms. The solution structure is nearly identical to the crystal structure. It is in agreement with previous NMR data and, in combination with these data, supports the current model of procolipase micelle interaction and the lipase activation by colipase.

Monoclonal antibodies to human pancreatic procolipase: production and characterization by competitive binding studies

Hybridomas secreting monoclonal antibodies (MAbs) specific for human pancreatic colipase were established and 11 clones were selected by using a dot immunobinding assay. Characterization of the MAbs was carried out by using direct and competitive epitope mapping methods, including ELISA and inactivation of colipase-dependent pancreatic lipase. Monoclonal antibodies showed four distinct patterns of reactivity. Monoclonal antibody 5.30 (group I) inhibited colipase-dependent lipase activity. The dissociation constant of the inactive antibody-antigen complex was 10(-9) M. Monoclonal antibodies 48.30, 66.24, and 153.23 (group II) had no effect on activity although they bound competitively with MAb 5.30 to antigen as shown by their capacity to displace MAb 5.30 from the antibody-antigen complex and by ELISA additivity test. Dissociation constants calculated from the displacement curves were 0.9 10(-9) M, 0.6 10(-9) M, and 2 10(-9) M, respectively. Noninhibitory MAbs 13.29, 16.25, and 33.30 bound competitively with MAbs of group II but not with MAb 5.30 (group I). Monoclonal antibodies of group IV (MAbs 17.6, 18.1, 37.39, and 169.29) had no effect on activity and did not react with immobilized antigen. None of the MAbs reacted in ELISA with reduced and carboxymethylated human procolipase, indicating that epitopes involved conformationally dependent determinants on protein antigen. Anti-human colipase MAbs showed no cross-reactivity with porcine or equine procolipases. Monoclonal antibodies described here appear to be useful tools for studying surface hydrophobic domain of colipase and/or interaction between colipase and lipase in its active conformation (open lid).

Direct involvement of the C-terminal extremity of pancreatic lipase (403-449) in colipase binding

After a selective cleavage of a lipase/colipase cross-linked complex, the colipase has been shown to be bound to a 5 kDa lipase fragment identified as the C-terminal extremity of the chain extending from residue 403 to the C-terminus (Cys 449). The colipase binding site on lipase is therefore localized in a restricted contact area. Moreover, from sequence comparison of lipase from various species, an acidic residue, Glu 440, is likely to be involved in ion pairing with colipase.

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.

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.

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.

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.

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.

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.

Pancreatic colipase: crystallographic and biochemical aspects

A detailed study of the crystallization of hog and horse colipases has been undertaken. Several crystallographic varieties have been obtained and a 0.3-nm resolution structure determination is actually in progress. The sequence of the A form of horse colipase (one methionine) is given. From spectrophotometric experiments and sequence comparisons, the involvement of the aromatic residue in position 52 in the micelle binding site has been demonstrated.

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.

[Effect of the addition of hog pancreatic colipase on the permeability to glucose and the phase transition of phosphatidyl choline liposomes]

An interaction between porcine pancreatic coli-pase and lecithin liposomes is demonstrated by gel filtration assays. The extent of the colipase penetration into the phospholipid bilayer was assessed by permeability and calorimetry studies carried out on the liposome colipase complex. The addition of colipase to liposomes induces a three fold increase in the permeability to [6-H3] glucose. This result reflects a perturbation in the bilayer which may be the consequence of the colipase interaction. The phase transition temperature is not modified by the added colipase. This observation suggests that the perturbation brought by the protein does not affect the acyl chain packing of the bulk lipid. On the other hand the enthalpy of transition (delta H) is decreased from 8.9 to 7.8 kcal/mole by the addition of colipase to the lipid. This could be explained by the interaction of the colipase with neighbouring acyl chains which do not participate in the cooperative melting of the bulk lipid. In agreement with previous spectrophotometric observations, the present results are indicative of hydrophobic interactions between colipase and bilayer hydrocarbon chains.

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.

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.

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

The secondary structure of porcine colipase (93 residues) was established according to the predictive method of Chou and Fasman (Chou, P.Y. and Fasman, G.D. (1974) Biochemistry 13, 211--222 and 222--245). The relative composition of the conformational regions was as follows: 5% alpha-helix (region 39--44), 25% beta-sheet (three regions, 7--11, 49--57 and 77--85) and eight beta-turns corresponding to 32% of the polypeptide. Colipase contains a large proportion (about 35%) of unordered structure. Estimated values for the alpha-helix and beta-sheet contents from the circular dichroism spectrum were in good accordance with the predicted model. A less satisfactory value was found for the beta-turns. A characteristic feature of the far ultraviolet dichroic spectrum is the presence of an unusual positive band at 225 nm that might be indicative of a particular spatial arrangement of the chromophores in the molecule. Two tyrosines (Tyr56 and Tyr57) and one histidine (His86) are at close vicinity in the three dimensional structure of the protein as shown by proton NMR studies. These residues are located at the end of two beta-sheet hydrophobic regions(49--57 and 77--85) which might play a role in the association of colipase with the lipid-water interface as indicated by results of the NMR studies of the taurodeoxycholate-colipase complex.