The effect of pancreatic procolipase and colipase on pancreatic lipase activation

Intestinal fat digestion is carried out by the concerted action of pancreatic lipase and its protein cofactor colipase. Colipase is secreted from pancreas as a procolipase and is transformed into colipase by the trypsin cleavage of the Arg5-Gly6 bond during liberation of an N-terminal pentapeptide. The kinetic parameters for the lipase-colipase system compared to the lipase-procolipase system has been compared using trioctanoin and Intralipid as substrates. It was found that at pH 7.0 the Kmapp using Intralipid as substrate was the same for procolipase and colipase, 0.06 mM and 0.05 mM, respectively. At pH 8.0, however, the Kmapp were different-0.23 mM for procolipase and 0.08 mM for colipase. In a similar way the binding between colipase and lipase had a dissociation constant of 2.4 x 10(-6) M at pH 7.0, while for procolipase--lipase binding the dissociation constant was 4.1 x 10(-6) M with no significant difference. At pH 8.0 the binding between colipase and lipase was stronger, Kd being 2.0 x 10(-7) M, while weaker for procolipase and lipase, Kd being 1.0 x 10(-5) M. It is concluded that at the physiological pH value as is found in the intestine, the activation of procolipase to colipase has no influence on the hydrolysis of trioctanoin or Intralipid in the presence of bile salt.     

A cross-linked complex between horse pancreatic lipase and colipase

The water soluble carbodiimide N-cyclohexyl-N'-2-morpholinoethyl-carbodiimide-methyl-p-toluolsulfona te was found to effectively covalently cross-link pancreatic colipase to lipase as evidenced by Western blotting experiments using antibodies directed either against lipase or colipase. Moreover the resulting covalent complex has a Mr consistent with a stoichiometry of 1 mol colipase per mol lipase. Cross-linked lipase and colipase retain their activity implying a correct covalent binding between the two proteins. The specificity of the lipase-colipase binding was further supported by the very low amount of cross-linked products when lipase or colipase alone were incubated in the presence of carbodiimide. The formation of a covalent lipase-colipase complex in the presence of carbodiimide clearly demonstrates that the binding between both proteins involves ion pairing. Furthermore, the formation of an active covalent complex strongly suggests that the lipase-colipase binding site is distinct from the colipase interfacial recognition site as well as from the lipase catalytic site.     

Binding of bile salts to pancreatic colipase and lipase.

The binding of conjugated bile salts to pancreatic colipase and lipase has been studied by equilibrium dialysis and gel filtration. The results indicate that at physiological ionic strength and pH, conjugated bile salts bind as micelles to colipase: 12-15 moles/mole of colipase for the dihydroxy conjugates and 2-4 for the trihydroxy conjugates. No binding of bile salt takes place from monomeric solutions. Under the same experimental conditions, only 1-2 moles of conjugated dihydroxy bile salts bind to pancreatic lipase.     

Development of pancreatic enzymes in fetal and suckling rats with emphasis on lipase and colipase

Pancreatic amylase, chymotrypsinogen, lipase and colipase were assayed, at intervals, in rats from day 16 of fetal life until weaning. In the fetus, amylase and chymotrypsinogen accumulated regularly, in parallel, until birth. Lipase and colipase accumulation slowed down between day 20 and birth. The ratio of colipase to lipase was extremely high (9.5) and decreased until weaning towards adult values. Enzyme contents of the pancreas were depleted after birth and remained low until day 14. Intestinal concentrations were equally low, showing that pancreatic depletion was not due to hypersecretion. Protein synthesis was very active, intermediate between that of the fetus and of the adult. It is concluded that in the early suckling phase the proteins synthesized are mainly constitutive and not enzymatic. Starvation followed by refeeding showed that secretion sensitivity to nutritional stimulation only appears at 14 days. During the suckling period amylase concentrations decreased, evidencing a degree of nutritional sensitivity to the low level of carbohydrate in the diet. The productive capacity for lipase underwent a slow maturation which was not even complete at weaning, since concentrations had not yet reached adult level despite the high fat content of milk. This was in part compensated for by the high proportion of colipase but shows that lipase was not adaptative during this phase and that pancreatic lipase can hardly account for lipid digestion before weaning.     

Porcine pancreatic lipase related protein 2 has high triglyceride lipase activity in the absence of colipase

Efficient dietary fat digestion is essential for newborns who consume more dietary fat per body weight than at any other time of life. In many mammalian newborns, pancreatic lipase related protein 2 (PLRP2) is the predominant duodenal lipase. Pigs may be an exception since PLRP2 expression has been documented in the intestine but not in the pancreas. Because of the differences in tissue-specific expression, we hypothesized that the kinetic properties of porcine PLRP2 would differ from those of other mammals. To characterize its properties, recombinant porcine PLRP2 was expressed in HEK293T cells and purified to homogeneity. Porcine PLRP2 had activity against tributyrin, trioctanoin and triolein. The activity was not inhibited by bile salts and colipase, which is required for the activity of pancreatic triglyceride lipase (PTL), minimally stimulated PLRP2 activity. Similar to PLRP2 from other species, PLRP2 from pigs had activity against galactolipids and phospholipids. Importantly, porcine PLRP2 hydrolyzed a variety of dietary substrates including pasteurized human mother's milk and infant formula and its activity was comparable to that of PTL. In conclusion, porcine PLRP2 has broad substrate specificity and has high triglyceride lipase activity even in the absence of colipase. The data suggest that porcine PLRP2 would be a suitable lipase for inclusion in recombinant preparations for pancreatic enzyme replacement therapy.     

Uncoupling of catalysis and colipase binding in pancreatic lipase by limited proteolysis.

In the intestine, the hydrolysis of triglycerides by pancreatic lipase is performed only in the presence of colipase, whose function is to anchor lipase to the bile-salt-coated lipid interface. Biochemical and crystallographic data on porcine and human lipases have shown that the molecule is made of two well-delimited domains. In order to get more information on the role of the domains in catalysis and colipase binding, we performed limited proteolysis on lipase from various species and obtained different patterns of cleavage. In the case of porcine and human lipases, only the C-terminal domain (12 kDa) could be obtained after chymotryptic attack, whereas in the horse enzyme the cleavage of the Leu410-Thr411 bond gave rise to a large N-terminal (45 kDa) and a small C-terminal (4 kDa) fragment. The isolated porcine and human C-terminal domains were completely inactive towards emulsified tributyrin, though were able to bind colipase. Conversely, the horse 45 kDa fragment retained the lipase activity but failed to correctly bind colipase. This work definitely proves that catalysis and colipase binding are separate events involving topographically distinct regions of the molecule and focuses attention on the role of the C-terminal domain in colipase binding.     

Human pancreatic lipase: colipase dependence and interfacial binding of lid domain mutants

Five key amino acid residues from human pancreatic lipase (HPL) are mutated in some pancreatic lipase-related proteins 2 (PLRP2) that are not reactivated by colipase in the presence of bile salts. One of these residues (Y403) is involved in a direct interaction between the HPL C-terminal domain and colipase. The other four residues (R256, D257, Y267, and K268) are involved in the interactions stabilizing the open conformation of the lid domain, which also interacts with colipase. Here we produced and characterized three HPL mutants: HPL Y403N, an HPL four-site mutant (R256G, D257G, Y267F, and K268E), and an HPL five-site mutant (R256G, D257G, Y267F, K268E, and Y403N), in which the HPL amino acids were replaced by those present in human PLRP2. Colipase reactivated both the HPL Y403N mutant and HPL, and Y403 is therefore not essential for lipase-colipase interactions. Both the HPL four-site and five-site mutants showed low activity on trioctanoin, were inhibited by bile salts (sodium taurodeoxycholate, NaTDC) and were not reactivated by colipase. The interfacial binding of the HPL four-site mutant to a trioctanoin emulsion was suppressed in the presence of 4 mM NaTDC and was not restored by addition of colipase. Protein blotting/protein overlay immunoassay revealed that the HPL four-site mutant-colipase interactions are not abolished, and therefore, the absence of reactivation of the HPL four-site mutant is probably due to a lid domain conformation that prevents the interfacial binding of the lipase-colipase complex. The effects of colipase were also studied with HPL(-lid), an HPL mutant showing an 18-residue deletion within the lid domain, which therefore has only one colipase interaction site. HPL(-lid) showed a low activity on trioctanoin, was inhibited by bile salts, and recovered its lipase activity in the presence of colipase. Reactivation of HPL(-lid) by colipase was associated with a strong interfacial binding of the mutant to a trioctanoin emulsion. The lid domain is therefore not essential for either the interfacial binding of HPL or the lipase-colipase interactions.     

Evolutionary studies on pancreatic colipase

In this evolutionary study the following criteria have been used to prove the existence of colipase: 1. It restores the activity of human and porcine pancreatic lipase inhibited by bile salt. 2. It cross-reacts with antisera to human and porcine colipases. 3. Its restoration of lipase activity, inhibited by bile salt, in the tributyrin assay system, is prevented by antiserum to colipase. 4. It is a heat-stable, low-molecular-weight protein (molecular weight about 10 000 by gel-filtration). The occurrence of colipase has been verified in the exocrine pancreatic cells from hagfish (Myxine glutinosa), ratfish (Chimaera monstrosa), rayfish (Raja radiata), Greenland shark (Somnius microcephalus) and dogfish (Squalus acanthius). No colipase activity could be found in the gastric juice of crayfish (Pacifastacus leniusculus). These results indicate that colipase envolved in the vertebrates before the organized exocrine pancreatic gland and occurred simultaneously with the bile salts/bile alcohols.     

Studies on the effect of bile salt and colipase on enzymatic lipolysis. Improved method for the determination of pancreatic lipase and colipase.

The rate of hydrolysis of long chain triglycerides by pure bovine pancreatic lipase has been determined in the presence of variable amounts of bile salts and colipase. Cofactor-free lipase is strongly inhibited by sodium taurodesoxycholate and by mixed bovine bile salts at concentrations higher than the critical micellar concentration. Bile salt inhibited lipase is reactivated by the addition of bovine colipase. Gel filtration of pancreatic juice from several species (Cow, dog, pig) on Sephadex G 100 allows the separation of lipase from colipase. It is found that the enzyme catalyzed hydrolysis of long chain triglycerides by pancreatic lipase from one species is activated by the addition of colipase from other species. Studies on the activation of pancreatic lipase by colipase in the presence of bile salts allowed the re-evaluation of optimal conditions for the determination of lipase and the development of a procedure to assay colipase.     

Inhibition of pancreatic lipase B activity by taurodeoxycholate and its reversal by colipase.

In our two-phase reaction system taurodexycholate prevents the adsorption of pancreatic lipase B to the nonaqueous phase. Our data are consistent with a mechanism for this reaction which involves the cooperative formation of an enzyme-(bile salt)4 complex in solution with a dissociation constant of 1.4 X 10(-15)M4. Whereas the free enzyme is readily adsorbed to a bile salt-substrate-covered surface, the complex is not. Thus, the "inhibition" of substrate hydrolysis occurs because enzyme and substrate are separated physically. The protein cofactor, colipase, reverses the inhibitory effects of bile salt by providing a high affinity binding site at the interface for the lipase-(bile salt)4 complex. Steady state and presteady state kinetic data are consistent with the formation of a complex with a 1/1, lipase/colipase, ratio, and a dissociation constant of 0.4 to 2.8 X 10(-9)M. The rate of adsorption of lipase to adsorbed colipase appears to be controlled by diffusion through the unstirred layer with a second order rate constant of 1.3 X 10(6)M-1S-1.     

The interaction between pancreatic lipase and colipase: a protein-protein interaction regulated by a lipid

The eukaryotic transposable element Ty1 is present in about 20-30 integrated copies per yeast aploid genome, variably localized in different strains. Here, we report the presence in yeast of circular extrachromosomal molecules homologous to Ty1, 6 kilobases in size (the same as integrated copies) present in about 1 circular copy/250-300 cells. This finding shows another analogy between eukaryotic-transposable elements and the pro-viral integrative form of retroviruses.