Effects of colipase on hydrolysis of monomolecular films by lipase

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.

Hydrolysis of emulsified mixtures of triacylglycerols by pancreatic lipase

Hydrolysis of the emulsified mixture of short-chain triacylglycerols by porcine pancreatic lipase in the presence of procolipase and micellar sodium taurodeoxycholate has been studied. Increase in the content of tributyrin and trioctanoin in the mixture with triacetin had highly cooperative effects on the formation of the interfacial lipase procolipase complex. Abrupt enhancement of the complex stability was observed in the presence of 0.4-0.6 mol mol-1 of tributyrin or 0.58 mol mol-1 of trioctanoin in the substrate phase. The affinity of lipase towards interfacially bound procolipase for the trioctanoin containing 0.07-0.42 mol mol-1 of triacetin was approximately three times higher than that for pure trioctanoin. The cooperative processes involved in complex formation did not contribute to the affinity of the interfacial lipase/(pro)colipase complex towards substrate molecules and its catalytic activity.

How colipase-fatty acid interactions mediate adsorption of pancreatic lipase to interfaces

Colipase is a cofactor protein which forms a 1:1 complex with pancreatic lipase. This facilitates lipase adsorption to phosphatidylcholine-rich interfaces, presumably as a consequence of the higher affinity of colipase for such interfaces. According to this model, the presence of colipase in an interface should be sufficient to enable lipase adsorption from the aqueous phase. To test this hypothesis, mixed monolayers of colipase, phosphatidylcholine, and fatty acid at the argon-buffer interface were exposed to lipase injected into the stirred aqueous subphase. Spread colipase remained associated with the lipid monolayer in a surface pressure- and lipid composition-dependent manner. For example, with diacylphosphatidylcholine alone, colipase remained in the lipid monolayer at surface pressures </=20 mN/m, but with pure fatty acid this was increased to approximately 40 mN/m. Contrary to the existing paradigm, the presence of colipase in a lipid monolayer was not sufficient to enable the adsorption of lipase to the interface. Fatty acid was also required, and its ability to enhance lipase adsorption over that observed in the absence of colipase was dependent on the fatty acid and colipase mole fractions. These results support the hypothesis that colipase concentrates fatty acids laterally at its periphery and suggest that, together with lipase-colipase interaction, the fatty acid-rich nano-domain surrounding colipase facilitates lipase adsorption in the 'flap-opened' conformation.

Effects of colipase and bile salts on the catalytic activity of human pancreatic lipase. A study using the oil drop tensiometer

Using the oil drop technique, we studied the effects of colipase and bile salts on the rate of hydrolysis of soybean oil by human pancreatic lipase (HPL) as well as on the interfacial binding. Upon continuously recording the decrease in the interfacial tension with time, a 10-15-fold increase in the HPL activity was found to occur in the presence of colipase. The catalytic rate constants of hydrolysis measured at the oil drop surface were found to be of the same order of magnitude as those obtained with monomolecular films spread at the air-water interface. Biotin-labeled HPL (HPL*) was used to determine the amount of adsorbed enzyme using an ELISA test. Less than 1% of the total amount of injected HPL* molecules was found to have adsorbed to the oil-water interface, and no significant effects of colipase on HPL* binding were observed. No significant changes in the hydrolysis rates or the binding of HPL* were detected in the presence of bile salts at concentrations ranging from below their critical micellar concentration (CMC) up to 100 microM. At the oil-water interface, in the absence or presence of bile salts below their CMC, it can be concluded that the colipase is a true lipase cofactor, i.e, it increases the enzyme turnover (approximately 10-15-fold) and does not affect the interfacial lipase adsorption.

Pancreatic lipase structure-function relationships by domain exchange

We designed chimeric mutants by exchanging the lid domains of the classical human pancreatic lipase (HPL) and the guinea pig pancreatic lipase related protein 2 (GPLRP2). This latter enzyme possesses naturally a large deletion within the lid domain and is not activated by lipid/water interfaces. Furthermore, GPLRP2 exhibits phospholipase A1 and lipase activities in the same order of magnitude, whereas HPL has no significant phospholipase activity and displays a clear interfacial activation. An HPL mutant [HPL(-lid)] with GPLRP2 mini-lid domain does not display interfacial activation. Its specific activity toward triglycerides is, however, dramatically reduced. A GPLRP2 mutant [GPLRP2(+lid)] with HPL full-length lid domain is not interfacially activated, and its lid domain probably exists under a permanent open conformation. Therefore, the phenomenon of interfacial activation in HPL is not only due to the presence of a full-length lid domain but also to other structural elements which probably allow the existence of stabilized closed and open conformations of the lid. GPLRP2(+lid) phospholipase activity is significantly reduced as compared to GPLRP2, whereas its lipase activity remains at the same level. Therefore, the lid domain plays a major role in substrate selectivity and can be considered as part of the active site. However, the presence of a full-length lid domain is not sufficient to explain the absence of phospholipase activity in HPL since HPL(-lid) does not display any phospholipase activity. We also produced a chimeric GPLRP2 mutant in which the C-terminal domain was substituted by the HPL C-terminal domain. The colipase effects, i.e., anchoring and stabilization of the lipase at the interface, are clearly observed with the chimera, whereas GPLRP2 is insensitive to colipase. The kinetic characterization of this chimera reveals for the first time that the interfacial stability of pancreatic lipases depends on the structure of the C-terminal domain.

Purification and characterization of a 28 kDa cytosolic inhibitor of cholesteryl ester hydrolases in rat testis

A 28 kDa inhibitory protein was purified from rat testis cytosol by sequential 40-65% ammonium sulfate precipitation, cation exchange chromatography, anion exchange chromatography, and preparative SDS-polyacrylamide gel electrophoresis. The heat-stable, trypsin-labile protein exhibited nonenzymatic, concentration-dependent inhibition of testicular and pancreatic cholesteryl ester hydrolases at all stages of purification. Copurifying at each stage was a 26.5 kDa protein which comprised 25% of the mass of the two proteins. Polyclonal antibodies raised to either or both 28 kDa and 26.5 kDa proteins by direct injection of excised electrophoretic bands cross-reacted with both proteins on western blots, immunoprecipitated both proteins, and neutralized inhibitory activity. Amino acid compositions of the individual proteins electroeluted from SDS-polyacrylamide gels were different from those of other surface-active proteins of similar molecular weights. Both proteins exhibited identical pl of 4.8 on chromatofocusing columns and two-dimensional gel electrophoresis. Although the subcellular distribution of the 28 kDa protein is unknown, its testicular cytosolic concentration, calculated from the purified protein mass, was 8 X 10(-9) mols/L, which probably underestimates the actual concentration by an order of magnitude. This is greater than the minimum concentration required for in vitro inhibition (10(-9) mols/L), consistent with a physiological role for this protein.

Kinetics of the two-step hydrolysis of triacylglycerol by pancreatic lipases

Pancreatic lipases catalyze the hydrolysis of triacylglycerol in a sequential manner. First, triacylglycerol is hydrolyzed to 1,2-diacylglycerol, which is subsequently converted to 2-monoacylglycerol. We studied the kinetics of trioleoylglycerol hydrolysis by rabbit and human pancreatic lipases. The products (acylglycerols and fatty acid) were analyzed by extraction from the reaction mixture, separation by thin-layer chromatography, and quantification by capillary gas chromatography. The first-order rate constants of trioleoylglycerol and dioleoylglycerol hydrolysis were calculated showing that both enzymes hydrolyze dioleoylglycerol faster than trioleoylglycerol. Using rabbit pancreatic lipase, we found that deoxycholate enhanced dioleoylglycerol hydrolysis to a higher degree than trioleoylglycerol hydrolysis. Colipase increased both rate constants similarly at high deoxycholate concentrations (35 mM), while at low concentrations (5 mM) a selectivity toward trioleoylglycerol was observed. From the variation of the rate constants with respect to temperature, we calculated the apparent activation energies of trioleoylglycerol and dioleoylglycerol hydrolysis to be 59.8 kJ.mol-1 and 53.5 kJ.mol-1, respectively. Upon storage, both rabbit and human pancreatic lipases showed a greater loss of activity toward dioleoylglycerol as compared to trioleoylglycerol, suggesting that different conformational elements of the enzyme molecule are responsible for the interaction with each substrate.

Lipase evolution: trout, Xenopus and chicken have lipoprotein lipase and apolipoprotein C-II-like activity but lack hepatic lipase-like activity

Lipoprotein lipase and hepatic lipase are members of a gene family which also contains pancreatic lipase. High activity of lipoprotein lipase is present in extrahepatic tissues in all mammals studied and also in birds. The activity of hepatic lipase varies more. To investigate the evolutionary relationship, lipase activities in tissues of some lower vertebrates were measured. In fish and in frog, low activities with the characteristics of lipoprotein lipase were found. Serum from frog and from fish, and plasma from chicken, stimulated lipoprotein lipase in vitro, indicating that these species contain analogues to human apolipoprotein C-II. Little or no hepatic lipase-like activity was found in post-heparin plasma or in liver homogenates of chickens. In fish liver, lipase activity with an apparent heparin affinity similar to, or even higher than lipoprotein lipase was found. Frog liver contained a small amount of lipase activity with high heparin affinity. This activity was inhibited both by apolipoprotein C-II and by 1 M NaCl. It is not clear whether the low lipase activities in livers from fish and from frog are variants of hepatic lipase. Since lipoprotein lipase and apolipoprotein C-II are already present in fish, this lipase probably evolved before hepatic lipase.

Lipase structures at the interface between chemistry and biochemistry

In this chapter we review recent molecular knowledge on two structurally related mammalian triglyceride lipases which have evolved from a common ancestral gene. The common property of the lipase family members is that they interact with non-polar substances. Pancreatic lipase hydrolyzes triglycerides in the small intestine in the presence of many dietary components, other digestive enzymes and high concentrations of detergents (bile salts). Lipoprotein lipase acts at the vascular side of the blood vessels where it hydrolyses triglycerides and some phospholipids of the circulating plasma lipoproteins. A third member of the gene family, hepatic lipase, is found in the liver of mammals. Also, this lipase is involved in lipoprotein metabolism. The three lipases are distantly related to some non-catalytic yolk proteins from Drosophila (Persson et al., 1989; Kirchgessner et al., 1989; Hide et al., 1992) and to a phospholipase A1 from hornet venom (Soldatova et al., 1993).

The activation of porcine pancreatic lipase by cis-unsaturated fatty acids

In the presence of taurodeoxycholate, cis-unsaturated fatty acids increase porcine pancreatic lipase activity 15-fold at pH 7.5. This effect is saturable with a low proportion of fatty acid to substrate. The overall angle of the fatty acid, the position of its double bond and the presence of a carboxyl group were critical factors in whether the fatty acid effectively increased lipase activity. When the substrate is emulsified by taurodeoxycholate, the pH optimum for lipase ranges from 6.2 to 7.0. In the presence of cis-unsaturated fatty acids, the overall activity of lipase increases, the pH optimum shifts, and the pH-activity curve becomes biphasic, with one optimum around pH 7.7, and the other around pH 8.8. Fluorescence studies indicate that fatty acids bind near aromatic residues in lipase, particularly tryptophan. Using the fluorescent fatty acid cis-parinaric acid, it was determined that multiple binding sites are present with Kd values of approx. 10(-6) M. Far-UV circular dichroism (CD) studies indicate that in addition to a high affinity fatty acid binding site with a Kd of approx. 10(-6) M, there is also a low affinity binding site with a Kd of approx. 10(-4) M. The far-UV CD data also show that cis-unsaturated fatty acids change the conformation of lipase. It is calculated that the percentage of alpha helix decreases, and the amount of beta sheet and beta turn structure increases. Because the three-dimensional crystal structure of lipase is known, a model is proposed to describe how cis-unsaturated fatty acids increase lipase activity.

A structural domain (the lid) found in pancreatic lipases is absent in the guinea pig (phospho)lipase

Typically pancreatic lipases are characterized by the following properties: (1) they are activated by lipid/water interfaces (interfacial activation), (2) they are inhibited by bile salts but reactivated by colipase (a small activator protein), and (3) they do not hydrolyze significantly phospholipids. A cDNA clone encoding a guinea pig pancreatic (phospho)lipase (GPL) has been sequenced and expressed. The enzyme (recombinant as well as native) differs from other pancreatic lipases in that (1) it is not interfacially activated, (2) its activity is unaffected by the presence of bile salts and/or colipase using tributyrin as substrate, and (3) it exhibits equally phospholipase A1 and lipase activities. The amino acid sequence of GPL is highly homologous to that of other known pancreatic lipases, with the exception of a deletion in the so-called lid domain that regulates access to the active centers of other lipases. We propose that this deletion is directly responsible for the anomalous behavior of this enzyme. Thus GPL challenges the classical distinction between lipases, esterases, and phospholipases.

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.

Role of a sulfhydryl group in gastric lipases. A binding study using the monomolecular-film technique

Native human and rabbit gastric lipases (HGL and RGL, respectively) were inactivated after modification of one sulfhydryl group/enzyme molecule. HGL and RGL were covalently labeled using 5,5'-dithiobis(2-nitro-[14C]benzoic acid) and the interaction of 2-nitro-5-thio-[14C]benzoic-acid-labeled lipases ([14C]Nbs-lipases) with monomolecular lipid films was investigated. Our results show that [14C]Nbs-lipases bind to lipid films as efficiently as native HGL or RGL. The critical surface pressure pi c and the maximal surface pressure (delta pi max) of [14C]Nbs-lipases were enhanced in comparison with those of native RGL and HGL. These changes in behavior were probably due to an increase in hydrophobicity brought about, directly or indirectly, by the binding of the Nbs radical. This chemical modification thus blocks the hydrolysis site and reinforces the hydrophobic character of the gastric lipases.

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.

Human gastric lipase. A kinetic study with dicaprin monolayers

The effects of several proteins on the hydrolysis at pH 3.0 of didecanoylglycerol monolayers by human gastric lipase were investigated. Among the six proteins tested (bovine serum albumin, myoglobin, a protein inhibiting lipase isolated from soya bean, melittin, beta-lactoglobulin and ovalbumin), only the first three proteins were found to inhibit lipase activity. The inhibition capacity of the proteins was not related to the decrease in interfacial tension or to their isoelectric points. However, inhibition of human gastric lipase by proteins may be correlated with the penetration power of the protein into the lipid interface. It is hypothesized that this lipase has a higher penetration power than that of pancreatic lipase, even though the former enzyme is more susceptible to interfacial denaturation.

Control of the action of phospholipases A by "vertical compression" of the substrate monolayer

Monolayers of rac-1,2-didodecanoyl-sn-glycero-3-phosphoglycerol at an air-water interface were "vertically compressed" by substituting an alkylated glass plate for air while maintaining a constant surface pressure of 15 mN m-1. At this surface pressure the overlaying of the lipid film by the alkylated surface resulted in an average increase of 16 A2/molecule in the mean molecular area of those phospholipid molecules residing at the interface between water and the alkylated glass. Subsequently, the activities of phospholipases A1 and A2 toward the monolayers were measured both in the presence and in the absence of the support. While phospholipase A1 activity was increased 4-fold by the support, the activity of phospholipase A2 was reduced to 15% of the activity measured in the absence of the alkylated surface. These findings indicate that such a "vertical compression" of the monolayer is likely to induce a conformational change in the phospholipid molecules, which in turn would cause the above reciprocal changes in the activities of phospholipases A1 and A2. A molecular model accounting to these findings is presented.

Inhibition of lipases by proteins: a binding study using dicaprin monolayers

We previously reported that the inhibition of pancreatic and Rhizopus delemar lipases by proteins is due to the protein associated with lipid and is not caused by direct protein-enzyme interaction in the aqueous phase [Gargouri, Y., Piéroni, G., Rivière, C., Sugihara, A., Sarda, L., & Verger, R. (1985) J. Biol. Chem. 260, 2268-2273]. In this study, using radiolabeled lipases, serum albumin, and beta-lactoglobulin A, we investigated their respective binding with respect to lipolysis of dicaprin monolayers. Lipase inhibition was found to be correlated with a lack of lipase binding to mixed protein-dicaprin films or to a desorption of lipase from the interface when inhibitory protein was added later. Since a large proportion of the lipid film remained potentially accessible to the enzyme in the presence of inhibitory protein, it was concluded that the observed decrease in lipase binding to the interface was due to a variation of the physiochemical properties of the lipid-water interface following binding of inhibitory protein. On the basis of the results presented here, it is proposed that mixed protein-glyceride films could be used to characterize the interaction of various lipases with lipid substrates and to classify these enzymes according to their penetration power.

Surface properties of bacterial sulfhydryl-activated cytolytic toxins. Interaction with monomolecular films of phosphatidylcholine and various sterols

Sulfhydryl-activated cytolysins are a group of bacterial protein toxins which, in the reduced state, lyse eukaryotic cells by disruption of the cytoplasmic membrane. Cell surface cholesterol is thought to be the target of the toxins. In the present work, the monolayer technique was used to investigate the interaction of four SH-activated toxins (streptolysin 0, alveolysin , perfringolysin 0, pneumolysin ) with various lipid films as a model for studying toxin-induced membrane disruption. A surface pressure increase up to very high values was elicited by reduced toxins (approximately equal to 10 nM) on films of cholesterol, other toxin-binding 3 beta-hydroxy-sterols, thiocholesterol and cholesterol-phosphatidylcholine mixtures suggesting deformation or penetration of the films. The surface-active potency of the toxins was of the same order as that of melittin and snake cardiotoxins at similar concentrations. No pressure increase was observed on films made of pure phosphatidylcholine, lanosterol and other sterols lacking the 3 beta-OH group. Optimal efficiency was at cholesterol/phosphatidylcholine molar ratio of 1 to 1. The critical pressures for toxin interaction with phosphatidylcholine and cholesterol monolayers were 25 mN X m-1 and 45 mN X m-1 respectively. Toxin interaction with phosphatidylcholine [14C]-cholesterol films did not modify monolayer radioactivity, indicating no cholesterol desorption. No pressure increase was elicited by toxins inactivated by SH-group reagents, heating or neutralization with antibody. Toxin effect was dependent temperature and pH. The overall potency of the four toxins tested was streptolysin 0 greater than alveolysin approximately equal to perfringolysin 0 greater than pneumolysin . The monolayer system mimicked in several respects toxin interaction with eukaryotic cells.

The influence of the internal content of negatively charged liposomes on their interaction with high-density lipoprotein

The release of the internal content of negatively charged phosphatidylcholine/phosphatidylserine vesicles under the influence of high density lipoprotein was studied. Under standard conditions (the same composition outside and inside the compartment) the leakage of negative liposomes increased significantly. However, a high internal concentration of calcein provoked a sealing effect, exhibited both in sucrose and in calcein release. This sealing effect is not related to the size of vesicles, the fluidity of the membrane, the distribution of phosphatidylserine molecules, or the membrane potential. Our data indicate that surface potential influences this effect, probably in addition to a lateral pressure effect such as with cholesterol. The surface potential, as measured by the water-lipid partition coefficient of fatty acids, is strongly affected by internal ionic strength when liposomes contain calcein as well as other polyanions (6-carboxyfluorescein, sodium citrate).

The temperature-dependent interfacial inactivation of porcine pancreatic lipase. Effect of colipase and bile salts

This paper confirms and extends the previous observation that colipase and bile salts stabilize pancreatic lipase against inactivation at its water/substrate interface. It is shown that colipase and bile salts above their critical micellar concentration offer better protection than either of them alone. Colipase has no effect on the catalytic efficiency of lipase against an emulsified substrate in the absence or presence of bile salts. Its reported activation of pancreatic lipolysis at high temperatures in the absence of bile salts is, most likely, fully explained by its protective effect on lipase inactivation. Colipase at high concentrations relative to lipase inhibits the enzyme activity in a competitive fashion. The temperature-dependent surface inactivation of lipase has certain consequences for the methodology of lipase activity determination.

[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.

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.

Lipoprotein lipase: some effects of activator proteins

This paper considers how apolipoprotein CII from human plasma lipoproteins and T1 and T2 proteins from egg yolk lipoproteins stimulate the activity of lipoprotein lipase. These activator proteins stabilized the enzyme much more effectively than a thousandfold higher concentration of albumin did, indicating a direct interaction with the enzyme. The effects of the activators were seen also at 1 M NaCl. Thus, forces other than electrostatic are implicated. Centrifugation experiments showed that 125I-labeled lipase bound equally well to the emulsion droplets in the absence of activator protein as in its presence. This was true even under conditions when the activator caused a severalfold increase in the rate of hydrolysis. Thus, the activator makes enzyme at the interface more effective in hydrolysis. By optimizing the conditions it was possible to obtain almost as high rates of triglyceride hydrolysis in the absence as in the presence of activator. Thus, the main effect of the activator protein is probably not on a rate-limiting chemical step. Under most conditions, the rate of hydrolysis was much below optimal and activator increased it. This was always the case with phosphatidyl-choline/triglyceride emulsions, where the activator enhanced hydrolysis of both lipids. Other experiments showed that the activator enhanced triglyceride hydrolysis in the absence of phospholipids and phospholipid hydrolysis in the absence of triglycerides. It is suggested that interaction with activator orientates the enzyme and/or the lipid substrate for effective hydrolysis at the surface of lipoproteins/model substrates.

The influence of bile salts and bile lipoprotein complex on pancreatic lipase hydrolysis of monomolecular films

We report a new technique which allows us to follow the lipolysis of monomolecular films in the presence of bile salts by using a 'zero-order' trough (Verger, R. and de Haas, G.H. (1973) Chem. Phys. Lipids 10, 127). The effects of bile salts, the bile lipoprotein complex and colipase on pancreatic lipase hydrolysis of rac-1,2-didodecanoylglycerol films were studied at different surface pressures. Taking into account previous studies, lipase activity was interpreted as a function of its degree of binding to the bile lipoprotein complex.

Effect of bile lipids on the adsorption and activity of pancreatic lipase on triacylglycerol emulsions

Emulsions of natural triacylglycerols obtained with different shear forces were used to study lipase adsorption and lipolysis. The influence of the bile lipoprotein complex on these two processes was determined. Optimal lipase activity was observed to occur with a given phospholipid : triacylglycerol ratio. This ratio depended on the degree of triacylglycerol emulsification and was accompanied by maximal adsorption of the bile lipoprotein complex. These results support our previous model for pancreatic lipolysis under physiological conditions, according to which colipase controls lipase binding to the bile lipoprotein complex and the resulting association directs enzyme adsorption to the acylglycerol particle (Lairon, D., Nalbone, G., Lafont, H., Léonardi, J., Domingo, N., Hauton, J.C. and Verger, R. (1978) Biochemistry 17, 5263--5269).

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.