Heterogeneous catalysis by phospholipase A2: formulation of a kinetic description of surface effects

An acidic phospholipase A(2) (RVVA-PLA(2)-I) purified from Daboia russelli venom exerts its anticoagulant activity by enzymatic hydrolysis of plasma phospholipids and by non-enzymatic inhibition of factor Xa in a phospholipids/Ca(2+) independent manner

A homodimeric acidic PLA(2) (RVVA-PLA(2)-I) of 58.0 kDa molecular weight purified from Russell's viper (Daboia russelli) venom demonstrated dose-dependent catalytic, strong anticoagulant and indirect hemolytic activities and inhibited blood coagulation cascade in both enzymatic and non-enzymatic mechanisms. In in vitro condition, RVVA-PLA(2)-I showed preferential hydrolysis of phosphatidylcholine with a K(m) and V(max) values of 0.65 mM and 28.9 μmol min(-1), respectively. Biochemical study and GC-analysis of plasma phospholipids hydrolysis by PLA(2) revealed that anticoagulant activity of RVVA-PLA(2)-I was partly attributed by the enzymatic hydrolysis of pro-coagulant phospholipids PC, followed by PS. The spectrofluorometric and gel-filtration analyses documented binding of RVVA-PLA(2)-I with activated factor X and PC; however, it does not bind with factor Va, prothrombin and thrombin. Therefore, this anticoagulant PLA(2) inhibits the blood coagulation cascade non-enzymatically by binding with coagulation factor Xa, even in the absence of phospholipids and Ca(2+) and thus slows down the blood coagulation by partially inhibiting the prothrombin activation. Chemical modification of essential amino acids present in the active site, neutralization with Azadirachta indica leaves extract (AIPLAI) and heat-inactivation study reinforce the association of catalytic and anticoagulant activity of RVVA-PLA(2)-I and also throw a light on its non-enzymatic mechanism of anticoagulant action.

Enhancement of Agkistrodon piscivorus piscivorus venom phospholipase A2 activity toward phosphatidylcholine vesicles by lysolecithin and palmitic acid: studies with fluorescent probes of membrane structure

The activity of phospholipase A2 from snake venom to hydrolyze bilayers of phosphatidylcholines is greatly enhanced by the presence of the hydrolysis products, lysolecithin and fatty acid, in the bilayer. The fluorescence of several probes of membrane structure was used to monitor changes in bilayer physical properties during vesicle hydrolysis. These changes were compared to emission spectra and fluorescence polarization results occurring upon direct addition of lysolecithin and/or fatty acid to the bilayer. The excimer to monomer ratio of 1,3-bis(1-pyrene)propane was insensitive to vesicle hydrolysis, suggesting that changes in the order of the phospholipid chains were not relevant to the effect of the hydrolysis products on phospholipase activity. The fluorescence of 6-propionyl-2-(dimethylamino)-naphthalene (Prodan) suggested that the polarity of the bilayer in the region of the phospholipid head groups increases as the hydrolysis products accumulate in the bilayer. The fluorescence of 6-dodecanoyl-2-(dimethylamino)naphthalene (Laurdan) confirmed that such effects were restricted to the bilayer surface. Furthermore, the lysolecithin appeared to be the product most responsible for these changes. These results suggested that lysolecithin increases the activity of phospholipase A2 during vesicle hydrolysis by disrupting the bilayer surface, making the phospholipid molecules more accessible to the enzyme active site.

Reversibility of the activation of soluble phospholipase A2 on lipid bilayers: implications for the activation mechanism

The time-courses of hydrolysis of large vesicles of dipalmitoylphosphatidylcholine were compared using four species of phospholipase A2 (Agkistrodon piscivorus piscivorus, Crotalus adamanteus and Naja naja venoms and porcine pancreatic). In all four cases, the hydrolysis rate suddenly increases 10 to 100-fold at the time (tau) when a specific mole fraction of reaction products has accumulated. The intrinsic fluorescence emission of the three venom enzymes also increases suddenly at time tau. Both the activation and the fluorescence change are reversible with a half-time of about 50 s for the activity and 2 to 6 s for the fluorescence. These reversal rates and the vesicle concentration dependence of tau are considered for monomer and dimer enzyme activation models. Apparently, at least three states of the enzyme exist beyond the initial unbound state: (1) inactive and bound, (2) inactive with high fluorescence and (3) active. The dimer model already contains the necessary number of states but requires that the activation rate be much lower than the reversal rate to account for the vesicle concentration dependence of tau. Success of the monomer model requires an enzyme state additional to those proposed previously. Although these results do not exclude either the monomer or dimer models conclusively, they do impose important constraints on each model.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.

Activation of phospholipase A2 on lipid bilayers

So far, three phospholipases A2 that display activation kinetics during the time course of hydrolysis of DPPC LUV have been found to undergo a fluorescence change coincident with the activation: the monomer (AppD49) and the dimer enzymes from A. piscivorus piscivorus and the dimer enzyme from C. atrox. The porcine pancreatic enzyme produces similar time courses of hydrolysis but does not display a concurrent fluorescence change. It is assumed that other phospholipases A2 will behave similarly in terms of the hydrolysis reaction. Which enzymes respond with a similar change in intrinsic fluorescence during the time course may well depend on the position of tryptophan residues and the amino acid sequence. Even though a given phospholipase A2 may not change its fluorescent properties on activation, the simultaneous monitoring of the hydrolysis reaction and the fluorescence of probes of the bilayer structure can be done with any phospholipase A2. A variety of probes exist which are sensitive to slightly different membrane properties and could be used as described here for TMA-DPH. For example, 1,3-dipyrenylpropane is sensitive to the apparent microviscosity of the bilayer is terms of the ability of molecules to translationally diffuse in the membrane. 6-Palmitoyl-2-[[2-(trimethylammonio)ethyl]methylamino]naphthalene chloride is sensitive to the ability of a molecule to rotate in the bilayer and displays large changes in its steady-state fluorescence as the anisotropy of the bilayer changes. 6-Propionyl-2-(dimethylamino)naphthalene is sensitive to the polarity and degree of hydration of its environment. Finally, a compound titled NK-529 has recently been introduced that apparently monitors the lateral phase separation of fatty acids in the bilayer. The fact that activation of phospholipase A2 can be monitored during the time course of hydrolysis of DPPCLUV makes this system an excellent choice for studying the mechanisms of activation and possible effects of various activators and inhibitors. The experimental system described here provides a way to determine whether such regulators exert their effects through alterations of the properties of the membrane and/or the enzyme. Importantly, this system allows one to seek temporal correlations of the various events in the process.

Segregation of anionic lipophiles in bilayers monitored by binding of cationic dye NK-529

Fluorescence emission properties of a cationic indodicarbocyanine dye, NK-529, bound to anionic and zwitterionic vesicles, are examined under a variety of conditions to monitor lateral distribution of anionic amphiphiles in bilayers as a function of their phase properties. The change in the fluorescence properties of NK-529 arises from the binding of the dye to the bilayer that is dominated by ionic interactions when possible, as well as from the self-quenching of the dye bound to bilayers when the surface density of the dye is high. The binding affinity of the dye to anionic interfaces is more than 100-fold higher compared to that in zwitterionic bilayers. The limiting phospholipid/dye ratio in anionic bilayers at low vesicle concentrations is about 3. Thus the density of the bound dye in anionic bilayers can be more than 40-fold higher than that in zwitterionic bilayers, and therefore under such conditions the bound dye is completely self-quenched in vesicles or micelles of anionic phospholipids. The change in the fluorescence emission intensity on incorporation of anionic amphiphiles in zwitterionic bilayers is used to monitor segregation of the anionic amphiphiles. The organizational features of bilayers that cause a change in the fluorescence properties of bound NK-529 show that the lateral distribution of anionic amphiphiles is appreciably influenced not only by the mole fraction of the amphiphile but also in the presence of other additives, and by the gel-fluid thermotropic transition. As shown in the following paper, the fluorescence changes related to self-quenching in anionic bilayers containing NK-529 can be used to understand the organizational changes that occur during the course of interfacial catalysis by phospholipase A2 on zwitterionic bilayers.

Binding of phospholipase A2 to zwitterionic bilayers is promoted by lateral segregation of anionic amphiphiles

Catalytic action of phospholipase A2 is appreciably influenced by the organization and dynamics of bilayers of glycerophosphocholines (Apitz-Castro et al. (1988) Biochim. Biophys. Acta 688, 341-348). However, such effects of the quality of the interface are not observed with bilayers of glycerophosphoryl methanol and other anionic phospholipids (Jain et al. (1986) Biochim. Biophys. Acta 860, 435-447). Such differences between the catalytic susceptibility of zwitterionic versus anionic bilayers are due to a large difference in the affinity of the enzyme for these interfaces. Binding to phospholipase A2 to zwitterionic interfaces can be promoted in the presence of certain anionic additives. For example in the pre-steady-state phase of hydrolysis, segregation of the nacently produced products of hydrolysis could promote binding of phospholipase A2 to regions of higher anionic charge density in the zwitterionic interface. In this paper we show that the dynamics of segregation of the nacently produced products of hydrolysis in zwitterionic bilayers can be readily followed by monitoring the fluorescence intensity of the cationic dye NK-529 (Yu and Jain (1989) Biochim. Biophys. Acta 980, 15-22). The fluorescence emission characteristics of NK-529 change appreciably due to self-quenching of the bound dye molecules as the fatty acid molecules segregate in the bilayer. The kinetics of segregation of fatty acids during the course of hydrolysis of bilayers of zwitterionic phospholipids by phospholipase A2 exhibits an unequivocal correlation with a variety of phenomena that are observed during the transition from the pre-steady-state phase to the steady-state phase of hydrolysis in the reaction progress curves as a function of temperature and in the presence of lipophilic additives.

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.

Kinetics of interfacial catalysis by phospholipase A2 in intravesicle scooting mode, and heterofusion of anionic and zwitterionic vesicles

In this and the following three papers we examine the kinetics of action of pig pancreatic phospholipase A2 on vesicles of anionic phospholipids without any additives. The results provide the first unequivocal demonstration of interfacial catalysis in intravesicle scooting mode. In this paper we describe the conditions in which the action of pig pancreatic phospholipase A2 on DMPMe (ester) vesicles in the absence of any additive commences without a latency. Under these conditions the free monomer substrate concentration is insignificant; the bilayer enclosed vesicle organization remains intact even when all the substrate in the outer monolayer has been hydrolyzed; the rate of intervesicle exchange and the rate of transbilayer movement (flip-flop) of molecules is negligibly slow; and the rate of fusion of vesicles is insignificant. Thus an enzyme molecule bound to one vesicle hydrolyzes all the DMPMe molecules in the outer monolayer of the vesicle by a first-order process with a rate constant of 0.6 per min at 30 degrees C; or viewed another way, one enzyme molecule in a DMPMe vesicle can hydrolyze all the available substrate molecules at the rate of 3000 per min. At low anion concentrations excess substrate vesicles are not hydrolyzed unless the rate of intervesicle exchange of the bound enzyme is stimulated by anions in the aqueous phase. Higher calcium concentrations promote not only homofusion of DMPMe vesicles but also heterofusion of DMPMe and DMPC vesicles. It is proposed that calcium-induced isothermal lateral phase separation in DMPMe vesicles induces defects in the bilayer organization, and such defects are the sites for phospholipase A2 binding and for heterofusion with DMPC (ester) vesicles which do not have such sites.

Action of phospholipase A2 on bilayers. Effect of fatty acid and lysophospholipid additives on the kinetic parameters

Action of pig pancreatic phospholipase A2 on the ternary codispersions of diacylphosphatidylcholine, 1-acyllysophosphatidylcholine and fatty acids is examined. The binding and kinetic constants are found to be the same under a variety of conditions. These parameters and the catalytic turnover number change with the phase-transition temperature of the ternary codispersions, and optimal binding, kinetic and catalytic constants are seen in the phase-transition range where an equilibrium exists between laterally separated phases. The effect of changing the structure of any of the three components is also via a change in the phase-transition temperature of their ternary codispersions. These observations suggest that the binding of pig pancreatic phospholipase A2 to the defect sites on the substrate interface determines the substrate concentration dependence of the initial rate of hydrolysis, and the catalytic turnover by the bound enzyme also depends upon the phase state of the bilayer. An additive-induced stabilization of the defects in the substrate bilayer is postulated to account for the enhanced binding of the enzyme to the bilayer.

Action of phospholipase A2 on bilayers. Effect of inhibitors

Action of several solutes on the kinetics of phospholipase-A2-catalyzed hydrolysis of the ternary codispersions containing dimyristoylphosphatidylcholine + 1-palmitoyllysophosphatidylcholine + palmitic acid is examined. The kinetics of hydrolysis is interpreted in terms of the ability of the enzyme to bind to the substrate interface. The inhibitory effect of these solutes is correlated with their ability to modify fluorescence intensity of the bound enzyme, to modify the phase-transition profile, and to inhibit aggregation/fusion of the ternary codispersions. Based on these observations, it is suggested that the solutes like n-alkanols, ketamine, alphadolone, alphaxalone, flufenamic acid, tobramycin, mepacrine, EMD 21657 and U-10029A modulate the phase equilibria in the codispersions and thus noncompetitively inhibit the phospholipase action. Inhibition by feverfew extract (Tanacetum parthemium) is also by a similar mechanism. Lipid-soluble drugs as indomethacin had little effect on the kinetics of hydrolysis. All these inhibitors decrease the total extent of hydrolysis of the available substrate. However, none of these inhibitors have any effect on the hydrolysis of monomeric substrate or on the inactivation of the phospholipase A2 by p-bromophenacylbromide. These observations suggest that all these inhibitors do not interact directly with the catalytic site of the free or the bound enzyme, and their effect is primarily on the enzyme-binding sites on the substrate vesicle, that is, by perturbation of lipid-protein interaction.

Interaction of phospholipase A2 and phospholipid bilayers

Binding of phospholipase A2 from porcine pancreas and from Naja melanoleuca venom to vesicles of 1,2-di(tetradecyl)-rac-glycero-3-phosphocholine (diether-PC14) is studied in the presence and absence of 1-tetradecanoyl-sn-glycero-3-phosphocholine and myristic acid. The bound enzyme coelutes with the vesicles during gel filtration through a nonequilibrated Sephadex G-100 column, modifies the phase transition behavior of bilayers, and exhibits an increase in fluorescence intensity accompanied by a blue shift. Using these criteria it is demonstrated that the snake-venom enzyme binds to bilayers of the diether-PC14 alone. In contrast, the porcine enzyme binds only to ternary codispersions of dialkyl (or diacyl) phosphatidylcholine, lysophosphatidylcholine and fatty acid. Binding of pig-pancreatic enzyme to vesicles of the diether-PC14 could not be detected even after long incubation (up to 24 H) below, at, or above the phase-transition temperature, whereas the binding in the presence of products is almost instantaneous and observed over a wide temperature range. Thus incorporation of the products in substrate dispersions increases the binding affinity rather than increase the rate of binding. The results are consistent with the hypothesis that the pancreatic enzyme binds to defect sites at the phase boundaries in substrate bilayers induced by the products. The spectroscopically obtained hyperbolic binding curves can be adequately described by a single equilibrium by assuming that the enzyme interacts with discrete sites. The binding experiments are supported by kinetic studies.

Origin of the latency phase during the action of phospholipase A2 on unmodified phosphatidylcholine vesicles

The reaction progress curve for the action of pig-pancreatic phospholipase A2 on dimyristoylphosphatidylcholine vesicles is characterized under a variety of conditions. The factors that regulate the rate of hydrolysis during the presteady-state phase determine the latency period. The results demonstrate that the accelerated hydrolysis following the latency phase of the reaction progress curve is due to the product-assisted binding of the enzyme to the substrate bilayer by chaning the number of bindings sites and therefore the binding equilibrium. A critical mole fraction of products appears to be formed in the substrate bilayers before the steady-state phase of hydrolysis begins. The latency phase shows a minimum at the phase-transition temperature of the substrate vesicles; however, we did not observe a significant binding of the enzyme to pure substrate bilayers even at the phase-transition temperature. The rate of binding of the enzyme is found to be fast and the rate of desorption of the bound enzyme is very slow compared to the latency phase. The rate of redistribution of products between substrate bilayers is rather slow. These observations demonstrate that during the latency phase of the action of phospholipase A2, a critical mole fraction of products is formed in the substrate bilayer.

Action of phospholipase A2 on unmodified phosphatidylcholine bilayers: organizational defects are preferred sites of action

The hydrolytic action of the bee venom phospholipase A2 on phosphatidylcholine bilayers is studied under a variety of conditions that introduce alterations in the packing, such as those induced by sonication, gel to liquid crystalline phase transition, and osmotic shock. Two phases of hydrolysis could be resolved under a wide range of experimental conditions. With the various forms of the bilayers one observes only a partial hydrolysis of the total available substrate during the first phase. However, the fraction of the substrate hydrolyzed in the first phase changes with the form of the available substrate, with the amount of the enzyme added, with the temperature, with the phase transition characteristics of the substrate, and by the sonication of the substrate. The second phase of hydrolysis is generally observed when a certain concentration of the products has been produced during the first phase of hydrolysis. These obervations are interpreted to suggest that the bee venom phospholipase A2 preferentially catalyzes hydrolysis of the substrate available at or near the defects in the organization of the substrate in the bilayers.

Intrinsic differences in the perturbing ability of alkanols in bilayer: action of phospholipase A2 on the alkanol-modified phospholipid bilayer

The kinetic parameters for the steady-state rate of hydrolysis of egg phosphatidylcholine in multilamellar vesicles by bee venom phospholipase A2 are measured in the presence of 27 alkanols and several organic solvents. In general, small nonpolar solutes like enflurane, tetrahydrofuran, benzene, chloroform and diethylether do not promote the hydrolysis of multilamellar vesicles. The rate of hydrolysis shows a biphasic dependence upon the alkanol concentration for all higher (C5-C9) alcohols examined, i.e., an optimal rate of hydrolysis is observed at a characteristic concentration for each alcohol. The alkanol to lipid mole ratio (D/L ratio) in the bilayer at the peak activating concentration of an alkanol was computed from its bilayer/water partition coefficient. The branched chain alcohols induce peak activation of hydrolysis at lower D/L ratios in the bilayer than the corresponding straight chain analogs. Similarly, the longer chain n-alkanols at peak activating concentration have a lower D/L ratio than the corresponding lower alcohols. Both the Km and Vm for phosphatidylcholine increase as a function of the chain length of the activating alcohol. These kinetic parameters also depend upon the position of the substituents on the activating alcohols. Both the D/L ratio and Vm for an alcohol are found to change with the cross-sectional area of the activating alcohol across its long axis: alcohols with a more asymmetric cross-section exhibit higher Vm and a lower D/L ratio. Such correlations of Vm and D/L ratio with the molecular parameters of the alkanols are interpreted to suggest that the accessibility of the substrate molecule in the bilayer to the phospholipase is modulated by the free space introduced by the alkanols in the bilayer. Effect of tetradecane derivatives and A2C (a membrane fluidizing agent) on the phase transition characteristics of DPPC bilayers, and their susceptibility to phospholipase A2 from bee venom and pig pancreas is also reported. These solutes cause a broadening of the transition profile and reduce the size of the cooperative unit and the enthalpy of transition. These effects depend upon the mole fraction of a solute in the bilayer; however, equal concentrations of these solutes do not induce equal response. Susceptibility of the modified bilayers to phospholipase A2 depends not only upon the structure of the solute but also upon the source of the enzyme. The data show that the activity of the membrane-bound enzyme is modulated to different extents by different solutes, and the bilayer perturbing ability of these solutes may be related to the asymmetry of their cross-sectional area and to the free space introduced by the alkanols in a bilayer.