Kinetics of hydrolysis of dispersions of saturated phosphatidylcholines by Crotalus atrox phospholipase A2

The Chemical Reactivity of Membrane Lipids

It is well-known that aqueous dispersions of phospholipids spontaneously assemble into bilayer structures. These structures have numerous applications across chemistry and materials science and form the fundamental structural unit of the biological membrane. The particular environment of the lipid bilayer, with a water-poor low dielectric core surrounded by a more polar and better hydrated interfacial region, gives the membrane particular biophysical and physicochemical properties and presents a unique environment for chemical reactions to occur. Many different types of molecule spanning a range of sizes, from dissolved gases through small organics to proteins, are able to interact with membranes and promote chemical changes to lipids that subsequently affect the physicochemical properties of the bilayer. This Review describes the chemical reactivity exhibited by lipids in their membrane form, with an emphasis on conditions where the lipids are well hydrated in the form of bilayers. Key topics include the following: lytic reactions of glyceryl esters, including hydrolysis, aminolysis, and transesterification; oxidation reactions of alkenes in unsaturated fatty acids and sterols, including autoxidation and oxidation by singlet oxygen; reactivity of headgroups, particularly with reactive carbonyl species; and E/Z isomerization of alkenes. The consequences of reactivity for biological activity and biophysical properties are also discussed.

Lipid bilayer heterogeneities and modulation of phospholipase A2 activity

The regulation of phospholipase A2 (PLA2) activity toward synthetic vesicular substrates is a model for the modulation c enzyme function by biological membranes. PLA2's catalytic rate toward membrane phospholipids can be modified by several order of magnitude by altering the membrane's composition and structure. The physical basis of this sensitivity is the subject of thi report. The results described here imply that the salient features of membrane-structure which modulate PLA2 activity include compositional phase separation; membrane curvature and, possibly, curvature-associated defects; and dynamic product inhibition due to limitations imposed by the rate of lateral diffusion of lipid in the membrane. Furthermore, it is shown that the effects of membrane structure on the catalytic rate are not exerted merely by enhancing association of PLA2 with the membrane surface: a membrane-bound inactive state is spectroscopically identified. Finally, these results are discussed in the context of some published models for the role of membrane structure in the regulation of membrane-bound enzymes.

Role of lateral phase separation in the modulation of phospholipase A2 activity

Phospholipase A2-catalyzed hydrolysis of phosphatidylcholine large unilamellar vesicles is characterized by a period of slow hydrolysis followed by a rapid increase in the rate of hydrolysis. The temporal relationship between the burst of PLA2 activity and the lateral distribution of substrate and product lipids was examined by simultaneously recording product accumulation and the fluorescence of 1-pyrenyldecanoate, a fatty acid derivative sensitive to lipid distribution and lateral diffusion. The excimer: monomer ratio of the probe changes slowly prior to the burst in activity and then abruptly at the time of the burst. A partial phase diagram for the ternary codispersion of substrate and products (dipalmitoylphosphatidylcholine and 1:1 monopalmitoylphosphatidylcholine/palmitic acid) was constructed by differential scanning calorimetry and suggests gel/gel immiscibility in this system. Thus, the changes in pyrene fluorescence during the time course of hydrolysis appear to be due to lateral phase separation. The critical mole fraction of product both for lateral phase separation in the gel state and for elimination of the lag phase is approximately 0.083. The simultaneous recordings of PLA2 activity and pyrene fluorescence show that the lateral rearrangement of lipids begins prior to and continues during the rapid activation process of PLA2. Two possible effects of lateral phase separation are that concentration of the protein in the product-rich regions promotes putative dimerization or that formation of phase interface regions promotes enzyme activation.

Hydrolysis of supported pyrenephospholipid monolayers by phospholipase A2

Hydrolysis by pancreatic and snake venom (Crotalus atrox) phospholipase A2 of fluorescent monolayers of pyrene-labelled phosphatidylglycerol on solid support was studied. We used a fluorescence microscope equipped with video camera, video recorder and an image analyzer to monitor changes in fluorescence. Decrease in pyrene excimer emission was evident when pyrene phosphatidylglycerol monolayers transferred onto quartz glass slides (at a surface pressure of 15 mN m-1) were subjected to enzymatic hydrolysis. Snake venom phospholipase A2 could hydrolyze the monolayers almost completely while pancreatic phospholipase A2 could cause only 50% decrease in fluorescence intensity. EDTA totally inhibited the action of both A2 phospholipases. When monolayers were transferred onto solid supports at a surface pressure of 31 mN m-1 C. atrox phospholipase A2 could still exert activity whereas porcine pancreatic phospholipase A2 was inactive.

Modulation of phospholipase A2 activity by the tumour promoters phorbol esters and teleocidin

The effects of tumour promoters, namely phorbol esters and teleocidin, on the activity of porcine pancreatic phospholipase A2 (PLA2) was investigated by using a system of small unilamellar vesicles composed of dipalmitoyl-phosphatidylcholine (DPPC). DPPC vesicles encapsulating Quin 2 (Quin 2/DPPC vesicles) were suspended in a medium containing Ca2+. The addition of PLA2 to Quin 2/DPPC vesicles increased the fluorescence intensity of Quin 2. This increase was due to chelation of Quin 2 with Ca2+, which resulted from an increase in the permeability of the phospholipid bilayer caused by the hydrolytic activity of PLA2. The tumour promoters phorbol 12-myristate 13-acetate (PMA) and teleocidin, at low concentrations, enhanced PLA2 activity at temperatures below the phase-transition temperature of the membrane, but, in contrast, high concentrations of the tumour promoters suppressed PLA2 activity. Phorbol 12-myristate (PM) also had a similar effect on PLA2 activity. PMA and PM disturbed the membrane structure markedly, which was indicated by the enhanced leakage of carboxyfluorescein (CF) from DPPC vesicles encapsulating CF. On the other hand, phorbol 12,13-didecanoate and 4 alpha-phorbol 12,13-didecanoate, which did not disturb the membrane structure to the same extent, had an insignificant effect on PLA2 activity. It is therefore concluded that PLA2 catalyses the hydrolysis of phospholipids in bilayer vesicles which contain a moderate degree of structural defects. However, the effects of tumour promoters on PLA2 activity was not related to their potencies as inflammatory and tumour-promoting agents.

Hydrolytic action of phospholipase A2 in monolayers in the phase transition region: direct observation of enzyme domain formation using fluorescence microscopy

Phospholipase A2, a ubiquitous lipolytic enzyme highly active in the hydrolysis of organized phospholipid substrates, has been characterized optically in its action against a variety of phospholipid monolayers using fluorescence microscopy. By labeling the enzyme with a fluorescent marker and introducing it into the subphase of a Langmuir film balance, the hydrolysis of lipid monolayers in their liquid-solid phase transition region could be directly observed with the assistance of an epifluorescence microscope. Visual observation of hydrolysis of different phospholipid monolayers in the phase transition region in real-time could differentiate various mechanisms of hydrolytic action against lipid solid phase domains. DPPC solid phase domains were specifically targeted by phospholipase A2 and were observed to be hydrolyzed in a manner consistent with localized packing density differences. DPPE lipid domain hydrolysis showed no such preferential phospholipase A2 response but did demonstrate a preference for solid/lipid interfaces. DMPC solid lipid domains were also hydrolyzed to create large circular areas in the monolayer cleared of solid phase lipid domains. In all cases, after critical extents of monolayer hydrolysis in the phase transition region, highly stabile, organized domains of enzyme of regular sizes and morphologies were consistently seen to form in the monolayers. Enzyme domain formation was entirely dependent upon hydrolytic activity in the monolayer phase transition region and was not witnessed otherwise.

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.

An enzymatic colorimetric assay of calcium-dependent phospholipases A

In this paper we describe a new method for the assay of calcium-dependent phospholipases A. In this method released fatty acids are quantitated by an enzymatic colorimetric reagent kit which is commercially available. We have tested the applicability of this assay with enzymes from porcine pancreas (phospholipase A2), snake venom (phospholipase A2), and Rhizopus arrhizus (a lipase with phospholipase A1-like activity) as well as with a phospholipase A2 activity of bovine seminal vesicle fluid. We conclude that the kit procedure is a convenient, rapid and sensitive endpoint assay for calcium-dependent phospholipases A.

Action of cobra venom phospholipase A2 on large unilamellar vesicles: comparison with small unilamellar vesicles and multibilayers

Phospholipase A2 (Naja naja naja) catalyzes the hydrolysis of dipalmitoyl phosphatidylcholine in small unilamellar vesicles (SUVs) with a faster initial rate than in large unilamellar vesicles (LUVs) and multilamellar vesicles (MLVs). For the SUVs, the hydrolysis was initially faster for gel phase than liquid crystalline phase phospholipid. For both LUVs and MLVs, hydrolysis was low except in a small temperature range around the thermotropic phase transition of the phospholipid. In this temperature range, the reaction time course of phospholipase action on dipalmitoyl phosphatidylcholine in LUVs and MLVs included a lag period. With SUVs, a lag period also was observed above the phase transition temperature, but it was not observed below it.

Hydrolysis of mixed monomolecular films of phosphatidylcholine/triacylglycerol by pancreatic phospholipase A2

We studied the effect of glycerides on pancreatic phospholipase A2 hydrolysis of mixed monomolecular films of trioctanoylglycerol/1,2-didodecanoyl-sn-glycero-3-phosphocholine with the technique of Piéroni and Verger [(1979) J. Biol. Chem. 254, 10090-10094]. The quantity of enzyme adsorbed to the interface was concomitantly determined with [3H]amidinated phospholipase. At phospholipid packing above the critical penetration pressure, triacylglycerol stimulates phosphatidylcholine hydrolysis to a great extent. On the other hand, the activity of pancreatic phospholipase A2 on a mixed film is inhibited by the action of pancreatic lipase. Interface binding of phospholipase A2 to the lipid substrate does not imply activity.

The mechanism of inhibition of phospholipase activity of crotoxin B by crotoxin A

In the crotoxin complex isolated from Crotalus durissus terrificus venom, the component A inhibits the phospholipase A2 activity of crotoxin B only when the substrate is in the aggregated form, preventing the interaction of the enzyme with lecithin--water interfaces. In contrast, with similar rates of hydrolysis of dihexanoyllecithin monomers, the activity of the crotoxin complex is lower than that of crotoxin B when the substrate is aggregated into micelles. Crotoxin B readily hydrolyses dimyristoyllecithin vesicles, the rate being modulated by the physical state of the phospholipid, suggesting that the enzyme is tightly bound to the interface. With the crotoxin complex the rate of vesicle hydrolysis is much slower (about 1/10 that of crotoxin B) and is little affected by the physical state of the lecithin. Direct binding experiments demonstrate that, in contrast to crotoxin B, the crotoxin complex is unable to interact with lecithin--water interfaces. Together with the free accessibility of the enzyme active site in the crotoxin complex, this evidence suggests that a specific area on the enzyme surface, different from the active site and shielded by crotoxin A in the complex, is responsible for the interaction of crotoxin B with lipid--water interfaces.

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.