Toward understanding interfacial activation of secretory phospholipase A2 (PLA2): membrane surface properties and membrane-induced structural changes in the enzyme contribute synergistically to PLA2 activation

Importance of membrane structural integrity for RPE65 retinoid isomerization activity

Regeneration of visual chromophore in the vertebrate visual cycle involves the retinal pigment epithelium-specific protein RPE65, the key enzyme catalyzing the cleavage and isomerization of all-trans-retinyl fatty acid esters to 11-cis-retinol. Although RPE65 has no predicted membrane spanning domains, this protein predominantly associates with microsomal fractions isolated from bovine retinal pigment epithelium (RPE). We have re-examined the nature of RPE65 interactions with native microsomal membranes by using extraction and phase separation experiments. We observe that hydrophobic interactions are the dominant forces that promote RPE65 association with these membranes. These results are consistent with the crystallographic model of RPE65, which features a large lipophilic surface that surrounds the entrance to the catalytic site of this enzyme and likely interacts with the hydrophobic core of the endoplasmic reticulum membrane. Moreover, we report a critical role for phospholipid membranes in preserving the retinoid isomerization activity and physical properties of RPE65. Isomerase activity measured in bovine RPE was highly sensitive to phospholipase A(2) treatment, but the observed decline in 11-cis-retinol production did not directly reflect inhibition by products of lipid hydrolysis. Instead, a direct correlation between the kinetics of phospholipid hydrolysis and retinoid isomerization suggests that the lipid membrane structure is critical for RPE65 enzymatic activity. We also provide evidence that RPE65 operates in a multiprotein complex with retinol dehydrogenase 5 and retinal G protein-coupled receptor in RPE microsomes. Modifications in the phospholipid environment affecting interactions with these protein components may be responsible for the alterations in retinoid metabolism observed in phospholipid-depleted RPE microsomes. Thus, our results indicate that the enzymatic activity of native RPE65 strongly depends on its membrane binding and phospholipid environment.

Interfacial binding dynamics of bee venom phospholipase A2 investigated by dynamic light scattering and quartz crystal microbalance

Bee venom phospholipase A(2) (bvPLA(2)) is part of the secretory phospholipase A(2) (sPLA(2)) family whose members are active in biological processes such as signal transduction and lipid metabolism. While controlling sPLA(2) activity is of pharmaceutical interest, the relationship between their mechanistic actions and physiological functions is not well understood. Therefore, we investigated the interfacial binding process of bvPLA(2) to characterize its biophysical properties and gain insight into how membrane binding affects interfacial activation. Attention was focused on the role of membrane electrostatics in the binding process. Although dynamic light scattering experiments indicated that bvPLA(2) does not lyse lipid vesicles, a novel, nonhydrolytic activity was discovered. We employed a supported lipid bilayer platform on the quartz crystal microbalance with dissipation sensor to characterize this bilayer-disrupting behavior and determined that membrane electrostatics influence this activity. The data suggest that (1) adsorption of bvPLA(2) to model membranes is not primarily driven by electrostatic interactions; (2) lipid desorption can follow bvPLA(2) adsorption, resulting in nonhydrolytic bilayer-disruption; and (3) this desorption is driven by electrostatic interactions. Taken together, these findings provide evidence that interfacial binding of bvPLA(2) is a dynamic process, shedding light on how membrane electrostatics can modulate interfacial activation.

Sequence of physical changes to the cell membrane during glucocorticoid-induced apoptosis in S49 lymphoma cells

During apoptosis, physical changes in the plasma membrane prepare the cell for clearance by phagocytes and hydrolysis by secretory phospholipase A(2) (sPLA(2)). The relationships among these changes have not been adequately established, especially for hormone-stimulated apoptosis. This study addresses these issues for glucocorticoid-induced apoptosis in S49 lymphoma cells. Flow cytometry, microscopy, and fluorescence spectroscopy were used to assess merocyanine 540 emission, laurdan generalized polarization, phosphatidylserine exposure, caspase activation, and membrane permeability to propidium iodide in the absence and presence of sPLA(2). The earliest event observed was activation of cellular caspases. Results with membrane probes suggest that interlipid spacing also increases early during apoptosis and precedes transbilayer migration of phosphatidylserine, DNA fragmentation, and a general increase in lipid order associated with blebbing and dissolution of the cells. The activity of sPLA(2) appeared to be linked more to lipid spacing than to loss of membrane asymmetry. The early nature of some of these events and their ability to promote activity of a proinflammatory enzyme suggests the possibility of an inflammatory response during T-lymphocyte apoptosis.

Activation of phospholipase A2 by temporin B: formation of antimicrobial peptide-enzyme amyloid-type cofibrils

Phospholipases A2 have been shown to be activated in a concentration dependent manner by a number of antimicrobial peptides, including melittin, magainin 2, indolicidin, and temporins B and L. Here we used fluorescently labelled bee venom PLA2 (PLA2D) and the saturated phospholipid substrate 1,2-dipalmitoyl-glycero-sn-3-phosphocholine (L-DPPC), exhibiting a lag-burst behaviour upon the initiation of the hydrolytic reaction by PLA2. Increasing concentrations of Cys-temporin B and its fluorescent Texas red derivative (TRC-temB) caused progressive shortening of the lag period. TRC-temB/PLA2D interaction was observed by Förster resonance energy transfer (FRET), with maximum efficiency coinciding with the burst in hydrolysis. Subsequently, supramolecular structures became visible by microscopy, revealing amyloid-like fibrils composed of both the activating peptide and PLA2. Reaction products, palmitic acid and 1-palmitoyl-2-lyso-glycero-sn-3-phosphocholine (lysoPC, both at >8 mol%) were required for FRET when using the non-hydrolysable substrate enantiomer 2,3-dipalmitoyl-glycero-sn-1-phosphocholine (D-DPPC). A novel mechanism of PLA2 activation by co-fibril formation and associated conformational changes is suggested.

Pro-inflammatory secretory phospholipase A2 type IIA binds to integrins alphavbeta3 and alpha4beta1 and induces proliferation of monocytic cells in an integrin-dependent manner

Secretory phospholipase A2 group IIA (sPLA2-IIA) plays an important role in the pathogenesis of inflammatory diseases. Catalytic activity of this enzyme that generates arachidonic acid is a major target for development of anti-inflammatory agents. Independent of its catalytic activity, sPLA2-IIA induces pro-inflammatory signals in a receptor-mediated mechanism (e.g. through the M-type receptor). However, the M-type receptor is species-specific: sPLA2-IIA binds to the M-type receptor in rodents and rabbits, but not in human. Thus sPLA2-IIA receptors in human have not been established. Here we demonstrated that sPLA2-IIA bound to integrin alphavbeta3 at a high affinity (K(D)=2 x 10(-7) M). We identified amino acid residues in sPLA2-IIA (Arg-74 and Arg-100) that are critical for integrin binding using docking simulation and mutagenesis. The integrin-binding site did not include the catalytic center or the M-type receptor-binding site. sPLA2-IIA also bound to alpha4beta1. We showed that sPLA2-IIA competed with VCAM-1 for binding to alpha4beta1, and bound to a site close to those for VCAM-1 and CS-1 in the alpha4 subunit. Wild type and the catalytically inactive H47Q mutant of sPLA2-IIA induced cell proliferation and ERK1/2 activation in monocytic cells, but the integrin binding-defective R74E/R100E mutant did not. This indicates that integrin binding is required, but catalytic activity is not required, for sPLA2-IIA-induced proliferative signaling. These results suggest that integrins alphavbeta3 and alpha4beta1 may serve as receptors for sPLA2-IIA and mediate pro-inflammatory action of sPLA2-IIA, and that integrin-sPLA2-IIA interaction is a novel therapeutic target.

The interaction of phospholipase A2 with a phospholipid bilayer: coarse-grained molecular dynamics simulations

A number of membrane-active enzymes act in a complex environment formed by the interface between a lipid bilayer and bulk water. Although x-ray diffraction studies yield structures of isolated enzyme molecules, a detailed characterization of their interactions with the interface requires a measure of how deeply such a membrane-associated protein penetrates into a lipid bilayer. Here, we apply coarse-grained (CG) molecular dynamics (MD) simulations to probe the interaction of porcine pancreatic phospholipase A2 (PLA2) with a lipid bilayer containing palmitoyl-oleoyl-phosphatidyl choline and palmitoyl-oleoyl-phosphatidyl glycerol molecules. We also used a configuration from a CG-MD trajectory to initiate two atomistic (AT) MD simulations. The results of the CG and AT simulations are evaluated by comparison with available experimental data. The membrane-binding surface of PLA2 consists of a patch of hydrophobic residues surrounded by polar and basic residues. We show this proposed footprint interacts preferentially with the anionic headgroups of the palmitoyl-oleoyl-phosphatidyl glycerol molecules. Thus, both electrostatic and hydrophobic interactions determine the location of PLA2 relative to the bilayer. From a general perspective, this study demonstrates that CG-MD simulations may be used to reveal the orientation and location of a membrane-surface-bound protein relative to a lipid bilayer, which may subsequently be refined by AT-MD simulations to probe more detailed interactions.

Amyloid-type fiber formation in control of enzyme action: interfacial activation of phospholipase A2

The lag-burst behavior in the action of phospholipase A(2) (PLA(2)) on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was investigated at temperatures slightly offset from the main phase transition temperature T(m) of this lipid, thus slowing down the kinetics of the activation process. Distinct stages leading to maximal activity were resolved using a combination of fluorescence parameters, including Förster resonance energy transfer between donor- and acceptor-labeled enzyme, fluorescence anisotropy, and lifetime, as well as thioflavin T fluorescence enhancement. We showed that the interfacial activation of PLA(2), evident after the preceding lag phase, coincides with the formation of oligomers staining with thioflavin T and subsequently with Congo red. Based on previous studies and our findings here, we propose a novel mechanism for the control of PLA(2), involving amyloid protofibrils with highly augmented enzymatic activity. Subsequently, these protofibrils form "mature" fibrils, devoid of activity. Accordingly, the process of amyloid formation is used as an on-off switch to obtain a transient burst in enzymatic catalysis.

Lipoprotein electrostatic properties regulate hepatic lipase association and activity

The effect of lipoprotein electrostatic properties on the catalytic regulation of hepatic lipase (HL) was investigated. Enrichment of serum or very low density lipoprotein (VLDL) with oleic acid increased lipoprotein negative charge and stimulated lipid hydrolysis by HL. Similarly, enrichment of serum or isolated lipoproteins with the anionic phospholipids phosphatidylinositol (PI), phosphatidic acid, or phosphatidylserine also increased lipoprotein negative charge and stimulated hydrolysis by HL. Anionic lipids had a small effect on phospholipid hydrolysis, but significantly stimulated triacylglyceride (TG) hydrolysis. High density lipoprotein (HDL) charge appears to have a specific effect on lipolysis. Enrichment of HDL with PI significantly stimulated VLDL-TG hydrolysis by HL. To determine whether HDL charge affects the association of HL with HDL and VLDL, HL-lipoprotein interactions were probed immunochemically. Under normal circumstances, HL associates with HDL particles, and only small amounts bind to VLDL. PI enrichment of HDL blocked the binding of HL with HDL. These data indicate that increasing the negative charge of HDL stimulates VLDL-TG hydrolysis by reducing the association of HL with HDL. Therefore, HDL controls the hydrolysis of VLDL by affecting the interlipoprotein association of HL. Lipoprotein electrostatic properties regulate lipase association and are an important regulator of the binding and activity of lipolytic enzymes.

Secretory phospholipase A2-α from Arabidopsis thaliana: functional parameters and substrate preference

The secretory phospholipase A2-alpha from Arabidopsis thaliana (AtsPLA2-alpha), being one of the first plant sPLA2s obtained in purified state, has been characterised with respect to substrate preference and optimum conditions of catalysis. The optima of pH, temperature, and calcium concentration were similar to the parameters of secretory PLA2s from animals. However, substrate preferences markedly differed. In contrast to pancreatic PLA2s, AtsPLA2-alpha preferred zwitterionic phospholipids, and showed lower activity toward anionic phospholipids. In substrates with two identical fatty acid chains, AtsPLA2-alpha showed optimum activity toward phospholipids with decanoyl groups. In substrates with palmitoyl groups in sn-1 position, acyl chains with higher degree of unsaturation in sn-2 position were preferred, excluding arachidonic acid, showing the evolutionary adaptation of the enzyme to substrate composition in plants. Km values for short chain phospholipids were comparable to sPLA2s from animals, whereas k cat values were much smaller and interfacial activation was less important.

Relationship between membrane physical properties and secretory phospholipase A2 hydrolysis kinetics in S49 cells during ionophore-induced apoptosis

During apoptosis, changes occur in lymphocyte membranes that render them susceptible to hydrolysis by secretory phospholipase A(2) (sPLA(2)). To study the relevant mechanisms, a simplified model of apoptosis using a calcium ionophore was applied. Kinetic and flow cytometry experiments provided key observations regarding ionophore treatment: the initial rate of hydrolysis was elevated at all enzyme concentrations, the total amount of reaction product was increased fourfold, and adsorption of the enzyme to the membrane surface was unaltered. Analysis of these results suggested that susceptibility during calcium-induced apoptosis is limited by availability of substrate rather than adsorption of enzyme. Fluorescence experiments identified three membrane alterations during apoptosis that might affect substrate access to the sPLA(2) active site. First, intercalation of merocyanine 540 into the membrane was improved, suggesting an increase in lipid spacing. Second, laurdan detected increased solvation of the lower headgroup region of the membrane. Third, the rate at which fluorescent lipids could be removed from the membrane by albumin was enhanced, implying greater vertical mobility of phospholipids. Thus, it is proposed that the membranes of apoptotic cells become susceptible to sPLA(2) through a reduction in lipid-neighbor interactions that facilitates migration of phospholipids into the enzyme active site.

Distribution of reaction products in phospholipase A2 hydrolysis

We have monitored the composition of supported phospholipid bilayers during phospholipase A(2) hydrolysis using specular neutron reflection and ellipsometry. Porcine pancreatic PLA(2) shows a long lag phase of several hours during which the enzyme binds to the bilayer surface, but only 5+/-3% of the lipids react before the onset of rapid hydrolysis. The amount of PLA(2), which resides in a 21+/-1 A thick layer at the water-bilayer interface, as well as its depth of penetration into the membrane, increase during the lag phase, the length of which is also proportional to the enzyme concentration. Hydrolysis of a single-chain deuterium labelled d(31)-POPC reveals for the first time that there is a significant asymmetry in the distribution of the reaction products between the membrane and the aqueous environment. The lyso-lipid leaves the membrane while the number of PLA(2) molecules bound to the interface increases with increasing fatty acid content. These results constitute the first direct measurement of the membrane structure and composition, including the location and amount of the enzyme during hydrolysis. These are discussed in terms of a model of fatty-acid mediated activation of PLA(2).

Optically trapping confocal Raman microscopy of individual lipid vesicles: kinetics of phospholipase A(2)-catalyzed hydrolysis of phospholipids in the membrane bilayer

Phospholipase A2 (PLA2)-catalyzed hydrolysis at the sn-2 position of 1,2-dimyristoyl-sn-glycero-3-phosphocholine in optically trapped liposomes is monitored in situ using confocal Raman microscopy. Individual optically trapped liposomes (0.6 microm in diameter) are exposed to PLA2 isolated from cobra (Naja naja naja) venom at varying enzyme concentrations. The relative Raman scattering intensities of C-C stretching vibrations from the trans and gauche conformers of the acyl chains are correlated directly with the extent of hydrolysis, allowing the progress of the reaction to be monitored in situ on a single vesicle. In dilute vesicle dispersions, the technique allows the much higher local concentration of lipid molecules in a single vesicle to be detected free of interferences from the surrounding solution. Observing the local composition of an optically trapped vesicle also allows one to determine whether the products of enzyme-catalyzed hydrolysis remain associated with the vesicle or dissolve into solution. The observed reaction kinetics exhibited a time lag prior to the rapid hydrolysis. The lag time varied inversely with the enzyme concentration, which is consistent with the products of enzyme-catalyzed lipid hydrolysis reaching a critical concentration that allows the enzyme to react at a much faster rate. The turnover rate of membrane-bound enzyme determined by Raman microscopy during the rapid, burst-phase kinetics was 1200 s(-1). Based on previous measurements of the equilibrium for PLA2 binding to lipid membranes, the average number of enzyme molecules responsible for catalyzing the hydrolysis of lipid on a single optically trapped vesicle is quite small, only two PLA2 molecules at the lowest enzyme concentration studied.

A micelle nucleation model for the interaction of dodecyl sulphate with Lys49–phospholipases A2

Bothropstoxin-I (BthTx-I) is a Lys49-PLA(2) from the venom of Bothrops jararacussu that lacks detectable catalytic activity, yet causes rapid Ca(2+)-independent membrane damage. With the aim of understanding the interaction between BthTx-I and amphiphilic molecules, we have studied the interaction of sodium dodecyl sulphate (SDS) with the protein. Circular dichroism and attenuated total reflection Fourier-transform infrared spectra of BthTx-I reveal changes in the alpha-helical organization of the protein at an SDS/BthTx-I molar ratio of 20-25. At SDS/BthTx-I ratios of 40-45 the alpha-helices return to a native-like conformation, although fluorescence emission anisotropy measurements of 2-amino-N-hexadecyl-benzamide (AHBA) demonstrate that the total SDS is below the critical micelle concentration when this transition occurs. These results may be interpreted as the result of SDS accumulation by the BthTx-I homodimer and the formation of a pre-micelle SDS/BthTx-I complex, which may subsequently be released from the protein surface as a free micelle. Similar changes in the alpha-helical organization of BthTx-I were observed in the presence of dipalmitoylphosphatidylcholine liposomes, suggesting that protein structure transitions coupled to organization changes of bound amphiphiles may play a role in the Ca(2+)-independent membrane damage by Lys49-PLA(2)s.

Structural and functional effects of tryptophans inserted into the membrane-binding and substrate-binding sites of human group IIA phospholipase A2

Phospholipase A(2) (PLA(2)) enzymes become activated by binding to biological membranes and hydrolyze phospholipids to free fatty acids and lyso-phospholipids, the precursors of inflammatory mediators. To understand the functional significance of amino acid residues at key positions, we have studied the effects of the substitution of Val(3) (membrane binding surface) and Phe(5) (substrate binding pocket) of human group IIA PLA(2) by tryptophan on the structure and function of the enzyme. Despite the close proximity of the sites of mutations, the V3W mutation results in substantial enhancement of the enzyme activity, whereas the F5W mutant demonstrates significantly suppressed activity. A structural analysis of all three proteins free in buffer and bound to membranes indicates that large differences in activities result from distinct conformational changes in PLA(2)s upon membrane binding. Although PLA(2) and the V3W mutant demonstrate a decrease in helical content and an increase in helix flexibility, the F5W mutant experiences partial distortion of the alpha-helical structure presumably resulting from the tendency of Trp(5) to insert into the membrane. Furthermore, whereas the PLA(2) and the V3W mutant bind to the membrane at similar and apparently productive-mode orientation, the F5W mutant binds to membranes with a distinctly different orientation. It is suggested that both the stimulatory effect of the V3W mutation and the inhibitory effect of the F5W mutation result from the high affinity of Trp for the membrane-water interface. Although Trp(3) at the membrane binding face of PLA(2) facilitates the proper membrane binding of the enzyme, Trp(5) in the internal substrate binding site causes partial unwinding of the N-terminal helix in order to interact with the membrane.

Isoform-specific membrane insertion of secretory phospholipase A2 and functional implications

Despite increasing evidence that the membrane-binding mode of interfacial enzymes including the depth of membrane insertion is crucial for their function, the membrane insertion of phospholipase A(2) (PLA(2)) enzymes has not been studied systematically. Here, we analyze the membrane insertion of human group IB PLA(2) (hIBPLA(2)) and compare it with that of a structurally homologous V3W mutant of human group IIA PLA(2) (V3W-hIIAPLA(2)) and with a structurally divergent group III bee venom PLA(2) (bvPLA(2)). Increasing the anionic charge of membranes results in a blue shift of the fluorescence of Trp(3) of hIBPLA(2), a decrease in quenching by acrylamide, and an increase in enzyme activity, reflecting an enhancement in the membrane binding of PLA(2). Fluorescence quenching by brominated lipids indicates significant penetration of Trp(3) into fluid POPC/POPG membranes but little insertion into the solid DPPC/DPPG membranes. Increased membrane fluidity also supports hIBPLA(2) activity, suggesting that membrane insertion of hIBPLA(2) is controlled by membrane fluidity and is necessary for the full activity of the enzyme. Trp fluorescence quenching of the V3W-hIIAPLA(2) and bvPLA(2) by water- and membrane-soluble quenchers indicates substantial membrane insertion of Trp(3) of V3W-hIIAPLA(2), similar to that found for hIBPLA(2), and no insertion of tryptophans of bvPLA(2). Our results provide evidence that (a) structurally similar group IB and IIA PLA(2)s, but not structurally diverse group III PLA(2), significantly penetrate into membranes; (b) membrane insertion is controlled by membrane fluidity and facilitates activation of IB and IIA PLA(2)s; and (c) structurally distinct PLA(2) isoforms may employ different tactics of substrate accession/product release during lipid hydrolysis.

A new hat for an old enzyme: Waste management

The history of research regarding secretory phospholipase A(2) (sPLA(2)) has often focused in one of two directions. Originally, the enzyme was studied biophysically in terms of its fundamental structure, enzymology, and the relationship between membrane physics and catalytic activity. More recently, a large and growing body of information has accumulated concerning regulatory factors, tissue distribution, and physiological/pathological roles of sPLA(2). Evidence is presented that suggests an additional function for the protein in which it helps to clear dead and damaged cells while avoiding digestion of those that are healthy. Apparently, the ability of the enzyme to discriminate between susceptible and resistant cells depends on physical properties of membrane lipids related to order, distribution, and neighbor/neighbor interactions. Investigations into this action of the enzyme offer the rare opportunity to apply biophysical approaches and principles to a physiological setting.

Characterization of the kinetics of phospholipase C activity toward mixed micelles of sodium deoxycholate and dimyristoylphosphatidylcholine

Phospholipase C catalyzed hydrolysis of dimyristoyl phosphatidylcholine (DMPC) in phospholipid-bile salt mixed micelles was studied with particular attention on the relationship between interfacial enzyme activity and the physicochemical properties of substrate aggregates. Steady state kinetics is observed and it is argued that conditions for steady state exist because the enzyme encounters a steady supply of substrate by hopping between micelles at a rate faster than the chemical reaction rate. An existing kinetic model is reformulated to a more usable form. This presents a new approach to treating the kinetic data and allows extraction of the kinetic parameters of the model from the activity dependence on micellar lipid substrate surface concentration. The kinetic parameters were found to depend on the physicochemical properties of substrate aggregates, but remain constant over a range of lipid and bile salt concentrations. The substrate aggregates were characterized by time-resolved fluorescence quenching (TRFQ). The activity values and the micelle sizes group into two sets: (i) larger micelles for bile salt/lipid <or=5 showing higher activity and shorter steady state duration (<or=4 min) and (ii) smaller micelles for bile salt/lipid > 5 with lower activity and longer steady state ( approximately 10 min). At least two sets of parameters, for bile salt/lipid <or=5 and >5, characterize the kinetics. Higher enzyme-micelle dissociation constant and lower catalytic rate are found for the group of smaller micelles. An explanation supporting our finding is that as micelles become smaller the overlap area for enzyme-micelle binding decreases, leading to weaker binding. Consequently the enzyme dissociation constant increases. Extension of the present approach to other phospholipases and substrates to establish its generality and correlation between micelle size and the catalytic rate are areas for future investigations.

Effects of interactions on the formation of mixed micelles of 1,2-diheptanoyl-sn-glycero-3-phosphocholine with sodium dodecyl sulfate and dodecyltrimethylammonium bromide

Mixed micelles of the phospholipid 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC) with sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB) in aqueous solutions and the effects of interactions between the components were studied by fluorescence and NMR measurements. The regular solution theory (RST) was applied to analyze the experimental critical micelle concentration values determined from the fluorescence spectra of pyrene in the mixed micelles. Negative values for the interaction parameter (beta12) were obtained for both DHPC + SDS and DHPC + DTAB mixtures, with the value being more negative in the former case. The negative beta12 values for the two systems imply that the interaction between the phospholipid and the two ionic surfactants is attractive in nature, being more intense in the case of DHPC + SDS. The interaction parameter, beta12, varies with composition of the mixtures indicating changes in packing. The proton NMR shifts are quite different for the two systems and also vary with composition. An interpretation of these experimentally determined chemical shifts in terms of the degree of compactness attributed to electrostatic and steric interactions in the mixed micelle supports the conclusions derived from the fluorescence cmc experiments.

Hydrolysis of fluid supported membrane islands by phospholipase A(2): Time-lapse imaging and kinetic analysis

The activity of phospholipase A(2) (PLA(2)) which catalyzes the hydrolysis of phospholipids into free fatty acids and lysolipids, depends on the structure and thermodynamic state of the membrane. To further understand how the substrate conformation correlates with enzyme activity, model systems that are based on time-resolved membrane microscopy are needed. We demonstrate a methodology for preparing and investigating the dynamics of fluid supported phospholipid membranes hydrolyzed by snake venom PLA(2). The method uses quantitative analysis of time-lapse fluorescence images recording the evolution of fluid bilayer islands during hydrolysis. In order to minimize interactions with the support surface, we use double bilayer islands situated on top of a complete primary supported membrane prepared by hydration of spincoated lipid films. Our minimal kinetic analysis describes adsorption of enzyme to the membrane in terms of the Langmuir isotherm as well as enzyme kinetics. We use two related models assuming hydrolysis to occur either at the perimeter or at the surface of the membrane island. We find that the adsorption constant is similar for the two cases, while the estimated turnover rate is markedly different. The PLA(2) concentration series is measured in the absence and presence of beta-cyclodextrin which forms water soluble complexes with the reaction products. The results demonstrate the versatility of double bilayer islands as a membrane model system and introduces a new method for quantifying the kinetics of lipase activity on membranes by directly monitoring the evolution in substrate morphology.

Computational Analysis of the Membrane Association of Group IIA Secreted Phospholipases A2: A Differential Role for Electrostatics

Secreted phospholipases A2 (sPLA2's) are enzymes that hydrolyze glycerophospholipids at the sn-2 position, which leads to the production of lipid mediators of many cellular processes. These interfacial enzymes are regulated by their lipid specificity at two levels: membrane binding and substrate recognition. Different sPLA2's utilize different combinations of electrostatic and hydrophobic interactions to adsorb to membrane surfaces, which results in the wide range of membrane binding behaviors observed. Here, the finite difference Poisson Boltzmann (FDPB) method is used to quantitatively analyze the contribution of electrostatic interactions to the membrane association of two highly basic group II sPLA2's: Agkistrodon piscivorus piscivorus (AppD49) sPLA2 and nonpancreatic human group IIA (hGIIA) sPLA2. The calculations predict how membrane binding is affected by ionic strength, membrane composition, substitutions of residues in the enzymes, and the presence of calcium in the active site. In addition, the results provide molecular models for the membrane-associated forms of the enzymes. Furthermore, these models account for (1) changes in orientation and protonation state of both the native and charge reversal forms of the enzymes at the membrane surface and (2) the effect of protein/vesicle aggregation, as observed for hGIIA sPLA2. Importantly, the modeling quantitatively describes the complex membrane binding behaviors of these interfacial enzymes in terms of simple physical forces and provides structural information that is difficult to obtain experimentally. The computational analysis shows that nonspecific electrostatic interactions not only play a major role in recruiting these enzymes to membrane surfaces but also orient the enzymes for productive catalysis at the membrane interface.

Evidence for the Regulatory Role of the N-terminal Helix of Secretory Phospholipase A2 from Studies on Native and Chimeric Proteins

The phospholipase A(2) (PLA(2)) enzymes are activated by binding to phospholipid membranes. Although the N-terminal alpha-helix of group I/II PLA(2)s plays an important role in the productive mode membrane binding of the enzymes, its role in the structural aspects of membrane-induced activation of PLA(2)s is not well understood. In order to elucidate membrane-induced conformational changes in the N-terminal helix and in the rest of the PLA(2), we have created semisynthetic human group IB PLA(2) in which the N-terminal decapeptide is joined with the (13)C-labeled fragment, as well as a chimeric protein containing the N-terminal decapeptide from human group IIA PLA(2) joined with a (13)C-labeled fragment of group IB PLA(2). Infrared spectral resolution of the unlabeled and (13)C-labeled segments suggests that the N-terminal helix of membrane-bound IB PLA(2) has a more rigid structure than the other helices. On the other hand, the overall structure of the chimeric PLA(2) is more rigid than that of the IB PLA(2), but the N-terminal helix is more flexible. A combination of homology modeling and polarized infrared spectroscopy provides the structure of membrane-bound chimeric PLA(2), which demonstrates remarkable similarity but also distinct differences compared with that of IB PLA(2). Correlation is delineated between structural and membrane binding properties of PLA(2)s and their N-terminal helices. Altogether, the data provide evidence that the N-terminal helix of group I/II PLA(2)s acts as a regulatory domain that mediates interfacial activation of these enzymes.

The Highly Selective Production of 2-Arachidonoyl Lysophosphatidylcholine Catalyzed by Purified Calcium-independent Phospholipase A2γ

Herein, we report the heterologous expression of the human peroxisomal 63-kDa calcium-independent phospholipase A2gamma (iPLA2gamma) isoform in Sf9 cells, purification of the N-terminal His-tagged enzyme by affinity chromatography, and the identification of its remarkable substrate selectivity that results in the highly selective generation of 2-arachidonoyl lysophosphatidylcholine. Mass spectrometric analyses demonstrated that purified iPLA2gamma hydrolyzed saturated or monounsaturated aliphatic groups readily from either the sn-1 or sn-2 positions of phospholipids. In addition, purified iPLA2gamma effectively liberated arachidonic acid from the sn-2 position of plasmenylcholine substrates. In contrast, incubation of iPLA2gamma with 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine resulted in the rapid release of palmitic acid and the selective accumulation of 2-arachidonoyl lysophosphatidylcholine (LPC), which was not metabolized further by iPLA2gamma. The putative regiospecificity of the 2-arachidonoyl LPC product was authenticated by its diagnostic fragmentation pattern during tandem mass spectrometric analysis. To identify the physiological relevance of iPLA2gamma-mediated 2-arachidonoyl LPC production utilizing naturally occurring membranes, we incubated purified rat hepatic peroxisomes with iPLA2gamma and similarly identified the selective accumulation of 2-arachidonoyl LPC. Furthermore, tandem mass spectrometric analysis demonstrated that 2-arachidonoyl LPC is a natural product in human myocardium, a tissue in which iPLA2gamma expression is robust. Because 2-arachidonoyl LPC represents a key branch point intermediate that can potentially lead to a variety of bioactive molecules in eicosanoid signaling (e.g. arachidonic acid, 2-arachidonoylglycerol), these results have uncovered a novel eicosanoid selective pathway through iPLA2gamma-mediated 2-arachidonoyl LPC production to amplify and diversify the repertoire of biologic lipid second messengers in response to cellular stimulation.

Phospholipid containing mixed micelles

Mixed micelles of l,2-diheptanoyl-sn-grycero-3-phosphocholine (DHPC) with ionic detergents were prepared to develop well characterized substrates for the study of lipolytic enzymes. The aggregates that formed on mixing DHPC with the anionic surfactant sodium dodecyl sulfate (SDS) and with the positively charged dodecyl trimethylammonium bromide (DTAB) were investigated using time-resolved fluorescence quenching (TRFQ) to determine the aggregation numbers and bimolecular collision rates, and electron spin resonance (ESR) to measure the hydration index and microviscosity of the micelles at the micelle-water interface. Mixed micelles between the phospholipid and each of the detergents formed in all compositions, yielding interfaces with varying charge, hydration, and microviscosity. Both series of micelles were found to be globular up to 0.7 mole fraction of DHPC, while the aggregation numbers varied within the same concentration range of the components less than 15%. Addition of the zwitterionic phospholipid component increased the degree of counterion dissociation as measured by the quenching of the fluorescence of pyrene by the bromide ions bound to DHPC/DTAB micelles, showing that at 0.6 mole fraction of DHPC 80% of the bromide ions are dissociated from the micelles. The interface water concentration decreased significantly on addition of DHPC to each detergent. For combined phospholipid and detergent concentration of 50 mM the interface water concentration decreased, as measured by ESR of the spin-probes, from 38.5 M/L of interface volume in SDS alone to 9 M/L when the phospholipid was present at 0.7 mole fraction. Similar addition of DHPC to DTAB decreased the interfacial water concentration from 27 M/L to 11 M/L. Determination of the physicochemical parameters of the phospholipid containing mixed micelles here presented are likely to provide important insight into the design of assay systems for kinetic studies of phospholipid metabolizing enzymes.

Mechanisms governing the level of susceptibility of erythrocyte membranes to secretory phospholipase A2

Although cell membranes normally resist the hydrolytic action of secretory phospholipase A(2) (sPLA(2)), they become susceptible during apoptosis or after cellular trauma. Experimentally, susceptibility to the enzyme can be induced by loading cells with calcium. In human erythrocytes, the ability of the calcium ionophore to cause susceptibility depends on temperature, occurring best above approximately 35 degrees C. Considerable evidence from experiments with artificial bilayers suggests that hydrolysis of membrane lipids requires two steps. First, the enzyme adsorbs to the membrane surface, and second, a phospholipid diffuses from the membrane into the active site of the adsorbed enzyme. Analysis of kinetic experiments suggested that this mechanism can explain the action of sPLA(2) on erythrocyte membranes and that temperature and calcium loading promote the second step. This conclusion was further supported by binding experiments and assessment of membrane lipid packing. The adsorption of fluorescent-labeled sPLA(2) was insensitive to either temperature or ionophore treatment. In contrast, the fluorescence of merocyanine 540, a probe sensitive to lipid packing, was affected by both. Lipid packing decreased modestly as temperature was raised from 20 to 60 degrees C. Calcium loading enhanced packing at temperatures in the low end of this range, but greatly reduced packing at higher temperatures. This result was corroborated by measurements of the rate of extraction of a fluorescent phosphatidylcholine analog from erythrocyte membranes. Furthermore, drugs known to inhibit susceptibility in erythrocytes also prevented the increase in phospholipid extraction rate. These results argue that the two-step model applies to biological as well as artificial membranes and that a limiting step in the hydrolysis of erythrocyte membranes is the ability of phospholipids to migrate into the active site of adsorbed enzyme.

Secondary structure, conformational stability and glycosylation of a recombinant Candida rugosa lipase studied by Fourier-transform infrared spectroscopy

The secondary structure of lipase 1 from Candida rugosa, a model system for large monomeric enzymes, has been studied by FTIR (Fourier-transform infrared) spectroscopy in water and 2H2O. The secondary structure content, determined by the analysis of the amide I band absorption through second derivative and curve fitting procedures, is in agreement with that estimated by X-ray data and predicts, in addition, the existence of two classes of alpha-helices. We have also investigated the enzyme stability and aggregation at high temperature by following the protein unfolding. The thermal stability determined by FTIR is in excellent agreement with the temperature dependence of the lipase activity. Furthermore, new insights on the glycosylation of the recombinant protein produced in Pichia pastoris and on its heterogeneity related to different fermentation batches were obtained by the analysis of the IR absorption in the 1200-900 cm(-1) carbohydrate region. A drastic reduction of the intensity of this band was found after enzymic deglycosylation of the protein. To confirm that the FTIR absorption in the 1200-900 cm(-1) region depends on the carbohydrate content and glycoform distribution, we performed an MS analysis of the protein sugar moieties. Glycosidic structures of the high mannose type were found, with mannoses ranging from 8 to 25 residues.

The N-terminal α-Helix of Pancreatic Phospholipase A2 Determines Productive-mode Orientation of the Enzyme at the Membrane Surface

Phospholipase A(2) (PLA(2)) hydrolyzes glycerophospholipids to free fatty acid and lyso-phospholipid, which serve as precursors for the biosynthesis of eicosanoids and other lipid-derived mediators of inflammation and allergy. PLA(2) activity strongly increases upon binding to the surface of aggregated phospholipid. The N-terminal approximately ten residue alpha-helix of certain PLA(2) isoforms plays important roles in the interfacial activation of the enzyme by providing residues for membrane binding of PLA(2) and by contributing to the formation of the substrate-binding pocket. However, the relative contributions of the N-terminal alpha-helix and the rest of the protein in membrane binding of PLA(2) and its productive-mode orientation at the membrane surface are not well understood. Here we use a variety of biophysical approaches to determine the role of the N-terminal helix in membrane binding strength, orientation, and activity of human pancreatic PLA(2). While the full-length PLA(2) binds to membranes with a defined orientation, an engineered PLA(2) fragment DeltaN10 that lacks the N-terminal ten residues binds to membranes with weaker affinity and at random orientation, and exhibits approximately 100-fold lower enzymatic activity compared to the full-length PLA(2), indicating the key role of the N terminus in PLA(2) function. The results of polarized infrared spectroscopic experiments permit determination of the orientation of membrane-bound PLA(2) and identification of its interfacial binding surface. Moreover, the full-length PLA(2) demonstrates increased conformational flexibility in solution and is stabilized upon membrane binding, while the DeltaN10 fragment is more rigid than the full-length PLA(2) both in free and membrane-bound states. Our results suggest that the N-terminal alpha-helix supports the activation of PLA(2) by (a) enhancing the membrane binding strength, (b) facilitating a productive-mode orientation of PLA(2) at the membrane surface, and (c) conferring conformational integrity and plasticity to the enzyme.

Activation of Ca2+-independent membrane-damaging activity in Lys49–phospholipase A2 promoted by amphiphilic molecules

Association of class-II phospholipase A(2) (PLA(2)) with aggregated phospholipid substrate results in elevated levels of the Ca(2+)-dependent hydrolytic activity. The Asp49 residue participates in coordination of the Ca(2+) ion cofactor, however, in Lys49-PLA(2) homologues (Lys49-PLA(2)s), substitution of the Asp49 by Lys results in loss of Ca(2+) binding and lack of detectable phospholipid hydrolysis. Nevertheless, Lys49-PLA(2)s cause Ca(2+)-independent damage of liposome membranes. Bothropstoxin-I is a homodimeric Lys49-PLA(2) from the venom of Bothrops jararacussu, and in fluorescent marker release and dynamic light scattering experiments with DPPC liposomes we demonstrate activation of the Ca(2+)-independent membrane damaging activity by approximately 4 molecules of sodium dodecyl sulphate (SDS) per protein monomer. Activation is accompanied by significant changes in the intrinsic tryptophan fluorescence emission (ITFE) and near UV circular dichroism (UVCD) spectra of the protein. Subsequent binding of 7-10 SDS molecules results in further alterations in the ITFE and far UVCD spectra. Reduction in the rate of N-bromosuccinimide modification of Trp77 at the dimer interface suggests that initial binding of SDS to this region accompanies the activation of the membrane damaging activity. 1-anilinonaphthalene-8-sulphonic acid binding studies indicate that subsequent SDS binding to the active site is concomitant with the second structural transition. These results provide insights in the structural basis of amphiphile/protein coupling in class-II PLA(2)s.

Membrane-related effects underlying the biological activity of the anthraquinones emodin and barbaloin

Commercial plant extracts containing anthraquinones are being increasingly used for cosmetics, food and pharmaceuticals due to their wide therapeutic and pharmacological properties. In this work, the interaction with model membranes of two representative 1,8-dihydroxyanthraquinones, barbaloin (Aloe) and emodin (Rheum, Polygonum), has been studied in order to explain their effects in biological membranes. Emodin showed a higher affinity for phospholipid membranes than barbaloin did, and was more effective in weakening hydrophobic interactions between hydrocarbon chains in phospholipid bilayers. Whereas emodin induced the formation of hexagonal-H(II) phase, barbaloin stabilized lamellar structures. Barbaloin promoted the formation of gel-fluid intermediate structures in phosphatidylglycerol membranes at physiological pH and ionic strength values. It is proposed that emodin's chromophore group is located at the upper half of the membrane, whereas barbaloin's one is in a deeper position but having its glucopyranosyl moiety near the phospholipid/water interface. Moreover, membrane disruption by emodin or barbaloin showed specificity for the two major phospholipids present in bacterial membranes, phosphatidylethanolamine and phosphatidylglycerol. In order to relate their strong effects on membranes to their biological activity, the capacity of these compounds to inhibit the infectivity of the viral haemorrhagic septicaemia rhabdovirus (VHSV), a negative RNA enveloped virus, or the growth of Escherichia coli was tested. Anthraquinone-loaded liposomes showed a strong antimicrobial activity whereas these compounds in their free form did not. Both anthraquinones showed antiviral activity but only emodin was a virucidal agent. In conclusion, a molecular mechanism based on the effect of these compounds on the structure of biological membranes is proposed to account for their multiple biological activities. Anthraquinone-loaded liposomes may suppose an alternative for antimicrobial, pharmaceutical or cosmetic applications.

Differential hydrolysis of erythrocyte and mitochondrial membrane phospholipids by two phospholipase A2 isoenzymes (NK-PLA2-I and NK-PLA2-II) from the venom of the Indian monocled cobra Naja kaouthia

We previously demonstrated that venom from the Indian monocled cobra Naja kaouthia is a rich source of phospholipase A2 enzymes, and we purified and characterized a major PLA2 isoenzyme (NK-PLA2-I) from N. kaouthia venom. In the present study, we report the purification and biochemical characterization of a second PLA2 isoenzyme (NK-PLA2-II) from the same venom. A comparison of the membrane phospholipid hydrolysis patterns by these two PLA2s has revealed that they cause significantly more damage to mitochondrial membranes (NK-PLA2-I > NK-PLA2-II) as compared to erythrocyte membranes due to more efficient binding of the enzymes to mitochondrial membranes. Fatty acid release patterns by these PLA2s from the membrane phospholipid PC-pools indicate that NK-PLA2-I does not discriminate between saturated and unsaturated fatty acids whereas NK-PLA2-II shows a preference for unsaturated fatty acids during the initial phase of attack. The current investigation provides new insight into the molecular arrangement of NK-PLA2-sensitive domains in erythrocyte and mitochondrial membranes and highlights the contribution of polar, but uncharged, amino acids such as serine and cysteine in NK-PLA2 induced membrane damage.

Characterization of Arabidopsis secretory phospholipase A2-gamma cDNA and its enzymatic properties

Plant secretory phospholipases A(2) (sPLA(2)s) probably play important roles in phospholipid signaling based on the data reported from other organisms, but their functions are poorly understood because of the lack of cloned sPLA(2) genes. In this study, we cloned and characterized an Arabidopsis secretory phospholipase A(2)-gamma (AtsPLA(2)-gamma) cDNA, and examined its enzymatic properties. The recombinant protein of AtsPLA(2)-gamma showed maximal enzyme activity at pH 8.0, and required Ca(2+) for activity. Moreover, AtsPLA(2)-gamma showed sn-2 position specificity but no prominent acyl preference, though it showed head group specificity to phosphatidylethanolamine rather than to phosphatidylcholine. AtsPLA(2)-gamma was found to predominate in the mature flower rather than in other tissues, and subcellular localization analysis confirmed that AtsPLA(2)-gamma is secreted into the intercellular space.

Different modulation of phospholipase A2 activity by saturated and monounsaturated N-acylethanolamines

The physiological functions of N-acylethanolamines (NAEs) are poorly understood, although many functions were suggested for these naturally occurring membrane components of plants and animals. The binding with cannabinoid receptors CB1 and CB2 was demonstrated for some NAEs, such as anandamide. However, the chemical nature of these molecules suggests that some of their biological effects on biomembranes could be related, at least partially, to physical interactions with the lipid bilayer. The present work studies the effect of saturated and monounsaturated NAEs on phospholipase A2 (PLA2) activity, which is dependent on lipid bilayer features. The present study, performed by 2-dimethylamino-(6-lauroyl)-naphthalene (Laurdan) fluorescence, demonstrates that the acyl chain length and the presence of a single double bond are crucial for the enzymatic activity modulation by NAEs. In fact, saturated NAEs with 10 carbon atoms don't affect the PLA2 activity, while NAEs with 12 and 16 carbon atoms largely activate the enzyme. On the other hand, an acyl chain length of 18 carbon atoms, with or without the presence of a double bond, only slightly affects the enzymatic activity. A structural model for NAE-lipid interactions is proposed in order to explain the differences in PLA2 activity modulation by these fatty acid derivatives.

Structural effects of covalent inhibition of phospholipase A2 suggest allosteric coupling between membrane binding and catalytic sites

Phospholipase A(2) (PLA(2)) binds to membranes and catalyzes phospholipid hydrolysis, thus initiating the biosynthesis of lipid-derived mediators of inflammation. A snake-venom PLA(2) was completely inhibited by covalent modification of the catalytic histidine 48 by p-bromophenacyl bromide. Moreover, His(48) modification affected PLA(2) structure, its membrane-binding affinity, and the effects of PLA(2) on the membrane structure. The native PLA(2) increased the order parameter of fluid membranes, whereas the opposite effect was observed for gel-state membranes. The data suggest membrane dehydration by PLA(2) and the formation of PLA(2)-membrane hydrogen bonding. The inhibited PLA(2) had lower membrane-binding affinity and exerted weaker effects on membrane hydration and on the lipid-order parameter. Although membrane binding resulted in formation of more flexible alpha-helices in the native PLA(2), which corresponds to faster amide hydrogen exchange, the modified enzyme was more resistant to hydrogen exchange and experienced little structural change upon membrane binding. The data suggest that 1), modification of a catalytic residue of PLA(2) induces conformational changes that propagate to the membrane-binding surface through an allosteric mechanism; 2), the native PLA(2) acquires more dynamic properties during interfacial activation via membrane binding; and 3), the global conformation of the inhibited PLA(2), including the alpha-helices, is less stable and is not influenced by membrane binding. These findings provide further evidence for an allosteric coupling between the membrane-binding (regulatory) site and the catalytic center of PLA(2), which contributes to the interfacial activation of the enzyme.

Influence of product phase separation on phospholipase A(2) hydrolysis of supported phospholipid bilayers studied by force microscopy

In situ atomic force microscopy studies reveal a marked influence of the initial presence of hydrolysis products on the hydrolysis of supported phospholipid bilayers by phospholipase A(2). By analysis of the nano-scale topography of a number of supported bilayers with different initial product concentrations, made by Langmuir-Blodgett deposition, we show that small depressions enriched in products are efficiently promoting enzyme degradation of the bilayer. These small depressions, which are indicative of phase separation, are initially present in samples with 75% products. The kinetics of phospholipase A(2) exhibit under certain conditions an initial phase of slow hydrolysis, termed the latency phase, followed by a marked increase in the hydrolysis rate. The appearance of the phase-separated bilayer is strikingly similar to that of bilayers at the end of the latency phase. By analysis of individual nano-scale defects we illustrate a quantitative difference in the growth rates of defects caused by product aggregation and other structural defects. This difference shows for the first time how the enzyme prefers one type of defect to another.

Monitoring the kinetics and thermodynamics of interfacial enzymatic catalysis by differential scanning calorimetry

Using phase transition profile as an indicator of thermodynamic property and phase transition heat as the second indicator of the percentage of substrates unhydrolyzed, differential scanning calorimetry has been used to observe in detail the kinetics and thermodynamics of phospholipase A(2)-catalyzed 1,2-dipalmitoyl-sn-glycero-3-phosphocholine large unilamellar vesicle (LUV) hydrolysis. Phase transition profiles show that the original LUV almost completely changes into a novel aggregate at the end of the latency, followed by an abrupt activation of the reaction. The phase transition profiles are asymmetric between the heating and cooling curves, indicating a thermodynamic mesostatic property of the system. The reaction in activated phase follows a single first-order kinetics and all of the substrates in vesicles can be hydrolyzed. All these evidences indicate that the products and substrates can freely exchange between the outer and the inner layers of the vesicles and the membrane of the vesicle in the activated phase is permeable. This permeability favors the exchange of the substrates and products, thus, resulting in the activation of the fast reaction.

Modulation of the susceptibility of human erythrocytes to snake venom myotoxic phospholipases A(2): role of negatively charged phospholipids as potential membrane binding sites

Cerrophidion (Bothrops) godmani myotoxins I (CGMT-I) and II (CGMT-II), Asp-49 and Lys-49 phospholipases A(2) (PLA2s), which drastically differ in enzymatic activity, were devoid of direct hemolytic effects on erythrocytes (RBC) from different species despite the fact that enzymatically active CGMT-I was able to hydrolyze RBC membrane phospholipids and disrupt liposomes prepared from RBC lipids. Human RBC did not become susceptible to the toxins after treatment with neuraminidase or after altering membrane fluidity with cholesterol or sublytic concentrations of detergent. Unlike normal RBC, significant hemolysis was induced by CGMT-II and another similar Lys-49 isoform, B. asper MT-II (BAMT-II), in RBC enriched with phosphatidylserine (PS). Hemolysis was greater in RBC preincubated with pyridyldithioethylamine (PDA), a potent inhibitor of aminophospholipid transport. RBC enriched with phosphatidic acid (PA) also became susceptible to the myotoxins but was unaffected by PDA. Cells enriched with phosphatidylcholine (PC) remained resistant to the action of the toxins. BAMT-II also induced damage in black lipid membranes prepared with PS but not PC alone. When RBC binding of BAMT-II was measured by enzyme-linked immunosorbent assay, it was observed that PS- and PA-enriched erythrocytes were always able to capture more toxin than normal and PC-enriched RBC. This effect was significantly improved by PDA (in the case of PS) and it was observed either in the presence or in the absence of calcium in the medium. These data suggest that negatively charged lipids in the outer leaflet of cell membranes constitute myotoxic PLA2 binding sites. The scarcity of anionic phospholipids in the outer leaflet of RBC could explain their resistance to the action of these PLA2s.