Phospholipases. I. Effect of n-alkanols on the rate of enzymatic hydrolysis of egg phosphatidylcholine

Secretory phospholipase A2 of group IIA: Is it an offensive or a defensive player during atherosclerosis and other inflammatory diseases?

Since its discovery in the serum of patients with severe inflammation and in rheumatoid arthritic fluids, the secretory phospholipase A2 of group IIA (sPLA2-IIA) has been chiefly considered as a proinflammatory enzyme, the result of which has been very intense interest in selective inhibitors of sPLA2-IIA in the hope of developing new and efficient therapies for inflammatory diseases. The recent discovery of the antibacterial properties of sPLA2-IIA, however, has raised the question of whether the upregulation of sPLA2-IIA during inflammation is to be considered uniformly negative and the hindrance of sPLA2-IIA in every instance beneficial. The aim of this review is for this reason, along with the results of various investigations which argue for the proinflammatory and proatherogenic effects of an upregulation of sPLA2-IIA, also to array data alongside which point to a protective function of sPLA2-IIA during inflammation. Thus, it could be shown that sPLA2-IIA, apart from the bactericidal effects, possesses also antithrombotic properties and indeed plays a possible role in the resolution of inflammation and the accelerated clearance of oxidatively modified lipoproteins during inflammation via the liver and adrenals. Based on these multipotent properties the knowledge of the function of sPLA2-IIA during inflammation is a fundamental prerequisite for the development and establishment of new therapeutic strategies to prevent and treat severe inflammatory diseases up to and including sepsis.

Excitement ahead: structure, function and mechanism of snake venom phospholipase A2 enzymes

Venom phospholipase A2 (PLA2) enzymes share similarity in structure and catalytic function with mammalian enzymes. However, in contrast to mammalian enzymes, many are toxic and induce a wide spectrum of pharmacological effects. Thus structure-function relationship of this group of small proteins is subtle, but complex puzzle to protein biochemists, molecular biologists, toxinologists, pharmacologists and physiologists. This review describes the present status of our understanding of their structure, function and mechanism. It was proposed that their unique ability to 'target' themselves to a specific organ or tissue is due to their high affinity binding to specific proteins which act as receptors (more precisely, acceptors). This specific binding of PLA2 is conferred by the presence of a 'pharmacological site' on its surface which is independent of the catalytic site. The high affinity interaction of PLA2 with its acceptor (or target protein) is probably due to the complementarity, in terms of charges, hydrophobicity and van der Waal's contact surfaces, between the pharmacological site and the binding site on the surface of the acceptor protein. Upon binding to the target, the PLA2 can induce its pharmacological effects by mechanisms either dependent on or independent of its catalytic activity. Because of the unprecedented wide spectrum of specific targeting to various tissues and organs, identification of the pharmacological sites has potential for exploitation in development of novel systems useful for 'delivering' specific proteins to a particular target tissue or organ. Thus research in this field will provide a lot of exciting opportunities.

Effect of diacylglycerols on the activity of cobra venom, bee venom, and pig pancreatic phospholipases A2

The effects of a series of diacylglycerols (DAGs) with varying acyl chain lengths and degree of unsaturation on the activity of cobra venom, bee venom, and pig pancreatic phospholipases A2 (PL-A2S) were studied using two lipid substrates: dipalmitoylphosphatidylcholine (DPPC) or bovine liver phosphatidylcholine (BL-PC). The activities of the phospholipases critically depended on the chain length and degree of unsaturation of the added DAGs and on the chemical composition of the substrate. The effects of DAGs on cobra or bee venom PL-A2S were similar, but significantly different from the pig pancreatic PL-A2. The data, taken together with our previous NMR studies on physicochemical effects of these DAGs on lipid bilayer structure [De Boeck, H., & Zidovetzki, R. (1989) Biochemistry 28, 7439; (1992) Biochemistry 31, 623], allowed detailed correlation of the type of a bilayer perturbation induced by DAG with the activation or inhibition of the phospholipase on the same system. In general, the activation of the phospholipases correlated with the DAG-induced defects of the lipid bilayer structure. The results, however, argue against general designation of DAGs as "activators" or "inhibitors" of PL-A2S. Thus, for example, diolein activated phospholipases with the BL-PC lipid substrate, but inhibited them with the DPPC substrate. Dihexanoylglycerol and dioctanoylglycerol inhibited pig pancreatic PL-A2 with both lipid substrates and inhibited cobra or been venom PL-A2 with the DPPC substrate, but activated the latter two enzymes with the BL-PC substrate. Longer-chain DAGs (C greater than 12), which induce lateral phase separation of the bilayers into the regions of different fluidities, activated all PL-A2S with both lipid substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetaminophen fails to inhibit ethanol-induced subjective effects in human volunteers

In animals, ethanol causes some of its CNS effects by releasing prostaglandins (PG); this is demonstrated by reports that prostaglandin synthetase inhibitors (PGSIs) diminish ethanol-induced effects. However, use of animals in these studies has precluded testing for subjective effects. We studied the interaction of ethanol and acetaminophen, a PGSI, in a double-blind crossover experiment. Six adult males were given no drug or acetaminophen (0, 325, 650, 1300 or 1950 mg) 75 min before ethanol (total dose = 0.625 g/kg; five divided doses). Physiologic, subjective and performance measures were collected. Compared to the no drug condition, ethanol significantly increased ratings of drug "liking," "drunk," "sluggish" and "drug strength" and decreased ratings of "sober." Ethanol increased heart rate and acetaminophen did not diminish or enhance this effect. The failure to antagonize ethanol-induced subjective and physiologic effects by acetaminophen in humans may be due to species differences or inadequate dosage of the PGSI. It is also possible that subjective and certain physiologic effects of ethanol in humans are not mediated by prostaglandin-dependent neural processes. Nevertheless, the finding that at greater than typical analgesic doses, acetaminophen failed to prevent subjective effects of ethanol is of clinical significance.

Ethanol inhalation and dietary n-6, n-3, and n-9 fatty acids in the rat: effect on platelet and aortic fatty acid composition

The effects of 18-carbon n-6, n-3, and n-9 fatty acid diets and ethanol exposure on the fatty acyl composition of platelets and vascular tissue were examined. An experimental design was devised to control the dietary content of 18-carbon fatty acids. The levels of 18:3n6, 18:3n3 and 18:1n9 were varied by a formulation of dietary oils which contained similar proportions of 18:2n6. Male Sprague-Dawley rats were fed a purified diet containing 11% by weight of either borage oil (BOR) rich in 18:3n6, linseed/safflower oil (LSO) rich in 18:3n3, or sesame oil (SES) rich in 18:1n9 for 7 weeks and exposed to ethanol vapors by means of inhalation for the final 6 days of the dietary regimen. Moderate blood ethanol levels of 118 +/- 6.6 mg/dl were obtained. Total lipids were extracted from platelets and aortae, and the fatty acid distributions were analyzed by gas chromatography. BOR feeding resulted in increases in the proportion of n-6 fatty acids (18:3n6, 20:3n6, 20:4n6) in platelets and aorta. Animals fed the LSO diet had increased levels of n-3 fatty acids (18:3n3, 20:5n3, 22:6n3). The SES-based diet resulted in an increase in 18:1n9 in both aorta and platelets. Following ethanol exposure alone, the most marked change in the fatty acid profile was a decrease in 20:4n6 in the platelet. This effect was not observed in rats supplemented with BOR. No significant changes were observed in the aortic fatty acid content at this level of ethanol exposure. The results suggested that, in the rat, a diet enriched with BOR effectively prevented ethanol-induced alterations in platelet fatty acid composition.

Phospholipase A2 at the bilayer interface

Interfacial catalysis is a necessary consequence for all enzymes that act on amphipathic substrates with a strong tendency to form aggregates in aqueous dispersions. In such cases the catalytic event occurs at the interface of the aggregated substrate, the overall turnover at the interface is processive, and it is influenced the molecular organization and dynamics of the interface. Such enzymes can access the substrate only at the interface because the concentration of solitary monomers of the substrate in the aqueous phase is very low. Moreover, the microinterface between the bound enzyme and the organized substrate not only facilitates formation of the enzyme-substrate complex, but a longer residence time of the enzyme at the substrate interface also promotes high catalytic processivity. Binding of the enzyme to the substrate interface as an additional step in the overall catalytic turnover permits adaptation of the Michaelis-Menten formalism as a basis to account for the kinetics of interfacial catalysis. As shown for the action of phospholipase A2 on bilayer vesicles, binding equilibrium has two extreme kinetic consequences. During catalysis in the scooting mode the enzyme does not leave the surface of the vesicle to which it is bound. On the other hand, in the hopping mode the absorption and desorption steps are a part of the catalytic turnover. In this minireview we elaborate on the factors that control binding of pig pancreatic phospholipase A2 to the bilayer interface. Binding of PLA2 to the interface occurs through ionic interactions and is further promoted by hydrophobic interactions which probably occur along a face of the enzyme, with a hydrophobic collar and a ring of cationic residues, through which the catalytic site is accessible to substrate molecules in the bilayer. An enzyme molecule binds to the surface occupied by about 35 lipid molecules with an apparent dissociation constant of less than 0.1 pM for the enzyme on anionic vesicles compared to 10 mM on zwitterionic vesicles. Results at hand also show that aggregation or acylation of the protein is not required for the high affinity binding or catalytic interaction at the interface.

A nuclear magnetic resonance study of the interactions of the antimalarials chloroquine, quinacrine, quinine and mefloquine with lipids extracted from normal human erythrocytes

The interaction of four antimalarials (chloroquine, quinacrine, mefloquine and quinine) with lipid membranes re-formed from lipid extracts of normal human erythrocytes was studied using 2H- and lipid extracts of normal human erythrocytes was studied using 2H- and 31P-nuclear magnetic resonance (NMR). Inclusion of small amounts of chain-perdeuterated dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylethanolamine (DPPE) as an 2H-NMR probe allowed us to study separately the effects of drugs on PC or PE components of the membranes. Only a very small decrease in the order parameters of the DPPE, but not the DPPC probe, was observed in the presence of chloroquine at a molar ratio of 1:5 of drug to lipid. Addition of quinacrine at the same molar ratio resulted in a small but significant decrease in the order parameters of the lipid side chains; identical effects were obtained with DPPC or DPPE perdeuterated probes. The presence of quinacrine did not induce non-bilayer lipid phases. In contrast, mefloquine and quinine produced a significant disordering of the lipid side chains; the effect was considerably larger with the DPPE probe. In addition, both mefloquine and quinine induced non-bilayer phases of the lipids; mefloquine induced formation of hexagonal and micellar lipid conformation, whereas addition of quinine resulted in the formation of lipid micelles only. The lipid polymorphism induced by either of these drugs was more pronounced when the DPPE component was observed, indicating that the non-bilayer phases were enriched in PE. The results suggest the presence of strong interactions between mefloquine and quinine with lipid bilayers, especially with the PE component.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol-induced modifications to membrane lipid structure: effect on phospholipase A2-membrane interactions

Chronic ethanol intoxication leads to the development of a resistance to lipid disordering by ethanol, a phenomenon known as "membrane tolerance". In the absence of the added ethanol, the lipid order, as measured by ESR and fluorescence techniques, does not necessarily change as a result of chronic ethanol ingestion (as in liver microsomes, for example). This suggests that the spectroscopic techniques detect tolerance somewhat indirectly, in that the modification responsible may reside in a region distinct from that being probed and also raises the question of whether membrane tolerance is necessarily associated with an alteration in the membrane lipid structure. Here we show that liver microsomes from rats treated chronically with ethanol are rendered relatively resistant to the hydrolytic action of exogenous phospholipase A2, compared to preparations from control animals. This resistance persists in reconstituted lipid vesicles prepared from extracted phospholipids. Since the same substrate (1-palmitoyl-2-N-(4-nitrobenzo-2-oxa-1,3-diazole)amino caproylphosphatidylcholine) was used in both membranes from ethanol-treated animals and controls, the modification appears to reside in the structure and/or organization of the membrane. Further evidence that the lipid structure is modified by chronic ethanol treatment is provided by the observation that perturbance of the membrane structural integrity by increasing levels of oleic acid led to a progressive loss of the ethanol-induced relative resistance to hydrolysis by phospholipase A2. The results of this study support the idea that membrane tolerance involves a modification to lipid structure probably at the bilayer surface. The use of exogenous phospholipase A2 provides a new method for probing the structural modifications induced by chronic ethanol ingestion.

Flow microcalorimetry investigation of the influence of surfactants on a heterogeneous aerobic culture

The influence of various surfactants on the biological activity of a mixed aerobic culture has been investigated by using flow microcalorimetry. The response of the culture to the addition of homologous n-alkylcarboxylates (C2 to C16) and n-alkylpyridinium bromides (C11 to C14) has been examined under endogenous and substrate saturation conditions, and inhibitory concentrations (MIC or the concentration which decreased the initial activity (heat flux) of the culture by 50%) were determined for each state. Under both conditions, the n-alkylpyridinium bromides were found to be more toxic than the n-alkylcarboxylates of identical chain length, thus confirming that the head group of the amphiphiles plays an important role in the microbial toxicity of surfactants. The relationship observed between the concentration at which 50% of the activity is lost and the chain length of the surfactant further confirms that cellular toxicity is also dependent on surfactant hydrophobicity. In relation to the biodegradability of surfactants in mixed aerobic cultures, the low concentration effects of n-alkylcarboxylates on endogenous culture were investigated in some detail. There appear to be compounded indications that these surfactants are rapidly metabolized by the microorganisms of the mixed culture, at least for homologs lower than C10.

Phospholipase C and the physical states of polar head groups of lipids

Activity of phospholipase C from Clostridium perfringens on liposomes made from sn-3-phosphatidylcholine, dimyristoyl (DMPC), dipalmitoyl (DPPC) or distearoyl (DSPC) was measured at various temperatures and was correlated with their gel/liquid-crystalline phase transitions (Tc:23, 41.5, 52 degrees C for DMPC, DPPC, DSPC, respectively). In all cases, the activity of phospholipase C was high in the gel phases of the substrates and was almost zero in their liquid-crystalline phases. Fluorescence depolarization measurements of N-dansyl-sn-3-phosphatidylethanolamine (DPE) and 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated into the liposomes showed that both the head group and the alkyl chains of the lipids were immobilized in the gel phases but were highly mobile in the liquid-crystalline phase. These results indicate that the rotational mobility of lipids (both of the head groups and the alkyl chains) was not a major factor in the phospholipase C reaction. It is inferred that some electrostatic and/or hydrophobic interactions might play important roles in regulation of the phospholipase C activity.

Hydrolysis of short-chain phosphatidylcholines by bee venom phospholipase A2

In order to find out the aggregation state of the substrate, preferred by bee venom phospholipase A2 (EC 3.1.1.4), its action on short-chain phosphatidylcholines with two identical (C6-C10) fatty acids has been tested. The rate of hydrolysis as a function of acyl chain length showed a maximum at dioctanoylphosphatidylcholine. The effects of alcohols, NaCl and Triton X-100, which affect the aggregation state of phospholipids in water, were also studied. The addition of n-alcohol led to a significant inhibition of the hydrolysis of the substrates present in micellar form and activated the hydrolysis of substrates which form liposomes. The inhibitory effect increased with increasing length of the aliphatic carbon chain of the alcohol. Triton X-100 at low Triton/phospholipid molar ratios enhanced enzyme activity. These results do not agree with the accepted idea that bee venom phospholipase A2 hydrolyzes short-chain lecithins in their molecularly dispersed form and that micelles cannot act as substrates. The data indicate that short-chain lecithins in the aggregated state are hydrolyzed and that the requirements of bee venom phospholipase A2 for the aggregation state of the substrate are not strict.

Effect of ethanol on prostacyclin and thromboxane A2 synthesis in rat aortic rings in vitro

Spontaneous 6-keto-PGF1 alpha and TXB2 levels in isolated rat aortic rings increased in a concentration dependent manner after a 0.5 hour incubation with moderate or high ethanol concentrations (11 mM to 218 mM). After a 1 hour incubation with moderate concentrations of ethanol less than or equal to 22 mM) spontaneous prostaglandin (PG) production did not increase although high concentrations (87 mM and 218 mM) increased both 6-keto-PGF1 alpha and TXB2 levels. Similarly, in the presence of 40 microM Na-arachidonate, high ethanol concentrations increased PG production after 0.5 and 1 hour incubation. In addition, either a 4 or an 8 hour exposure to high ethanol concentrations increased spontaneous PG production. A moderate concentration of ethanol (22 mM) increased the 6-keto-PGF1 alpha/TXB2 ratio whereas high levels (greater than or equal to 87 mM) depressed the ratio after 0.5 and 1 hour exposure. This effect was short-lived since after 4 or 8 hours incubation with high ethanol concentrations the 6-keto-PGF1 alpha/TXB2 ratio was markedly increased. The alcohol-induced changes in both spontaneous and arachidonate-stimulated PG levels were concentration dependent and related to the incubation time. Furthermore, these data suggest that there may be unbalanced production of PGI2 and thromboxane A2 in vascular tissue exposed to alcohol.

Effect of antituberculous calixarenes on phospholipase A2 susceptibility and on fusion of phospholipid bilayers

Some homologous calixarenes or polyoxyethylene ethers that are known to suppress or enhance experimental tuberculous infection (depending on their polyoxyethylene chain lengths) were examined for their effects on phospholipid bilayers. The effect of these solutes is seen at 0.5-50p.p.m., and their effect depends upon their structure as well as that of the phospholipid substrate. The antituberculous compound HOC-12.5 (Macrocyclon) inhibits susceptibility to pig pancreatic phospholipase A2 action and to aggregation/fusion of the ternary co-dispersions of dimyristoyl phosphatidylcholine + 1-palmitoyl lysophosphatidylcholine + palmitic acid (50:11:11 molar proportions). In contrast, the protuberculous compound HOC-60 stimulates these effects. Differential scanning calorimetry suggests that these effects are probably due to modulation of the phase equilibrium in substrate bilayers by these polyethers.

Bone marrow genotoxicity of N-methyl, N-nitrosourea (NMU): n-alkanols as sister chromatid exchange (SCE) anti-inducers

The in vivo SCE test was used to demonstrate significant inhibition of NMU bone marrow genotoxicity by pretreatment of Chinese hamsters with n-alkanols. Our findings exclude a loss of intracellular DNA alkylation potential through a competitive direct reaction of NMU with the weakly nucleophilic polar end of the n-alkanols, but not through methylations of nucleophilic membrane sites possibly liberated by structural modifications which the membrane-active amphiphilics induce.

The alkaline phospholipase A1 of rat liver cytosol

1. Rat liver cytosol contains a heat-sensitive phospholipase A1 active against phosphatidylethanolamine, 1-acylglycerophosphoethanolamine and, to a very much lesser extent, phosphatidylcholine and phosphatidylinositol. 2. Activity towards a pure phosphatidylethanolamine substrate is invoked by the presence of water-soluble cations that do not precipitate at the pH optimum of the enzyme (9.5). In this activation bivalent cations, e.g. Mg2+, Ca2+, Mn2+, Sr2+ and Ba2+, are effective at much lower concentrations (2.5-5 mM) than univalent cations K+, Na+ and NH4+ (100 mM). 3. In the absence of such cations the enzyme can be activated by cationic amphiphiles containing quaternary nitrogen or by basic proteins. 4. It is concluded that these agents activate the enzyme by reducing the negative zeta potential on the substrate at the high pH optimum (9.5) and allow interaction with the enzyme whose isoelectric point is at 7.15. 5. The activated enzyme is markedly inhibited by mixing the phosphatidylethanolamine substrate with many other phospholipids that exist in cell membranes, e.g. phosphatidylcholine, phosphatidylinositol. On the other hand, both phosphatidylcholine and phosphatidylinositol can be hydrolysed much more readily if they are mixed with an excess of phosphatidylethanolamine. 6. Such results on the inhibition and substrate specificity of the enzyme, coupled with birefringence measurements, allow the tentative conclusion that phospholipid substrates are only attacked when they exist in a hexagonal or non-bilayer structure and not in the bilayer (lamellar) form.

Use of phospholipase A to compare phospholipid organization in synaptic membranes, myelin, and liposomes

The pattern of fatty acid release from rat synaptic membranes in the presence of phospholipase A2 (Vipera russelli) was compared to that from liposomes comprised of phospholipids. Phospholipase A2 more readily attacked myelin and synaptic membranes than liposomes prepared from total phospholipids derived from myelin. Although hydrolysis of liposomal phospholipids occurred in the absence of added calcium, the presence of 2 mM CaCl2 or 2% bovine serum albumin significantly enhanced the phospholipase attack of liposomes, but not synaptic membranes of myelin. Phospholipase exhibited a marked preference for phospholipids containing docosahexaenoic acid (22:6) in the synaptic membranes, while with liposomes the pattern of released fatty acid reflected the fatty acid composition in the two-position of the phospholipids. Although either calcium or albumin markedly increased the phospholipase hydrolysis of liposomes, neither affected the hydrolysis of synaptic membranes or the pattern of fatty acid release from liposomes. It was concluded that the nonlipid constituents, particularly the proteins, of biomembranes were responsible for the organization of the phospholipids and accounted for the observed differences between liposomes and synaptic membranes with respect to enzymic accessibility.

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.

Steady-state fluorescence polarization (FP) of 1,6-diphenyl-1,3,5-hexatriene (dph)-labelled platelets in the presence of bradykinin (bk)

Steady-state fluorescence polarization (FP) decreases, when 1,6-diphenyl-1,3,5-hextriene (dph) labelled platelets are exposed to bradykinin (bk). At pH 8, the dose-response curve is bell-shaped with an optimum bk effect at 10(-7) M. In contrast to the ricinoleic-acid ester of glycerin-polyethyleneglycol, cremophor EL (CEL), bk is no more effective when platelets are pretreated with 10(-5) M p-bromophenacylbromide (B phi B). These results suggest that platelets are target cells for the peptide bk, which induces an FP decrease indirectly by stimulating the release of non-saturated fatty acids in the platelet membrane.

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.

Initial membrane reaction in peptidoglycan synthesis: perturbation of lipid-phospho-N-acetylmuramyl-pentapeptide translocase interactions by n-butanol

Phospho-N-acetylmuramyl-pentapeptide translocase, the initial membrane enzyme in the biosynthesis of peptidoglycan, requires a lipid microenvironment for function. n-Butanol was reversibly intercalated into membranes to perturb the hydrophobic interactions in this microenvironment in order to define further the role of lipid. In the concentration range for maximal stimulation of enzymic activity (0.12-0.18 M), n-butanol causes a 40% decrease in the fluorescence emission of the dansylated product, undecaprenyl diphosphate-(N epsilon-dansyl)pentapeptide. Since no change in emission maximum occurs below 22 degrees C in the presence of 0.12 M n-butanol, it is concluded that intercalation of this alkanol causes an increase in fluidity. Above 22 degrees C this concentration of n-butanol causes both a decrease in the fluorescence emission and a red shift in the emission maximum. It is concluded that a polarity change as well as fluidity change occurs above 22 degrees C. n-Butanol also causes a significant change in the phase transition experienced by the dansylated lipid product. Thus, it is possible with n-alkanols, e.g. n-butanol, to perturb lipid-translocase interactions resulting in an increase in fluidity in the microenvironment of the enzyme. This change in fluidity correlates with a stimulation of enzymic activity.

On the question of an electrokinetic requirement for phospholipase C action

The phospholipase C of clostridium welchii (alpha toxin) has an absolute requirement for trace quantities of Ca2+. It attacks pure phosphatidylcholine particles (smectic mesophases) having a close-packed bilayer structure only when their surface zeta potential is made positive by the addition of certain divalent ions (e.g., Ca2+) to the aqueous phase or by the presence of low concentrations of long chain cations to the lipid. Alternatively, if the rotational freedom of individual phospholipid molecules is increased by the insertion of short n-alkanols (e.g., hexanol) into the bilayer or when a monolayer of the substrate at an air/water interface is expended, enzymic hydrolysis can occur without any requirement for a net postive charge on the surface.

Parenteral administration of phospholipase C in the rat. Distribution, elimination, and lethal doses

Infusions of tissue thromboplastin induce intravascular coagulation in animals. Phospholipase C (PLC) (EC 3.1.4.3) from Bacillus cereus has a marked protective effect against such infusions and might be of value in the therapy of certain types of intravascular coagulation. We have therefore studied the toxicity, half-life, and effect on lipolysis of purified PLC in rats, using parenteral administration of 14C-labelled enzyme. Following intravenous injection, the plasma half-life was 5.2-5.4 min, and LD50 was approximately 1.6-1.7 mg/kg. The effect of PLC on lipolysis was moderate. The enzyme does not appear to be bound to any plasma macromolecules, and there was no accumulation of labelled enzyme in tissues other than kidney.

Hydrolysis of phosphatidylcholine liposomes by phospholipases A2. Effects of the local anesthetic dibucaine

(1) Dibucaine evokes a downward shift in the phase transition temperature of saturated phosphatidylcholines, while it also affects the pretransition. (2) The binding of dibucaine to phosphatidylcholine liposomes increases sharply when the lipid is transformed from the gel phase to the liquid-crystalline phase. (3) The activity of Naja naja phospholipase A2 towards dimyristoyl phosphatidylcholine liposomes is either stimulated or inhibited by dibucaine, depending on whether the substrate is in the gel or the liquid-crystalline state, respectively, whereas the activity of pancreatic phospholipase A2 is inhibited by the anesthetic irrespective of the physical state of the substrate. This observation is further substantiated by the results of studies on liposomes prepared from mixtures of dimyristoyl and dipalmitoyl phosphatidylcholine or dilauroyl and distearoyl phosphatidylcholine. (4) The uptake of dibucaine by positively charged liposomes composed of phosphatidylcholine and stearylamine is considerably reduced in comparison with pure phosphatidylcholine liposomes. This decrease is paralleled by a reduction of the inhibitory and stimulatory effects of dibucaine on the hydrolysis of such liposomes by pancreatic and Naja naja phospholipase, respectively. (5) The inhibitory action of dibucaine towards the pancreatic phospholipase is lowered by increasing CaCl2 concentrations. This reduction is accompanied by a decreased uptake of anesthetic by the liposomes.

Phospholipases. III. Effects of ionic surfactants on the phospholipase-catalyzed hydrolysis of unsonicated egg lecithin liposomes

Apparent values of Km and Vmax have been measured for catalysis of hydrolysis of unsonicated egg lecithin liposomes, activated through addition of 0.4 M n-hexanol, by phospholipases A2 from bee and snake venoms and by phospholipase C from Clostridium welchii as a function of the concentration of three surfactants: hexadecylamine, hexadecyltrimethylammonium bromide, and dihexadecyl phosphate. For all three enzymes, values of Km and Vmax show little or no dependence on the concentration of these ionic surfactants, demonstrating that the liposomal surface charge is not a crucial factor in determining susceptibility to phospholipase-catalyzed hydrolysis.