Subcellular localization of the phospholipases A of rat heart: evidence for a cytosolic phospholipase A1

Lipids in muscles and adipose tissues, changes during processing and sensory properties of meat products

Dry-cured meat products represent a large part of the meat products on the European market. The technologies develop for these products lead to the production of a large scale of meat products with typical sensory traits. Numerous studies have been devoted to optimise the quality traits of these products which are considered as traditional products by the consumer and provide a high added value to the producer. Among the components of the raw material, lipids play a key role in the final quality of these products. Many sensory traits of dry-cured meat products depend on lipid traits of muscle and adipose tissues of fresh meat and on their degradation through a complex set of lipolytic and oxidative reactions during processing. Lipid traits of both muscle and adipose tissues of fresh meat are strongly related to pig rearing conditions, mainly genotype and feeding strategy. During processing, lipids undergo intense lipid hydrolysis controlled by both lipases and phospholipases, which remain active all along the process. Lipids are also subjected to oxidation, which generates numerous volatile compounds. These volatiles contribute to some typical aroma notes of dry-cured meat products such as rancid, aged ham and dry-cured odours. This paper reviews the recent knowledge on the influence of lipid traits of fresh meat, lipid hydrolysis and oxidation on the development of sensory traits of dry-cured meat products.

Studies on the endogenous phospholipids of chick embryo myocardium and theirin vitro hydrolysis by endogenous phospholipases during embryogenesis

The phospholipid profiles of the myocardium (from 10- and 18-day old chick embryos and 13-day old chick) and their in vitro response to the endogenous lipolytic enzymes (mainly of the phospholipase group) at pH 7.4 and 38 degrees C for 60 min were analyzed by TLC technology and densitometry. Cardiolipin (CL) was shown to be one of the major phospholipids of the chick embryo myocardium and its concentration increased as the chick embryo advanced in development. Monolysocardiolipin (MLCL) was produced subsequent to in vitro incubation of whole tissue homogenates in all myocardia studied as well as a concurrent reduction in CL. This deacylation of CL increased in magnitude as the chick embryo advanced in development indicating its age relatedness. The level of phosphatidyl ethanolamine (PE) plasmalogen was also high in all myocardia studied. Lyso alkenyl PE (LPE) was produced subsequent to in vitro incubation and its level increased as the chick embryo advanced in development, indicating PLA(2) action on the sn-2 fatty acid of PE. Phosphatidyl choline (PC) plasmalogen was also present in the chick embryo myocardium and its level increased gradually as the chick embryo advanced in development. In contrast, yolk-sac membrane contains very minute amounts of CL and PE. No PC was detected and no LPE was formed following in vitro incubation. The yolk of the unfertilized chicken egg has no CL and has very minute amounts of PE, no PC and no lysophospholipids were detected following in vitro incubation in all samples analyzed.

On the differential lipolytic capabilities of rat spleen and cardiac muscle. An in vitro incubation in conjunction with chromatographic and densitometric analysis

The phospholipid profiles of newborn, young adult and aged rat heart and spleen and their in vitro response to endogenous phospholipases at pH 7.4 and 38 degrees C for 60 min were analysed by thin layer chromatography (TLC) technology and densitometry measurement. The noticeable high level of cardiolipin (CL) and its preferential deacylation, as detected by the formation of monolysocardiolipin (MLCL) and concurrent reduction of CL level were the most prevalent lipolytic events of rat cardiac muscle (newborn, young adult and aged) but the least prevalent in rat spleen. The level of ethanolamine plasmalogen (PE) was high in both the rat spleen and cardiac muscle (newborn, young adult and aged). Following in vitro incubation, the reduction in the level of PE and the high level of lyso alkenyl PE produced were most conspicuous in rat spleen (newborn, young adult and aged) and noticeably less in rat cardiac muscle. These data clearly illustrate the differential response of the endogenous substrates (phospholipids) to the endogenous phospholipases of these two tissues, and probably are related to their physiological activities in vivo.

Comparative studies of the endogenous phospholipids and theirin vitro hydrolysis by endogenous phospholipases of various tissues from 7-day-old chicks: a thin layer chromatographic and densitometric analysis

The phospholipid profiles of heart, kidney, and pectoral muscle of 7-day-old chicks and their in vitro response to the endogenous lipolytic enzymes (mainly in the phospholipase group) at pH 7.4 and 38 degrees C for 60 min were analysed by TLC technology and densitometry. The noticeable preferential deacylation of cardiolipin (CL) as detected by the formation of monolysocardiolipin (MLCL) and concurrent reduction of CL level were the most prevalent lipolytic events of chick cardiac muscle, but the least prevalent in chick pectoral muscle. Deacylation of ethanolamine plasmalogen (PE) as revealed by the formation of the corresponding lyso alkenyl derivative was also prominent in cardiac muscle, but much less so in kidney and none at all was detected in pectoral muscle. The level of sphingomyelin (SM) was much higher in kidney than heart and pectoral muscle. Following in vitro incubation, the reduction in the level of SM and the high level of ceramide (Cer) production were most conspicuous in kidney, less in cardiac muscle and least in pectoral muscle. The hydrolysis of PE and SM confirm the action of endogenous PLA(2) and endogenous sphingomyelinase on PE and SM respectively. These data clearly illustrate the differential response of the endogenous substrates (phospholipids) to the endogenous phospholipases of the tissues studied and are probably related to their physiological activities in vivo.

Age-related changes of the endogenous cardiolipin and plasmalogens of guinea pig kidney and theirin vitro hydrolysis by endogenous phospholipases: a thin layer chromatographic analysis in conjunction with densitometric measurement

The phosphoglycerides profile of guinea pig kidney, fetal, young adult, and aged, and their in vitro response to the endogenous lipolytic enzymes, mainly in the phospholipase group were determined by TLC technology in conjunction with densitometric measurement. Changes in phosphoglycerides profile subsequent to in vitro incubation of these tissues at pH 7.4, and 38 degrees C for 45 min and prior to phospholipid extraction has provided evidence relating to their respective lipolytic enzymes capabilities and age. These changes are mainly related to endogenous cardiolipin (CL), alkenyl phospholipids (phosphatidyl ethanolamine and phosphatidyl choline) and their endogenous deacylation to their respective lyso derivatives monolysocardiolipin (MLCL), lyso alkenyl phosphatidyl ethanolamine (LPE), and lyso alkenyl phosphatidyl choline (LPC) by endogenous phospholipases. The hydrolysis of the plasmalogen confirms the action of endogenous PLA(2) on sn-2 fatty acids of these compounds.

Activities of phospholipases A and lysophospholipases in glycolytic and oxidative skeletal muscles in the rabbit

Oxidative muscles contain more free fatty acids than glycolytic muscles, which could explain in part their higher sensitivity to oxidation. These fatty acids are partly the result of phospholipid hydrolysis catalysed by phospholipases A and lysophospholipases. Up to now, very little is known on the activities of these enzymes in skeletal muscles. This study deals with the activities of phospholipases A and lysophospholipases in five rabbit muscles covering a large range of metabolic types (oxidative Soleus and Semimembranosus proprius muscles, glycolytic Psoas major and Longissimus lumborum muscles and intermediate Gastrocnemius laterale muscle). The results showed that (a) phospholipases A and lysophospholipases had maximal activity at pH 8-9; (b) phospholipases A and lysophospholipases retained more than 50% of their maximal activity at pH 5.5-6, the ultimate pH of muscles; (c) lysophospholipases exhibited a higher activity than phospholipases A (4-7-fold higher in the oxidative muscles, 11-fold higher in the intermediate muscle and 18-23-fold higher in the glycolytic muscles); and (d) phospholipase A and lysophospholipase activities were higher in oxidative muscles than in glycolytic muscles (10-25-fold higher for phospholipases A and 4-5-fold higher for lysophospholipases). Thus oxidative muscles have a higher potential activity for post-mortem hydrolysis of phospholipids. © 2000 Society of Chemical Industry.

Purification of a lysophospholipase from bovine brain that selectively deacylates arachidonoyl-substituted lysophosphatidylcholine

A high activity lysophospholipase A (lysoPLA) was purified from the soluble fraction of bovine brain. The separation included sequential DEAE-Sephacel, phenyl-Sepharose FF, heparin-Sepharose CL-6B, and Q-Sepharose FF column chromatography. Mono Q, Sephacryl S300HR, and hydroxylapatite column chromatography in the presence of the detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate) and glycerol further purified the activity to 17,000-fold. The enzyme was purified to homogeneity by polyacrylamide gel electrophoresis using nondenaturing conditions. The pure enzyme migrated as a single polypeptide of 95 kDa mass by SDS-polyacrylamide gel electrophoresis and deacylated arachidonoyl-lysophosphatidylcholine (ara-lysoPC) at rate of 70 micromol/(min mg). The enzyme showed selectivity for arachidonoyl-substituted lysoPC, since palmitoyl-lysoPC was deacylated at a much lower rate (7 micromol/(min mg)). LysoPLA activity was maximal at pH 7.4-8.0 and was increased 1.3-fold by MgCl2 (5 mM). By including MgCl2, however, the range of optimal activity was expanded to pH values up to 9.0. The 95-kDa protein also deacylated arachidonoyl groups from 1-O-hexadecyl-2-arachidonoyl-PC (PLA2 activity) at a rate of 15 micromol/(min mg). Moreover, the deacylation of arachidonoyl groups from diacylPC was greatly increased by including purified bovine brain PLA1 in the reaction mixture. Thus, the same 95-kDa polypeptide catalyzed both lysoPLA and PLA2 activities, but the rate of arachidonoyl group deacylation was increased by prior sn-1 deacylation. Finally, the 95-kDa polypeptide cross-reacted with antibodies raised against a human recombinant cPLA2, implying that the 95-kDa protein is structurally similar to cPLA2. Additionally, these data suggest that the combined actions of PLA1 and the 95-kDa protein generate significant amounts of free arachidonic acid in the brain.

Purification and properties of a phosphatidic acid-preferring phospholipase A1 from bovine testis. Examination of the molecular basis of its activation

We recently identified a cytosolic phospholipase A1 activity in bovine brain and testis that preferentially hydrolyzes phosphatidic acid substrates. We also showed that the enzyme displays sigmoidal kinetics toward phosphatidic acid substrates in Triton X-100 mixed micelle assay system (Higgs, H.N., and Glomset J.A. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 9574-9578). In the present work we purified the bovine testis enzyme 14,000-fold and used a combination of size exclusion chromatography, labeling with the phospholipase A inhibitor, methyl arachidonyl fluorophosphonate, and SDS-polyacrylamide gel electrophoresis to provide evidence that it is a homotetramer of 110-kDa subunits. Studies of the molecular basis of the enzyme reaction in Triton micelles revealed that (a) a nonhydrolyzable sn-1-alkyl-2-oleoyl-analogue of phosphatidic acid activated the enzyme 30-fold in a sigmoidal fashion (Hill coefficient 3.2, EC50 4 mol %) without substantially affecting its preference for specific diacyl phosphoglyceride substrates, (b) the activator promoted tight binding of the enzyme to micelles, and (c) the enzyme's activity toward unsaturated phosphatidic acid substrates was affected by the location and nature of the fatty acyl chain double bonds.

Subcellular fractions of bovine brain degrade phosphatidylcholine by sequential deacylation of the sn-1 and sn-2 positions

Phosphatidylcholine (PC) metabolism was investigated using cytosol (fraction I) and particulate fractions of bovine brain that were enriched with microsomes (fraction II), plasma membranes (fraction III) or mitochondria (fraction IV). Fractions I-III incubated with 1-palmitoyl-2-[14C]arachidonoyl-sn-glycero-3-phosphocholine yielded [14C]arachidonic acid at near equal rates, whereas only fraction I accumulated significant amounts of 2-[14C]arachidonoyl-sn-glycero-3-phosphocholine. Much slower rates of arachidonic acid release were observed using an ether PC (1-O-hexadecyl-2-[3H]arachidonoyl-sn-glycero-3-phosphocholine). Moreover, arachidonic acid yield from the diacyl, but not ether PC was slowed by pretreating fractions I-III, but not IV, with phenylmethylsulfonyl fluoride (PMSF). Coincident with this decreased arachidonic acid, 2-[14C]arachidonoyl-sn-glycero-3-phosphocholine was increased, indicating high PLA1 activity. Taken together these data suggest that arachidonic release was largely dependent on initial deacylation of position sn-1. Incubating each untreated fraction with 2-[3-H]arachidonoyl-sn-glycero-3-phosphocholine yielded [3H]arachidonic acid (lysophospholipase A2 activity) at rate that was substantially greater than that using the comparable PMSF-treated fraction. Thus, the large effect of PMSF on arachidonic acid release can be accounted for if much of the fatty acid formation arose from the sequential sn-1 and sn-2 deacylation of diacyl-PC by phospholipase A1 and lysophospholipase A2. When PMSF-treated fractions were incubated with 2-[3H]arachidonoyl-sn-glycero-3-phosphocholine, [3H]PC accumulated at low rates that were enhanced by adding coenzyme A or stearoyl-coenzyme A. Thus, the lysophospholipid was also reacylated to form PC, but this reaction was negligible in the absence of PMSF and added cofactors. In summary, we conclude that, in brain subcellular fractions, deacylation of the sn-1 position of diacyl-PC proceeded more rapidly than sn-2 hydrolysis. There was substantial further metabolism of 2-acyl lysophospholipids due to the combined activities of a PMSF-sensitive and -insensitive lysophospholipase. Finally, the sequential deacylation of diacyl-PC by phospholipase A1 and lysophospholipase A2 probably accounted for the major portion of arachidonic acid produced.

Phospholipase A stimulation in tumor cells by subtoxic concentration of tert-butyl hydroperoxide

Previous studies have shown an increase in the intracellular free arachidonic acid content associated with a disturbance in phospholipid metabolism in P815 tumor cells exposed to subtoxic concentration of tert-butyl hydroperoxide. The present study was to determine the respective contribution of the major phospholipid-metabolizing enzymes that could be involved in this process. The enzymes (phospholipase A, lysophospholipase, acylCoA:lysophosphatidylcholine acyltransferase and acylCoA synthetase) were studied under their respective optimal conditions. When P815 cells were treated with 50 microM of tert-butyl hydroperoxide, a significant stimulation (x 2.5) of phospholipase A was observed after 15 min of treatment. The activity of the acyltranferase tended to be higher in cells treated by tert-butyl hydroperoxide while the other enzyme activities (lysophospholipase and acyl CoA synthetase) were not affected. t-BHP did not significantly induce higher levels of lipid peroxides in P815 cells. These results show that, in the tumor cell line P815, the disturbance of phospholipid and arachidonate metabolism induced by t-BHP is linked to phospholipase A, the activation of which seems independent of oxidative stress.

The hydrolysis of glycerol-3-phosphate into glycerol in cardiac tissue: possible consequences for the validity of glycerol release as a measure of lipolysis

Glycerol release has been generally accepted as an index of lipolysis in the intact heart. The glycerol moiety of glycerol-3-phosphate (glycerol-3-P) is incorporated into triacylglycerols, which are then hydrolysed with release of glycerol. This study investigates the possibility that glycerol may be derived directly from glycerol-3-P instead of passing through the triacylglycerol pool. The cardiac capacity for hydrolysis of glycerol-3-P into glycerol was determined in homogenates of rat hearts. Glycerol-3-P hydrolysis activity in homogenates increased with decreasing pH. The activity was approximately four times higher at pH 5.0 than at pH 7.2 (0.94 +/- 0.11 and 0.25 +/- 0.03 mumol.g wet weight-1.min-1 respectively). The substrate concentration at which half-maximal glycerol-3-P hydrolysis activity was reached did not significantly differ at pH 5.0 and pH 7.2 (4.2 +/- 1.1 mM and 2.9 +/- 1.0 mM respectively). In the intact heart, the pH and substrate conditions found under ischaemia are favourable for direct conversion of glycerol-3-P into glycerol. The glycerol-3-P hydrolysis activity measured in vitro was sufficiently high to account for glycerol production in the ischaemia heart. However, the lack of a stoichiometric relation between cardiac glycerol-3-P and glycerol levels in ischaemia indicates that production of glycerol cannot be explained solely by hydrolysis.

Effect of anthracyclines on phospholipase A2 activity and prostaglandin E2 production in rat gastric mucosa

The purpose of this study was to investigate in rats the effects of three anthracyclines, pirarubicin, doxorubicin and epirubicin on gastric prostaglandin E2 (PGE2) metabolism and phospholipase A2 (PLA2, EC 3.1.1.4) activity. The level of the membrane precursor, arachidonic acid, and the stability of the membrane were investigated by analysis of the composition of fatty acids. Enzymatic activities involved in the turnover of membrane phospholipids such as lysophospholipase (LPase, EC 3.1.1.5) and acyl-CoA lysophosphatidylcholine: acyltransferase (ACLAT, EC 2.3.1.23), and in the detoxification of lipid hydroperoxides, selenium-dependent glutathione-peroxidase (GSH-PX, EC 1.11.1.9) were measured after injection of the drugs for 4 consecutive days. Pirarubicin does not give rise to any changes in these activities but doxorubicin and epirubicin decreased PGE2 production and the activities of PLA2, LPase and ACLAT. GSH-PX activity was not changed by any of the drugs. The decrease in PLA2 activity does not seem to be related to variations in membrane lipid composition because the total phospholipids content was unchanged. The P/S (polyunsaturated/saturated) ratio increased in the doxorubicin group and decreased in the epirubicin group, and the unsaturation index was moderately modified. Arachidonic acid was increased only in the doxorubicin group. In vitro, PLA2 activity was not inhibited by the three drugs in the micromolar range. A marked inhibition was observed at 2.5 mM for pirarubicin and at 1.0 mM for doxorubicin and epirubicin. The Lineweaver-Burk representation showed that these inhibitions were of an uncompetitive type. Pirarubicin may therefore be considered to be an anthracycline without marked side-effects on gastric mucosa. However, the in vitro inhibition of PLA2 activity by anthracyclines does not fully explain the in vitro decrease in PLA2 specific activity observed after doxorubicin and epirubicin treatment, and in this context membrane structure modifications unconnected with the lipid composition can not be excluded. In vivo these phenomena may affect PGE2 synthesis, whose level was lower in the doxorubicin and epirubicin groups than in control group.

Phosphatidylethanolamine derived from phosphatidylserine is deacylated and reacylated in rat hepatocytes

The metabolism of phosphatidylserine (PS), phosphatidylethanolamine (PE) and phosphatidylcholine (PC), derived from [3H]serine, has been studied in rat hepatocytes. After an initial pulse with radioactivity for 10 min and a chase for up to 240 min, cells were harvested and PS, PE and PC isolated. At the end of the pulse, greater than 90% of [3H]serine derived phospholipid radioactivity was associated with PS. In the subsequent chase, newly-made PS was degraded rapidly with less than 25% of the label lost from PS appearing in the PE and PC pools. In contrast, [3H]serine-labeled PE turnover was not detectable. Very little newly-made PS was converted to PC. PE and PC were further fractionated into molecular species by high-performance liquid chromatography. We report that [3H]serine-labeled PE is deacylated/reacylated with the major product of remodeling being 18:0-20:4 PE. In contrast, [3H]serine-labeled PC is not significantly remodeled.

Heart and liver membrane phospholipid homeostasis during acute administration of various antitumoral drugs to the rat

The purpose of this study was to investigate in the rat heart and liver the effects of an acute administration of three anthracyclines, doxorubicin, epirubicin and pirarubicin, and an anthracenedione, mitoxantrone, on the membrane peroxidative status, which was estimated by the composition of polyunsaturated fatty acids (PUFA), and on the activities of the enzymes involved in membrane repair processes and lipid hydroperoxide detoxification. Rats were injected for four consecutive days with the drugs or saline (control) and killed 24 hr after the last injection. All the drugs induced an increase in plasma thiobarbituric reactive substances and alpha-tocopherol concentrations, both expressed per milligram of plasma lipids. Plasma vitamin A was decreased by about a factor of two by all the drugs. The fatty acid profile in the heart lipids showed that the polyunsaturated species (20:4 n-6, 22:6 n-3) remained at the same or even higher levels after anthracycline treatment. This can be explained by the fact that the activities of the enzymes involved in either the recycling of membrane phospholipids, such as phospholipases A1 and A2 (EC 3.1.1.4 and EC 3.1.1.32), lysophospholipases (EC 3.1.1.5) and acylCoA:lysophosphatidylcholine acyltransferases (EC 2.3.1.23), or hydroperoxide detoxification, such as selenium-dependent glutathione peroxidase (GSH-PX, EC 1.11.1.9) and glutathione S-transferases (GSH-T, EC 2.1.5.18), were maintained at the same level of activity after the antitumoral treatment. In liver, membrane phospholipid levels of PUFA were maintained as well as the activities of phospholipid-metabolizing enzymes. GSH-PX activity was not affected whereas that of GSH-T was slightly lowered by the drugs. These results suggest that during acute antitumoral-induced lipid peroxidation of membranes, the multi-enzymatic complex of the immediate processes of repair and detoxification is fully operational, allowing the membrane to rapidly recover its functional status. The results are discussed in the context of the equivocal relationships between antitumoral-induced lipid peroxidation and cardiac disturbances.

Identification and characterization of human myocardial phospholipase A2 from transplant recipients suffering from end-stage ischemic heart disease

Although numerous studies have implicated accelerated phospholipid catabolism during myocardial ischemia as an important contributor to ischemic membrane dysfunction, no information is currently available on the subcellular distribution, physical properties, or kinetic characteristics of human myocardial phospholipase A2. In this report, we demonstrate that the overwhelming majority (98%) of total phospholipase A2 activity in human myocardium (obtained from transplant recipients) is calcium independent, plasmalogen selective, and is distributed between the microsomal (60-70% of total activity) and cytosolic (30-40% of total activity) fractions. Both human myocardial microsomal and cytosolic phospholipase A2 enzymes 1) preferentially hydrolyze plasmalogen molecular species containing arachidonic acid at the sn-2 position, 2) are recalcitrant to chemical inactivation by the indole-reactive agent parabromophenacyl bromide, 3) are irreversibly inhibited by covalent modification of an essential thiol residue by 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB), and 4) are exquisitely sensitive to mechanism-based inhibition by (E)-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one (bromoenol lactone). In sharp contrast, human mitochondrial phospholipase A2 1) accounts for only a diminutive amount of total myocardial phospholipase A2 activity (1-2%), 2) is augmented by calcium ion, 3) exhibits a higher reaction velocity using phosphatidylcholine in comparison with plasmenylcholine substrate, and 4) is not substantially inhibited by either DTNB or bromoenol lactone. Collectively, these results demonstrate that the majority of phospholipase A2 activity in human myocardium is catalyzed by a novel class of calcium-independent plasmalogen-selective phospholipases A2 and underscore the potential importance of this class of enzymes in mediating membrane dysfunction during myocardial infarction in humans.

The catabolism of exogenous lysophosphatidylcholine in isolated perfused rat and guinea pig hearts: a comparative study

Lysophosphatidylcholine (lysoPC) is an arrhythmogenic phospholipid metabolite which accumulates in the ischemic myocardium. Reduced catabolism of lysoPC has been proposed to be one of the biochemical mechanisms responsible for the increase in lysoPC content. In this investigation we compared the microsomal catabolism of exogenous labeled lysoPC in isolated perfused rat and guinea pig hearts. Analysis of the amount of radioactivity in microsomal phosphatidylcholine (PC) and free fatty acid (FFA) was used as an index of the participation in lysoPC clearance by acylation catalyzed by acyl-CoA:lysoPC acyltransferase and deacylation catalyzed by lysophospholipase, respectively. There was no significant difference in the incorporation of radioactivity into rat and guinea pig heart microsomes; however, the patterns of radioactivity in lysoPC metabolites were notably different. Equal participation by deacylation and reacylation was observed in rat microsomes, whereas deacylation was clearly the preferred route for lysoPC clearance in guinea pig microsomes. Modulation of enzyme activity by treatment of the isolated heart with pHMB, a sulfhydryl agent, was used to probe the relationship among acylation, deacylation and the extent of lysoPC clearance. In guinea pig microsomes impairment of lysoPC acylation was not associated with any change in the amount of radioactivity in lysoPC because of a compensatory increase in deacylation. In contrast, impaired deacylation in rat microsomes led to significant elevations in the amount of radioactivity in lysoPC. We conclude, therefore, that in intact perfused rat and guinea pig hearts the relative participation of acylation and deacylation in lysoPC clearance differs. Moreover, we propose that the level of deacylation by lysophospholipase is an important factor in the extent of clearance of lysoPC.

Subcellular localization of rat gastric phospholipase A2

In the present study, we have performed experiments to gain some insight into the subcellular localization and biochemical properties of gastric mucosal phospholipase A2. After classical subcellular fractionation of whole glandular stomach mucosa, we found that gastric phospholipase A2 was essentially enriched in the 105,000 x g pellet that contains microsomes and plasma membranes. Except for the cytosol, all the subcellular fractions exhibited similar phospholipase A2 activity (i.e., optimum of pH, calcium dependence, apparent Km and positional specificity). The high-speed pellet was further characterized by ultracentrifugation on a sucrose gradient. Data showed that the sedimentation profile of phospholipase A2 was quite similar to those of plasma membrane markers and more specifically to an apical membrane marker. These results, taken together, showed that a gastric phospholipase A2 is distributed among the various subcellular fractions (as a result of cross-contamination) together with the membrane fraction on which it is associated. It is proposed that this fraction is the apical plasma membrane which would be the main site of phospholipase A2 action for arachidonic acid release. Lysophospholipase showed the same sedimentation profile as phospholipase A2, whereas acyl CoA-lysophosphatidylcholine: acyltransferase mainly sedimented with heavy microsomes. The substrate specificity of the enzyme was assessed by endogenous hydrolysis of gastric mucosal phospholipids. We were able to show that the enzyme acts at nearly the same rate on two major gastric membrane phospholipids, namely phosphatidylcholine and phosphatidylethanolamine.

Assay of phospholipases C and D in presence of other lipid hydrolases

The activity of a phospholipase C or phospholipase D may be assessed by measuring the radioactivity or phosphate released into the aqueous phase of a lipid extract. However, in crude enzyme fractions, this type of analysis may not be possible due to formation of water-soluble metabolites by other enzymatic reactions, as demonstrated here with a crude lysosomal enzyme fraction. In such instances, analysis of both water-soluble and lipid-soluble metabolites, at various times of incubation, may still provide clear identification of phospholipases C or D, even when a variety of lipases and other hydrolases are present.

Mechanism of lysophosphatidylcholine accumulation in the ischemic canine heart

The metabolism of lysophosphatidylcholine (LPC) in non-ischemic and ischemic canine heart was investigated by in vitro enzyme analysis. Selected subcellular fractions were assayed for the LPC-producing enzyme phospholipase A and the LPC-eliminating enzymes LPC:acyl-CoA acyltransferase, LPC:LPC transacylase and lysophospholipase. The canine heart was found to contain all enzymes differing, however, in subcellular distribution and specific activity. Phospholipase A activity did not change significantly in any of the fractions prepared from the ischemic tissue of hearts rendered ischemic for 1, 3 or 5 hr when compared to non-ischemic tissue. Changes in the activity of the microsomal LPC:acyl-CoA acyltransferase over the course of 5 hr of ischemia were observed. Significant decreases in the activity of the cytosolic and microsomal lysophospholipases were detected especially after 3 and 5 hr of ischemia. Similarly, a decrease in the activity of the microsomal LPC:LPC transacylase was noted after 3 and 5 hr of ischemia. Our results suggest that impaired catabolism of LPC rather than an enhanced production of LPC is the principal mechanism for the increase in LPC levels in the ischemic canine heart.

Phospholipase A activity of cultured rat ventricular myocyte is affected by the nature of cellular polyunsaturated fatty acids

Fatty acid composition of membrane phospholipids of cultured cardiomyocytes can be modified by the type of polyunsaturated fatty acids (n-3 or n-6 PUFA) constituting the culture medium. In this study, we investigated the effect of fatty acid modification on the activities of the key enzymes involved in the deacylation-reacylation cycle of membrane phospholipids. Results showed that cardiomyocytes grown in the presence of n-6 PUFA exhibited a higher specific alkaline phospholipase A (mainly A2) activity (+34%) and a moderately lower lysophospholipase activity (-17%) than when incubated with n-3 PUFA. AcylCoA:lysophosphatidylcholine acyltransferase, acid lysosomal phospholipase A1 and acylCoA synthetase activities were not significantly altered by changes in cellular PUFA composition. It was demonstrated that the differences between phospholipase A activities of the two types of cultured cells were linked neither to a differential leakage of enzyme nor to oxidative injury to the enzyme through blockage of essential sulfhydryl groups. One likely explanation is that the PUFA-induced changes in membrane composition alter membrane physical properties which, in turn, affect membrane-bound phospholipase A activity. Possible beneficial effects of the n-3 PUFA-induced changes on membrane stability are discussed.

Phosphatidylcholine metabolism in isolated rat heart: modulation by ethanol and vitamin E

Prolonged ethanol administration has been reported to cause defects in cardiac performance and abnormal cardiac lipid contents. However, little is known regarding the short-term administration of ethanol to the perfused heart and its effect on cardiac phospholipid metabolism. In this study, the isolated Langendorff heart perfusion was used as a model to study the effects of ethanol and a combination of ethanol and vitamin E (DL-alpha-tocopherol) on phospholipid metabolism. When perfused with 1% ethanol for 4 h, the major cardiac phospholipids were not altered but a 60% increase in lysophosphatidylcholine level was observed. Studies on the lysophosphatidylcholine metabolic enzymes revealed that phospholipase A (both phospholipase A1 and A2) activity was enhanced in the ethanol-perfused heart, but lysophospholipase and acyltransferase activities were unaffected by ethanol treatment. When the heart was perfused with 1% ethanol in the presence of 50-100 microM vitamin E, the ethanol-induced lysophosphatidylcholine accumulation was completely abolished. This was largely attributed to the attenuation of phospholipase A activities by vitamin E. In order to delineate the opposing effects of ethanol and vitamin E on phospholipid metabolism in the heart, phospholipase A activities in the subcellular fractions were determined in the presence of 0.5-2.0% ethanol or a combination of 1% ethanol and 0-100 microM vitamin E. Ethanol alone exhibited a biphasic effect on phospholipase A activity with maximum stimulation of enzyme activities at 1% concentration. When phospholipase A was assayed in 1% ethanol and vitamin E (25-100 microM), its activity was inhibited by vitamin E in a dose-dependent manner. The mechanism by which ethanol enhanced phospholipase A activities was further investigated with a partially purified enzyme from the rat heart cytosol. Kinetic studies with different concentrations of phosphatidylcholine revealed that at low substrate concentrations, ethanol was inhibitory to the reaction, whereas at high substrate concentrations, the reaction was enhanced by ethanol. Vitamin E (50 microM) completely abolished the ethanol-induced enhancement of enzyme activity in a noncompetitive manner. Since lysophosphatidylcholine is cytolytic at high concentration and its accumulation in the heart has been postulated as a biochemical cause of cardiac dysfunction, the level of the lysolipid in the heart must be under rigid control. Our result suggest that the modulation of cardiac phospholipase A activity is an important mechanism for the the regulation of lysophosphatidylcholine levels in the rat heart.

Lipid alterations in isolated, working rat hearts during ischemia and reperfusion: its relation to myocardial damage

Disturbances in lipid metabolism may play an important role in the onset of irreversible myocardial damage. To investigate the effect of ischemia and reperfusion on lipid homeostasis and to delineate its possible consequences for myocardial damage, Krebs-Henseleit-perfused, working rat hearts were subjected to various periods of no-flow ischemia (10 to 90 minutes) with or without 30 minutes of reperfusion. During ischemia, the rise in nonesterified fatty acids (NEFAs) was preceded by the accumulation of substantial amounts of glycerol, indicating the presence of an active triacylglycerol-NEFA cycle. The subsequent rise in NEFAs (from 0.25 to 1.64 mumol/g dry residue wt after 90 minutes [means]) coincided with the reduction of ATP to values lower than 10 mumol/g dry wt and the rise of AMP, a potent inhibitor of acyl-coenzyme A synthetase, to values exceeding 2 mumol/g dry wt, making the latter compound a good candidate to hamper the turnover of endogenous lipids during prolonged ischemia. Reperfusion resulted in an additional rise in NEFAs (up to 4.1 mumol/g dry residue wt after 60 minutes of ischemia). Neither ischemia nor reperfusion resulted in significant decreases in the tissue content of triacylglycerols and the various phospholipids. During reperfusion recovery of stroke volume was still adequate at tissue NEFA levels thought to be incompatible with normal mitochondrial function. A positive correlation (r = 0.81) was found between NEFA content of reperfused hearts and cumulative release of lactate dehydrogenase during reperfusion. Accordingly it is concluded that 1) reperfusion results in additional changes in myocardial lipid homeostasis, 2) the accumulating NEFAs are compartmentalized, possibly at the cellular level, and 3) the accumulation of NEFAs is a sensitive marker for myocardial cell damage.

Interleukin-1 stimulation of phospholipase activity in rat synovial fibroblasts. Possible regulation by cyclooxygenase products

Tritiated arachidonic acid (3H-AA)-labeled rat synovial fibroblasts stimulated with human recombinant interleukin-1 beta (rIL-1 beta) released incorporated radiolabel in a time-dependent and dose-dependent manner, with labeled prostaglandins representing 29% of the released radiolabel. Treatment of the cells with dibutyryl cAMP or prostaglandin E2 enhanced both spontaneous and rIL-1 beta-induced 3H-AA release; treatment with indomethacin or naproxen inhibited the response. The effects of these cyclooxygenase inhibitors on 3H-AA release were not reversed by the addition of prostaglandin E2. The activities of phospholipase A, phospholipase C, and diglyceride lipase were detected in the homogenates of rat synovial fibroblasts. Pretreatment of synovial cells with rIL-1 beta resulted in a threefold stimulation of phospholipase A activity and a slight increase in phospholipase C activity in cell homogenates. These data show that rIL-1 beta stimulates phospholipase activities in rat synovial fibroblasts and that at least one of these activities may be regulated by either prostaglandins or cAMP.

Solubilization and partial characterization of phospholipase A from rat heart sarcoplasmic reticulum

Phospholipase A has been solubilized from the sarcoplasmic reticulum of rat heart by treatment with Tris buffer, potassium chloride, taurodeoxycholate or octyl glucoside. On HPLC gel permeation, two phospholipases were identified at the void volume of a TSK 3000 column and at an apparent molecular mass of 60 kDa. The two activity peaks exhibited a predominance of phospholipase A1 activity (83-91%) and a lesser phospholipase C activity (4-9%) using sonicated 1-palmitoyl-2[1-14C]oleoylphosphatidylcholine liposomes as substrate. The voiding phospholipase A peak, which represented the bulk of the recovered activity, exhibited a requirement for calcium ions in the 0.3-3 microM range. The heat stability and response to mercuric ions was studied and some similarities were noted between the solubilized sarcoplasmic reticulum phospholipases A and the cytosolic phospholipases A of rat heart. It is speculated that the cytosolic phospholipase A which we reported earlier may represent in part phospholipase A released from sarcoplasmic reticulum during isolation of the subcellular membrane fractions.

Lipid peroxidation and phospholipase A2 activity in liposomes composed of unsaturated phospholipids: a structural basis for enzyme activation

The effect of lipid peroxidation on membrane structure and phospholipase A2 activity was studied using liposomes composed of bovine liver phosphatidylcholine (PC) and phosphatidylethanolamine (PE). The phospholipids were mixed at set ratios and sonicated to yield small unilamellar vesicles. The liposome preparations were subjected to lipid peroxidation as induced by cumene hydroperoxide and hematin. Under these conditions, a sharp increase in lipid peroxidation was noted over a 30 min incubation period and was accompanied by loss of polyunsaturated fatty acids (PUFA). Liposomes enriched in PE were most extensively peroxidized with a preferred oxidation of this phospholipid. The extent of PC oxidation was also greater in liposomes containing the largest proportions of PE. Analysis of liposome anisotropy, via steady-state fluorescence polarization of diphenylhexatriene indicated that progressive increases in either PE content or the level of lipid peroxidation increased the apparent microviscosity of the vesicles. Moreover, lipid peroxidation increased anisotropy more effectively than variations in the ratios of PE vs. PC. Thus, peroxidation of 5-10% of the phospholipids produced the same anisotropy increase as a 20% increase in the ratio of PE vs. PC. Analysis of vesicle turbidity suggested that fusion was also more readily achieved through lipid peroxidation. When liposomes were incubated with 0.4 U/ml of snake venom phospholipase A2, a direct correlation was found between the degree of lipid peroxidation and the extent of phospholipid hydrolysis. The more unsaturated phospholipid, PE, was most extensively hydrolyzed following peroxidation. Increasing the proportion of PE also resulted in more extensive phospholipid hydrolysis. These findings indicate that lipid peroxidation produces a general increase in membrane viscosity which is associated with vesicle instability and enhanced phospholipase A2 attack. A structural basis for membrane phospholipase A2 activation as a consequence of lipid peroxidation is discussed in light of these findings.

Use of a silicic acid microcolumn to assay acyl-CoA: lysophosphatidylcholine acyltransferase

A simple and rapid method for assaying acyl-CoA:lysophosphatidylcholine acyltransferase is described. This method is based on silicic acid microcolumn chromatography using labelled lysophosphatidylcholine (lysoPC) as substrate. The reaction was stopped by conventional Folch extraction. The chloroform extract (2 ml) was deposited on the silica gel and pushed through with air, and then elution was performed with methanol/water (50:50, v/v). Under these conditions, only the labelled phosphatidylcholine (PC) synthesized was retained on the gel, and this was then removed from the column and counted immediately. This method gave enzyme activities comparable to those obtained with the TLC method, and has proved to be reproducible. The new method, however, is both faster and safer than the classical TLC method.

In vitro inhibition of lysosomal phospholipase A1 of rat lung by amiodarone and desethylamiodarone

Amiodarone causes phospholipid storage in the lysosomes of various types of lung cell in animals and man. It has been proposed that this is due to its ability to inhibit lysosomal phospholipase A. To investigate this further, a crude lysosomal fraction from rat lung was prepared and phospholipase A was isolated and its positional specificity was determined. Analysis of the products formed after incubation with 2-[1-14C]oleoylphosphatidylcholine showed that only phospholipase A1 activity is present. This soluble preparation of lung lysosomal phospholipase A1 was used to study inhibition by amiodarone and desethylamiodarone, in vitro. Both were extremely potent inhibitors of the lung acid phospholipase A1. To evaluate the levels of amiodarone in lung lysosomes, rats were treated with the agent for 3 days and the combined mitochondrial/lysosomal fraction of lung tissue was prepared by differential centrifugation. This fraction had been shown previously to be highly enriched in amiodarone. Purified mitochondria and lysosomes were isolated from the combined mitochondrial/lysosomal fraction with Percoll gradients and analyzed for their drug content by HPLC. Amiodarone and desethylamiodarone were present in roughly equal amounts, relative to protein, in mitochondria and lysosomes, respectively. Amiodarone appears to differ from other cationic amphiphilic drugs which cause lipidosis because the latter are more highly lysosomotropic. Although amiodarone does not appear to be highly lysosomotropic in lung, it causes lysosomal phospholipid storage because of its ability to concentrate in lung and because it inhibits lysosomal phospholipase A to a much greater extent than other cationic amphiphiles such as diethylaminoethoxyhexestrol, chloroquine and chlorphentermine.

Activities of some enzymes of phospholipid metabolism in cultured rat ventricular myocytes in normoxic and hypoxic conditions

Phospholipid catabolism is thought to be one of the critical events in membrane injury during heart ischemia. In this work, the enzymes involved in phospholipid metabolism were studied in purified cultured ventricular myocytes in normoxic and hypoxic conditions. Purified ventricular myocytes exhibited an alkaline phospholipase A activity which had sn-2 specificity and which was calcium dependent, and an acid phospholipase A activity with sn-1 specificity. These cells also exhibited lysophospholipase and acyl-CoA/lysophosphatidylcholine acyltransferase activities. Oxygen deprivation of the myocardial cells for 4 h resulted in a sharp reduction of both phospholipase A2 and A1 activities. The activities of the other lipolytic enzymes were unaffected by hypoxia. Although hypoxia resulted in a marked increase of lactate dehydrogenase leakage in the bathing fluid, no additional release of the lipolytic enzymes and mitochondrial enzyme was observed. However, we noted an important alkaline phospholipase A2 leakage during normoxia. It is suggested that ventricular myocytes, under hypoxia, tend to prevent phospholipid degradation by reducing their phospholipase A activities.

Propranolol inhibition of the neutral phospholipases A of rat heart mitochondria, sarcoplasmic reticulum and cytosol

Membrane damage caused by phospholipase A action is thought to be an important factor in ischemic myocardial injury. Propranolol has been shown previously to have beneficial effects in both animal experiments and clinical trials, and it has membrane-stabilizing properties in vitro. To investigate the possibility that these effects might be due, in part, to effects on phospholipases, we determined the effects of propranolol on rat heart phospholipases A at physiological pH using small unilamellar liposomes of di[1-14C]oleoylphosphatidylcholine as substrate. Propranolol inhibited heart phospholipases A in vitro. The concentration required to give 50% inhibition was 0.2 mM for the mitochondrial and cytosolic phospholipases A and 2.9 mM for sarcoplasmic reticulum phospholipase A. The binding of [4-3H]propranolol to fresh membrane preparations was studied using an ultracentrifugation method. Propranolol bound readily to both membrane fractions in vitro with no significant difference in the saturation number (0.20 to 0.28 mol drug per mol phospholipid) but the association constant, KA, was higher for mitochondrial membranes (3760 +/- 350) than for the sarcoplasmic reticulum (2190 +/- 390). Our results show that propranolol inhibited heart phospholipases A in vitro at physiological pH. The mitochondrial and cytosolic phospholipases A were more susceptible to inhibition than the phospholipase A of sarcoplasmic reticulum. Propranolol bound to mitochondria and sarcoplasmic reticulum in vitro, suggesting the possibility that propranolol binding to heart membranes in vivo could result in drug concentrations in these membranes high enough to inhibit phospholipase A. This could represent an additional mechanism by which propranolol exerts beneficial effects in myocardial ischemia.

Phospholipase A2 activity of rat stomach

Phospholipase A activity in rat stomach wall and in gastric content was studied using [1-14C]dioleoylphosphatidylcholine as substrate. The optimum activity of the stomach wall was found to take place at pH 7.0. During optimal phospholipase action about 40% of the [1-14C]oleic acid released was due to an active intracellular lysophospholipase. The gastric phospholipase required 5 mM Ca2+ for full activity and is inhibited by EDTA. It specifically hydrolyzed the sn-2 position of the phospholipid molecule. The enzyme was heat labile and inactivated by acidification at pH 3.0. The gastric content enzyme had a lower specific activity and an optimum pH of 8.0. It was heat stable and was not inactivated by acidification. These results indicate that gastric content phospholipase A is of pancreatic origin, via a duodenal reflux. By ligating the stomach we were able to further confirm that the gastric wall phospholipase was different from that of the gastric content. It originated from the stomach mucosa. Subcellular fractionation suggests that the gastric phospholipase A2 is essentially bound to the plasma membrane. About 6% of the activity was found to be soluble. Biopsies of human gastric mucosa displayed a phospholipase A activity which had similar properties to that of rat gastric enzyme. The physiological function of this enzyme is discussed in terms of prostaglandin synthesis via the release of arachidonic acid.

Effect of dietary lipids on the lipid composition and phospholipid deacylating enzyme activities of rat heart

Rats were fed lard-enriched (17%) or corn oil-enriched (17%) diets and were compared with rats fed a low fat (4.5%) diet. Cardiac protein, DNA, phospholipid (PL) and fatty acid (FA) compositions were analyzed. Neutral phospholipase A, lysophospholipase and creatine kinase activities in the membrane and cytosolic compartments were also investigated. No significant modification of cardiac protein, DNA nor PL was observed among the three groups. Some alterations appeared in the FA composition. A lard-enriched diet induced a significant increase of 22:5n-3 and 22:6n-3 in heart phosphatidylcholine (PC) and phosphatidylethanolamine (PE), whereas a linoleic acid-rich diet induced a specific increase of 22:4n-6 and 22:5n-6 in these two major PL. Compared to rats fed the low fat diet, membrane-associated phospholipase A activity, measured by endogenous hydrolysis of membrane PC and PE, showed a significant increase (+45%) for both PL in rats fed corn oil. However, the activity of membrane-associated phospholipases, measured with exogenous [1-14C]dioleoyl PC, was not different among the three groups of rats. Cytoplasmic activity was decreased in rats fed corn oil, and lysophospholipase and creatine phosphate kinase activities were not significantly affected by diet. FA modification of the long chain n-6 FA induced by corn oil may be responsible for the observed increase in phospholipase activity. Physiological implications are suggested in terms of membrane degradation and prostaglandin production.

Differential effects of manoalide on secreted and intracellular phospholipases

Manoalide, a novel nonsteroidal sesterterpenoid, is a potent inhibitor of phospholipase A2 isolated from bee and cobra venoms. This report compares the inhibition by manoalide of phospholipase A2 in crude cytosol fractions from four mammalian tissues with that of four purified extracellular phospholipase A2's. Phospholipase A2 isolated from bee venom (Apis mellifera) was the most sensitive to inactivation by manoalide (IC50 approximately equal to 0.12 microM). Extracellular phospholipase A2 from rattlesnake and cobra venom was intermediate in sensitivity to manoalide (IC50 values of 0.7 and 1.9 microM respectively). Porcine pancreatic phospholipase A2 was relatively resistant to inactivation by manoalide (IC50 approximately equal to 30 microM). The phospholipase A2 assayed in crude cytosol fractions from four mammalian tissues exhibited IC50 values of 30 microM or greater. Cytosolic proteins as well as bovine serum albumin and poly-L-lysine (Mr = 57,000) protected purified bee venom phospholipase A2 from inactivation by manoalide. In contrast, amino acids such as lysine and alanine failed to protect the purified enzyme from inactivation. Proteins and certain amino acids, such as lysine, formed a chromogenic product when incubated with manoalide. These data suggest that lysine is capable of reacting with manoalide, but only when it is present in macromolecules is it capable of protecting phospholipase A2 from inactivation by manoalide. Because cellular proteins protect PLA2 from inactivation by manoalide, high concentrations of manoalide must be applied topically to produce statistically significant inactivation of intracellular phospholipase A2. Finally, a chemical model is presented which explains the formation of a chromogenic product when manoalide is incubated with proteins and amino acids.

Accumulation of lipids and lipid-intermediates in the heart during ischaemia

The content of non-esterified fatty acids (NEFA) and their CoA and carnitine esters is low in normoxic cardiac tissue. The majority of fatty acids is esterified in the triacylglycerol and phosphoglyceride pool. During myocardial ischaemia beta-oxidation of fatty acids is inhibited. In addition, turnover of the esterified fatty acid pools is most likely disturbed. Accumulation of hydroxy fatty acids, acylCoA and acylcarnitine rapidly occurs after the onset of ischaemia. The accumulation of NEFA is a slower process. In addition to extracellular sources, NEFA originate also from intracellular lipid pools, most likely from phosphoglycerides. Although it has been suggested that activation of phospholipase A2 occurs in ischaemic tissue, the mechanism underlying the enhanced degradation of phosphoglycerides ist still incompletely understood.

Membrane phospholipid metabolism during myocardial ischaemia: past, present and future

Alterations in myocardial membrane phospholipids may play an important role in the pathogenesis of ischaemic myocardial cell injury. Studies in canine myocardium, perfused rat heart, and cultured myocardial cells have demonstrated that the accumulation of free arachidonic acid correlates with the development of irreversible cell injury. Accumulation of other phospholipid hydrolysis products, including amphiphilic compounds such as lysophosphatidylcholine, has also been reported. The biochemical mechanisms which are responsible for phospholipid hydrolysis and arachidonic acid accumulation during ischaemia are unknown. This manuscript provides a synopsis of previous work in this field and suggests new directions for the field of myocardial phospholipid metabolism.

Purification of lysosomal phospholipase A and demonstration of proteins that inhibit phospholipase A in a lysosomal fraction from rat kidney cortex

Phospholipase A has been isolated from a crude lysosomal fraction from rat kidney cortex and purified 7600-fold with a recovery of 9.8% of the starting activity. The purified enzyme is a glycoprotein having an isoelectric point of pH 5.4 and an apparent molecular weight of 30,000 by high-pressure liquid chromatography gel permeation. Naturally occurring inhibitors of lysosomal phospholipase A are present in two of the lysosomal-soluble protein fractions obtained in the purification. They inhibit hydrolysis of 1,2-di[1-14C]oleoylphosphatidylcholine by purified phospholipase A1 with IC50 values of 7-11 micrograms. The inhibition is abolished by preincubation with trypsin at 37 degrees C, but preincubation with trypsin at 4 degrees C has no effect, providing evidence that the inhibitors are proteins. The results suggest that the activity of lysosomal phospholipase A may be regulated in part by inhibitory proteins. Lysosomal phospholipase A from rat kidney hydrolyzes the sn-1 acyl group of phosphatidylcholine, does not require divalent cations for full activity, and is not inhibited by ethylenediaminetetraacetic acid. It has an acid pH optimum of 3.6-3.8. Neither p-bromophenacyl bromide, diisopropyl fluorophosphate, nor mercuric ion inhibits phospholipase A1. In contrast to rat liver, which has two major isoenzymes of acid phospholipase A1, kidney cortex has only one isoenzyme of lysosomal phospholipase A1.

Partial characterization of rat heart cytosolic phospholipase A1 and demonstration of essential sulfhydryl groups

The cytosol of rat heart has been previously shown to contain phospholipase A activity in substantial amounts. This paper describes the isolation and partial purification of rat heart cytosolic phospholipase A. After homogenization of rat heart followed by centrifugation to remove membraneous material, the phospholipase A activity was isolated by ammonium sulfate precipitation and further purified by gel permeation chromatography with Sephadex G-100 in the presence of 5 mM taurodeoxycholate. Two peaks were isolated: a minor peak at the void volume and major peak corresponding to a molecular weight of 45,000. The molecular weight observed in HPLC gel permeation chromatography experiments was also Mr 45,000 and was not significantly affected by the nature of the detergent used. Phospholipase A was purified 77-fold over the crude cytosol. Further purification could not be attained due to lability of the phospholipase A activity. The enzyme is a phospholipase of the A1 type which does not require Ca2+ and lacks lipase or transacylase activity. It is unusual for the phospholipases A described to date, since it is inhibited by thiol reagents and is protected by beta-mercaptoethanol, suggesting the presence of essential sulfhydryl residues.