Purification and characterization of phospholipase A2 from rat stomach

Phospholipid hydroperoxides are detoxified by phospholipase A2 and GSH peroxidase in rat gastric mucosa

The aim of this study was to determine the metabolic fate of phospholipid hydroperoxides (PLOOH) in rat gastric mucosa. Here we report evidence concerning the mechanism for PLOOH detoxification in gastric mucosa homogenate. Analysis by the TLC blot technique showed that the gastric mucosa has the highest potential to eliminate 1-palmitoyl-2-linoleoyl-phosphatidylcholine hydroperoxides (PL-PtdChoOOH) compared with the intestinal mucosa and liver. Major products detected after incubation with gastric mucosa were the partially reduced linoleic acid hydroperoxides (LAOOH) and lysophosphatidylcholine, indicating the involvement of phospholipase A2 (PLA2) in the elimination pathway. Using unilamellar vesicles, we demonstrated that gastric mucosal PLA2 does not distinguish between PLOOH and intact phospholipids. Although gastric mucosal PLA2 activity efficiently eliminated excess amounts of PLOOH, the complete reduction of LAOOH was dependent on the supply of exogenous GSH. In a separate experiment, administration of egg yolk PtdChoOOH to rats for 6 d significantly elevated GSH peroxidase (GPx) activity in the gastric mucosa. We concluded that excess amounts of PLOOH are efficiently eliminated through the hydrolysis by PLA2, and the subsequent reduction of FA hydroperoxide by GPx is the critical step for complete detoxification of oxidized phospholipids in the stomach.

Helicobacter infection and phospholipase A2 enzymes: effect of Helicobacter felis-infection on the expression and activity of sPLA2 enzymes in mouse stomach

The murine gastric mucosa possesses very high secretory type phospholipase A2 activity. Northern and Western blots indicated that the pancreatic-type, sPLA2-IB represents the predominant form of sPLA2 enzymes present in the gastric mucosa. Both sPLA2-IB mRNA and protein in the gastric mucosa exceeded levels found in the pancreas, and in contrast to the pancreatic enzyme it was present primarily in the active state. The sPLA2-IB gene is not expressed in the murine small intestine and colon. Infection by the gastritis-inducing bacteria, Helicobacterfelis (H. felis) dramatically and time dependently decreased the PLA2 activity in the glandular stomach of the mouse strain, C57BL/6, sensitive to the organism, which appeared to be related to a decrease in the percentage of sPLA2-IB present in the active form. This bacterial-induced reduction in PLA2 activity was not observed in BALB/c mice that fail to develop gastritis in response to H. felis infection. C57BL/6 mice do not, while BALB/c mice express, the PLA2-II enzyme. The H. felis-induced reduction in sPLA2-IB activity may weaken the gastric barrier by reducing the local concentration of arachidonic and linoleic acid, liberated from membrane phospholipids, the major precursors of 'cytoprotective' prostaglandins. Data presented here suggest that both sPLA2-IB and sPLA2-II enzymes may contribute to the gastric response to Helicobacter infection.

Phospholipase A2: its usefulness in laboratory diagnostics

Phospholipase A2 (PLA2) is an enzyme that catalyzes the hydrolysis of membrane phospholipids. This article reviews the source and structure of PLA2, the involvement of the enzyme in various biological and pathological phenomena, and the usefulness of PLA2 assays in laboratory diagnostics. Of particular importance is the role of PLA2 in the cellular production of mediators of inflammatory response to various stimuli. Assays for PLA2 activity and mass concentration are discussed, and the results of enzyme determinations in plasma from patients with different pathological conditions are presented. The determination of activity and mass concentration in plasma is particularly useful in the diagnosis and prognosis of pancreatitis, multiple organ failure, septic shock, and rheumatoid arthritis. A very important result is the demonstration that PLA2 is an acute phase protein, like CRP. Indeed, there is a close correlation between PLA2 mass concentration and CRP levels in several pathological conditions. Although the determination of C-reactive protein is much easier to perform and is routinely carried out in most clinical laboratories, the assessment of PLA2 activity or mass concentration has to be considered as a reliable approach to obtain a deeper understanding of some pathological conditions and may offer additional information concerning the prognosis of several disorders.

Melanocyte-stimulating properties of secretory phospholipase A2

Phospholipase A2 (PLA2) catalyzes the release of free fatty acids from membrane phospholipids, and its products derived from these fatty acids, such as prostaglandins and leukotrienes, significantly up-regulate the key melanogenic enzyme, tyrosinase, in melanocytes. This has led to suggestions that PLA2 itself triggers melanin synthesis in melanogenesis following UV irradiation or inflammation. We have examined the effect of secretory PLA2 (sPLA2) on melanogenesis in cultured human melanocytes. Secretory PLA2 stimulated DNA synthesis and melanin synthesis, and these phenomena were completely inhibited by treatment with a phospholipase inhibitor, p-bromophenacyl bromide, demonstrating that the catalytic activity of sPLA2 is required for melanogenesis. Secretory PLA2 also stimulated tyrosinase activity, increased the amount of tyrosinase-related protein-1 and up-regulated the expression of both mRNA. These findings suggest that sPLA2 is an important mediator of UV-induced or postinflammatory pigmentation.

Arachidonic acid up-regulates and prostaglandin E2 down-regulates the expression of pancreatic-type phospholipase A2 and prostaglandin-endoperoxide synthase 2 in uterine stromal cells

It is well known that arachidonic acid, as a substrate of prostaglandin G/H synthase (PGHS), is converted into prostaglandins of the two-series. In this work, we attempted to determine whether arachidonic acid and prostaglandin E2 might regulate the expression of PGHS and the pancreatic-type phospholipase A2 (PLA2I), which may be involved in the liberation of arachidonic acid from membrane phospholipids. For this purpose, we used the uterine stromal cell line UIII, which produces prostaglandin E2 and expresses both the constitutive and inducible PGHS enzymes (PGHS1 and PGHS2) and PLA2 I. The results show that PGHS1, which is expressed at a high level in UIII cells, was not modified by arachidonic acid. The expression of PGHS2 and PLA2 I was up-regulated by increasing arachidonate concentrations (1-10 microM). The maximal response was obtained at 24 h, reaching a 2.3-fold and 2.6-fold increase for PGHS2 and PLA2 I expression, respectively, compared to the control level. To discriminate between the effect of arachidonic acid and that of prostaglandins, which are highly increased in the presence of exogenous arachidonic acid, we treated the cells with two inhibitors of PGHS activity, aspirin and meclofenamic acid. Both inhibitors failed to suppress the arachidonate-induced increase of PLA2 I and PGHS2 expression and even enhanced it either in the presence or absence of arachidonic acid. In contrast, the addition of prostaglandin E2 to the culture medium decreased the expression of both enzymes in a dose-dependent manner, the maximal response being reached at 1 microM. We conclude that arachidonic acid up-regulates the expression of PLA2 I and PGHS2 in the uterine stromal cells, independently of prostanoids, and that prostaglandin E2 is capable of down-regulating enzyme expression.

Purification and properties of lysophospholipase isoenzymes from pig gastric mucosa

Two lysophospholipases, named gastric lysophospholipases I and II (enzymes I and II), were purified 3730- and 2680-fold from pig gastric mucosa. The preparations showed 22 and 23 kDa single protein bands on SDS/PAGE respectively. Both enzymes lacked transacylase activity and appeared to exist as monomers. Their activities were not affected by Ca2+, Mg2+ or EDTA. Enzyme I was most active at pH 8.5 and hydrolysed a variety of lysophospholipids including acidic lysophospholipids and the acyl analogue of platelet-activating factor, whereas enzyme II was most active at pH 8 and its activity was confined to lysophosphatidylcholine and lysophosphatidylethanolamine. When 1-palmitoylglycerophosphocholine was used as substrate, enzymes I and II showed half-maximal activities at 11 and 12 microM respectively. The enzymes exhibited no phospholipase B, lipase or general esterase activity. Enzyme II was significantly inhibited by lysophosphatidic acid whereas enzyme I was only moderately inhibited. Peptide mapping with V8 protease and papain revealed structural dissimilarity between the two enzymes. Antiserum raised against enzyme I did not recognize enzyme II, but did recognize the small-sized lysophospholipase purified from rat liver. Anti-(enzyme II) consistently did not cross-react with enzyme I or the liver enzyme. These antisera specifically recognized neither the 60 kDa lysophospholipase transacylase purified from liver nor any peritoneal macrophage protein. Thus gastric mucosa contains two different small-sized lysophospholipases: one is closely related to the small-sized lysophospholipase of liver, but the other appears to be a novel isoform.

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.

Responses of purified phospholipases A2 to phospholipase A2 activating protein (PLAP) and melittin

The role of the phospholipase A2 (PLA2) stimulating protein PLAP in the regulation of PLA2 activity was assessed by determination of the effects of PLAP on two purified PLA2s. An approx. 14 kDa enzyme was purified from mouse thymoma cells, EL-4 cells, by cation ion exchange HPLC and immunoaffinity HPLC (with antiserum to the N-terminal sequence of an inflammatory exudate PLA2). An approx. 110 kDa enzyme was purified from mouse mammary carcinoma derived cells by sequential hydrophobic, anion exchange, hydroxyapatite and gel filtration HPLC. Neither PLAP nor melittin, an immunologically related PLA2 stimulating peptide from bee venom, increased the activity of the high molecular weight enzyme. In contrast, there was more than a 20-fold stimulation of the low molecular weight PLA2 by PLAP and an approx. 5-fold stimulation by melittin. The stimulation of enzyme activity by PLAP was observed at a protein to phospholipid ratio of 1:10(6) while the ratio of melittin to phospholipid was 1:3. Thus, PLAP mediated stimulation of PLA2 activity may include an interaction between PLAP and the enzyme, in contrast to melittin stimulation, which involves interactions between melittin and phospholipid.

Purification and characterization of a high-affinity binding protein for pancreatic-type phospholipase A2

A high-affinity and specific binding site for mammalian group I phospholipase A2 (PLA2-I) was found on the membranes of bovine corpus luteum. Affinity labeling experiments revealed that PLA2-I binds to a single polypeptide with a mass of 190-200 kDa. The PLA2-I binding protein in the membranes was solubilized in an active form with n-octyl beta-D-thioglucoside, and then purified approx. 16,000-fold. The purification procedures consisted of diethylaminoethyl-Sephacel chromatography, PLA2-I-affinity gel chromatography and gel-filtration high-performance liquid chromatography on a TSKgel G3,000SWXL column. The final preparation migrated as a single molecular species of 190 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and identification of the 190 kDa protein as the PLA2-I binding protein was demonstrated by ligand blotting analysis. The purified protein possessed a binding capacity with high affinity and specificity for a mammalian mature type of PLA2-I. Treatment of the purified material with N-glycosidase F resulted in increased mobility of the protein on SDS-PAGE as well as considerable abolition of the PLA2-I binding activity, thus suggesting the requirement of the carbohydrate moiety of the PLA2-I binding protein for receptor-ligand interactions.

Inhibition of phospholipase A2 by cis-unsaturated fatty acids: evidence for the binding of fatty acid to enzyme

Calcium-dependent phospholipases A2 are markedly inhibited in vitro by cis-unsaturated fatty acids (CUFAs) and to a much lesser extent by trans-unsaturated or saturated fatty acids. Thus, CUFAs may function as endogenous suppressors of lipolysis. To better understand the mechanism of inhibition, kinetic analysis, fluorescence spectroscopy and gel permeation chromatography were employed to demonstrate that CUFAs interact with a highly purified Ca(2+)-dependent phospholipase A2 from Naja mossambica mossambica venom. Arachidonate inhibited hydrolysis of both [1-14C]oleate-labelled, autoclaved Escherichia coli and [1-14C]linoleate-labelled phosphatidylethanolamine in an apparent competitive manner. When subjected to gel permeation chromatography, [3H]arachidonate, but not [3H]palmitate, comigrated with the enzyme. Arachidonic and other CUFAs increased the fluorescence intensity of the enzyme almost 2-fold in a dose-dependent fashion (50 microM = 180% of control); methyl arachidonate was without effect. Saturated fatty acids had only a modest effect on enzyme fluorescence (50 microM = 122% of control). Concentrations of arachidonate that inhibited in vitro enzymatic activity by almost 80% did not alter binding of phospholipase A2 to the E. coli substrate. Collectively, these data demonstrate that, while CUFAs selectively bind to the enzyme, they do not influence phospholipase A2-substrate interaction. Inhibition of in vitro phospholipase A2 activity by CUFAs may be mediated by the formation of an enzymatically inactive enzyme-substrate-inhibitor complex.

Contraction of guinea pig lung parenchyma by pancreatic type phospholipase A2 via its specific binding site

Porcine pancreatic group I phospholipase A2 (PLA2-I) induced contraction of guinea pig parenchyma in a concentration-dependent manner. Its EC50 value was similar to the Kd value calculated from the specific binding of 125I-labeled porcine PLA2-I in the membrane fraction of guinea pig lung. Type-specific action of PLA2's and homologous desensitization strongly implicated the involvement of PLA2-I-specific sites in the activation process. Thromboxane A2 was found to be the main product from lung tissue by PLA2-I action and the contractile response by PLA2-I was specifically suppressed by thromboxane A2 receptor antagonists and cyclooxygenase inhibitor, but not by leukotriene receptor antagonist and H1 blocker. These findings indicate that PLA2-I-induced contractile response may depend on the secondarily produced thromboxane A2, thus providing a new aspect of PLA2-I from the pathophysiological standpoint.

Group I phospholipase A2 mRNA expression in rat glandular stomach and pancreas. Ontogenic development and effects of cortisone acetate

The postnatal development of group I phospholipase A2 (group I PLA2) in the glandular stomach and pancreas of neonatal rats was investigated. The amounts of group I PLA2 mRNA (and also the PLA2 enzymatic activity) in the glandular stomach mucosa increased with age in 3-60-day-old animals. This postnatal development of rat stomach group I PLA2 mRNA agreed with that of group I PLA2 mRNA of the rat pancreas, and thus seems to follow the general development of the gastrointestinal tract during the neonatal period. The latter was further supported by the finding that maturation of group I PLA2 in both the stomach and pancreas was induced precociously in rats treated with cortisone acetate. It is suggested that the stomach group I PLA2 is involved in mucosal eicosanoid production.

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.