Myocardial phospholipase A of microsomal and mitochondrial fractions

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.

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.

Evidence for receptor and G-protein regulation of a phosphatidylethanolamine-hydrolysing phospholipase A1 in guinea-pig heart microsomes: stimulation of phospholipase A1 activity by DL-isoprenaline and guanine nucleotides

While evidence has been presented for the receptor-mediated activation of phospholipases A2, C and D, the activation of phospholipase A1 subsequent to receptor activation has not been established. Phospholipase A1-catalysed hydrolysis of 1-palmitoyl-2-linoleoyl-glycerophosphoethanolamine (GPE) by guinea-pig heart microsomes was stimulated 40-60% by isoprenaline. This isoprenaline-mediated increase in activity was blocked by propranolol and butoxamine, a specific beta 2-adrenergic antagonist, but not by atenolol, a specific beta 1-adrenergic antagonist. Neither clonidine nor phenylephrine, alpha 1- and alpha 2-adrenergic agonists respectively, had a stimulatory effect on the hydrolysis of the PE substrate. Guanosine 5'(-)[gamma-thio]triphosphate (GTP[S]) and guanosine 5'(-)[beta,gamma-imido]triphosphate, but not guanosine 5'(-)[beta-thio]diphosphate (GDP[S]) or adenosine 5'(-)[gamma-thio]triphosphate, stimulated the hydrolysis of 1-palmitoyl-2-linoleoyl-GPE by phospholipase A1. GDP[S] inhibited the isoprenaline-mediated stimulation of phospholipase A1 activity. Phospholipase A1 hydrolysis of 1-palmitoyl-2-linoleoyl-GPE was not dependent on cations; however, the stimulatory effects of isoprenaline and GTP[S] on the hydrolytic activity were abolished by cation chelators. The above data suggest that phospholipase A1 activity in guinea-pig heart microsomes is activated by the binding of isoprenaline to beta 2-adrenergic receptors. Furthermore the stimulation of phospholipase A1 activity by the agonist may be mediated via activation of G-proteins.

Cloning, expression and partial characterization of a novel rat phospholipase A2

We report the cloning of a novel rat cDNA encoding a Ca(2+)-dependent, low molecular weight phospholipase A2 (PLA2). A rat RNA blot hybridized with the cDNA exhibited a putative 2.4 kb transcript in heart. When the cDNA was expressed in human 293s cells, PLA2 activity accumulated in the culture medium. This conditioned medium optimally hydrolyzed the phospholipids of [1-14C]oleate-labeled Escherichia coli at neutral to alkaline pH with 10 mM or greater Ca2+. When single substrates were tested, L-alpha-palmitoyl-2-oleoyl phosphatidylcholine was more efficiently hydrolyzed than L-alpha-1-palmitoyl-2-arachidonyl phosphatidylcholine, L-alpha-1-palmitoyl-2-arachidonyl phosphatidylethanolamine or L-alpha-1-stearoyl-2-arachidonyl phosphatidylinositol.

Distribution of pancreatic (group I) and synovial-type (group II) phospholipases A2 in human tissues

The catalytic activity of phospholipase A2 (cat-PLA2) and the concentration of pancreatic (group I) phospholipase A2 (pan-PLA2) and synovial-type (group II) phospholipase A2 (syn-PLA2) were studied in 19 human tissues in order to find potential sources of circulating phospholipase A2. Five specimens of each tissue were collected at autopsies or from normal deliveries (placentas and amnionic membranes). The concentrations of pan-PLA2 and syn-PLA2 were measured by specific time-resolved fluoroimmunoassays. The cat-PLA2 was measured by radioactive dipalmitoyl phosphatidylcholine as substrate. The concentration of pan-PLA2 was negligible in all tissue homogenates except the pancreas. Immunoreactive syn-PLA2 was found in the homogenates of the digestive tract, cartilage, and prostatic and parotid glands. By immunohistochemistry, syn-PLA2 was localized in Paneth cell secretory granules, chondrocytes, cartilage matrix, and glandular cells of prostate.

Cell-associated and soluble phospholipases A2 increase during carrageenan and zymosan-induced pleurisy in rat

Extra-cellular and cell-associated Ca(2+)-dependent phospholipases A2 and released thromboxane B2 were correlated to exudation and cell migration during rat pleurisy induced by carrageenan or zymosan. Extra-cellular phospholipase A2 was delayed with respect to acute inflammation, while cell-associated phospholipase A2 closely correlated with cell migration and thromboxane B2 levels. This confirms that the subcellular localization of phospholipases A2 is linked to their physiological action and, in particular, suggests that the cell-associated, rather than the extracellular enzyme, accounts for the production of eicosanoids.

Serum phospholipase A2 and lysolecithin changes following myocardial infarction

We have examined changes in the activities of phospholipase A2 (PLA2) and the concentrations of total choline-phospholipids and lysolecithin, in serum from patients following a myocardial infarction by comparison with patients suffering from unstable angina. A significant increase in PLA2 activity was found after myocardial infarction. The peak increase occurred approximately 36 h after infarction. No significant PLA2 change was found in the patients with unstable angina. Concentrations of lysolecithin, the major metabolite of PLA2 activity, were high in the admission samples from the infarction patients, followed by an overall fall during the first 24 h: the concentrations in the patients with angina were normal. PLA2 and lysolecithin changes post-infarction showed they were involved in processes not occurring in angina.

Phospholipases in biology and medicine

Phospholipases, a group of enzymes that catalyze the hydrolysis of membrane phospholipids, are classified according to the bond cleaved in a phospholipid into PLA1 (EC 3.1.1.3), PLA2 (EC 3.1.1.4), PLB (EC 3.1.1.5), PLC (EC 3.1.4.3), and PLD (EC 3.1.4.4). This paper reviews source and structure of PLA2 and the involvement of PLA2 and PLC in several biological phenomena, such as, signal transduction, photoreception, biosynthesis of lung surfactant, sperm motility, and fertilization. New assays for PLA2 activity and concentration in biological fluids are discussed. Phospholipases are involved in many inflammatory reactions by making arachidonate available for eicosanoid biosynthesis. The determination of PLA2 activity and mass concentration in plasma is useful in the diagnosis and prognosis of pancreatitis and of septic shock. Naturally occurring phospholipase inhibitors, such as lipocortins act as second messengers in the anti-inflammatory response to steroids. Lipocortins may be valuable therapeutic agents, because they are more specific in their anti-inflammatory action than glucocorticoids; therefore, they are less likely to produce harmful side effects.

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.

Age-related changes of lipid fractions and total fatty acids in liver lipids and heart lipids of female and male rats aged 37-1200 days (liver) and 331-1200 days (heart)

1. Total lipids and the lipid fractions cholesterol ester, triacylglycerol, free cholesterol, free fatty acids and phospholipids, as well as the fatty acid patterns of total lipids, were measured in liver homogenates of female and male rats (Wistar SPF, strain Hannover) aged 37-1213 days. 2. The same parameters were measured in the apex of the heart in female and male rats aged 331-1213 days. 3. All parameters were monitored every 49th day. Five female and five male animals were used in each experiment. 4. The lipid fractions in liver showed a positive linear regression vs age, whereas all lipids in rat heart showed a negative regression vs age in both sexes. 5. The significance of regression vs age of fatty acids was much less than that in the lipid fractions of liver and heart of these animals.

Calcium and pH in anoxic and toxic injury

The critical events that lead to the transition from reversible to irreversible injury remain unclear. Studies are reviewed that have suggested that a rise in cytosolic free Ca2+ initiates plasma membrane bleb formation and a sequence of events that leads ultimately to cell death. In recent studies, we have measured changes in cytosolic free Ca2+, mitochondrial membrane potential, cytosolic pH, and cell surface blebbing in relation to the onset of irreversible injury and cell death following anoxic and toxic injury to single hepatocytes utilizing multiparameter digitized video microscopy (MDVM). MDVM is an emerging new technology that permits single living cells to be labeled with multiple probes whose fluorescence is responsive to specific cellular parameters of interest. Fluorescence images specific for each probe are collected over time, and then digitized and stored. Image analysis and processing then permits quantitation of the spatial distribution of the various parameters within the single living cells. Our results indicate the following: (1) formation of plasma membrane blebs accompanies all types of injury in hepatocytes; (2) cell death is a rapid event, initiated by rupture of a plasma membrane bleb, and is coincident with the onset of irreversible injury; (3) an increase of cytosolic free Ca2+ is not the stimulus for bleb formation or the final common pathway leading to cell death; (4) a decrease of mitochondrial membrane potential precedes loss of cell viability; (5) cytosolic pH falls by more than 1 pH unit during chemical hypoxia. This acidosis protects against the onset of cell death.

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.

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.

Prevention by amiodarone of phospholipid depletion in isoproterenol-induced ischemia in rats

This work was performed to study phospholipid metabolism in isoproterenol-induced ischemic heart and the possible protective effect of amiodarone (Am) and chlorpromazine (CPZ). Heart weight increased 24 h after subcutaneous injection of isoproterenol (40 mg/kg) whereas myocardial phospholipid content and creatine kinase activity decreased without modification of the cholesterol content. The phospholipid content was significantly correlated with creatine kinase activity (P less than 0.001). Phosphatidylcholine, phosphatidylethanolamine and cardiolipin decreased significantly (P less than 0.001) in the isoproterenol group whereas the lysophosphatidylcholine and lysophosphatidylethanolamine content increased. The lysophosphatidylcholine/phosphatidylcholine and lysophosphatidylethanolamine/phosphatidylethanolamine ratios consequently increased to a significant degree (P less than 0.01) suggesting indirectly the activation of phospholipases A in the ischemic myocardium. Free fatty acid content increased, indicating hydrolysis of phosphatidylcholine, phosphatidylethanolamine, lysophosphatidylcholine and lysophosphatidylethanolamine. Intravenous injection of Am (20 mg/kg) or intraperitoneal injection of CPZ (30 mg/kg) prior to isoproterenol injection provided complete protection against phospholipid depletion and against increase of the lysophosphatidylcholine/phosphatidylcholine and lysophosphatidylethanolamine/phosphatidylethanolamine ratios which returned to control values. Neither substance had any effect on the heart weight increase due to an edematous and inflammatory process. The total protection by both substances against phospholipid depletion was not sufficient to prevent the creatine kinase activity decrease. The improved phospholipid degradation in the ischemic myocardium is discussed in relation to the in vitro inhibitory effect of Am or CPZ on phospholipases A activity.

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.

Cholesterol and myocardial membrane function

The incorporation of cholesterol into phospholipid membranes changes the physical properties of the membranes, such as their phase transition, fluidity and homogeneity. In cholesterol-containing phospholipid membranes, integral proteins are surrounded by "annular" phospholipids which exclude cholesterol. Cellular cholesterol is supplied by circulating low-density lipoproteins, and by intracellular de novo synthesis. Cholesterol removal is predominantly handled by circulating high-density lipoproteins. In cardiomyopathic hamsters, myocardial membranes (sarcolemma, mitochondria, sarcoplasmic reticulum) have an increased cholesterol content. In ischaemic myocardium, cholesterol content of sarcolemma fell and that of mitochondria rose. Apparently, cholesterol is redistributed within the ischaemic heart cell. Phospholipids are degraded in sarcolemma, mitochondria and sarcoplasmic reticulum of ischaemic heart cells, probably by activation of phospholipases present in these membrane systems.

The effects of ischaemia, lysophosphatidylcholine and palmitoylcarnitine on rat heart phospholipase A2 activity

Phospholipase A2 activity was studied in the isolated rat heart following coronary artery ligation. In both the homogenate and mitochondrial fractions phospholipase A2 activity was significantly depressed at 20 min post ligation in the ischaemic region only. This is at a time of peak lysophospholipid concentration and severity of arrhythmias. No such depression of activity was seen in a crude sarcolemmal fraction, possibly due to washout of inhibitory factors during isolation. Lysophosphatidylcholine and palmitoylcarnitine, two amphiphiles known to accumulate during ischaemia, were both shown to be capable of inhibiting phospholipase A2. It is suggested that lysophospholipid and palmitoylcarnitine accumulation during ischaemia may contribute to the depression of phospholipase A2 activity seen and that the decreased metabolism of lysophospholipids may be of more importance in their accumulation than increased production by phospholipase A2.

Effects of chronic administration of doxorubicin on heart phospholipase A2 activity and in vitro synthesis and degradation of prostaglandins in rats

Male rats received doxorubicin (DXR) 2 mg/kg or phosphate buffered saline (PBS) weekly by the SC route for 13 weeks and were sacrificed at 14 and 19 weeks, one and six weeks, after the last dose, respectively. Heart phospholipase A2 activity in the 1000 X g supernatant was unchanged between DXR and PBS-treatment groups at both 14 and 19 weeks. In vitro heart microsomal syntheses of PGD2, PGE2, PGF2 alpha, and 6-keto-PGF1 alpha were significantly elevated in DXR-treated rats over controls at 14 weeks. In contrast, syntheses of TxB2, PGE2, and PGF2 alpha in DXR-treated rats were significantly depressed from controls at 19 weeks. In vitro heart metabolism of PGF2 alpha in the 100,000 X g supernatant fraction was significantly elevated in DXR treated rats over controls at 14 weeks, but unchanged from controls at 19 weeks. It was concluded from the findings of the present study that increased prostaglandin synthesis may play a role in the mediation of cardiac injury induced by doxorubicin.

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.

Phospholipases of the myocardium

The myocardium contains diverse cellular components and heterogeneous phospholipid-containing membranes. The major phospholipids are phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositnol, sphingomyelin and cardiolipin. The phospholipases capable of hydrolyzing these membrane lipids include phospholipase A, lysophospholipase, and phosphatidylnositol-specific phospholipase C. Early studies revealed that myocardial phospholipase A with an acid pH is localized to lysosomes; those with more alkaline and neutral activities are present in cytosol, microsomes, mitochondria and sarcolemma. Recently, we have identified phosphatidylinositol-specific phospholipase C activity in bovine myocardium with molecular weights ranging from 40,000 to 271,000. Interestingly, forms I, II and III, had pH optima ranging from 4.5 to 5.5; form III also had significant activity at pH 7.0. All activities were stimulated by calcium, suggesting that they are different from calcium-independent phospholipases C found in liver and brain. The pathophysiological significance of these four cytosolic forms of phospholipase C remains to be determined. Thus, under injury-promoting conditions, phospholipase C appears capable of hydrolyzing membrane-associated phosphatidylinositol and the polyphosphoinositides, whereas phospholipases A and lysophospholiphases appear to prefer non-inositol containing phospholipids. Finally, very recent studies suggest "free radical-triggered lipolysis" by phospholipases as a possible mechanism for production of lysophospholipids in myocardial membranes.

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.

Fatty acyl chain composition in the determination of renal membrane order

The relative roles of phospholipid fatty acyl chain length and phospholipid fatty acyl chain unsaturation in the determination of rat renal brush border membrane order were examined using multilamellar liposomes. Exposure of brush border membranes to sphingomyelinase resulted in a time- and concentration-dependent decrement in sphingomyelin content. Liposomes prepared from lipid extracts of these membranes were reconstituted to defined phosphatidylcholine (PC)/sphingomyelin (SPH) ratios with pure synthetic PCs of defined chain length and degrees of unsaturation. Mixed-acid PCs from bovine liver, egg, and the rat renal brush border membrane were also examined. The steady state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) at 37 degrees C was used to reflect acyl chain packing. The steady state anisotropy of DPH in liposomes isolated from the rat renal brush border membrane averaged 0.205 +/- 0.001, n = 8. When liposomes were reconstituted to PC/SPH ratios of 1.1, 1.6, and 2.4 with saturated PCs of acyl chain length 16 to 22, differences in anisotropy between groups were not observed. However, when PCs containing unsaturated or mixed-acid fatty acyl chains were introduced, anisotropy decreased in a concentration dependent fashion. These data suggest that phospholipid fatty acyl chain unsaturation, but not acyl chain length, has a powerful influence on renal brush border membrane order and the PC/SPH ratio is an important determinant of renal membrane order by virtue of the unsaturated fatty acids normally present with these phospholipids.

Accumulation of unesterified arachidonic acid in ischemic canine myocardium. Relationship to a phosphatidylcholine deacylation-reacylation cycle and the depletion of membrane phospholipids

Studies in ischemic canine left ventricle have shown that the depletion of membrane phospholipids is a critical event in the development of a sarcolemmal calcium permeability defect and associated irreversible myocyte injury. The mechanism of phospholipid loss is unclear, but may be due to the activation of endogenous phospholipases. Since arachidonic acid is a fatty acid found almost entirely in phospholipid, increases in arachidonate provide evidence for increased phospholipase activity. The present study was designed to examine the temporal relationship of the accumulation of free arachidonate with the onset of phospholipid depletion during fixed ligation of the left anterior descending coronary artery in canine myocardium. The following results were demonstrated in ischemic canine myocardium: (1) the accumulation of unesterified arachidonate is minimal during 10-30 minutes of ischemia, but is significantly increased after prolonging the duration of ischemia to 1-3 hours; (2) significant increases in arachidonate precede the development of a significant decrease in total phospholipid content; (3) the decrease in the arachidonate content of phosphatidylcholine is accompanied by similar decreases in all of the fatty acyl moieties; (4) the arachidonate content of lysophosphatidylcholine and diacylglycerol are unchanged during myocardial ischemia; (5) there is evidence of a deacylation-reacylation cycle in phosphatidylcholine prior to the accumulation of free arachidonate; (6) the fatty acyl specificity of the lysophosphatidylcholine acyltransferase corresponds to the pattern of fatty acyl remodeling of phosphatidylcholine during early myocardial ischemia. These data suggest that the accumulation of arachidonate may be a more sensitive measure of phospholipid degradation than the decrease in total phospholipid content in ischemic canine myocardium. It is postulated that the defective reacylation of arachidonate into phosphatidylcholine may contribute to the net loss of membrane phospholipid during myocardial ischemia.

Phospholipid metabolism in the rat renal inner medulla

In view of the importance of phospholipids as a source of precursor fatty acids for the high prostaglandin synthesis in the renal inner medulla, we studied pathways of phospholipid esterification and degradation in the rat inner medulla. De novo acylation of [14C]arachidonate occurred predominantly in position 2 of phosphatidylcholine in the microsomal fraction. This newly esterified [14C]arachidonate was accessible to deacylation by a microsomal phospholipase A2 (EC 3.1.1.4) with alkaline optimum which was Ca2+-dependent and resistant to 0.1% deoxycholate. No phospholipase A1 (EC 3.1.1.32) activity against endogenous labeled phosphatidylcholine could be demonstrated in the microsomal fraction. When exogenous phosphatidylcholine labeled at position 2 was deacylated by renomedullary homogenates, labeled free fatty acid but no labeled lysophosphatidylcholine was recovered in the reaction products. This could be attributed to further degradation of generated lysophosphatidylcholine by a cytosolic lysophospholipase (EC 3.1.1.5). Sodium deoxycholate at a concentration of 0.1% or higher inhibited the lysophospholipase and allowed the demonstration of both A2 and A1 alkaline phospholipase activities in the homogenate. The major in vitro pathway of lysophosphatidylcholine disposition is further degradation by a cytosolic lysophospholipase, while reutilization for phosphatidylcholine synthesis through the action of a predominantly microsomal acyltransferase appears to be a minor pathway. In the presence of several acyl-CoAs, reutilization of lysophosphatidylcholine is significantly increased by an acyl-CoA:lysophosphatidylcholine acyltransferase (EC 2.3.1.23) but there is no preferential transfer of arachidonyl-CoA compared to other acyl-CoAs.

Fatty acylcarnitine accumulation and membrane injury in ischemic canine myocardium

Although previous work has shown that long chain fatty acylcarnitine derivatives accumulate in ischemic canine myocardium, their role in the production of irreversible injury and associated sarcolemmal membrane injury is undefined. The present study examines the temporal and topographic relationship of the accumulation of long chain acylcarnitine with the uptake of technetium pyrophosphate and tissue calcium content during ligation of the left anterior descending coronary artery (LAD) in canine myocardium. After 60 minutes of fixed LAD ligation, there was no significant increase in long chain acylcarnitine content in the ischemic subendocardium compared with the corresponding nonischemic value. However, the ischemic subendocardium was irreversibly injured at this time, as assessed by a 4-fold increase in tissue calcium content and a 20-fold increase in technetium-99m pyrophosphate uptake after reperfusion. The ischemic subepicardium showed a 41% increase in long chain acylcarnitine content compared with the corresponding nonischemic subepicardium. However, the ischemic subepicardium contained only 50% of the calcium content and 10% of the technetium-99m pyrophosphate uptake found in the ischemic subendocardium. It is concluded that increases in fatty acylcarnitine can be dissociated from the development of irreversible ischemic injury during fixed LAD occlusion in ischemic canine myocardium.

Homogenized milk and atherosclerotic disease: a review

The theory that consumption of homogenized milk containing active xanthine oxidase is a causative factor in development of atherosclerosis is reviewed. Biologically available xanthine oxidase in consumed milk products may be absorbed in the small intestine and enter the blood stream. However, there appears to be no unequivocal evidence that the absorbed enzyme has any pathological effects that may contribute to development of atherosclerotic heart disease.

Phospholipase A2 activity in carrageenin-induced inflammatory tissue of rats

Phospholipase A2 activity was detected in 7-day-old carrageenin-induced inflammatory tissue of rats using a synthetic substrate. 1-acyl-2-[3H]arachidonyl-phosphatidylcholine. The inflammatory tissue was homogenized in saline containing 1 M KCl, and the 105,000 g supernatant fraction was placed on a Sephadex G-100 column. The partially purified phospholipase A2 had a pH optimum at 6-7 and was Ca2+ dependent. p-Bromophenacyl bromide was strongly inhibitory to the partially purified phospholipase A2 (IC50 = 1.44 x 10(-5) M). A moderate inhibition was observed with indomethacin. Cycloheximide and dexamethasone, which inhibit prostaglandin production in inflammatory tissue, exerted no direct inhibitory action on the phospholipase A2. There were no direct inhibitory effects of quinacrine, bradykinin, or actinomycin D. The cell-free supernatant fraction of the inflammatory exudate of 7-day-old carrageenin-induced granulation tissue was found to have no phospholipase A2 activity.

Purification of rat liver mitochondrial phospholipase A2

Rat liver mitochondrial phospholipase A2 was purified to near homogeneity by a combination of gel-filtration, hydroxyapatite and Matrex gel Blue A column chromatography. The absolute positional specificity of the enzyme for acylester bonds at the sn-2-position was established in experiments with 1-[9,10-3H2]palmitoyl-2-[1-14C]linoleoylphosphatidylethanolamine. Molecular weight estimations revealed Mr values of 15000 by SDS-polyacrylamide gel electrophoresis and of 9700 gel by gel-filtration over Ultrogel AcA 54 columns. The enzyme is unaffected by diisopropylfluorophosphate and thiol reagents such as 5,5'-dithiobis(2-nitrobenzoic acid), N-ethylmaleimide and iodoacetamide, but is completely inhibited by the alkylating reagent p-bromophenacylbromide.

Studies on the liberation of fatty acids from 2-lysophosphatidylcholine by a liver lysosomal enzyme activity from chloroquine-treated rats

A lysophospholipase activity was observed in cell fractions from rat liver without and after treatment with chloroquine following incubation with 1-[1(-14)C]palmitoyl-sn-glycero-3-phosphocholine. The specific activity of the enzyme (optimum pH at 4.2) is highly enriched in lysosomes but decreases after prolonged treatment of the animals with chloroquine. Some of the properties of the enzyme were studied.

Changes in enzyme activities of glycerolipid metabolism of guinea-pig cerebral hemispheres during experimental hypoxia

Hypoxic treatment causes changes of some enzymatic activities involved in the glycerolipid metabolism in subcellular fractions of guinea pig cerebral hemispheres. The activity of lysophosphatidylcholine acyltransferase, choline phosphotransferase, glycerol-3-phosphate acyltransferase(s), as well as the activity of triacylglycerol lipase significantly decreased in the microsomes of cerebral hemispheres of animals intermittently exposed to hypoxic treatment for eighty hours. At the same time, a marked activation of microsomal and mitochondrial phospholipase A2 occurred. The changes of the above-mentioned enzymatic activities after the hypoxic treatment correlated well with the increase in the level of brain and blood free fatty acids. The changes also correlated with the decrease of labeled lipid precursors incorporated into lipids of the cerebral hemispheres, observed during oxygen insufficiency.

Lipids of myocardial membranes: susceptibility of a fraction enriched in sarcolemma to hydrolysis by an exogenous mammalian phospholipase A2

A myocardial membrane fraction enriched in sarcolemma was used to determine the susceptibility of the lipids to hydrolysis by a phospholipase A2 from granulocytes. After incubation (37 C, pH 7.0, 5 mM Ca2+) with the phospholipase A2 for 30 min, a more than 3-fold increase in unesterified fatty acids was found (up to 47 nmol/mg protein; P < 0.001) relative to a control incubated without phospholipase A2 or Ca2+. This included a 10-fold increase in the arachidonic acid content (up to 42 mol%) and at least a 7-fold increase in lysophosphatidylethanolamine (up to 7.4 mol% total phospholipid-P). However, the exogenous phospholipase did not augment the quantity of lysophosphatidylcholine produced by endogenous phospholipases in the presence of Ca2+ (5 mM). These results demonstrate the in vitro susceptibility of phospholipids of myocardial membranes, particularly phosphatidylethanolamine, to the neutral-active, Ca2+-dependent phospholipase A2 from granulocytes. This work may be relevant to myocardial inflammation and tissue damage during ischemia, where heterolytic injury of the myocardium may occur subsequent to the accumulation of granulocytes.

Lipolytic enzymes in bovine thyroid tissue. III. Lysophospholipase activity

Lysophospholipids are formed during phospholipid breakdown as a result of the action of phospholipases A. At certain concentrations these lysoderivatives destabilise biological membranes. Therefore, their concentration is of critical importance for membrane integrity. Prevention of lysophosphoglycerides accumulation may be the important role for lysophospholipases and is probably the explanation for their widespread occurrence in nature. Lysophospholipase activities were found in molds (Fairbairn, 1948), rice bran (Contardi & Ercoli, 1933), several microorganisms (Brockerhoff & Jensen, 1974), snake and bee venoms (Doery & Pearson, 1964; Mohamed et al., 1969; Shiloah et al., 1973), insects (Khan & Hodgson, 1967; Rao & Subrahmanyam, 1969), fish muscle (Yurkovski & Brockerhoff, 1965; Cohen et al., 1967) and in various animal tissues (Marples & Thompson, 1960). In mammalian tissue the enzyme was first described in beef pancreas (Shapiro, 1953). Relatively high levels were detected in intestine, lung, spleen, liver and pancreas, while lower levels were present in muscle, kidney, testes, brain and blood (Marples & Thompson, 1960). The presence of lysophospholipase activity in both supernatant and sediment of bovine thyroid was reported previously in relation to possible interference of this enzyme with the phospholipase A activity assay (De Wolf et al., 1976). The subcellular localization of bovine thyroid lysophospholipase and some properties of the membrane bound enzyme activity are discussed in this paper.

Phospholipases A1 and A2 in bovine thyroid

In both supernatant and sediment of thyroid tissue homogenate phospholipase and lysophospholipase activities were demonstrated. In the supernatant, using 1-acyl-2[1-14C]linoleoyl-sn-glycero-3-phosphorocholine in the presence of sodium taurocholate, phospholipase A1 activity with pH optima at 3.6 and 4.8 and phospholipase A2 activity with pH optima at 3.6 and 5.7 were found. The sediment showed mainly phospholipase A2 activity with a pH optimum at pH 6.5. Lysophospholipase activity (optimum pH 7--8), USING 1-[9,10-(3)H]stearyl-sn-glycero-3-phosphorocholine as a substrate was present in both supernatant and sediment. Enzyme assays performed on subcellular fractions suggest the soluble phospholipases to be of lysosomal origin and the solubilized phospholipase A2 activity of homogenate sediment to be of microsomal origin. Incubations with 3H-14C mixed labelled phosphatidylcholine further confirmed the above observations.