Influence of plasma proteins on activity of proinflammatory enzyme phospholipase A2

LY315920NA/S-5920, a selective inhibitor of group IIA secretory phospholipase A2, fails to improve clinical outcome for patients with severe sepsis

OBJECTIVE

Group IIA secretory phospholipase A2 (sPLA2-IIA), released during inflammation, is increased in severe sepsis, and plasma levels are inversely related to survival. In a previous study, a selective inhibitor of sPLA2-IIA (LY315920NA/S-5920) was well tolerated and appeared to improve survival in a subgroup of patients who received the drug within 24 hrs of first sepsis-induced organ failure. This study was designed to determine whether improvement in survival could be confirmed in a larger patient population meeting the characteristics of that subgroup.

DESIGN

Multicenter, double-blind, placebo-controlled, parallel-group clinical trial of LY315920NA/S-5920 in patients with severe sepsis.

SETTING

Seventy-five institutions worldwide.

PATIENTS

A total of 373 patients with at least two sepsis-induced organ failures.

INTERVENTIONS

Patients were randomized 1:1 to receive LY315920NA/S-5920 (target plasma concentration of 800 ng/mL; n = 188) or placebo (n = 185). Study medication was administered as a continuous intravenous infusion for 168 hrs.

MEASUREMENTS AND MAIN RESULTS

The study was terminated after data on 250 patients suggested a significant improvement in 28-day all-cause mortality would not be found if the trial continued as planned. The mortality rate was 39.4% in the LY315920NA/S-5920 group, compared with 31.9% in the placebo group (p = .092). The negative trend in mortality was most pronounced among patients with cardiovascular failure at baseline (41.6% vs. 28.7%; p = .008) and patients whose culture data at baseline were negative (42.9% vs. 22.7%; p = .045). The negative trend in mortality is not explained by adverse events, microbiology, or laboratory data.

CONCLUSIONS

Continuous 7-day infusion of an inhibitor of sPLA2-IIA had no beneficial effect on 28-day all-cause mortality among severe sepsis patients with at least two organ failures. This study did not confirm earlier promising subgroup results with LY315920NA/S-5920, which provides a reminder that subgroup effects should be viewed cautiously, especially when primary effects are not significant.

Efficacy and safety of LY315920Na/S-5920, a selective inhibitor of 14-kDa group IIA secretory phospholipase A2, in patients with suspected sepsis and organ failure

OBJECTIVE

Concentrations of group IIA secretory phospholipase A, an inflammatory response mediator, are increased in the plasma of patients with sepsis and septic shock, and the extent of elevation is correlated with mortality. LY315920Na/S-5920 is a selective inhibitor of group IIA secretory phospholipase A that has been shown to inhibit serum group IIA secretory phospholipase A enzyme activity in patients with severe sepsis. The primary objectives of this study were to determine whether there was a dose-response relationship between two doses of LY315920Na/S-5920 compared with placebo in the reduction of 28-day all-cause mortality in patients with severe sepsis and to determine whether LY315920Na/S-5920 had an acceptable safety profile.(2) (2) (2)

DESIGN

Multicenter, double-blind, placebo-controlled trial of two doses of LY315920Na/S-5920 in a parallel design.

PATIENTS

A total of 586 patients with severe sepsis at 72 institutions in the United States. INTERVENTIONS Patients enrolled within 72 hrs from onset of first sepsis-induced organ failure were randomized (1:1:1) to low-dose LY315920Na/S-5920 (target plasma concentration of 200 ng/mL, n = 196), high-dose LY315920Na/S-5920 (800 ng/mL, n = 194), or placebo (n = 196). Study medication was administered as a constant-rate intravenous infusion for 168 hrs.

MEASUREMENTS AND MAIN RESULTS

The study was stopped prematurely because it was unlikely that a statistically significant difference in mortality between LY315920Na/S-5920 and placebo would be found. There was no effect of LY315920Na/S-5920 on the primary end point of 28-day all-cause mortality across the entire study population. The 28-day all-cause mortality was distributed as follows: placebo group, 33.2% (65/196 patients); low-dose LY315920Na/S-5920, 37.2% (73/196); and high-dose LY315920Na/S-5920, 36.1% (70/194); p = .525. However, in a prospectively planned analysis, there was a favorable overall dose-response effect on 28-day all-cause mortality in patients administered LY315920Na/S-5920 within 18 hrs of onset of the first sepsis-induced organ failure. Among these patients, 28-day all-cause mortality was distributed as follows: placebo group, 43.5% (20/46 patients); low-dose LY315920Na/S-5920, 31.4% (16/51); and high-dose LY315920Na/S-5920, 20.8% (10/48); p = .018.

CONCLUSIONS

Administration of LY315920Na/S-5920 had an acceptable safety profile in patients with severe sepsis. There was no overall survival benefit associated with the use of LY315920Na/S-5920 in this study. However, prospectively planned secondary analyses suggested that treatment with LY315920Na/S-5920 was associated with an improvement in survival in patients treated within 18 hrs of the first sepsis-induced organ failure.

Lipid mediators in the pathophysiology of critical illness

Inflammatory lipid mediators are produced by the metabolism of membrane phospholipids following a number of different stimuli. These mediators lead to a variety of cellular and systemic responses which contribute to the manifestations of the systemic inflammatory response syndrome in the critically ill patient. These mediators include platelet-activating factor and the eicosanoids, including prostaglandins, thromboxanes, leukotrienes, and HETEs. This review seeks to evaluate the current role of these mediators in the pathophysiology of critical illness. We will focus on recent studies concerning the modulation of these pathways as a potential therapeutic strategy for management of these critically ill patients. This includes the gamut from nutritional strategies to alter the cellular membrane lipid composition, thereby effecting the substrate available to produce these lipid byproducts, to intracellular inhibitors to alter production of these mediators, to receptor blockage and enhanced clearance to inhibit their effects.

The role of inflammation in disk herniation-associated radiculopathy

OBJECTIVE

The causes and physiopathology of low-back pain and acute lumbar radiculopathy remain unclear. A compression of the nerve root by protruded disk has been suggested but explains only partially the physiopathology of radicular pain. This article provides an overview of the role of inflammation in disk herniation-associated radiculopathy.

METHODS

A review of the relevant literature in American and European medical journals was performed.

RESULTS

Several studies have identified inflammatory mediators (phospholipase A2, prostaglandin E2, leukotrienes, nitric oxide, immunoglobulins, pro-inflammatory cytokines such as interleukin [IL]-1alpha, IL-1beta, IL-6, and tumor necrosis factor alpha [TNFalpha]) and autoimmune reaction (macrophages expressing IL-1beta, intercellular adhesion molecules) in disk herniation. An appealing hypothesis is that the leakage of these agents may produce an excitation of the nociceptors, a direct neural injury, a nerve inflammation, or an enhancement of sensitization to other pain-producing substances (such as bradykinin), leading to the nerve root pain. However, the role of these inflammatory mediators in the pathophysiology of lumbar radiculopathy has not been proven. Several findings suggest that this inflammatory response, which occurs in the early stage of disk herniation, is transient. Indeed, most studies of chronic disk herniation samples failed to demonstrate inflammation.

CONCLUSION

Although inflammation may partially explain lumbar radiculopathy, involvement of inflammatory mediators in the physiopathology of disk herniation-associated radiculopathy has not been proven.

Transpedicle instrumentation in the degenerative spine

Controversy continues regarding the use of pedicle instrumentation in fusion surgery on the degenerative lumbar spine. Recently, however, published reports are increasingly demonstrative of the positive effect of pedicle fixation on fusion and outcome success in the management of these disorders. Recent developments in surgical technique and implant design have improved operative risk and implant related complications. However, patient selection and demonstration of the underlying pathophysiology of degenerative lumbar motion segment pain remain 2 of the main challenges to clinical practice. The importance of well designed clinical studies are of paramount consequence in the solution of the existing controversies surrounding surgical treatment of the degenerative lumbar spine. Equally significant is the need to establish general and accepted measures of outcome to assess and compare treatment options. Combining study design and accepted outcome assessment will permit the resolution of the remaining controversies. The future of pedicle instrumentation of the degenerative lumbar spine is of necessity tied to the resolution of these remaining points of controversy-only thereafter can the current rate of advances continue.

Inhibition of LDL-associated phospholipase A activity in human plasma by albumin

LDL has been previously shown to exhibit activity of phospholipase A1 and phospholipase A2 as measured by the hydrolysis of NBD caproic phosphatidylcholine (C6-NBD-PC), which is considered to represent the phospholipase activity towards short chain or oxidized fatty acyl chains/3/. In the present study we show that in whole plasma the LDL-associated phospholipase A activity, measured by the hydrolysis of C6-NBD-PC, is inhibited by albumin due to its binding of the substrate. The inhibition depends on the molar ratio between the substrate, albumin and the enzyme. C6-NBD-PC and other synthetic phospholipid analogues have been used previously to determine plasma phospholipase A2 activity in various pathological states. This study suggests that when using this kind of substrate to measure plasma PLA activity, the choice of substrate and the experimental conditions should be carefully considered. Determination of the appropriate ratio between the three reaction components--enzyme, substrate and albumin--is required for reliable and consistent determination of plasma phospholipase activity.

Vasoactive intestinal peptide and helodermin inhibit phospholipase A2 activity in vitro

We recently reported that the widely distributed neuropeptide vasoactive intestinal peptide (VIP) reduces inflammatory lung injury due to a variety of agents and inhibits the associated generation of cyclo-oxygenase and lipoxygenase products. We therefore investigated whether VIP may inhibit phospholipase A2 activity, thus reducing the release of arachidonic acid, the common precursor of all eicosanoids. VIP dose-dependently inhibited PLA2 of porcine pancreas and of Naja naja venom, as assessed by the release of free [3H]oleic acid from labeled Escherichia coli phospholipids. The potency of VIP was similar to that of mepacrine, with 50% inhibition at 400-500 microM. The closely related peptide helodermin produced 50% inhibition at 200 microM, but secretin and peptide histidine isoleucineamide produced little or no inhibition. The results suggest that VIP and helodermin selectively inhibit PLA2 in vitro. If this activity is exerted in vivo, it may contribute to the anti-inflammatory activity of these two peptides.

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.

Recombinant human type II phospholipase A2 lacks edema producing activity in rat

The rat paw edema-inducing, acute inflammatory activity of four snake venom phospholipase A2S (PLA2) (Naja naja, Naja mocambique mocambique, Crotalus atrox and recombinant Naja naja naja) and of recombinant human type II PLA2 (rh-PLA2) found in rheumatoid synovial fluid, were compared after a bolus subplantar injection. The snake venom-derived PLA2s, including the recombinant Naja naja naja, were potent inducers of paw edema. On the other hand, when given in similar amounts (protein and/or enzymatic activity), the rh-PLA2 did not produce paw edema. Furthermore, the addition of Naja naja PLA2, blood plasma from rats with adjuvant arthritis or the E. coli-based enzymatic incubation mixture used to measure PLA2 activity to the injection mixture containing rh-PLA2, did not result in paw edema-inducing activity. These results suggest that the lack of paw edema-inducing activity of rh-PLA2 may be due to significant structural differences between snake venom PLA2s and human PLA2 which allow snake venom PLA2s, but not the human group II PLA2, to express inflammatory activity as measured by paw edema in rat induced by a bolus injection.

Enzymatic activity and immunoreactivity of extracellular phospholipase A2 in inflammatory synovial fluids

Synovial fluid PLA2 concentration was measured by an ELISA technique using monoclonal antibodies raised against human recombinant "synovial-type" group II phospholipase A2. This ELISA was specific for synovial-type PLA2 and did not detect pancreatic (group I) PLA2. In all synovial fluids examined, including rheumatoid, osteoarthritic, psoriatic, and gouty fluids, synovial fluid PLA2 enzyme activity significantly correlated with PLA2 immunoreactivity (P < 0.001). Within the limits of the ELISA technique, there was no evidence for the presence of specific or nonspecific modulation of PLA2 activity by either putative PLA2 activating or inhibitory proteins.

Phospholipase A2 is associated with albumin in patients with acute pancreatitis

Evidence for the association of serum phospholipase A2 (PLA2) (EC 3.1.1.4) and serum proteins was examined. The effect of this on the PLA2 results from patients diagnosed as having acute pancreatitis was investigated. Two distinct zones of PLA2 activity were found on agarose electrophoresis of purified human PLA2 in the presence of albumin, and in the sera from the acute pancreatitis patients. One of the zones was coincident with albumin. To investigate this finding, a comparison of the PLA2 activity in sera and protein-free ultrafiltrates prepared from the sera of the same patients, showed that PLA2 was not completely ultrafiltered as would be expected from its molecular weight. The PLA2 method used employed a radiolabelled E. coli membrane-phospholipid substrate. It has been previously shown that there is an association between PLA2 and albumin and there is good evidence that albumin interferes with certain methods used for the measurement of PLA2. The recovery of PLA2 activity from the ultrafiltrates of patients' serum was highly variable and it may be that serum proteins, in particular albumin, provide a protective 'buffer' against small increases in PLA2 activity. Inhibitor proteins such as lipocortin which were originally postulated as binding to the enzyme, have been subsequently shown to be substrate-binding agents. However, direct protein effects may be a factor in the inconsistent PLA2 results reported in studies of patients with acute pancreatic disease.

Eicosanoid production and phospholipase A2 secretion by peritoneal macrophages from rats with adjuvant-induced arthritis

The effect of adjuvant-induced arthritis on rat peritoneal macrophage (RPM) function with respect to [14C]arachidonic acid (AA) release, leukotriene B4 (LTB4) and prostaglandin E2 (PGE2) production, and secreted phospholipase A2 (PLA2) activity was investigated. Twice as many cells were lavaged from the peritoneal cavity of arthritic rats 21 days post-adjuvant injection than were lavaged from normal rats. PGE2 production was increased two-fold in Ca++ ionophore-stimulated RPM from the adjuvant animals as compared with RPM from control animals. However, PLA2 secretion, LTB4 production and [14C]AA release were unchanged. These results suggest that PGE2 production, rather than LTB4 production or PLA2 secretion, is preferentially enhanced in Ca++ ionophore-stimulated RPM from arthritic rats and may, therefore, reflect a major role for PGE2 in adjuvant-induced arthritis. However, the presence of increase numbers of macrophages and their associated products, including PLA2 and LTB4, may also contribute to the inflammatory process in this disease.

Cytosolic calcium ion and arachidonic acid release and metabolism in macrophages

The aim of the present work was to elucidate the role of cytosolic calcium ions, [Ca2+]i, in the control of arachidonic acid release and metabolism. [Ca2+]i was measured in resident peritoneal rat macrophages loaded with Fura2, and compared with the release of leukotriene B4(LTB4) and prostaglandin l2 (PGL2, assayed through its hydrolysis product 6-keto-PGF1 alpha). The calcium ionophore A 23187 stimulated both an increase in [Ca2+]i and the release of LTB4 and 6-keto-PGF1 alpha. On the contrary, zymosan and opsonized zymosan, while stimulating eicosanoid release to an extent only slightly lower than A 23187, did not affect [Ca2+]i. Lipopolysaccharide stimulated 6-keto-PGF1 alpha, but not LTB4, release, without affecting [Ca2+]i. In parallel experiments, macrophages were prelabelled with [3H]arachidonic acid and the release of total 3H-products was assayed and taken as an index of phospholipase activity. A 23187, zymosan and opsonized zymosan increased the release of 3H-products in the presence of Ca2+. When extracellular Ca2+ was removed, the ionophore-induced 3H-products release was greatly blunted, while the release induced by zymosan was actually augmented. Our data indicate that a generalized [Ca2+]i increase is not necessary for arachidonic acid release and metabolism in rat peritoneal macrophages.

Retinoids inhibit phospholipase A2 in human synovial fluid and arachidonic acid release from rat peritoneal macrophages

Retinoids have demonstrated antiinflammatory activity in certain animal models and human disease states. The mechanism by which retinoids elicit this activity is unknown. Some retinoids are known to inhibit arachidonic acid (AA) release and metabolism in intact cells in vitro. Retinoids may exert their antiinflammatory effects by inhibiting phospholipase A2 (PLA2) and the resultant production of inflammatory AA metabolites. Retinoids were evaluated in vitro as inhibitors of the PLA2 activity in human synovial fluid (HSF-PLA2). Of the naturally occurring, nonaromatic retinoids tested, all-trans-retinal, all-trans-retinoic acid (all-trans-RA) and 13-cis-RA were the most potent inhibitors (IC50 S 6-15 microM), whereas all-trans-retinol was much less potent. Of the synthetic aromatic retinoids and arotinoids examined, the free carboxylic, sulfonic, and sulfinic acid forms were more than 15-fold more potent inhibitors of HSF-PLA2 than their corresponding ethyl esters. These retinoids also were evaluated as inhibitors of calcium ionophore A23187-induced AA release from rat peritoneal macrophages. All-trans-RA and 13-cis-RA were potent inhibitors of AA release from these cells (IC50 S 4 microM), while the other natural retinoids were inactive. Of the aromatic retinoids and arotinoids tested, the free acid forms (IC50 S 2-6 microM) were 5- to 21-fold more potent inhibitors of AA release from the macrophages than their corresponding ethyl esters. The potencies of the arotinoids as inhibitors of HSF-PLA2 appeared to correlate with their potencies as inhibitors of AA release from A23187-stimulated rat peritoneal macrophages. These data support the hypothesis that one possible mechanism for the known antiinflammatory activity of some retinoids may be by inhibition of phospholipase A2.

Compartmental heterogeneity of soluble phospholipases A

Multiple forms of soluble phospholipase A2 (PLA2) are known to coexist in venoms of individual reptilian species. While similar observations in several mammalian species suggest that this is a common phenomenon, the functional implications are not yet understood. In attempting to devise therapeutic strategies for treatment of inflammatory disorders by inhibition of PLA2, it is imperative to define the various PLA2 species in the relevant compartments. Herein, we report the presence of three PLA2 isotypes in rheumatoid arthritis serum, one pancreatic and two nonpancreatic phospholipases A2. The pancreatic and one of the nonpancreatic forms were optimally active in 7 mM calcium at pH 7.5. The other nonpancreatic form was calcium-independent and optimally active at pH 7.0. Only the calcium-dependent nonpancreatic form was observed in rheumatoid synovial fluid. Of the three serum isotypes, only the calcium-dependent nonpancreatic form correlated with markers of disease activity, such as the joint count and Landsbury index. Therefore, not all soluble or circulating phospholipases A2 are relevant to inflammatory processes. Selective inhibition of the proinflammatory form of PLA2 may prove to have some therapeutic benefit while minimizing the possible adverse effects of this form of intervention.

Apolipoprotein C-1 inhibits the hydrolysis by phospholipase A2 of phospholipids in liposomes and cell membranes

A small polypeptide isolated from human serum inhibits the action of phospholipase A2 on dipalmitoylglycerol phosphocholine vesicles. Sequence analysis revealed the protein to be apolipoprotein C-1, a major component of very light-density lipoprotein. The inhibiting efficiency is increased by one order of magnitude after 10 min preincubation of the protein with the substrate, but not the enzyme. It also depends on the concentration of the phospholipid. IC50 is about 0.5 microM at 0.2 mM DPPC and 1 microM at 1 mM DPPC. Apolipoprotein C-1 is also inhibitory in a more physiological system: in broken human leukemia cells (HL-60 cells) it inhibits the release by endogenous phospholipases of arachidonic acid from membrane phospholipids. The effective concentrations correspond to those found in the serum. It is concluded that apolipoprotein C-1 and similar phospholipid-binding proteins may act as phospholipase inhibitors by blocking the access to the substrate.

Phospholipase A2 activation is the pivotal step in the effector pathway of inflammation

Our understanding of the mechanisms of initiation and propagation of local and systemic inflammatory processes is clearly imperfect if one uses the available therapeutic modalities as a yardstick. While glucocorticoids are potent anti-inflammatory drugs, the pharmacologic target of this class of agents has not been identified with certainty, and the use of steroids is fraught with the risk of considerable and potentially dangerous side effects. On the other hand, non-steroidal anti-inflammatory drugs (NSAIDS), while more specific, are relatively weak anti-inflammatory compounds and frequently require the addition of more potent agents. Cytotoxic drugs or anti-metabolites effectively suppress acute and chronic inflammatory reactions, but also predispose to infection and initiate the development of neoplasms following long-term exposure. The inadequacy and relative non-specificity of these approaches underscore the deficiencies in our understanding of the principles that govern these responses. A better understanding of these processes will be applicable to broad categories of human disease including autoimmunity, the collagen vascular diseases, aberrations in host defense and the response to trauma and infection.

Cellular and extracellular phospholipase A2 activity in zymosan pleurisy in rat

The pleural exudate from rats treated intrapleurally with zymosan contains phospholipase A2 (PLA2) activity which is Ca2(+)-independent and optimally active at a neutral pH. This PLA2 activity was found in approximately equal amounts in both the cellular and extracellular fractions of the exudate. The Ca2(+)-independency of the PLA2's in the pleural exudate distinguishes them from plasma PLA2's and this suggests that the source of the exudate PLA2's is not plasma. The appearance of PLA2 activity in zymosan-induced pleural exudate correlates temporally with increases in exudate volume and pleural cell number. In all cases, the maximum response was seen 24 hr after zymosan challenge. All parameters of pleurisy and PLA2 activity are similarly sensitive to the steroid dexamethasone which has been hypothesized to act, in part, through the synthesis of PLA2 inhibitory peptides. In its entirety, this information suggests that there is a relationship between pleural PLA2 activity and the appearance of pleural inflammation (exudate volume and cells) and that PLA2 may play an important role in the initiation and propagation of this inflammatory process in rats. Furthermore, the zymosan-induced pleurisy model may serve as a useful model for the identification of PLA2 inhibitors with antiinflammatory activity.

Mechanism for the inhibitory and stimulatory actions of proteins on the activity of phospholipase A2

The influence of proteins on phospholipase A2 was found to depend strongly on the enzyme assay system. We have used three different systems to measure phospholipase A2 which represent the different assay conditions used by a number of previous investigators. Two distinct stimulatory and two distinct inhibitory effects of proteins were observed. (1) A number of proteins - such as albumin, gamma-globulin and lysozyme - were found to inhibit phospholipase A2 activity only at very low substrate concentrations. This 'substrate depletion' was recently proposed as the mode of action for lipocortin. We therefore suggest that substrate depletion is not sufficiently specific to serve as a physiological regulatory mechanism and that the observed inhibition by lipocortin and other proteins more recently reported to mimic it are unlikely to be of physiological significance. (2) Use of liposomes at higher concentrations led to a nonlinear time-course. In this assay system, albumin (and other protein) stimulation can be accounted for as relief of product inhibition. (3) With high concentrations of phospholipids in the presence of cholate (mixed micelles), the behavior of proteins in the assay was complex. The assay time-course appeared linear in the absence of added protein, but at concentrations of added albumin up to 1 mg/ml, stimulation of phospholipase A2 activity was observed. Concentrations greater than this led to diminution of enzyme activity to the original activity. No effect whatever was observed when lysozyme was substituted for albumin. Since this biphasic result was not observed with liposomes, we suggest that the product whose inhibition is being relieved is the lysophosphatidylcholine, and not the free fatty acid. The inhibitory effect at high albumin concentrations is probably the result of removal of free fatty acids from the micelle: fatty acids are known to cause stimulation of phospholipase A2 by providing a negative charge to the lipid/water interface. (4) A different type of phospholipase A2 stimulation was apparent with melittin. This was found to be more specific than generally believed: we found no melittin stimulation of pancreatic phospholipase A2, yet confirmed a several-fold stimulation of bee venom phospholipase A2. We also found that high (millimolar) concentrations of calcium suppressed the melittin stimulation of bee venom phospholipase A2, and that a cationic detergent mimicked the stimulation by melittin. (5) We conclude that the effects of proteins on phospholipase A2 studied here can all be explained by proteins binding to substrate or product rather than enzyme-protein interactions.

Hydrolysis of 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine, a common precursor of platelet-activating factor and eicosanoids, by human platelet phospholipase A2

The metabolism of platelet-activating factor (PAF) and arachidonic acid is linked through the common intermediate 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine (alkylarachidonoyl-GPC). Hydrolysis of alkylarachidonoyl-GPC by phospholipase A2 may initiate the biosynthesis of both PAF and eicosanoids, since alkyllyso-GPC is formed for acetylation to PAF and arachidonic acid is liberated for conversion to biologically active metabolites. In order to elucidate the regulation and functional role of human platelet phospholipase A2 in the pathway leading to the formation of both classes of lipid mediators, we have characterized its action upon alkylarachidonoyl-GPC. Human platelet phospholipase A2 was solubilized and then partially purified in the presence of n-octyl-beta-D-glucopyranoside (octyl glucoside). Hexadecylarachidonoyl-GPC was prepared biosynthetically using platelet sonicates, purified by two-step high-performance liquid chromatography (HPLC) and suspended in buffer by sonication. Our results indicate that deacylation of alkylarachidonoyl-GPC by platelet phospholipase A2 has an absolute requirement for Ca2+. It occurs at submicromolar concentrations of free Ca2+ and exhibits a biphasic Ca2+-dependence with activity plateaus at 10 microM and 2 mM. Phospholipase A2-mediated hydrolysis of alkylarachidonoyl-GPC is increased 2-fold by albumin and is enhanced 5-fold if 1,2-dioleoylglycerol is incorporated into the substrate dispersion. The substrate dependence and specificity of platelet phospholipase A2 for 1-alkyl- vs. 1-acyl-linked subclasses of arachidonic acid containing phosphatidylcholine was examined with 1-O-hexadecyl-2-arachidonoyl-sn-glycero-3-phosphocholine (hexadecylarachidonoyl-GPC) and 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (palmitoylarachidonoyl-GPC). We found that the substrates were deacylated equivalently. We conclude that, in stimulated platelets, in the presence of increased levels of cytoplasmic free Ca2+ and newly generated diacylglycerol, alkylarachidonoyl-GPC may be rapidly hydrolyzed by phospholipase A2 and may serve as a precursor of both PAF and eicosanoids.

A novel binding assay for phospholipase A2

We have devised a rapid and simple assay for estimating the binding of pancreatic phospholipase A2 to a bilayer lipid membrane. The binding was observed to be extremely rapid at 37 degrees and was absolutely dependent upon Ca2+. Amongst several drugs known to inhibit the catalytic activity of phospholipase only mepacrine at high concentrations (500 microM) and chlorpromazine (100 microM) were active. Treatment of the enzyme with p-bromophenacylbromide did not inhibit binding. Several alcohols potentiated binding whereas detergents tended to inhibit. Amongst several purified proteins tested, only the steroid-induced anti-phospholipase protein lipocortin prevented binding. The use of this assay in screening for antiphospholipase agents is discussed.

Potential therapeutic efficacy of inhibitors of human phospholipase A2 in septic shock

Soluble phospholipase A2 has been implicated in the pathogenesis of local and systemic inflammatory reactions. Elevated levels of circulating phospholipase A2 (PLA2) correlate with the severity of circulatory collapse and pulmonary dysfunction in gram-negative septic shock. Characterization of septic shock serum PLA2 revealed a calcium-dependent enzyme with absolute 2-acyl specificity with a pH optimum of 7.5. We tested a number of therapeutic agents for their ability to inhibit PLA2 from human septic shock serum. Chloroquine, chlorpromazine, dexamethasone base, dexamethasone sodium phosphate, indomethacin, lidocaine, oleic acid, palmitic acid, promethazine, trans-retinoic acid, rutin and dl-alpha-tocopherol were all studied over the range of 10(-2) to 10(-7) M. All agents, with the sole exception of dexamethasone base, inhibited PLA2 activity at concentrations greater than 10(-3) M. PLA2 inhibition by dexamethasone sodium phosphate was factitious, due to the formation of calcium-phosphate complexes. Of the 11 agents studied, chlorpromazine was the most effective, with an IC50 of 7.5 X 10(-5) M, a membrane concentration achievable within its therapeutic range. Inhibition was non-competitive with an apparent Ki of 5 nM. Since serum PLA2 levels correlate with mortality in both experimental endotoxemia and clinical gram-negative septic shock, and chlorpromazine was previously shown to improve survival in these conditions, we postulate that its therapeutic efficacy resides at least in part in its PLA2-inhibitory activity.