The kinetics of the Ca2+-dependent, alkaline pH optimum, membrane-bound phospholipase A2 from the P388D1 macrophage-like cell line were studied using various phosphatidylcholine (PC) and phosphatidylethanolamine (PE) substrates. This enzyme exhibits "surface dilution kinetics" toward PC in Triton X-100 mixed micelles, and the "dual phospholipid model" was found to adequately describe its kinetic behavior. With substrate in the form of sonicated vesicles, the dual phospholipid model should give rise to Michaelis-Menten type kinetics. However, the hydrolysis of dipalmitoyl-PC, 1-palmitoyl-2-oleoyl-PC, and 1-stearoyl-2-arachidonoyl-PC vesicles exhibited two distinct activities. Below 10 microM, the data appeared to follow Michaelis-Menten behavior, while at higher concentrations, the data could best be fit to a Hill equation with a Hill coefficient of 2. These PCs had Vmax values for the low substrate concentration range of 0.2-0.6 nmol min-1 mg-1 and Km values of 1-2 microM. At the high substrate concentration range, the Vmax values were between 5 and 7 nmol min-1 mg-1. PC containing unsaturated fatty acids had an apparent Km, determined from the Hill equation, of about 15 microM, while the apparent Km of dipalmitoyl-PC was 0.6 microM. When 70% glycerol was included in the assays, a single Michaelis-Menten curve was obtained for both dipalmitoyl-PC and 1-stearoyl,2-arachidonoyl-PC. Possible explanations for these kinetic results include reconstitution of the membrane-bound phospholipase A2 in the phospholipid vesicle or the enzyme has tow distinct phospholipid binding function. The kinetics for both dipalmitoyl-PC and dipalmitoyl-PE hydrolysis in vesicles was very similar, indicating that the enzyme does not greatly prefer one of these head groups over the other. The enzyme also showed no preference for arachidonoyl containing phospholipid. Enzymatic activity toward PC containing saturated fatty acids was linear to about 15% hydrolysis while the hydrolysis of PC containing unsaturated fatty acids was linear to only about 5%. This loss of linearity was due to inhibition by released unsaturated fatty acids. Arachidonic acid was found to be a competitive inhibitor of dipalmitoyl PC hydrolysis with a K1 of 5 microM. This tight binding suggests a possible in vivo regulatory role for arachidonic acid. Three compounds of the arachidonic acid cascade, prostaglandin F2 alpha, 6-keto-prostaglandin F1 alpha, and thromboxane B2, showed no inhibition of enzymatic activity.

In order to ascertain the role of phospholipase A2 (PLA2) in the release of arachidonic acid for eicosanoid biosynthesis, we have characterized a Ca2+-dependent PLA2 from P388D1 cells, evaluated inhibitors of its activity, and correlated the effects of these inhibitors on prostaglandin (PG) E2 production in the intact cell. The Ca2+-dependent PLA2 has little preference for the polar head group or sn-2 fatty acid of phospholipids, and we have now found that it will hydrolyze 1-alkyl,2-acyl phospholipids, but it does not show a preference for this substrate over other phospholipids. Inhibitor studies with the Ca2+-dependent PLA2 have shown that arachidonic acid is an effective inhibitor. The analogs of natural fatty acids, eicosatetraynoic acid and octadecyleicosaynoic acid, were ineffective as inhibitors of the P388D1 PLA2. However, 7,7-dimethyl-5,8-eicosadienoic acid was as effective an inhibitor (IC50 = 16 microM) as arachidonic acid. Manoalide and its analog, manoalogue, were found to be good inhibitors of the P388D1 PLA2 (IC50 = 16 and 26 microM, respectively). The irreversible inhibitor of the extracellular PLA2, p-bromophenacyl bromide, was a very poor inhibitor of the P388D1 PLA2, apparent IC50 = 500-600 microM. Quinacrine was also ineffective as an inhibitor as was the cyclooxygenase inhibitor indomethacin. On the cellular level, the P388D1 cells respond to various stimuli to produce PGD2 and PGE2 as the major cyclooxygenase products with minor production of PGI2 and thromboxane A2. Similar arachidonic acid metabolite profiles were seen for calcium ionophore A23187, melittin, and platelet-activating factor. Manoalide, manoalogue, and 7,7-dimethyl-5,8-eicosadienoic acid, effective inhibitors of the isolated PLA2, inhibited PGE2 production in intact P388D1 cells 40-85% in the concentration range studied. In contrast, p-bromophenacyl bromide, which is ineffective as an inhibitor of the P388D1 PLA2, did not significantly effect PGE2 production in the concentration ranges used. These results demonstrate that there may be important differences between the intracellular P388D1 PLA2 and the more commonly studied extracellular forms of PLA2. These differences are also observed in the intact cell studies and emphasize the need for the evaluation of inhibitors both in vitro and in vivo using the isolated enzyme and intact cell. This is the first example of studies aimed at correlating the inhibition of a purified intracellular PLA2 with inhibition of prostaglandin production in the intact cell from which it is derived.

Bacterial endotoxin (lipopolysaccharide [LPS]) causes fatal shock in humans and experimental animals. The shock is mediated by cytokines released by direct LPS stimulation of cells of monocytic origin (monocyte/macrophage [MO]). Recent studies have supported the concept that the plasma protein, LPS binding protein (LBP), plays an important role in controlling MO responses to LPS. Specifically, evidence has been presented to suggest that CD14, a membrane protein present in MO, serves as a receptor for complexes of LPS and the plasma protein LPS binding protein (LBP). In this function CD14 mediates attachment of LPS-bearing particles opsonized with LBP and appears to play an important role in regulating cytokine production induced by complexes of LPS and LBP. The CD14-, murine pre-B cell line 70Z/3 responds to LPS by synthesis of kappa light chains and consequent expression of surface IgM. To better understand the role of CD14 in controlling cellular responses to LPS, we investigated the effect of transfection of CD14 into 70Z/3 cells on LPS responsiveness. We report here that transfection of human or rabbit CD14 cDNA into 70Z/3 cells results in membrane expression of a glycosyl-phosphatidylinositol-anchored CD14. When LPS is complexed with LBP, CD14-bearing 70Z/3 cells bind more LPS than do the parental or 70Z/3 cells transfected with vector only. Remarkably, the expression of CD14 lowers the amount of LPS required to stimulate surface IgM expression by up to 10,000-fold when LPS dose-response curves in the CD14-, parental and CD14-bearing, transfected 70Z/3 cells are compared. In contrast, the response of CD14-bearing 70Z/3 cells and the parental 70Z/3 cell line (CD14-) to interferon gamma is indistinguishable. LPS stimulation of the parental and CD14-bearing 70Z/3 cells results in activation of NF-kB. These data provide evidence to support the concept that the LPS receptor in cells that constitutively express CD14 may be a multiprotein complex containing CD14 and membrane protein(s) common to a diverse group of LPS-responsive cells.

Cobra venom factor (CoF), the anticomplementary protein in Naja naja cobra venom, is usually purified by sequential ion exchange and gel filtration chromatography. CoF prepared in this manner contains small but significant quantities of phospholipase A2 activity. This acyl hydrolase activity can be simply and efficiently removed on a large scale by treatment of CoF with p-bromophenacyl bromide (BPB), an irreversible modifier of the histidine residue in the active site of phospholipase A2. BPB treatment does not alter the anticomplementary activity of CoF. In vivo experiments utilizing intratracheal injections of control and BPB-treated CoF, as well as pure phospholipase A2, revealed that contaminating phospholipase A2, and not the anticomplementary protein, was responsible for the observed acute neutrophil-associated lung injury. However, phospholipase A2 had no effect on the hypotensive and thrombocytopenic effects of CoF infected intravenously into rabbits. Depletion of circulating C3-C9 by intraperitoneal injections of CoF was not altered by removal of phospholipase A2 activity with BPB.

The release of free arachidonic acid from membrane phospholipids is believed to be the rate-controlling step in the production of the prostaglandins, leukotrienes, and related metabolites in inflammatory cells such as the macrophage. We have previously identified several different phospholipases in the macrophage-like cell line P388D1 potentially capable of controlling arachidonic acid release. Among them, a membrane-bound, alkaline pH optimum, Ca2+-dependent phospholipase A2 is of particular interest because of the likelihood that the regulatory enzyme has these properties. This phospholipase A2 has now been solubilized from the membrane fraction with octyl glucoside and partially purified. The first two steps in this purification are butanol extractions that yield a lyophilized, stable preparation of phospholipase A2 lacking other phospholipase activities. This phospholipase A2 shows considerably more activity when assayed in the presence of glycerol, regardless of whether the substrate, dipalmitoylphosphatidylcholine, is in the form of sonicated vesicles or mixed micelles with the nonionic surfactant Triton X-100. Glycerol (70%) increases both the Vmax and the Km with both substrate forms, giving a Vmax of about 15 nmol min-1 mg-1 and an apparent Km of about 60 microM for vesicles and a Vmax of about 100 nmol min-1 mg-1 and an apparent Km of about 1 mM for mixed micelles. Vmax/Km is slightly greater for vesicles than for mixed micelles. The lyophilized preparation of the enzyme is routinely purified about 60-fold and is suitable for evaluating phospholipase A2 inhibitors such as manoalide analogues. Subsequent steps in the purification are acetonitrile extraction followed by high performance liquid chromatography on an Aquapore BU-300 column and a Superose 12 column. This yields a 2500-fold purification of the membrane-bound phospholipase A2 with a 25% recovery and a specific activity of about 800 nmol min-1 mg-1 toward 100 microM dipalmitoylphosphatidylcholine in mixed micelles. When this material was subjected to analysis on a Superose 12 sizing column, the molecular mass of the active fraction was approximately 18,000 daltons.