Ca2+ effect on ATP-independent lysophospholipid transacylases is indissociable from Ca2+ effect on phospholipase A2 activity in rat platelet sonicates

Biosynthesis of platelet-activating factor in cultured mast cells. Involvement of the CoA-independent transacylase demonstrated by analysis of the molecular species of platelet-activating factor

We have recently demonstrated that arachidonate [20:4(5,8,11,14)] was primarily linked to the hexadecyl (16:0) and octadecenyl (18:1) species of alkylacyl derivatives of glycerolphosphocholine (GroPCho). Consistent with the involvement of arachidonate-specific CoA-independent transacylase in the synthesis of platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-GroPCho), 16:0 and 18:1 PAF species were formed upon antigen stimulation [Joly, F., Breton, M., Wolf, C., Ninio, E. & Colard, O. (1992) Biochim. Biophys. Acta 1125, 305-312]. In the present work, addition of lyso-PAF to mast cells resulted in PAF production. We analyzed the PAF species formed in the presence of a defined lyso-PAF molecular species in order to differentiate between either direct acetylation or involvement of the membrane precursor. The 18:1 lyso-PAF was more effective than the 16:0 in producing PAF which was composed of 95% 18:1 PAF, the balance being 16:0, indicating that part of the acetylated lyso-PAF originated from the cellular pool of alkyl-arachidonyl-GroPCho in resting cells. Consistent with alkyl-arachidonyl-GroPCho species content and acetyltransferase specificity, similar amounts of 16:0 and 18:1 PAF species were formed when mast cells were stimulated with antigen. Supplemented with 16:0 or 18:1 lyso-PAF, antigen-stimulated mast cells responded by 230% and 125% increase in PAF synthesis, respectively. As expected, the amount of the PAF species corresponding to the added lyso-PAF was increased. More interestingly, addition of 16:0 lyso-PAF almost doubled the amount of 18:1 PAF content as compared to antigen alone, thus indicating that the lyso-PAF formed via the CoA-independent transacylase was significantly used for PAF synthesis, despite a large excess of exogenous lyso-PAF. The CoA-independent transacylase, measured using [3H]lyso-PAF as a substrate in sonicates from antigen-stimulated cells, was decreased concurrently with PAF formation. In conclusion, we show that when lyso-PAF is added to mast cells, a direct acetylation may occur. However, PAF is preferentially synthesized through a mechanism involving the CoA-independent transacylase reaction.

Inhibition of parathyroid hormone secretion correlates with increased incorporation of 32P into phosphatidylinositol and lysophosphatidylinositol

We have studied the incorporation of radioactive P (32P) into lipids of bovine parathyroid tissue under conditions of stimulated and inhibited hormone secretion. Utilizing low (0.5 mM) and high (3.0 mM) concentrations of calcium to regulate parathyroid hormone secretion, we initially found that the labeling of the cellular phospholipids with 32P was greater in those tissues incubated in high-calcium medium. Thin-layer chromatography of lipid extracts prepared from tissue incubated in either low- or high-calcium media revealed that the increased incorporation of 32P (high or low) was localized primarily to two phospholipids. To determine whether the increases were due directly to the different calcium concentrations, the experiments were performed in media containing normal calcium concentrations (1.25 mM) and low (0.5) or high (3.0) magnesium concentrations to modulate hormone secretion. The results were identical to those obtained using low and high calcium, indicating that the increased 32P incorporation was not an effect of high calcium but rather correlated with the inhibition of hormone secretion. The use of other secretagogues confirmed this correlation. The identity of the two phospholipids was established, by two-dimensional thin-layer chromatography, to be phosphatidylinositol (PI) and lysophosphatidylinositol (LPI). The correlation of increased 32P incorporation with inhibition of secretion led us next to examine isolated secretory granules from tissues exposed to either high-or low-calcium conditions. Thin-layer chromatography of granule lipid extracts yielded chromatograms containing PI and LPI, and the radioactivity of each was greater in the high-calcium sample than in the low-calcium sample.(ABSTRACT TRUNCATED AT 250 WORDS)

Very high proportion of disaturated molecular species in rat platelet diacyl-glycerophosphocholine: involvement of CoA-dependent transacylation reactions

The molecular species composition of rat platelet diacyl-glycerophosphocholine (GPC) was investigated by reverse-phase HPLC and by mass spectrometry. The two methods gave the same very high proportion of fully saturated phospholipids, the 16:0-16:0 and 16:0-18:0 species representing together about 40% of the overall molecular species. [14C]Palmitoyllyso-GPC was found to be acylated by resting platelets in equal amounts into 16:0-16:0 and into 16:0-20:4 species. The acylation rate of this lysophospholipid was increased by 3-fold and 14-fold when platelets were stimulated for 10 min with thrombin and the ionophore A23187, respectively. Essentially the same two molecular species were synthesized upon stimulation but with a higher preference for arachidonate than for palmitate. We investigated the mechanisms responsible for the incorporation of palmitate and arachidonate by examining the enzymatic acylation of [14C]palmitoyllyso-GPC by platelet homogenates. The percentage of the various molecular species formed when CoA, ATP, and Mg2+ were added excludes the CoA, ATP-dependent pathway as being involved in the acylation reactions previously observed. In the absence of ATP, CoA-independent transacylations appear to play a crucial role in the synthesis of the 16:0-20:4 species whereas the addition of CoA greatly favored dipalmitoyl-GPC synthesis. The involvement of CoA-dependent mechanisms in the synthesis of dipalmitoyl-GPC was demonstrated as follows: (i) the labeling in the sn-2 position of the dipalmitoyl-GPC synthesized in the presence of CoA was not modified when free unlabeled palmitic acid was added to the incubation medium and (ii) platelet homogenates were unable to esterify lysolecithin with added labeled palmitic acid in the presence of CoA only.

Arachidonate cannot be released directly from diacyl-sn-glycero-3-phosphocholine in thrombin-stimulated platelets

The origin of the arachidonate released from platelets on stimulation with thrombin was investigated by comparing the specific activities of released arachidonate and of arachidonoyl-containing phospholipids using rat platelets prelabelled with arachidonate. Quantification of the released arachidonate was determined in the presence of BW 755 C, a dual cyclo-oxygenase/lipoxygenase inhibitor, which was found not to modify the arachidonate mobilization between the platelet phospholipids. The phospholipid molecular species were analysed by h.p.l.c. of diradylglycerol benzoate derivatives of diacyl, alkylacyl and alkenylacyl classes. The labelled/unlabelled arachidonate ratio varied greatly in the phospholipids depending on whether an ether or acyl bond was present in sn-1 position of the glycerol, on the length and degree of unsaturation of this fatty chain and on the polar head group. Between 15 s and 5 min of stimulation by thrombin, the released arachidonate kept a constant specific activity which was considerably lower than the specific activity of diacyl-GPC. The specific activity of the released arachidonate was intermediate between the specific activities of the 16:0-20:4 and 18:0-20:4 species of diacyl-GPI and diacyl-GPE, and corresponded to the mean specific activity of alkylacyl-GPC. The data indicate that the released arachidonate cannot come directly from diacyl-GPC, and that two phospholipids in particular can act as direct precursors of the released arachidonate. These are (1) the alkylacyl-GPC and (2) the diacyl-GPE whose hydrolysis would induce an arachidonate transfer from diacyl-GPC.