Primary stimuli of icosanoid release inhibit arachidonoyl‐CoA synthetase and lysophospholipid acyltransferase

Icosanoid formation in platelets depends on the concentration of free arachidonate that is mainly liberated from membrane phospholipids by phospholipase A 2 . The concentration of free arachidonate is also controlled by the activities of the reacylating enzymes arachidonoyl‐CoA synthetase and lysophospholipid acyltransferase. In human platelet microsomes we determined the high enzyme activities of 5.9 nmol · min −1 · (10 9 platelets) −1 for the arachidonoyl‐CoA synthetase and 37 nmol · min −1 · (10 9 platelets) −1 for the lysophospholipid acyltransferase. The activities of these reacylating enzymes were strongly reduced by hydrogen peroxide (H 2 O 2 ) and methyl mercury that are primary stimuli of arachidonate release in intact platelets. H 2 O 2 inhibited the arachidonoyl‐CoA synthetase with an IC 50 of 3.3 nmol/1 without affecting the lysophospholipid acyltransferase. Sulfhydryl group protection by 3‐mercapto‐1,2‐propanediol did not overcome the inhibition but glutathione prevented the inhibition of the arachidonoyl‐CoA synthetase by H 2 O 2 . This suggests that glutathione by virtue of the glutathione peroxidase reduces H 2 O 2 rather than that it protects free sulfhydryl groups of the arachidonoyl‐CoA synthetase. Methyl mercury left the arachidonoyl‐CoA synthetase activity unaffected but inhibited the lysophospholipid acyltransferase activity with an IC 50 of 3.4 · mol/l. The inhibition is probably evoked by the blockade of sulfhydryl groups of the lysophospholipid acyltransferase because it disappeared when 3‐mercapto‐1,2‐propanediol was added at a concentration higher than that of methyl mercury. Thrombin as a physiological full agonist, Ca 2+ · 1 mmol/1, the calcium ionophore A23187 and phorbol 12‐myristate 13‐acetate (TPA) and 1‐oleoyl‐2‐acetyl‐glycerol as model stimuli of protein kinase C neither influenced arachidonoyl‐CoA synthetase nor lysophospholipid acyltransferase. It is concluded that the inhibitory effect of H 2 O 2 and methyl mercury on the arachidonate‐reacylating enzymes arachidonoyl‐CoA synthetase or lysophospholipid acyltransferase, respectively, are responsible for their capacity to stimulate icosanoid release in intact cells. Thrombin and its intracellular messengers Ca 2+ and diacylglycerol do not directly affect arachidonoyl‐CoA synthetase and lysophospholipid acyltransferase.