The inhibition of arachidonic acid metabolism in human platelets by RHC 80267, a diacylglycerol lipase inhibitor

The diacylglycerol Lipase inhibitor, RHC 80267, 1,6-di(O-(carbamoyl)cyclohexanone oxime)hexane, was tested for its ability to block the release of arachidonic acid from human platelets. At a concentration (10 microM) reported to completely inhibit diacylglycerol Lipase in fractions of broken platelets, RHC 80267 had no effect on diacylglycerol Lipase activity or the release of arachidonic acid from washed human platelets stimulated with collagen. At a high concentration (250 microM), the compound inhibited the formation of arachidonyl-monoacylglycerol by 70% and the release of arachidonate by 60%. However, at this concentration RHC 80267 was found to inhibit cyclooxygenase activity, Phospholipase C activity and the hydrolysis of phosphatidylcholine (PC) (presumably by inhibiting Phospholipase A2). The Phospholipase C inhibition was attributed to the inhibition of prostaglandin H2 formation, as it was alleviated by the addition of the endoperoxide analog, U-46619. PC hydrolysis was only partially restored with U-46619, suggesting that RHC 80267 directly alters Phospholipase A2 activity. The inhibition of arachidonate release observed was accounted for by the inhibition of PC hydrolysis. We conclude that RHC 80267, because of its lack of specificity at concentrations needed to inhibit diacylglycerol Lipase, is an unsuitable inhibitor for studying the release of arachidonic acid in intact human platelets.