Characterization of 1,2-diacylglycerol hydrolysis in human platelets. Demonstration of an arachidonoyl-monoacylglycerol intermediate.

When platelets are stimulated by thrombin, a phosphatidylinositol-specific phospholipase C produces a transient rise in 1,2-diacylglycerol. We have now characterized the hydrolysis of diacylglycerol by platelet membranes using doubly isotopically labeled substrates of defined fatty acid composition. We find that the fatty acid at sn-1 is hydrolyzed faster than that at sn-2 thereby producing a 2-monoacylglycerol intermediate. If hydrolysis had occurred at either position randomly, 1-monoacylglycerol would also be produced. That none was detected indicates that either the sn-1 fatty acid must be cleaved first or that 1-monoacylglycerol is hydrolyzed by monoacylglycerol lipase much faster than 2-monoacylglyceol. The latter possibility was excluded by the finding that 1-monoacylglycerol and 2-monoacylglycerol are hydrolyzed at equal rates by platelet membranes. The diacylglycerol lipase cleaves diacylglycerols with sn-1 palmitate as rapidly as those with sn-1 stearate. Arachidonate at sn-2 is cleaved twice as fast as sn-2 oleate by monoacylglycerol lipase. The two activities probably represent discrete enzymes since monoacylglycerol lipase activity can be separated from diacylglycerol lipase by fractionation on DEAE-Sepharose, although both are contained in the membrane fraction of platelets. That the sequential breakdown of 1,2-diacylglycerol also occurs in intact platelets is indicated by our finding of a transient rise in arachidonoyl-monoacylglycerol in thrombin-stimulated platelets. This provides further evidence for a role of the phospholipase C-diacylglycerol lipase pathway in the release of arachidonic acid.