Vitamin E inhibits platelet phospholipase A2.

One of the most important functions of phospholipase A2 is the release of arachidonic acid from membrane phospholipids for the synthesis of biologically active eicosanoids. We have demonstrated in our laboratory that vitamin E inhibits platelet phospholipase A2 in a dose-dependent manner. Rats fed a 100 ppm or a 1000 ppm vitamin E diet exhibit diminished phospholipase A2 activity compared to those fed a vitamin E-free diet. Addition of vitamin E to a sonicated platelet suspension resulted in further suppression of the phospholipase A2 activity in all groups of rats. In order to gain insight into the mechanism of vitamin E inhibition of platelet phospholipase A2, we partially purified this enzyme by gel filtration chromatography. Enzyme activity was localized in the soluble supernatant fraction of a high-speed spin. This partially purified rat platelet phospholipase A2 had an absolute requirement for Ca2+ and was inhibited by various forms of tocopherol. Tocol inhibited the enzyme to a greater extent than either D- or DL-alpha-tocopherol, while there was little or no effect from DL-alpha-tocopherol acetate. These results emphasize the importance of the hydroxyl moiety on the chromanol of the vitamin E molecule for its inhibitory action, compared to that of the methyl groups which are absent in tocol. This inhibitory action of vitamin E on platelet phospholipase A2 suggests a crucial function for vitamin E in regulating arachidonate release from the membrane phospholipids and its subsequent metabolism.