Phospholipase A
2 when activated by hormonal stimuli via receptor-mediated process, releases arachidonic acid from membrane phospholipids to produce biologically active eicosanoids. This activity has been known to be modulated by a variety of factors, such as G-protein, protein kinase A and also C, lipocortin, and Na
+/H
+ exchanger, other than by intracellular Ca
2+. However, we here demonstrate in platelet that decrease in lipid phase fluidity of the membranes, induced by cholesterol-enrichment or lipid peroxidation enhances the phospholipase A
2 activity, whereas the increase by incorporation of unsaturated fatty acids into the membranes exerts the opposite effect. Consequently, the lipid fluidity appears to affect the receptor-enzyme coupling rather than the enzyme molecule itself, and hence to be implicated in the factors modulating the enzyme activity.
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