Phosphoinositide fatty acids regulate phosphatidylinositol 5-kinase, phospholipase C and protein kinase C activities

PtdIns(4,5)P-2 generally results from phosphorylation of PtdIns(4)P by the phosphatidylinositol 5-kinase (PtdIns5-K). Its hydrolysis by phospholipase C (PLC) yields inositol 1,4,5-trisphosphate and diacylglycerol, which stimulates protein kinase C (PKC). We show that epithelial cells of the cockroach rectum contain three different inositol lipids: PtdIns(4,5)P-2, PtdIns(4)P, and PtdIns. They are composed of six major fatty acids: palmitic (16:0) stearic (18:0), oleic (18:1n-9), linoleic (18:2n-6), linolenic (18:3n-3), and arachidonic (20:4n-6) acids. The fatty acid preference of each of the above enzymes was evaluated by incorporating different fatty acids in pairs into membrane lipids. Incorporation of 16:0 plus 18:1n-9 provoked an increase in PtdIns(4,5)P-2-PLC activity and a decrease in PtdIns5-K activity. In contrast, incorporation of 16:0 plus 18:3n-3 led to a potentiation of PtdIns5-K activity and a decrease in PtdIns(4,5)P-2-PLC activity. Furthermore, PLC and PtdIns5-K acted preferentially on substrates containing 18:3n-3, and 18:3n-3-containing diacylglycerol specifically potentiated PKC activity. Thus, we propose that the fatty acids that make up the phosphoinositides function as intracellular modulators of the activity of certain enzymes.