Stimulation of Phosphatidylinositol Phosphorylation in the Sarcoplasmic Reticular Ca2+-Transport ATPase by Vanadate

Abstract

Vanadate increases the initial phosphatidylinositolphosphate formation rate as well as the steady state level of the above lipid phosphate when phosphatidylinositol associated with the isolated Ca2+ transport ATPase is phosphorylated either by the membrane bound endogeneously present phosphatidylinositol-kinase or by exogeneously added lipid kinase. Employing an ultrasonicated mixture of pure phosphatidylinositol and Triton X 100 (without membrane proteins) no vanadate effect can be seen. This vanadate effect is probably not mediated through the lipid kinase activity, but more likely, through conformational changes of the Ca2+ transport ATPase protein. Such conformational changes would lead to a higher degree of phosphatidylinositol exposed on the Ca2+ transport ATPase and thus a higher substrate concentration. Consequently, the initial phosphatidylinositolphosphate formation rate and steady state level increase.