Effect of vanadate on brain protein phosphorylation.

The effect of vanadate on the phosphorylation of synaptosomal membrane proteins prepared from rat cerebral cortex was studied. Vanadate concentrations of 10(-6), 10(-5), and 10(-4) M increased the endogenous phosphorylation activity by 25%, 37%, and 75%, respectively. Increasing the ATP concentration in the assay medium from 50 to 500 microM did not influence the above effect. A commercial preparation of the purified protein kinase was stimulated 40% by 10(-3) M vanadate. Calcium-calmodulin dependent activity was stimulated only 20% by 10(-5) M vanadate. The effect was not enhanced by further increasing vanadate concentration. Addition of calcium ions (above 50 microM) suppressed the vanadate effect, while an inhibition was observed at high Ca2+ concentration (2.5 mM). Below 50 microM calcium ions stimulated phosphorylation activity in the absence of vanadate and did not affect the stimulatory action of vanadate. Cyclic AMP-dependent endogenous phosphorylation was also stimulated by vanadate. Activation by cAMP could not be observed at vanadate concentrations above 10(-6) M. Possible mechanisms of the vanadate effect are discussed.