Effect of vanadate on brain protein phosphorylation

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Abstract
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 μM 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 μM) suppressed the vanadate effect, while an inhibition was observed at high Ca2+ concentration (2.5 mM). Below 50 μM 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.