Vanadate increases protein kinase C-induced phosphorylation of endogenous proteins of liver in vitro

In vitro effects of sodium orthovanadate on protein kinase C induced phosphorylation of rat liver cytosolic and particulate proteins were examined. Vanadate enhanced the phosphorylation of six liver cytosolic proteins (Mr 170K, 150K, 80K, 34K, 25K and 19K daltons), the probable substrates for protein kinase C. There was a 2.5-fold increase in total endogenous protein phosphorylation at 2.0 mM concentration which was abolished in the presence of protein kinase C inhibitors such as 1-(5-isoquinolinyl-sulfonyl-2-methylpiperazine (H-7), N-[2-(methylamine)-ethyl]-5-isoquinolinesulfonamide (H-8) and polymyxin B. Metavanadate showed a similar stimulatory effect whereas vanadyl sulfate was inhibitory. These differential effects of vanadium salts were also observed with the particulate fraction. The results suggest that some of the effects of vanadate could be mediated through protein kinase C-induced phosphorylation of endogenous proteins.