Inhibition by Vanadate of the Reactions Catalyzed stimulated ATPase

The interaction of vanadate with the (Na' + K')st imulated ATPase from electric organ was investigated using the acid quench-flow technique. A t 21"C, incubation of the enzyme with 1.3 to 1.6 p~ vanadate in the presence of 75 m~ Na' and 25 mM K' strongly inhibits phosphorylation by ATP. Enzyme activity remaining under these conditions shows no change in the apparent rates of phosphorylation or dephosphorylation, although effects were noted which suggest that vanadate increases the reverse rate of dephosphorylation. Ten micromolar vanadate, sufficient to inhibit the (Na' + K+)-stimulated ATPase by more than 98%, has no effect on phosphorylation in the presence of Na' alone. Phosphoenzyme formed in the presence of Na' and K' consists of rapidly and slowly decaying components which differ in sensitivity to vanadate . Up to 2 p~ vanadate suppresses predominantly the rapidly decaying phosphoenzyme, while at higher concentrations vanadate inhibits both the rate and level of formation of the slowly decaying phosphoenzyme. These results indicate that vanadate is a useful reagent for distinguishing between these two phosphorylation reactions.