Factors Affecting the Inhibition of Yeast Plasma Membrane ATPase by Vanadate

Inhibition of yeast plasma membrane ATPase by vanadate occurs only if either Mg2+ or MgATP2- is bound to the enzyme. The dissociation constant of the complex of vanadate and inhibitory sites is 0.14-0.20 microM in the presence of optimal concentrations of Mg2+ and of the order of 1 microM if the enzyme is saturated with MgATP2-. The dissociation constants of Mg2+ and MgATP2- for the sites involved are 0.4 and 0.62-0.73 mM, respectively, at pH 7. KCl does not increase the affinity of vanadate to the inhibitory sites as was found with (Na+ + K+)-ATPase. On the other hand, the effect of Mg2+ upon vanadate binding is similar to that upon (Na+ + K+)-ATPase, and the corresponding affinity constants of Mg2+ and vanadate for the two enzymes are of the same order of magnitude.