REGULATION OF PLASMA-MEMBRANE H-ATPASE FROM CORN-ROOT BY MG2+ AND PH()

The plasma membrane H+-ATPase of corn-root is activated by free Mg2+ a t pH 6.0 (K-S = 2.9 mM) but not at pH 7.0. As a result, the pH depende nce of the enzyme varies depending on the Mg2+ concentration of the me dium. The activation by Mg2+ is promoted by an increase in V-max with no effect on the apparent K-m for Mg . ATP. Different from Mg2+, free Mn2+, Co2+ and Ni2+ do not activate at pH 6.0 and inhibit the H+-ATPas e at pH 7.0. The effects of divalent cations on the corn ATPase observ ed in this report are different from those previously described for th e yeast enzyme (Brooker, R.J. and Slayman, C.W. (1983), J. Biol, Chem. 258, 8833-8838), suggesting different mechanisms of regulation for th e isoforms of yeast and corn H+-ATPase.