Electrostatic analysis of effects of ions on the inhibition of corn root plasma membrane Mg2+-ATPase by the bivalent orthovanadate.

The microelectrophoretic mobility of corn root plasma membranes and the inhibition of the Mg+2-ATPase by vanadate were investigated under different ionic conditions. The Mg2+-ATPase was uncompetitively inhibited and a 10-fold variation of the apparent inhibition constant was observed, depending on the addition of K+ and Mg2+. The determination of the zeta potential indicated that a 5-fold decrease of the apparent inhibition constant was due to aspecific electrostatic interactions of the vanadate anion and the negative charge of the membrane. The screening and masking effects of 6 mM free Mg2+ totally abolished electrostatic interactions and allowed the direct determination of the intrinsic vanadate inhibition constant (KIi). On the other hand, a specific, non-electrostatic, effect of K+ caused a 2-fold decrease of the inhibition constant in addition to the electrostatic effect. Finally, the electrostatic analysis indicates that the Mg2+-ATPase is inhibited by the monomeric bivalent anion HVO4(2-).