ADP Is a Competitive Inhibitor of ATP-Dependent H⁺ Transport in Microsomal Membranes from Zea mays L. Coleoptiles

An anion-sensitive ATP-dependent H(+) transport in microsomal membranes from Zea mays L. coleoptiles was partially characterized using the pH gradient-dependent decrease of unprotonated neutral red. The following criteria strongly suggest a tonoplast origin of the H(+) transport observed: strict dependence on Cl(-); inhibition by SO(4) (2-) and NO(3) (-); insensitivity against vanadate, molybdate, and azide; reversible inhibition by CaCl(2) (H(+)/Ca(2+) antiport); inhibition by diethylstilbestrol. The substrate kinetics revealed simple Michaelis Menten kinetics for ATP in the presence of 1 millimolar MgCl(2) with a K(m) value of 0.56 millimolar (0.38 millimolar for MgATP). AMP and c-AMP did not influence H(+) transport significantly. However, ADP was a potent competitive inhibitor with a K(i) value of 0.18 millimolar. The same inhibition type was found for membranes prepared from primary roots by the same procedure.