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

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Abstract
An anion-sensitive ATP-dependent H+ transport in microsomal membranes from Z. 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 SO42- and NO3-; insensitivity against vanadate, molybdate and azide; reversible inhibition by CaCl2 (H+/Ca2+ antiport); inhibition by diethylstilbestrol. The substrate kinetics revealed simple Michaelis Menten kinetics for ATP in the presence of 1 mM MgCl2 with a Km value of 0.56 mM (0.38 mM for MgATP). AMP and cAMP did not influence H+ transport significantly. ADP was a potent competitive inhibitor with a Ki value of 0.18 mM. The same inhibition type was found for membranes prepared from primary roots by the same procedure.