Inhibition of plant vacuolar H+-ATPase by diethylpyrocarbonate

Abstract

Treatment of the tonoplast H(+)-ATPase from mung bean seedlings (Vigna radiata L.) with histidine-specific modifier, diethyl pyrocarbonate (DEP), caused a marked loss of the ATP hydrolysis activity and the proton translocation in a concentration-dependent manner. The reaction order of inhibition was calculated to be 0.98, suggesting that at least one histidine residue of vacuolar H(+)-ATPase was modified by DEP. The absorbance of the vacuolar H(+)-ATPase at 240 nm was progressively increased after incubation with DEP, suggesting that N-carbethoxyhistidine had been formed. Hydroxylamine, which could break N-carbethoxyhistidine, reversed the absorbance change and partially restored the enzymic activity. The pK(a) of modified residues of vacuolar H(+)-ATPase was kinetically determined to be 6.73, a value close to that of histidine. Thus, it is assuredly concluded that histidine residues of the vacuolar H(+)-ATPase were modified by DEP. Kinetic analysis showed that V(max) but not K(m) of vacuolar H(+)-ATPase was decreased by DEP. This result is interpreted as that the residual activity after DEP inhibition was primarily due to the unmodified enzyme molecules. Moreover, simultaneous presence of DEP and DCCD (N,N'-dicyclohexyl-carbodiimide), an inhibitor modified at proteolipid subunit of vacuolar H(+)-ATPase, did not induce synergistic inhibition, indicating their independent effects. The stoichiometry studies further demonstrate that only one out of four histidine residues modified was involved in the inhibition of vacuolar H(+)-ATPase by DEP. Mg(2+)-ATP, the physiological substrate of vacuolar H(+)-ATPase, but not its analogs, exerted preferentially partial protection against DEP, indicating that the histidine residue involved in the inhibition of enzymatic activity may locate at/or near the active site and directly participate in the binding of the substrate.