Incubation of maize (Zea mays) leaf NADP-malic enzyme with monofunctional and bifunctional N-substituted maleimides results in an irreversible inactivation of the enzyme. Inactivation by the monofunctional reagents,N-ethylmaleimide (NEM) andN-phenylmaleimide, followed pseudo-first-order kinetics. The maximum inactivation rate constant for phenylmaleimide was 10-fold higher than that for NEM, suggesting a possible hydrophobic microenvironment of the residue(s) involved in the modification of the enzyme. In contrast, the inactivation kinetics with the bifunctional maleimides,ortho-,meta-, andpara-phenylenebismaleimide, were biphasic, probably due to different reactivities of the groups reacting with the two heads of these bifunctional reagents, with a possible cross-linking of two sulfhydryl groups. The inactivation by mono and bifunctional maleimides was partially prevented by Mg2+andl-malate, and NADP prevented the inactivation almost totally. Determination of the number of reactive sulfhydryl groups of the native enzyme with [3H]NEM in the absence or presence of NADP showed that inactivation occurred concomitantly with the modification of two cysteinyl residues per enzyme monomer. The presence of these two essential residues was confirmed by titration of sulfhydryl groups with [3H]NEM in the enzyme previously modified byo-phenylenebismaleimide in the absence or presence of NADP.