Selective Chemical Modification of Malate Dehydrogenase

Mitochondrial malate dehydrogenase (l-malate:NAD+ oxidoreductase, EC 1.1.1.37) from porcine heart is irreversibly inactivated by N-ethylmaleimide. At pH 5.0, only a limited number of residues of the enzyme (2) are chemically modified by interaction with this reagent. The residues modified have been identified as cysteine. Chemical modification of these essential residues is prevented by the presence of the coenzyme, NADH, suggesting that the site of interaction is located at or near the coenzyme binding site and hence at or near the enzymatic center of this enzyme.