CHEMICAL MODIFICATION OF RAT-LIVER CYTOSOLIC NADP-LINKED ISOCITRATE DEHYDROGENASE BY N-ETHYLMALEIMIDE - EVIDENCE FOR ESSENTIAL SULFHYDRYL-GROUPS()

Incubation of rat liver cytosolic isocitrate dehydrogenase with N-ethy lmaleimide (NEM) resulted in the inactivation of the enzyme following pseudo-first order kinetics. Isocitrate affords considerable protectio n against inactivation whereas NADP+ enhances modification of the enzy me, suggesting localization of the modified group at the active site. Correlation of loss of activity with incorporation of (C-14]NEM indica ted that two sulphydryl residues/sub-unit are modified of which only o ne is shown to be involved in catalysis. pH dependence of the inactiva tion process implicates a reactive group of pK(a) 8.1 in catalysis. We conclude that a unique cysteine residue is essential for maximal cata lytic activity of isocitrate dehydrogenase.