Reactivity of essential cysteine and lysine residues present at the catalytic domain of pig heart mitochondrial malate dehydrogenase.

Pig heart mitochondrial malate dehydrogenase was inactivated very rapidly by omicron-phthalaldehyde as compared to 5,5'-dithio bis(2-nitrobenzoic acid) and pyridoxal 5'-phosphate. The omicron-phthalaldehyde reaction followed pseudo first order kinetics, and a second order rate constant of 38 M-1 S-1 was obtained. Cysteine and lysine residues participating in the omicron-phthalaldehyde reaction are located in the NADH binding region of malate dehydrogenase as shown by protection experiments. The decrease in the rate of inactivation in the presence of NADH was used to determine the dissociation constant of the enzyme-NADH complex. pH dependent studies and molar transition energy calculations of the omicron-phthalaldehyde-inactivated enzyme have indicated that cysteine and lysine residues involved in the isoindole derivative formation are located in a hydrophobic environment at the coenzyme binding site.