A non-redox role of the coenzyme in the catalysis of isocitrate dehydrogenase

Abstract

NADP-dependent isocitrate dehydrogenase (EC 1 .l .1.42) catalyzes the oxidative decarboxylation of threo-D-isocitrate in two steps: (1) Dehydrogenation of isocitrate into oxalosuccinate. (2) Decarboxylation of oxalosuccinate into 2-oxoglutarate. Our purpose was to determine whether the coenzyme was involved only in the redox step or if it also interfered in other steps of the catalytic cycle. It is known [ 1 ] that mitochondrial isocitrate dehydrogenase catalyzes hydrogen exchange between [3H]2-oxoglutarate and water. This reaction is likely to be the first step in the carboxylation of 2oxoglutarate. NADPH is required for this reaction without playing a redox role. Rippa et al. indeed have shown that NADPH could be replaced by 1,4,5,6 tetrahydro NADP which is devoid of redox power [2]. It has also been shown that isocitrate dehydrogenase exhibited an autocatalytic behaviour due to activation by NADPH produced during the enzymatic oxidative decarboxylation of isocitrate (results submitted for publication). In this report, we show that NADPH can be replaced by 1,4,5,6 tetrahydro NADP in the catalytic activation reaction.