NADP-Isocitrate Dehydrogenases in Higher Plants

Abstract

The keto-acid, 2-oxoglutarate, provides the carbon skeleton implicated in the biosynthesis of glutamate. For higher plants, it is widely accepted that this oxo-acid is derived from the Krebs Cycle where it is produced through the action of the mitochondrial NAD-isocitrate dehydrogenase (NAD-IDH, E.C. 1.1.1.41) (Metzler 1977). The resulting keto-acid then traverses the mitochondrial and chloroplast envelopes to reach the chloroplast stroma where it is used by glutamate synthase (EC. 1.1.7.1). However, there is increasing evidence suggesting that the reaction catalyzed by NADP-isocitrate dehydrogenase (NADP-IDH, E.C. 1.1.1.42) provides an alternative pathway for the production of 2-oxoglutarate for ammonia assimilation (Elias and Givan 1977; Randall and Givan 1981; Chen et al. 1988, 1989 a,b; Chen and Gadal 1989). The significance of this scheme in ammonia assimilation is sustained by the fact that an increase in the NADP-IDH activity coincides with the onset of nitrogen fixation in nodules where a high supply for 2-oxoglutarate is required (Nautiyal and Modi 1987). In this review, we summarize the recent data obtained in our laboratory on this enzyme.