Plant NADP-dependent isocitrate dehydrogenases are predominantly localized in the cytosol.

The isoenzyme patterns of NADP-isocitrate dehydrogenase (NADP-IDH; EC 1.1.1.42) have been investigated in 15 species of higher plants using dietylaminoethyl ion-exchange chromatography and immunological techniques. The obtained results unambiguously demonstrate that the cytosolic enzyme is the predominant form in leaf extracts of all the surveyed plant species. The chloroplastic isoenzyme, previously reported in pea (Pisum sativum L.) leaves (R.D. Chen et al., 1989, Planta 178; 157-163), is a minor form in ferns and dicotyledonous angiosperms and is undetectable in gymnosperms and monocotyledonous angiosperms. Comparison of immunological relatedness suggests that the proteins of cytosolic isoenzymes have been highly conserved in the course of plant evolution. The data support the previously proposed idea that the cytosol is the major site for alpha-ketoglutarate production to be used for nitrogen assimilation.