Plant NADP-dependent isocitrate dehydrogenases are predominantly localizedin 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 dietylamin oethyl ion-exchange chromatography and immunological techniques. The obtain ed results unambiguously demonstrate that the cytosolic enzyme is the predo minant form in leaf extracts of all the surveyed plant species. The chlorop lastic 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 dic otyledonous angiosperms and is undetectable in gymnosperms and monocotyledo nous 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.