Cytosolic and chloroplastic NADP-dependent isocitrate dehydrogenases in Spirodela polyrhiza. I. Regulation of activity by metabolitesin vitro

In the duckweed Spirodela polyrhiza, NADP-dependent isocitrate dehydrogenases were detected and partially purified from cytosol (ICDH1; approximately 80 % of the total activity) and from chloroplasts (ICDH2; 20 %). The affinities of ICDH2 to isocitrate, NADP, Mn2+ and Mg2+ were higher than those of ICDH1. A possible metabolic regulation was tested by investigating the influence of several metabolites on the activity of both isoforms in vitro. Intermediates of the citrate cycle from the substrate side of ICDH (i.e. citrate, cis-aconitate, isocitrate) showed the tendency to stimulate both isoforms (with the exception of aconitate which inhibited ICDH1) whereas those intermediates from the product side (2-oxoglutarate, succinate) showed the opposite tendency. The strong inhibiting effect of 2-oxoglutarate may slow down the formation of this substrate of amino acid synthesis under conditions of a low nitrogen assimilation rate. Glutamate showed contrasting effects on the two isoforms: whereas ICDH1 was strongly stimulated (by more than 60 % at 300 μmol L−1), ICDH2 was strongly inhibited (e.g. by 60 % at 300 μmol L−1 and 80 % at 2 mmol L−1). The strong effects of glutamate clearly show the influence of an intermediate of the nitrogen metabolism on the rate of 2-oxoglutarate formation and thereby demonstrate possible interactions of nitrogen and carbon metabolisms by metabolic control of ICDHs.