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, N

NADP-dependent isocitrate dehydrogenase (ICDH) catalyses the formation of 2-oxoglutarate from isocitrate. Red light, applied to fronds of Spirodela polyrhiza, stimulated the ICDH enzyme capacity (activity assayed under optimal conditions in vitro). Anion exchange chromatography showed that this effect of light was only seen on the cytosolic isoform, ICDH1 (increase by 16 %). Subsequent Western analysis indicated that this increase of the ICDH1 capacity was caused by an increase of the enzyme concentration. The stimulation effect of light was blocked by the addition of the inhibitor of the photosynthetic electron transport, 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (10 μmol L−1). Moreover, single as well as repeated red light pulses applied each hour (5 min each) did not have any effect. These results indicate that the influence of light on ICDH1 capacity is most likely mediated by the effect of photosynthesis. Both ICDH1 and chloroplastic isoforms were stimulated by exogenously applied glucose (50 mmol L−1). However, no influence of exogenously applied nitrate was detectable. This indicates that the coordination of the carbon and nitrogen metabolism in S. polyrhiza does not include the regulation of ICDH capacity.