Isocitrate dehydrogenase as a potential electron donor to nitrogenase of nostocmuscorum

The presence of a physiological dark reduction mechanism of NADPf in isolated heterocysts of Nostoc muscorum was investigated by testing reactions with substrates and enzymes of the oxidative pentose phosphate pathway and the Krebs cycle. Activities of dehydrogenases were 5 -20 fold higher in the isolated heterocysts than in the vegetative cells, using glucose-6-P, fructose-6-P and isocitrate as substrates. Isocitrate dehydrogenase (IDH) tested in crude extracts was found to be very unstable, specific only to NADPf, and to have higher affinity for Mn+ than for Mg2. The response of IDH from N. muscorum to known effectors and regulators was very different from the response of this enzyme from other sources - activity was not stimulated by AMP, ADP and glyoxylate, and the inhibition by ATP and a KG was relatively low.