Effect of adenine nucleotides on NAD-dependent isocitrate dehydrogenases in rhizobia and bacteroids of legume root nodules.

ATP, ADP, AMP and cyclic AMP inhibit NAD-dependent isocitrate dehydrogenase (L-s-isocitrate : NAD-+ oxidoreductase, EC 1.1.1.41) from rhizobia but have no effect on the enzyme from corresponding bacteroids. This was observed using three rhizobial strains two of which are effective, and one ineffective, with Lotus pedunculatus. Using partially purified enzyme from each of the three rhizobial strains it was found that the adenine nucleotides inhibit the enzyme by competing with NAD-+, not with isocritrate. The rate of reaction catalysed by the enzyme (expressed as activity per mg protein) in cell-free extracts of each of the effective rhizobial strains was about three times that of the reaction in extracts of the corresponding bacteroids. No correlation was found between effectiveness and NAD-dependent isocitrate dehydrogenase activity in the rhizobial cells.