Interaction between NAD-dependent isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase complex, and NADH:ubiquinone oxidoreductase

Interaction between the alpha-ketoglutarate dehydrogenase complex and NAD+-dependent isocitrate dehydrogenase was detected with a variety of techniques including polyethylene glycol precipitation, ultracentrifugation, and centrifugal gel filtration on a Sepharose 6B column. The interaction was specific in that citrate synthase, cytosolic malate dehydrogenase, and NADP-dependent isocitrate dehydrogenase did not interact with alpha-ketoglutarate dehydrogenase complex. The interaction was not inhibited by either 0.1 M KCl or 0.4 M (NH4)2SO4, but was completely prevented by 5% glycerol. A new method for the preparation of NADH: ubiquinone oxidoreductase resulted in an enzyme having a protein subunit composition similar to that of classical complex I preparation. Evidence is given for the existence of ternary complexes containing NADH:ubiquinone oxidoreductase-alpha-ketoglutarate dehydrogenase complex-NAD-dependent isocitrate dehydrogenase and NADH: ubiquinone oxidoreductase-alpha-ketoglutarate dehydrogenase complex-succinate thiokinase. These data suggest that a part of the citric acid cycle may be located in the vicinity of NADH: ubiquinone oxidoreductase. These complexes may facilitate the transport of metabolites among these enzymes without their equilibrating with the whole compartment.