A recent claim in the literature (Moyle, J. and Mitchell, P. (1973) Biochem. J. 132, 571–585) that the NAD-dependent isocitrate oxidation observed in extracts of ratliver mitochondria proceeds entirely via the NADP-linked isocitrate dehydrogenase and nicotinamide nucleotide transhydrogenase, and that rat-liver mitochondria contain no NAD-linked isocitrate dehydrogenase, has been examined by using palmityl-CoA as a selective inhibitor of transhydrogenase and ADP as a selective activator of the NAD-linked isocitrate dehydrogenase. The results unambiguously demonstrate that the NAD-dependent oxidation of isocitrate observed under the conditions employed by Moyle and Mitchell proceeds predominantly via the NAD-linked isocitrate dehydrogenase. It is also shown that, by an unfortunate choice of assay conditions, these authors have considerably overestimated the rate of the transhydrogenase reaction.
