A Kinetic Study of the α-Keto Acid Dehydrogenase Complexes from Pig Heart Mitochondria1

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Abstract
The kinetic mechanisms of the 2-oxoglutarate and pyruvate dehydrogenease com plexes from pig heart mitochondria were studied at pH 7.5 and 25°. A three-site ping-pong mechanism for the actin of both complexes was proposed on the basis of the parallel lines obtained when 1/v was plotted against 2-oxoglutarate or pyruvate concentration for various levels of CoA and a level of NAD+ near its Michaelis constant value. Rate equations were derived from the proposed mechanism. Michaelis constants for the reactants of the 2-oxoglutarate dehydrogenase com plex reaction are: 2-oxoglutarate, 0.220 mM; CoA, 0.025 mM; NAD+ 0.050 mM. Those of the pyruvate dehydrogenase complex are: pyruvate, 0.015 mM; CoA, 0.021 mM; NAD+ 0.079mM. Product inhibition studies showed that succinyl-CoA or acetyl-CoA was competitive with respect to CoA, and NADH was competitive with respect to NAD in both overall reactions, and that succinyl-CoA or acetyl-CoA and NADH were uncompetitive with respect to 2-oxoglutarate or pyruvate, respectively. However, noncompetitive (rather than uncompetitive) inhibition patterns were observed for succinyl-CoA or acetyl-CoA versus NAD+ and for NADH versus CoA. These results are consistent with the proposed mechanisms.

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