The inhibition of alpha-ketoglutarate oxidation by fatty acids in rat liver mitochondria.

The oxidation of a-ketoglutarate in rat liver mitochondria incubated in State 3 and in the uncoupled state in the absence and presence of oligomycin was inhibited by the simultaneous oxidation of fatty acids. Palmitylcarnitine inhibited a-ketoglutarate oxidation by up to 80% and octanoate by up to 30%. Various possibilities for the mechanism of this inhibition were experimentally evaluated. The inhibition was not associated with the damage of mitochondrial membranes due to the detergent action of fatty acids or with a restriction of a-ketoglutarate translocation. Also, a severe depletion of intramitochondrial CoASH did not occur. a-Ketoglutarate dehydrogenase extracted from liver mitochondria which was stabilized with rabbit serum was inhibited by (listed from the highest to the lowest potency) palmityl-CoA, NADH, succinyl-CoA, GTP, and ATP. The intramitochondrial content of these inhibitory metabolites was examined in mitochondria oxidizing a-ketoglutarate in the absence and presence of fatty acids. The addition of octanoate increased the content of only one inhibitor, NADH. The addition of palmitylcarnitine resulted in an accumulation of long chain acyl-CoA and the elevation of the succinyl-CoA/CoASH ratio while the NADH content increased less than in the case of octanoate. A definitive correlation between the rate of a-ketoglutarate oxidation and the succinyl-CoA/CoASH ratio was not found at similar NADH levels in various energetic states of mitochondria. The significant suppression of the process by palmitylcarnitine was not necessarily associated with the high succinyl-CoA/CoASH ratio but it was correlated with the level of long chain acyl-CoA. Hence, the strong inhibition of the a-ketoglutarate oxidation in rat liver mitochondria by long chain fatty acids is postulated to be primarily due to a direct effect of long chain acyl-CoA on a-ketoglutarate dehydrogenase. The elevation of the succinyl-CoA/CoASH ratio and NADH level as well as the competition for CoASH have only secondary importance for this inhibition. The much weaker inhibition of the process by medium chain fatty acids is mediated exclusively by the increase of NADH content.