Control of Ketogenesis from Amino Acids: II. Ketone Bodies Formation from α-Ketoisocaproate, the Keto-analogue of Leucine, by Rat Liver Mitochondria

Abstract

α-Ketoisocaproate (KIC), the keto-analogue of leucine, was metabolized to ketone bodies by rat liver mitochondria in vitro on addition of ADP, Mg²⁺, and an oxidizable substrate, such as succinate or α-ketoglutarate. Formation of ketone bodies from KIC showed a biphasic increase with time, being slow in the first 20 min and then rapid and linear with time. The disappearance of KIC was proportional to ketone bodies formation. No slow phase was seen using mitochondria that had been pre-incubated with the above energy generating system. The two phases were not affected by addition of CoA, NAD⁺, or carnitine, but increase of succinate concentration over 2.5 mM prolonged the slow phase. The time course of ATP formation and succinate oxidation suggested that both decrease of oxidizable substrate and increase of ATP concentration may be necessary for the rapid phase. During oxidation of KIC, cytochrome b was reduced in the presence of ADP and oxidation of added succinate predominated over that of KIC. Addition of an ATP generating system (phosphoenolpyruvate, pyruvate kinase [EC 2.7.1.40], and ATP) with dinitrophenol partially abolished the slow phase. Addition of ATP alone did not enhance the oxidation.

From these results it is likely that KIC oxidation is controlled by the concentration of oxidizable substrates, which may monopolize the electron transport system in the first phase, but provide the necessary amount of ATP for the later phase.