The synthesis of
ketone bodies by intact isolated rat-liver mitochondria has been studied at varying rates of
acetyl-CoA production and of
acetyl-CoA utilization in the Krebs cycle. Factors which enhanced the rate of
acetyl-CoA production caused an increase in the fraction of
acetyl-CoA which was incorporated into
ketone bodies. On the other hand, it was found that factors which stimulated the formation of
citrate lowered the relative rate of ketogenesis. It is concluded that
acetyl-CoA is preferentially used for
citrate synthesis, if the level of
oxaloacetate in the mitochondrial matrix space is adequate. The intramitochondrial level of
oxaloacetate, which is determined by the
malate concentration and the ratio of
NADH over
NAD+, is the main factor controlling the rate of
citrate synthesis. The
ATP/
ADP ratio per se does not affect the activity of
citrate synthase in this in vitro system. Ketogenesis can be described as an overflow of acetyl-groups:
Ketone-body formation is stimulated only when the rate of
acetyl-CoA production increases beyond the capacity for
citrate synthesis. The interaction between
fatty acid oxidation and
pyruvate metabolism and the effects of
long-chain acyl-CoA on mitochondrial metabolism are discussed.
Ketone bodies which were generated during the oxidation of [1-14C]
fatty acids were preferentially labelled in their carboxyl group. This carboxyl group had the same specific activity as the
acetyl-CoA pool, whereas the specific activity of the
acetone moiety of
acetoacetate was much lower, especially at low rates of
ketone-body formation. The activities of
acetoacetyl-CoA deacylase and the
hydroxymethylglutaryl-CoA (
HMG-CoA) pathway were compared in soluble and mitochondrial fractions of rat- and cow-liver in different ketotic states. In rat-liver mitochondria, both pathways of
acetoacetate synthesis were stimulated upon
starvation or in
alloxan diabetes. In cow liver, only the
HMG-CoA pathway was increased during
ketosis in the mitochondrial as well as in the soluble fraction.