Role of Free Oxaloacetate in Ketogenesis: Derivation from the Direct Measurement of Mitochondrial [3‐Hydroxybutyrate]/[Acetoacetate] Ratio in Hepatocytes

Abstract

The distribution of ketone bodies between the cytosolic and mitochondrial spaces of isolated hepatocytes from 48‐h‐starved rats incubated at oleate concentrations of 0−0.8 mM has been investigated with respect to the rclationship between free mitochondrial oxaloacetate and the rate of ketone body formation. Some 25% of the total cellular 3‐hydroxybutyrate was found to be located intramitochondrially, independent of the oleate concentration used. Acetoacetate, however, was almost equally distributed between the two compartments. In contrast to acetoacetate, 3‐hydroxybutyrate distribution followed the transmitochondrial pH gradient. The mitochondrial concentrations of 3‐hydroxybutyrate and acetoacetate were higher (by a factor of about 2 and 4, respectively) than those in the cytosol. No concentration gradient was found between the cytosol and the incubation medium. At 0.8 mM oleate the mitochondrial concentration of 3‐hydroxybutyrate amounted to 4.5 ± 0.6 mM (n = 9), the [3‐hydroxybutyrate]/[acetoacetate] ratio being 1.4. Considerably higher values for this ratio resulted from the rate of ketone body production. Mitochondrial concentrations of malate were increased by oleate concentrations between 0.1 −0.8 mM, while mitochondrial concentrations of citrate were not significantly changed. Mitochondrial concentrations of free oxaloacetate calculated from the contents of malate and ketone bodies measured in the mitochondrial compartment was found to be correlated in an inverse manner with thc rate of ketone body production. The results are discussed with regard to the current concepts for the control of ketogenesis. Our data strongly support the classical view of the oxaloacetate concentration playing a key role in the regulation of production of ketone bodies. Copyright © 1982, Wiley Blackwell. All rights reserved