Studies on the conversion of radioactive leucine to acetoacetate.

Although it is well established that leucine gives rise to ketone bodies in the course of its metabolism in the mammalian organism, the mechanism of this transformation has not been fully elucidated. As early as 1906, Embden and his colleagues (2) presented evidence from perfusion studies that isovaleric acid is a ketogenic substance, and they proposed it as a possible intermediate in the biological degradation of leucine. Subsequently, Ringer, Frankel, and Jonas (3) reported that the administration of isovaleric acid to phlorhizinized dogs results in the excretion of extra ketone bodies. The latter investigators suggested that the compound undergoes reductive demethylation to form butyric acid, which is then oxidized directly to acetoacetic acid. More recently, Bloch (4) has found that deuterium-labeled leucine or isovaleric acid is partially converted in the intact animal to “acetate,” which can be utilized for acetylations as well as for cholesterol formation. These experiments are of particular interest in that they permit a calculation of the extent to which acetate was formed from the administered compounds. Bloch’s data indicate that only half a mole of acetoacetate may be formed from a mole of leucine. The experiments described below were undertaken in the hope that the mechanism of these reactions could be established with the aid of the Cl4 isotope as a tracer. For this purpose, two preparations of leucine were synthesized, with the tag in the /? and the y positions, respectively. From a study of the metabolism of the two compounds in liver slices, it was possible to demonstrate that the LYand p-carbons of leucine furnish a 2carbon fragment capable of condensing to form acetoacetate. The isopropyl group of the amino acid does not yield a similar a-carbon compound to an appreciable extent.