Citrate and the Conversion of Carbohydrate into Fat

Abstract

A cell-free system, consisting of particle-free cytoplasm and mitochondria prepared from rat liver, has been used to study the transfer of acetyl groups from the intramitochondrial to the extramitochondrial space during fatty acid synthesis. 14C-Pyruvate, generated from 14C-alanine, was used to generate intramitochondrial 14C-acetyl-CoA. Fatty acid synthesis was followed by measuring the incorporation of 3H2O and 14C. Pool dilution experiments showed that the acetyl group of intramitochondrial acetyl-CoA leaves the mitochondria in the form of citrate, or of a closely related tricarboxylate. (-)-Hydroxycitrate, an inhibitor of citrate cleavage enzyme, strongly inhibits fatty acid synthesis from 14C-alanine. On the other hand, (-)-hydroxycitrate has little or no effect on respiration, phosphorylation, and citrate accumulation during the course of the experiments. n-Butylmalonate, an inhibitor of malate permease, also inhibits fatty acid synthesis. This inhibition is reversed by malate. n-Butylmalonate probably exerts its effect by preventing the activation of α-ketoglutarate and tricarboxylate permeases by malate. It is concluded that citrate is the major carrier in the transfer of acetyl groups from the mitochondrial matrix to the cytoplasm.