Citrate and the conversion of carbohydrate into fat. Fatty acid synthesis by a combination of cytoplasm and mitochondria.

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 intramitochond&l to the extramitochondrial space during fatty acid synthesis. 14C-Pyruvate, generated from W-alanine, was used to generate intramitochondrial 14C-acetyl-CoA. Fatty acid synthesis was followed by measuring the incorporation of 3Hz0 and W. 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. nButyhnalonate probably exerts its effect by preventing the activation of cr-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.