Fatty acid biosynthesis. VIII. The fate of malonyl-CoA in fatty acid biosynthesis by purified enzymes from lactating-rabbit mammary gland

- 1. We have investigated the formation and utilization of malonyl-CoA in fatty acid synthesis catalysed by preparations of partially purified acetyl-CoA carboxylase and purified fatty acid synthetase from lactating-rabbit mammary gland. - 2. Carboxylation of [1-14C]acetyl-CoA was linked to fatty acid synthesis by the presence of fatty acid synthetase and NADPH. The rate of fatty acid formation was equal to that of acetyl-CoA carboxylation, without the accumulation of free malonyl-CoA to a concentration required to obtain the same rate of fatty acid synthesis from added [1,3-14C2]malonyl-CoA. - 3. The preparations of acetyl-CoA carboxylase and fatty acid synthetase were each able to decarboxylate [1,3-14C2]malonyl-CoA. - 4. Both enzyme preparations acted as competitive inhibitors of 14CO2 fixation into acetyl-CoA catalysed by acetyl-CoA carboxylase in the absence of NADPH. This is attributed to a mechanism of product inhibition. The effect on the apparent activity of acetyl-CoA carboxylase assayed by malonyl-CoA formation is discussed. - 5. Our results suggest a metabolic compartmentation of the carboxylation step which facilitates the incorporation of carboxylated acetyl-CoA into fatty acids in the presence of NADPH and prevents its catabolism by side reactions.