An investigation into the role of malonyl-coenzyme A in isoprenoid biosynthesis.

1. [(14)C]Malonyl-CoA was incorporated into isoprenoids by cell-free yeast preparations, by preparations from pigeon and rat liver, and by Hevea brasiliensis latex. 2. In agreement with previous reports the incorporation of acetyl-CoA into isoprenoids was not inhibited by avidin and was not stimulated by HCO(3) (-). In a cell-free yeast preparation addition of HCO(3) (-) stimulated the formation of fatty acids from acetyl-CoA and decreased the incorporation into unsaponifiable lipids. 3. The labelling patterns of beta-hydroxy-beta-methylglutaryl-CoA formed from [2-(14)C]- and [1,3-(14)C]-malonyl-CoA in rat and pigeon liver preparations were those that would be expected if malonyl-CoA underwent decarboxylation to acetyl-CoA before incorporation. 4. The labelling pattern of ergosterol formed by cell-free yeast preparations from [2-(14)C]malonyl-CoA was also consistent with decarboxylation of malonyl-CoA before incorporation. 5. The incorporation of [2-(14)C]malonyl-CoA into mevalonate by rat liver preparations was related to the malonyl-CoA decarboxylase activity present in the preparation.