Synthesis of Triacetic Acid Lactone by the Pigeon Liver Fatty Acid Synthetase Complex

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The synthesis of triacetic acid lactone (TAL) is effected by purified soluble pigeon liver fatty acid synthetase. This enzyme system synthesizes TAL from acetyl coenzyme A and malonyl coenzyme A in the absence of TPNH, and synthesizes palmitic acid in the presence of TPNH. The major product of the reaction in the absence of TPNH is TAL, and not free triacetic acid. Presumably triacetic acid, bound as a thioester to the 4′-phosphopantetheine prosthetic group of the fatty acid synthetase, is an intermediate in the formation of TAL. The latter compound is formed from 1 mole of acetyl-CoA and 2 moles of malonyl-CoA in the sequence, acetate-malonate-malonate (proceeding left to right from the methyl end of the molecule). The synthesis of TAL is inhibited by either TPN+ or TPNH. The very strong inhibition by both forms of the nucleotide implies that TAL is not synthesized in vivo by the avian soluble fatty acid synthetase.