Evidence that the coenzyme A requirement for avian fatty acid synthase is not for the termination reaction.

1. Fatty acid synthase from goose uropygial gland was inhibited when CoA was scavenged with ATP citrate lyase. 2. This inhibition was reversed by the addition of CoA, 3'-dephospho-CoA, 1,N6-etheno-CoA and pantetheine but not by desulfo-CoA, pantethine or mercaptoethanol suggesting that the structural features of pantetheine including the free thiol group are essential for the reversal. 3. The S-acyl fatty acid synthase thioester hydrolase from the uropygial glands of mallards, which hydrolytically removes acyl chains from fatty acid synthase of goose uropygial gland in the absence of CoA, did not reverse this inhibition, suggesting that the CoA depletion does not inhibit termination reaction. 4. However, triacetic acid lactone synthesis by the fatty acid synthase was inhibited by the scavenging of CoA, raising the possibility that the condensation and/or the transacylase reactions may require CoA.