Fatty Acid Synthetase from Lactating Rat Mammary Gland

Homogeneous preparations of fatty acid synthetase from mammary glands of lactating rats were used to prepare an antiserum and a γ-globulin fraction was isolated from the antiserum. The antiserum and the γ-globulin fraction were shown to be specific against the fatty acid synthetase multienzyme complex. Addition of the γ-globulin to homogenate fractions of lactating rat mammary glands, resulted in complete inhibition of synthesis, from acetate, of fatty acids of all chain lengths. These results show that the multienzyme complex is an obligatory enzyme for the synthesis of all chain length fatty acids in the mammary glands of lactating rats. The fatty acid synthetase from mammary gland produces almost exclusively palmitic acid from malonyl-CoA when acyl-CoA derivatives of chain length C2 to C8 were used as starters. Butyryl-CoA was found to be the most efficient starter, having the highest Vmax and the lowest Km of all acyl-CoA derivatives tested. The specificity of the thioesterase activity associated with the synthetase was also studied with acyl-CoA derivatives as model substrates. Activity was maximal for palmityl and stearyl thioesters, but some activity was indicated with thioesters of chain length C2 to C8. The chain length specificity of thioesterase activity was quite similar to that reported for the pigeon liver enzyme, despite the fact that the liver and mammary gland synthetases produce different chain length fatty acids in homogenate or tissue slice preparations, and presumably, therefore, in vivo. The results are discussed in relation to possible mechanisms for termination of growth of the carbon chain in fatty acid synthesis.