Fatty acid biosynthesis VI. Specificity for termination of fatty acid biosynthesis by fatty acid synthetase from lactating-rabbit mammary gland

Abstract

1. 1. The extent to which the pattern of fatty acids synthesised by fatty acid synthetase from rabbit mammary gland could be modified by acetyl-CoA and malonyl-CoA concentrations was examined and the results compared with the composition of rabbit milk triglycerides, which contain a high proportion of medium-chain fatty acids. 2. 2. Evidence for two pathways of butyrate synthesis by fatty acid synthetase has been obtained, since butyrate was synthesised both from acetyl-CoA alone and acetyl-CoA plus malonyl-CoA. 3. 3. Increasing the malonyl-CoA concentration and the ratio of concentrations of malonyl-CoA to acetyl-CoA increased the proportion of long-chain (C 14:0–C 18:0) and decreased the proportion of short-chain (C 4:0 and C 6:0) fatty acids formed. Long-chain fatty acids were synthesised mainly as free acids and the short-chain fatty acids were predominantly in an esterified form. Medium-chain fatty acids (C 8:0–C 12:0) were only formed in these experiments in amounts approximately equimolar to the fatty acid synthetase protein. The modification in the pattern of fatty acids synthesised with increasing acetyl-CoA carboxylase activity was analogous to that with increasing concentrations of added malonyl-CoA. 4. 4. A specificity for the in vivo synthesis of medium-chain fatty acids by rabbit mammary gland must be superimposed on the specificity of isolated fatty acid synthetase for chain termination.