Fatty acid biosynthesis: X. Specificity for chain-termination of fatty acid biosynthesis in cell-free extracts of lactating rabbit mammary gland

Abstract

1. 1. The pattern of fatty acids synthesised from [1− 14C]acetate by subcellular fractions of lactating rabbit mammary gland fortified by appropriate cofactors was found to be dependent on the concentration of the fractions as measured by protein. 2. 2. At high concentrations (14–22 mg protein/ml), the cell-free homogenate, the microsomal plus particle-free supernatant, and the particle-free supernatant fractions all synthesised a high proportion of the medium-chain fatty acids ( C 8:0) and C 10:0) which are characteristic of rabbit milk. 3. 3. Dilution of these fractions to less than 1 mg protein/ml caused a progressive increase in the proportion of long-chain acids ( C 14:0 and C 16:0) synthesised. 4. 4. With concentrated subcellular fractions, synthesis of medium-chain fatty acids was not dependent on the rate of fatty acid synthesis, the concentration of malonyl-CoA, or the amount of fatty acid synthetase present. 5. 5. Though there was a correlation between the synthesis of medium-chain fatty acids and of triglyceride by particulate fractions, esterification of synthesised fatty acids did not influence the proportion of medium-chain acids formed. 6. 6. Evidence is presented that chain termination at C 8:0 and C 10:0 acids is controlled by a factor (or factors) present in the particle-free supernatant fraction. This factor is not lost on dialysis, and can be precipitated between 40 and 100% saturation with (NH 4) 2SO 4 or by heating.