The interaction of fatty acid synthetase with cytoplasmic protein in the control of the chain-length of fatty acids synthesised by the lactating rabbit mammary gland.

1. Fatty acid synthetase from rabbit mammary gland can be separated from the protein which modifies the chain-length at which fatty acids are released from the enzyme complex in the soluble fraction. This can be achieved by ultracentrifugation, precipitation with specific antibody or ammonium sulphate. 2. The chain-length modifying protein in the supernatant fraction from rabbit mammary gland was less active towards cow mammary gland fatty acid synthetase than rabbit mammary gland fatty acid synthetase in the synthesis of medium-chain fatty acids. The fatty acid synthetases from these two tissues are also immunologically non-identical. 3. It is proposed that there is a loose but specific interaction of rabbit mammary gland fatty acid synthetase with the chain-length modifying protein in regulating product chain length which is dependent on the concentration of interacting proteins. 4. The chain-lengthening effect of added malonyl-CoA decreases with increasing concentration of interacting proteins, but differences in the fatty acid chain-length with malonyl-CoA synthesised in situ by acetyl-CoA carboxylase and with added malonyl-CoA indicate that the product chain-length is sensitive to the availability of malonyl-CoA for enlongation in all but the most tightly coupled situations.