Affinity labeling of fatty acid synthetase from lactating rat mammary gland with S-(4-bromo-2,3-dioxobutyl)-CoA: evidence for a "half-of-the-sites" catalytic mechanism.

The classical affinity label, S-(4-bromo-2,3-dioxobutyl)-CoA, rapidly and irreversibly inhibits fatty acid synthetase from lactating rat mammary gland. The limit stoichiometry of incorporated label and the kinetics of inactivation indicate that two sites can be labeled per enzyme dimer. Strong evidence of site-site interaction (weak negative cooperativity) was observed. At relatively low concentrations, the affinity label inhibits acetyl transacylase whereas the malonyl transacylase activity is enhanced. We propose that fatty acid synthetase from lactating rat mammary gland catalyses a “half-of-the-sites” mechanism.