The interaction of dansylpenicillin with the class A Staphylococcus aureus PCI beta-lactamase yielded an accumulating intermediate with fluorescence enhanced beyond that of the substrate. Acid quenching of the reaction mixture yielded a denatured enzyme with 1 molar equivalent of dansyl group covalently bound to it. A similar quenching experiment with the PC1 beta-lactamase and [14C]benzylpenicillin yielded an enzyme with 1 molar equivalent of 14C covalently bound. These data indicate that in turnover of S-type penicillins by the PC1 beta-lactamase deacylation is rate-determining. This has not indicate previously been demonstrated for a class A beta-lactamase.
