Inactivation of Bacillus cereus beta-lactamase by halogenated isoxazolylpenicillins

Upon preincubation with halogenated isoxazolyl-penicillins, the activity of Bacillus cereus beta-lactamase is decreased by a factor proportional to the inhibitor concentration. This inactivation, which concerns essentially the catalytic rate constant of the enzyme, occurs only at or above 37 degrees C with cloxacillin and dicloxacillin, and even at 25 degrees C with flucloxacillin. The dependence of the inactivation rate upon inhibitor concentration is consistent, in the former case, with the occurrence of a bimolecular reaction, and in the latter case, with a bimolecular process followed by a monomolecular step.