PS-5 inhibition of a beta-lactamase from Proteus vulgaris.

Inhibition of Proteus vulgaris beta-lactamase by a new beta-lactam antibiotic, PS-5 was studied kinetically. There were two stages of inhibition. In the early stage, PS-5 inhibited the beta-lactamase by formation of a Michaelis-complex, and showed a competitive inhibition pattern with Ki-value of 0.22 microM (substrate, cephaloridine). After the formation of a Michaelis-complex between PS-5 and the enzyme, PS-5 showed a characteristic progressive inhibition pattern with time. Maximum inactivation was obtained after several minutes of preincubation of the enzyme with PS-5; as hydrolysis of PS-5 progressed, the enzyme activity was gradually recovered. Reactivation by an excess of substrate (cephaloridine) was not substantially realized in the presence of PS-5. PS-5 was very slowly hydrolyzed by the enzyme, showing a triphasic pattern in its reaction curve.