Affinity of temocillin for Escherichia coli K-12 penicillin-binding proteins

Temocillin is a 6 alpha-methoxy penicillin which shows poor affinity for the penicillin-binding proteins (PBPs) of Escherichia coli K-12 when tested by competition with [14C]penicillin G or [125I]penicillin X. When the reaction conditions for the radiolabeled penicillin used in this procedure were modified by lowering the temperature (2 degrees C) and reducing the incubation time (3 min), temocillin showed a much higher affinity for PBP-3 and improved affinity for the other PBPs, with the exception of PBP-2. Direct labeling procedures with [14C]temocillin also showed that the compound had affinity for PBPs 1a and 3. These results are more consistent with the effects of temocillin on the morphology of E. coli than the poor affinity values obtained by the classical competitive procedure. Reasons for the disparity between these assay systems are discussed.