The ld ‐Carboxypeptidase Activity in Gaffkya homari

β‐Lactam antibiotics were found that inhibit the synthesis in vitro of wall‐bound peptidoglycan formed from UDP‐MurNAc‐tetrapeptide. With this precursor it is possible to exclude that the inhibition of the dd ‐carboxypeptidase by the antibiotics exerts an effect on the formation of wall‐ bound peptidoglycan. One group comprises cephalosporin C and nocardicin A. These antibiotics bear as substituents d ‐α‐aminoadipic acid and d ‐homoserine, respectively, and are suggested to affect ld ‐carboxypeptidase activity and to inhibit the formation of wall‐bound peptidoglycan like d ‐amino acids. Within a concentration range in which d ‐amino acids are also effective, the antibiotics cause an enhancement of the release of the C‐terminal d ‐alanine from the tetrapeptide subunits, which is suggested to be catalyzed by the d ‐amino acid exchange activity of the ld ‐carboxypeptidase. Cephalo‐ sporin C also inhibited dd ‐carboxypeptidase activity, whereas nocardicin A was not inhibiting to a significant extent. The antibiotics of the second group, penicillin G and thienamycin, inhibited ld ‐carboxypeptidase activity with thienamycin being extraordinarily effective (ID 50 values = 1 μmol/l and 5.5 pmolll, respectively). The inhibition of the formation of wall‐bound peptidoglycan required slightly higher concentrations of antibiotics (3.1 μmol/l and 11 pmol/l, respectively). Penicillin G and thienamycin also inhibit dd ‐carboxypeptidase activity very efficiently (ID 50 values = 0.7 μmol/l and 0.75 μmol/l, respectively). By different mechanisms both pairs of antibiotics inhibited the formation of tripeptide subunits in the nascent peptidoglycan which are assumed to be essential for transpeptidase activity.