d‐Alanyl‐d‐Alanine Carboxypeptidase in the Bacterial Form and L‐Form ofProteus mirabilis

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Abstract
Membranes of the bacterial form and the stable and unstable L‐forms ofProteus mirabiliscontain LD and DD‐carboxypeptidase. The DD‐carboxypeptidase is inhibited non‐competitively by penicillin G. The enzyme of the bacterial form is highly penicillin‐sensitive (Ki= 4 × 10−9M penicillin G). Inhibition is only partly reversible by treatment with penicillinase or by dialysis against buffer. In contrast, the DD‐carboxypeptidase of the unstable L‐form, grown in the presence of penicillin, is 175‐fold less penicillin‐sensitive (Ki= 7 × 10−7M penicillin G). Inhibition is completely reversed by penicillinase or dialysis. After inhibition by penicillin and subsequent reactivation the penicillin sensitivity of the bacterial DD‐carboxypeptidase is similar to the sensitivity of the enzyme of the unstable L‐form. The hypothesis is proposed thatP. mirabiliscontains two DD‐carboxypeptidases of different penicillin sensitivity and with different mechanisms of penicillin binding. Peptidoglycan synthesis in the cell walls of the unstable L‐form is probably carried out with the help of only one DD‐carboxypeptidase,viz. the completely reactivatable enzyme with the lower penicillin sensitivity.