Clazamycin A

Clazamycin, a novel pyrrolizidine antitumor antibiotic, exists in aqueous solution as a mixture of two epimers, clazamycins A and B [1A, 1B], the ratio of which is pH dependent. Several lines of evidence are presented, including the results of trapping experiments and a study demonstrating base promoted interconversion of the two forms, that implicate an azacyclooctenone species [3] as an intermediate in the interconversion process. This result supports a previous observation, that the C6a carbinolamidine hydroxyl of clazamycin is unreactive towards nucleophiles and may be significant in helping to elucidate the mechanism of action of this antibiotic.

Growth of Escherichia coli in a nutrient medium was inhibited by 100 micrograms/ml of clazamycin and at this concentration, the viable cell number decreases slowly. Elongated cells were observed in the treated cultures. The bactericidal activity was abolished by high concentrations of either sucrose or sorbitol but not by chloramphenicol. Non-growing cells suspended in a medium devoid of both carbon and nitrogen sources were killed by clazamycin more rapidly than cells in a rich medium. Incorporation of radioactive thymidine, uridine, leucine and N-acetylglucosamine into cellular macromolecules was inhibited to a similar extent. Permeability of N-acetylglucosamine and leucine was blocked by clazamycin . On the other hand, membrane transport of thymidine was only slightly inhibited. Thymidine-derived radioactivity accumulated as dTTP in the cells suggesting that DNA synthesis was blocked at the polymerization step. DNA synthesis in toluene-treated cells was also sensitive to clazamycin while the repair DNA synthesis induced by bleomycin in these cells was not. DNA-repair deficient mutants of E. coli were as sensitive to clazamycin as their DNA-repair proficient counterparts.