INFLUENCE OF THE ANTIOXIDANT N-ACETYLCYSTEINE AND ITS METABOLITES ON DAMAGE-INDUCED BY BLEOMYCIN IN BACTERIOPHAGE-PM2 DNA

Bleomycin is considered to be a useful model compound for studying env ironmental carcinogenesis, due to its broad spectrum of DNA damaging p roperties. In addition, bleomycin is a useful antitumor drug because o f its cytotoxic properties. To investigate the influence of the antiox idant N-acetylcysteine and its metabolites glutathione and cysteine on bleomycin-induced DNA damage and more importantly to gain insight int o the biological relevance of such damage, PM2 DNA was exposed to Cu2-bleomycin in the presence and absence of the thiols N-acetylcysteine, glutathione and cysteine. It was found that the presence of these thi ols led to a considerable enhancement of bleomycin-induced single- and double-strand breaks and a concomitant decrease in the biological act ivity of PM2 DNA in a dose-dependent way. A similar observation was ma de when ascorbic acid was used. Bleomycin showed no DNA damaging activ ity when PM2 DNA was pretreated with the strong Fe ion chelator desfer al and its activity was strongly inhibited by the addition of Cu2+ ion s or under hypoxic (N-2) conditions, Cu2+-bleomycin under our conditio ns is not active by itself, but most probably after binding to DNA exc hanges Cu2+ for Fe3+ bound to DNA. Fe3+-bleomycin is then reduced to F e2+-bleomycin, a process potentiated by the added antioxidants, and su bsequently activated by O-2. The contribution to biological inactivati on of bleomycin alone or in the presence of ascorbic acid is only simi lar to 15%. The contribution to lethality in the presence of thiols is higher. These results indicate that ascorbic acid only enhances the D NA damaging properties of bleomycin, whereas the thiol compounds in ad dition influence the type of DNA damage. The remainder of the biologic al inactivation is probably caused by double damage, such as single-st rand breaks with closely opposed alkali-labile sites or base damage.