Studies related to antitumor antibiotics. Part XIV. Reactions of mitomycin B with DNA

Abstract

The reactions of the antitumor antibiotic mitomycin B with DNA were examined using ethidium fluorescence assays. The following three aspects of mitomycin B action have been studied to compare its behavior with that of mitomycin C: (a) interstrand cross-linking events, (b) alkylation without necessarily cross-linking, and (c) strand breakage. The greater pKa value of 4.3 found for mitomycin B compared with that of mitomycin C, i.e., 3.2, together with the greater pH dependence of DNA alkylation and interstrand cross-linking and the faster and more extensive cross-linking by mitomycin B at low pH in the absence of reduction, support the suggestion that the aziridine moiety is involved in the initial alkylation of DNA. Mitomycin B, reduced in situ with NaBH4, nicks covalenty closed circular (CCC) PM2 DNA rapidly but less efficiently than mitomycin C in a reaction which is suppressed by (i) superoxide dismutase, (ii) catalase, and (iii) free radical traps showing the intermediacy of O−2∙, H2O2, and OH∙. DNA is cleaved by mitomycin B to which it is covalently attached as well as by the free antibiotic. The addition of intercalated ethidium bromide to DNA prior to treatment with reduced mitomycin B inhibits interstrand cross-linking but not strand scission. The reduced aziridine ring-opened mitomycin B (which lacks the 7-NH2 group of mitomycin C) alkylates DNA and thus provides evidence confirming a previous suggestion that the second covalent link to the DNA is formed at position 10 of the antibiotic.