Reaction of DNA with chemically or enzymatically activated mitomycin C: isolation and structure of the major covalent adduct.

The antitumor antibiotic mitomycin C is shown to form a covalent complex with calf thymus DNA under anaerobic conditions in the presence of either NADPH cytochrome c reductase/NADPH, xanthine oxidase/NADH, or the chemical reducing system H2/PtO2. Digestion of the complex with DNase I/snake venom diesterase/alkaline phosphatase yields a single mitomycin deoxyguanosine adduct as the major DNA alkylation product, identified as N2-(2'' beta,7''-diaminomitosen-1'' alpha-yl) 2'-deoxyguanosine (Structure 2). Two minor adducts, 2-5% each of the total adduct pool, are isolated and identified as the 1'' beta stereoisomer of 2 (Structure 3), and 10''-decarbamoyl-2 (Structure 7). The same results were obtained with M13 DNA and poly(dG-dC).poly(dG-dC); however, in the latter case, a minor adduct apparently possessing two deoxyguanosine and one mitomycin unit is isolated. Digestion of the covalent mitomycin-calf thymus DNA complex with nuclease P1 yields four dinucleotide adducts, all of which consist of 2 linked at its 3' end to each of the four possible 5' nucleotides (A, T, G, and C). Upon treatment of each dinucleotide adduct with snake venom diesterase/alkaline phosphatase, 2 is released along with the corresponding free nucleoside. In apparent conflict with the present results, previous reports from another laboratory have indicated that modification of calf thymus DNA by mitomycin C under conditions identical to those described here result in the isolation of three mitomycin C mononucleotide adducts possessing linkages of the drug to N2 and O6 of guanine and N6 of adenine. Evidence is shown suggesting that the latter adducts are actually three of the above four dinucleotide derivatives of 2 obtained independently by us and, thus, all of them in fact possess an identical N2-mitosenylguanine adduct moiety. Model-building studies indicate an excellent fit of the guanine N2-linked drug molecule inside the minor groove of B-DNA with no appreciable distortion of the DNA structure.