Selective Activation of Mitomycin A by Thiols To Form DNA Cross-links and Monoadducts:  Biochemical Basis for the Modulation of Mitomycin Cytotoxicity by the Quinone Redox Potential

Mitomycin A (MA) but not mitomycin C (MC) cross-linked linearized 32P-pBR322 DNA in the presence of dithiothreitol (DTT) or glutathione (GSH), as shown by a sensitive DNA cross-link assay. Incubation of calf-thymus DNA with MA and DTT or mercaptoethanol (MER) resulted in the formation of MA−DNA adducts, which were isolated from nuclease digests of the drug−DNA complexes by HPLC. The adducts were characterized by their UV absorption spectra, electrospray ionization mass spectrometry (ESIMS), and facile conversion from 7-methoxy- to 7-amino-substituted mitosene type adducts upon 10% NH4OH treatment, which were identical with known adducts of MC. Both DNA interstrand and intrastrand cross-link adducts, linking two deoxyguanosine residues at N2, as well as several deoxyguanosine-N2 monoadducts of MA, were identified. No DNA adducts were formed with MC under the same conditions. A specificity of DNA cross-link formation for the CpG sequence was observed using 12-mer synthetic oligodeoxyribonucleotides as substrates and as DNA sequence models, in analogy to the known CpG sequence specificity of MC-induced DNA cross-links. MA is known to be more cytotoxic by 2−3 orders of magnitude than MC, and this property correlates with redox potentials of MA (−0.19 V) and MA analogues that are higher than those of MC (−0.40 V) and its analogues. It is suggested that the biochemical basis for the higher cytotoxic potency of MA is MA's propensity to be reductively activated by cellular thiols while MC is resistant to thiol activation. This distinction is probably derived from the large difference between the quinone redox potentials of the two drugs.