Using a gel electrophoresis assay, we show that the target DNA sequence cross-linked by N-methylmitomycin A, its aziridinomitosene, and mitomycin C is CpG, in strong preference over GpC. The yield per CpG site increases as the number of successive CpG sequences increases. Molecular modeling reveals no systematic difference between the energies of mitomycin cross-links at CpG in comparison with GpC. However, the distance between guanine amino groups in CpG sequences is nearly the same as the distance in the cross-linked adduct, whereas the amino group separation at GpC sites is substantially larger in the starting DNA than in the adduct. We suggest that the favorable placement of the second reaction center in CpG greatly accelerates the second step in the cross-linking reaction. As shown by a competition assay, mitomycins bind A-T and G-C sequences noncovalently equally well, even though the only sequence that yields appreciable cross-linking is CpG. N-Methylmitomycin A and its aziridinomitosene are found to be better cross-linking agents than is mitomycin C.

We have assayed the cross-linking of oligonucleotides containing repeated mitomycin-reactive CpG sites in order to assess the factors that enhance activation of the carbamoyl function at C10, yielding efficient mitomycin cross-linking. Drugs studied include mitomycin C (MC), N-methylmitomycin A (NMA), and the aziridinomitosene of NMA (MS). Drugs were reduced both by catalytic hydrogenation and by diothionite. We find that cross-linking by fully reduced NMA can be increased severalfold by addition of either excess dithionite reductant or the oxidant FeCl3. Enhancement by FeCl3 is not seen with MC or MS, but excess dithionite increases cross-linking by all three compounds. We explain the action of Fe3+ by postulating production of the semiquinone of the monoadduct of mitomycin reacted at the C1-position; according to this mechanism, departure of the carbamate from C10 is more efficient for the semiquinone than for the hydroquinone. However, our results imply that the hydroquinone can also function as a cross-linking agent. Excess dithionite, beyond that required for stoichiometric reduction, activates the carbamate 2-3-fold for cross-linking. We find that the fully reduced leucoaziridinomitosene is highly unstable in solution, yet it produces efficient cross-liking. Hence, this compound is highly reactive in DNA alkylation and a good candidate for the role of primary alkylating agent.