Inhibition of DNA cross-linking by mitomycin C by peroxidase-mediated oxidation of mitomycin C hydroquinone

Mitomycin C requires reductive activation to crosslink DNA and express anti cancer activity. Reduction of mitomycin C (40 muM) by sodium borohydride (2 00 muM) in 20 nM Tris-HCl, 1 mM EDTA at 37 degreesC, pH 7.4, gives a 50-60% yield of the reactive intermediate mitomycin C hydroquinone. The hydroquin one decays with first order kinetics or pseudo first order kinetics with a t(1/2) of similar to 15 s under these conditions. The cross-linking of T7 D NA in this system followed matching kinetics, with the conversion of mitomy cin C hydroquinone to leuco-aziri-dinomitosene appearing to be the rate-det ermining step. Several peroxidases were found to oxidize mitomycin C hydroq uinone to mitomycin C and to block DNA cross-linking to various degrees. Co ncentrations of the various peroxidases that largely blocked DNA crosslinki ng, regenerated 10-70% mitomycin C from the reduced material. Thus, signifi cant quantities of products other than mitomycin C were produced by the per oxidase-mediated oxidation of mitomycin C hydroquinone or products derived therefrom. Variations in the sensitivity of cells to mitomycin C have been attributed to differing levels of activating enzymes, export pumps, and DNA repair. Mitomycin C hydroquinone-oxidizing enzymes give rise to a new mech anism by which oxic/hypoxic toxicity differentials and resistance can occur .