Activation of Mitomycin C by NADPH:Cytochrome P-450 ReducÃ-ase1

Mitomycin C is an alkylating agent used in cancer chemotherapy that shows some specificity towards hypoxic cells. The therapeutic effects of this compound are thought to result from its metabolic activation by enzymes such as NADPH:cytochrome P-450 reducÃ-ase.In a previous report we described a Chinese hamster ovary cell line resistant to mitomycin C, which had a decreased NADPH:cytochrome P-450 reductase activity coupled with a lower rate of mitomycin C metabolism. In order to provide further evidence that the lower reducÃ-aseactivity is a factor in the resistance mechanism, we incorporated NADPH:cytochronte P-450 reducÃ-aseinto cytotoxicity assays and showed that it significantly sensitizes cells to mitomycin C. Also, the difference in drug sensitivity between the wild-type and drug-resistant Chinese hamster ovary cells was no longer observed. In addition to these studies, we expressed a rat liver NADPH:cytochrome P-450 reducÃ-asecDNA in a Salmonella typhimurium strain, LR5000. The bacteria expressing the rat NADPH: cytochrome P-450 reducÃ-aseshowed increased sensitivily to mitomycin C when incubated with Ihis compound under aerobic condilions. However, under hypoxic condilions increased sensitivity was not observed. This parallels Ihe previous finding wilh milomycin ( -resistant Chinese ham ster ovary cells. These dala provide direct evidence for Ihe role of NADPH:cylochrome P-450 reducÃ-asein Ihe cyloloxic aclion of Ihis milomycin C under aerobic bui noi hypoxic condilions and suggesl thai reduced levels of Ihis enzyme can lead lo drug resislance. P-450 reducÃ-ase expressed in S. typhimurium may provide a valuable tool for evaluating the role of Ihis enzyme in Ihe loxicily of drugs activaled Ihrough a one electron reduction palhway.