Current issues in the enzymology of mitomycin C metabolic activation.

1. Mitomycin C (MMC) is considered to be the prototype bioreductive drug undergoing activation to toxic species preferentially under hypoxic conditions. Therefore a proper understanding of the enzymology of bioreduction in tumor tissue is of great importance. 2. DT-diaphorase and NADPH:cytochrome P-450 reductase (quinone reductases) are believed to have established roles in this activation pathway, but these roles are now undergoing revision. 3. It is emerging, however, that different reductases prevail under different physiological conditions. Indeed, DT-diaphorase has been found to protect cells from the hypoxic cytotoxicity of MMC in cell lines expressing high levels of the enzyme. 4. A novel mitochondrial reductase(s) has been identified in solid tumor tissue and is active only under hypoxic conditions and is more efficient at metabolizing MMC than are the other reductases identified. 5. Thus, this newly identified mitochondrial reductase(s) is a potential new target for enzyme-directed bioreductive drug therapy if tumor hypoxia can be achieved. However, because most tumors overexpress DT-diaphorase, this enzyme may prove optimal for MMC drug therapy.