Relative contributions of mouse liver subcellular fractions to the bioactivation of mitomycin C at various pH levels.

Mitomycin C (MMC) is a clinically active anticancer drug that requires reductive activation to exert its toxicity. The enzymes currently recognized as capable of activating MMC cannot account for all of the toxicity of the drug. These studies were conducted to identify and compare the subcellular compartments where MMC reduction can take place under different physiological conditions. Subcellular fractionation of mouse liver was achieved using differential centrifugation and isopycnic equilibrium gradient centrifugation. Nuclear, mitochondrial, microsomal, lysosomal, peroxisomal, and cytosolic fractions were assayed for their ability to reductively activate MMC at pH 6.0 and 7.4. MMC reductive activation was determined by its ability to generate reactive oxygen species. The results of these studies showed that MMC reductive activation by the various fractions was pH dependent. At pH 7.4, the microsomal fraction accounted for approximately 78% of the total MMC reductive activation. The peroxisomal fraction accounted for 12% and the nuclear and lysosomal fractions each accounted for 5% of the total reductive activation. At pH 6.0, the microsomes accounted for 51% and the peroxisomes for 34% of the total reductive activation. The mitochondrial fraction, which did not reductively activate MMC at pH 7.4, accounted for 9% of the total activation at pH 6.0. These results suggested that peroxisomes may be important in MMC activation at either pH and that at pH 6.0 the mitochondrial fraction may also be important for MMC reductive activation.