Production of Large Amounts of Hydrogen Peroxide by Human Tumor Cells1

Few nonphagocytic cells are known to generate reactive oxygen inter mediates. Based on horseradish peroxidase-dependent, catalase-inhibitable oxidation of fluorescent scopoletin, seven human tumor cell lines constitutive!)' elaborated H2O2 at rates (up to 0.5 nmol/104 cells/h) large enough that cumulative amounts at 4 h nere comparable to the amount of IM)., produced by phorbol ester-triggered neutrophils. Superoxide dismutase-inhibitable ferricytochrome c reduction was detectable at much lower rates. IM), production was inhibited by diphenyleneiodonium, a flavoprotein binder (concentration producing 50% inhibition, 0.3 Õ/M). and diethyldithiocarbamate, a divalent cation chelator (concentration producing 50% inhibition, 3 MM),but not by cyanide or azide, inhibitors of electron transport, or by agents that inhibit xanthine oxidase, pol\ aniini1oxidase, or cytochrome P450. Cytochrome ¿559, present in human phagocytes and lymphocytes, was undetectable in these tumor cells by a sensitive spectrophotometric method. Mouse fibroblasts transfected with human tyrosinase complementary DNA made melanin, but not IM).,. Constitutive generation of large amounts of reactive oxygen intermedi ates, if it occurs in vivo, might contribute to the ability of some tumors to mutate, inhibit antiproteases, injure local tissues, and therefore pro mote tumor heterogeneity, invasion, and metastasis.