Metabolism of the carcinogen 1,2-dimethylhydrazine by isolated human colon microsomes and human colon tumor cells in culture

Abstract

Human colon microsomes catalyze the metabolism of the model colon carcinogen 1,2‐dimethylhydrazine. Activity appears to be distributed in a gradient towards the lower end of the colon. Highest activities were observed for microsomes prepared from the descending segment of the colon with the transverse segment exhibiting lower activities, while the ascending segment showed the lowest rate of metabolism. Dimethylhydrazine metabolism in each segment is inhibited significantly by inhibitors of the cytochrome P‐450‐dependent mixed function oxidase system. Microsomes prepared from a human colon tumor cell also catalyze the metabolism of 1,2‐dimethylhydrazine. Metabolic activity in the cell line can be induced two‐fold by treatment of cells with phenobarbital and three‐fold by treatment of the cells with phenobarbital plus hydrocortisone. These results show that human colon activates 1,2‐dimethylhydrazine and suggest that the human colon may be capable of activating other carcinogens in situ.

Role of Hepatic and Intestinal P450 Enzymes in the Metabolic Activation of the Colon Carcinogen Azoxymethane in Mice

“…40 Hence, the metabolic mechanisms of AOM may have broad implications. In that regard, while several studies had reported efficient in vitro metabolism of AOM by colon microsomes from rodents, hamsters, and humans, 8,9,11,12 others failed to detect such activity in the colon. 41 When non-P450-mediated bioactivation of MAM in colon was investigated in vitro, alcohol dehydrogenase (ADH) was implicated; 42 however, a role for ADH was not confirmed in vivo.…”

“…MAM formed from AOM in microsomal incubations was detected as formaldehyde using the chromotropic acid method. 9 The microsomal incubations were performed at 37 °C, using 500 μM AOM and 0.2 mg/mL microsomal protein for 10 min. Control assays were performed in the absence of either AOM or microsomes to correct for any nonenzymatic conversion of AOM to formaldehyde.…”

“…MAM formed from AOM in microsomal incubations was detected as formaldehyde using the chromotropic acid method . The microsomal incubations were performed at 37 °C, using 500 μM AOM and 0.2 mg/mL microsomal protein for 10 min.…”

“…Previous in vitro studies have demonstrated that colon epithelial cells are capable of metabolizing DMH into carcinogenic metabolites, without the need for prior metabolism by other tissues or colonic bacteria. − However, the prevailing hypothesis is that the liver plays a critical role in DMH/AOM bioactivation in vivo and that the reactive intermediates produced by the liver are transported to the colon via the blood or bile to induce carcinogenicity. − This is a plausible hypothesis, given that the colon generally possesses much lower levels of P450 enzymes, relative to the liver. Nonetheless, the relative contributions of the liver and the intestine to DMH/AOM-induced DNA damage in the colon have not been directly determined, and the bioactivation in the target organ may also explain the organ-specific induction of tumors in the distal colon by AOM.…”

Identification of Inducible Mixed Function Oxidase System in Rat Glioma C6 Cell Line

“…Cells were lysed by sonication with a Heat Systems ultrasonifier (model WI 85) by using a stepped microtip at 40 W for 1 min at 4°C. After differential centrifugation, the products were stored at -78°C in 20% glycerol and 10 mM EDTA (Newaz et al ., 1983) . Rat liver microsomes were prepared as previously described (Saito and Strobel, 1981) .…”