Metabolic conversion and distribution of the products of 7,12-dimethylbenz(a)anthracene (DMBA) were analyzed in the mouse mammary cell transformation model in organ culture. In order to determine the levels of uptake of DMBA, the glands were exposed to the transforming dosage of the procarcinogen (7.8 µm, 20 µCi/ml) for 24 h, and the level of uptake was determined to be 8 × 104 cpm/mg of tissue. Subsequently, the glands were incubated in DMBA-free medium, and distribution of the radioactivity in DNA and in the acid-insoluble materials was measured. Data showed that, in addition to the 24-h DMBA treatment period, the initiation stage extends for another 3 h when the incubation is continued in DMBA-free medium. A saturation level of uptake of [3H]DMBA into the whole gland was observed at 12 h, while DMBA was continually metabolized with the products being bound to DNA and to the acidinsoluble fractions throughout the entire incubation period with or without DMBA. The three major adducts identified were anti-DMBA-3,4-diol-1,2-epoxide (DMBADE):deoxyguanosine, syn-DMBADE:deoxyadenosine, and anti-DMBADE:deoxyadenosine. Qualitatively the adduct profiles remained similar. However, with the additional 3-h incubation in DMBA-free medium, the three major DMBA-DNA adducts increased slightly by 6.5 to 7.5%. Thus the total 27-h time period can be considered as the duration of the initiation stage in the DMBA-induced carcinogenesis of mouse mammary cells in organ culture. Therefore the subsequent 6-h incubation in DMBA-free medium may be considered as within the promotional stage, and at the same time period the levels of the three DNA adducts decreased significantly by 67.5 to 84.1% (P < 0.001).

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This investigation was supported by Grant CA25304, awarded by the National Cancer Institute, Department of Health and Human Services.

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