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Metabolism and Macromolecular Binding of the Carcinogen Benzo(a)pyrene and Its Relatively Inert Isomer Benzo(e)pyrene by Hamster Embryo Cells¹

Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830

The metabolism and macromolecular binding of the carcinogen benzo(a)pyrene [B(a)P] and its relatively inert structural isomer benzo(e)pyrene [B(e)P] have been studied in order to determine if a metabolic basis exists for their very different biological activities.

B(a)P and B(e)P are metabolized by hamster embryo cells to organic solvent-soluble and water-soluble metabolites. Significant quantitative and qualitative differences are observed in the nature of the metabolites from the different hydrocarbons and the distribution of these metabolites between the cytoplasm and the extracellular medium. B(a)P is metabolized more extensively than B(e)P to both ethyl acetate-soluble and water-soluble metabolites. The major ethyl acetate-soluble metabolite in the medium after 24-hr culture with B(a)P (4 µM) is the bay-region 9,10-dihydro-9,10-dihydroxybenzo(a)pyrene (66.7% of total metabolites in medium). Rapid excretion of this product from the cells is indicative of its observed subsequent biological inactivity. Smaller amounts of 7,8-dihydro-7,8-dihydroxybenzo(a)pyrene were found, but more of this dihydrodiol than the 9,10-dihydrodiol was retained intracellularly, where it could be metabolized to an active diol-epoxide. The major metabolites found in the cytoplasm are 9-hydroxybenzo(a)pyrene and 3-hydroxybenzo(a)pyrene (40.7 and 16.5% of metabolites in cytoplasm, respectively) and 7,8-dihydro-7,8-dihydroxybenzo(a)pyrene (18.4% of metabolites in cytoplasm) with smaller amounts of 9,10-dihydro-9,10-dihydroxybenzo(a)pyrene (9.2% of metabolites in cytoplasm). The low amounts of phenols in the medium are due to their relatively rapid removal as water-soluble, glucuronide conjugates.

The major ethyl acetate-soluble metabolite formed in the extracellular medium, after 24 hr of culture with B(e)P (4 µM), is the K-region dihydrodiol, 4,5-dihydro-4,5-dihydroxybenzo(e)pyrene (69.6% of organic solvent-soluble metabolites in medium), with only small amounts of monohydroxybenzo(e)pyrenes (21.9% of organic solvent-soluble metabolites in medium) being formed. Most of the monohydroxybenzo(e)pyrenes formed and significant amounts of 4,5-dihydro-4,5-dihydroxybenzo(e)pyrene are metabolized to their respective water-soluble glucuronide conjugates. The much higher binding of B(a)P than B(e)P to both DNA and RNA of isolated nuclei from cells treated with the parent hydrocarbons reflects the higher biological activity of B(a)P. The metabolic formation of 7,8-dihydro-7,8-dihydroxybenzo(a)pyrene from B(a)P and the apparent lack of formation of 9,10-dihydro-9,10-dihydroxybenzo(e)pyrene from B(e)P suggest a metabolic basis for the relative biological activities of the parent hydrocarbons. While B(a)P forms a dihydrodiol which may be metabolized on an adjacent olefinic double bond to a potentially reactive diolepoxide adjacent to the bay-region, it is apparent that B(e)P does not enzymatically favor this mechanism and forms an inert K-region dihydrodiol.

¹ Research sponsored jointly by the Environmental Protection Agency under Interagency Agreement 40-516-75, the National Cancer Institute under Interagency Agreement 40-5-63, and the Division of Biomedical and Environmental Research, United States Department of Energy, under Contract W-7405-eng-26 with the Union Carbide Corporation.

² Permanent address: Department of Biochemistry, University of Surrey, Guildford GU2 5XH, Surrey, England.

³ To whom requests for reprints should be addressed, at Biology Division, Oak Ridge National Laboratory, P. O. Box Y, Oak Ridge, Tenn. 37830.

Received 12/27/78. Accepted 5/31/79.