Characteristics of benzo(a)pyrene metabolism by kidney, liver and lung microsomal fractions from rodents and humans.

The time course of metabolism of benzo(a)pyrene by liver microsomes from rats given injections of corn oil, pheno barbital, or 3-methylcholanthrene was followed by high pressure liquid chromatography. This method showed non linear time courses of benzo(a)pyrene metabolism after 2 mm of reaction at low protein concentrations. The forma tion of all of the benzo(a)pyrene metabolites measured by high-pressure liquid chromatography was not linear with time but was inducible upon pretreatment of rats with either phenobarbital or 3-methylcholanthrene. The benzo(a) pyrene metabolism activity of hamster liver was not induc ible by pretreatment with 3-methylcholanthrene. Hamster and rat lung microsomal fractions metabolized benzo(a)pyrene at 0.5% of the rate of liver microsomal fractions, and the rate of formation of metabolites was increased only by animal pretreatment with 3-methylchol anthrene. The time course of metabolism by hamster and rat lung microsomes was linear up to 15 mm. The metabolism of benzo(a)pyrene by microsomal frac tions from kidney, liver, and lung of a number of human subjects was studied; the reaction conditions required to limit metabolism to mainly the primary metabolites were established. The microsomal fractions from human kidney and lung were stable to freezing and had metabolic activi ties which were 40 to 60% as large as those obtained with rodent lung. In addition, lung samples obtained either at resection or autopsy metabolized benzo(a)pyrene at rates which were not statistically different. The activity of human liver microsomes was 15 to 20% of that obtained with rodents. Subjects who had ingested large doses of barbitu rates before death had significantly higher levels of benzo(a)pyrene metabolism activity; no correlation between liver or lung monooxygenase activity and smoking habits was readily discernible from the metabolic data. An analysis of the product distribution of the benzo-ring metabolites showed that the human liver and kidney had product ratios which were qualitatively similar to those obtained with rodent tissue. In addition, higher proportions of quinones were formed at the expense of the phenols which cochro matograph with 3-hydroxybenzo(a)pyrene. However, the human lung microsomal fractions catalyzed the formation of a significantly higher proportion of benzo-ring metabo lites than did the other human or rodent organs, with the possible exception of hamster lung. I Recipient of National Cancer Institute Contract NOl CP 33362 and NIH Grant HL 19654. Research Career Development Awardee, HLCA 00255. To whom requests for reprints should be addressed. 2 Recipient of NIH Grant GM16488. 3 Recipient of National Cancer Institute Contract NOl CP 33363. Received June 7, 1978; accepted December 21, 1978. INTRODUCTION Interest in the molecular mechanism of PAH4-mediated carcinogenesis has prompted the development of tech niques to evaluate the total profile of PAH metabolites and the factors which may regulate their formation. It has been established that the endoplasmic reticulum fraction of nu merous mammalian organs such as liver (4, 18, 22, 36, 44), kidney (7, 18, 43), intestinal mucosa (18), skin (19, 44), placenta (12, 43, 45), and lung (18, 22, 41, 44) catalyze the oxidative metabolism of at least one PAH, B(a)P. The identification and quantitation of the products of the cyto chrome P-450-mediated biological oxidation of B(a)P and other PAH's have utilized several analytical techniques which include fluorometric methods which measure phenol formation (5, 23, 43), radiometric methods which measure total metabolism (6, 8), HPLC or thin-layer chromatography which measures all of the various classes of metabolites (9, 29, 32), and direct spectrophotometric methods which measure phenol formation (27). The ability to determine accurately the structures and quantities of B(a)P metabolites has aided in the evaluation of the carcinogenicity, cytotoxicity, and mutagenicity of these compounds. While most of the primary metabolites are only weakly mutagenic, the epoxides appear to be uniquely active as mutagens to bacterial or cultured ham ster cell test systems (47, 48). Upon further metabolism, several B(a)P derivatives can be metabolically activated to reactive intermediates which are highly mutagenic (11, 39, 49). In addition, the results obtained from either giving mice i.p. injections or skin painting them with these compounds indicate that one possible proximal carcinogen is the (±)trans-7,8-dihydro-7,8-dihydroxy-B(a)P (20) which can be converted to a 7,8-dihydroxy-9,10-epoxy-7,8,9,1 0-tetrahy dro-B(a)P, a possible ultimate carcinogen (13, 16, 35). The lung is an organ which has direct contact with the environment and is readily susceptible to aryl hydrocarbon induced carcinogenesis (30, 40). Considerable effort has gone into the development of animal models of polycyclic hydrocarbon-induced lung tumorigenesis; hamsters and rats have been studied extensively due to the similarity between the lung tumors of these rodents and humans. The ability of rodent lung to metabolize B(a)P has been studied by several research groups and noted to be low relative to liver (18, 22, 44). The relationship between the lower rates of B(a)P metabolism and susceptibility to carcinogenesis in lung remains to be rationalized. 4 The abbreviations used are: PAH, polycyclic aromatic hydrocarbons; B(a)P, benzo(a)pyrene; HPLC, high-pressure liquid chromatography; 3-MC, 3-methylcholanthrene; PB, phenobarbital; 7.8-dihydrodiol, 7.8-dihydro-7,8dihydroxybenzo(a)pyrene; 9,10-dihydrodiol, 9.10-dihydro-9,1 0-dihydroxy benzo(a)Dvrene : 4.5-dihvdrodiob. 4.5-dihvdro-4.5-dihvdroxvbenzo(a)ovrene.