Genetic differences in the metabolism of carcinogens and in the binding of benzo[a]pyrene metabolites to DNA

Abstract

1. 1.|Aromatic hydrocarbon “responsiveness,” or the Ah locus, controls in the mouse the induction of cytochrome P 1-450 and more than ten associated monoxygenase activities by polycyclic hydrocarbons. One of the induced activities is aryl hydrocarbon hydroxylase (EC 1.14.14.2), which metabolizes BP and other chemical carcinogens. 2. 2.|For certain substrates, cytochrome P 1-450 is known to produce a predominance of reactive intermediates and products that differ from those formed by other forms of cytochrome P-450. The aromatic hydrocarbon-treated responsive mouse therefore may produce, in various tissues throughout the body, quantitative and qualitative differences in reactive intermediates of a given test compound. 3. 3.|Numerous conditions in the mouse, which include an increased susceptibility to chemical carcinogenesis and drug toxicity, are therefore directly associated with the Ah locus — considered to be a single gene or a small number of genes. 4. 4.|The various forms of cytochrome P-450 in mouse liver or skin microsomes produce reactive metabolites of BP that bind to DNA in vitro. These results confirm the work of other laboratories in which rat liver microsomes or cell cultures were used. 5. 5.|Nine peaks, reflecting BP metabolites bound to DNA nucleosides, can be reproducibly identified from C3H, C57BL/6, and DBA/2 mice. When either the C3H or C57BL/6 strain is compared with the DBA/2 strain, a positive general correlation is found between the carcinogenic index in vivo and each of three determinations in vitro: the size of eight of the nine peaks, the hepatic aryl hydrocarbon hydroxylase activity, and the total hepatic cytochrome P-450 content. 6. 6.|Peaks D, E, H, and I demonstrate relative differences among the three strains that are in the same direction as the carcinogenic index in vivo, i.e. the peaks are greater in C3H than in C57BL/6 and both are considerably greater than those in DBA/2 mice. When C3H mice are compared with C57BL/6 mice, however, no quantitative correlation is found between the carcinogenic index and the size or shape of any of the nine peaks, the hepatic hydroxylase activity, or the total hepatic cytochrome P-450 content. Hence, the total quantity of nucleoside-BP metabolites in each of the nine peaks is not necessarily associated with biological activity (i.e. cancer in vivo). It is suggested that a defect in the general immune competence of the C3H inbred strain may be an important factor in its markedly increased susceptibility to BP-initiated tumorigenesis, compared with the C57BL/6 strain. 7. 7.|With hepatic microsomes, peak G is associated, at least in part, with nucleoside-metabolite complexes involving BP-4,5-oxide, whereas the other eight peaks are associated predominantly with metabolites formed by cytochrome P 1-450 rather than other forms of P-450. With skin microsomes, all peaks including peak G appear to be associated with metabolites formed predominantly by cytochrome P 1-450. Peak E is associated predominantly with BP-7 8-diol-9,10-epoxide bound to one or more nucleosides. This reactive intermediate, oxygenated in the non-K region, is associated with genetically mediated increases in aromatic hydrocarbon-inducible cytochrome P 1-450. 8. 8.|Cultured human lymphocytes and microsomes from a human liver biopsy are both capable of metabolizing BP to intermediates that bind to DNA and cause peaks similar to those found with mouse liver or skin microsomes. (Permission to reprint the previously published material — Figs. 3, 4, 5, 6, 7, 8, and 9 and Tables 1 and 2 — has been obtained from the publishing house indicated in the references indicated).