Metabolism of aflatoxin B1 by rabbit and rat nasal mucosa microsomes and purified cytochrome P450, including isoforms 2A10 and 2A11.

Abstract

The nasal mucosa of some mammalian species are susceptible to the toxicity of aflatoxin B1 (AFB1), a potent hepatocarcinogen, but little is known about the nasal enzymes involved in the metabolic activation of AFB1 or the metabolites produced. In the present study, the metabolism of AFB1 was studied with nasal microsomes from rats and rabbits and with several purified isozymes of rabbit P450 in a reconstituted enzyme system. The rates of AFB1-N7-guanine DNA adduct formation with rabbit and rat nasal microsomes are over 3- and 10-fold higher, respectively, than with liver microsomes from the same species. On the other hand, the rates of formation of AFM1 (9a-hydroxy-AFB1) and AFQ1 (3-hydroxy-AFB1) products known to be less toxic, are lower with nasal than with liver microsomes. Of particular interest, nasal microsomes produce high levels of six unidentified polar metabolites that are not formed by microsomes from liver or several other tissues. These same products are also generated by P450 NMa purified from rabbit nasal microsomes in a reconstituted system, but not by five other isozymes of cytochrome P450 (1A2, 2B4, 2E1, 2G1, 3A6) that are known to be present in nasal microsomes. AFB1-DNA adducts are formed by P450 NMa at a rate 3-fold higher than that by nasal microsomes. The DNA adducts are formed at much slower rates by P450s 2G1, 2B4, and 1A2, and adducts are not formed at measurable rates by P450s 2E1 and 3A6. Moreover, AFB1-DNA adduct formation is also catalyzed by cDNA-derived, heterologously expressed P450s 2A10 and 2A11, both of which are known to be present in the purified P450 NMa preparation. The Km and Vmax values of the two isozymes for DNA adduct formation are comparable to those for nasal microsomes. Furthermore, the formation of AFB1-DNA adducts by nasal microsomes is decreased by nicotine, a known inhibitor of P450 NMa. These data indicate that members of the P450 2A gene subfamily play an important role in the metabolic activation of AFB1 in rabbit and rat nasal mucosa and suggest a molecular basis for assessing the health risk associated with inhalation exposure to this procarcinogen in humans.