Identification of Aflatoxin M1-N7-Guanine in Liver and Urine of Tree Shrews and Rats Following Administration of Aflatoxin B1

Abstract

Epidemiological studies have shown that exposure to aflatoxin B₁ (AFB₁) and concurrent infection with hepatitis B lead to a multiplicative risk of developing liver cancer. This chemical−viral interaction can be recapitulated in the tree shrew (Tupia belangeri chinensis). As an initial characterization of this model, the metabolism of AFB₁ in tree shrews has been examined and compared to a sensitive bioassay species, the rat. Utilizing LC/MS/MS, an unreported product, aflatoxin M₁-N⁷-guanine (AFM₁-N⁷-guanine), was detected in urine and hepatic DNA samples 24 h after administration of 400 μg/kg AFB₁. In hepatic DNA isolated from tree shrews, AFM₁-N⁷-guanine was the predominant adduct, 0.74 ± 0.14 pmol/mg DNA, as compared to 0.37 ± 0.07 pmol/mg DNA of AFB₁-N⁷-guanine. Conversely, in rat liver, 6.56 ± 2.41 pmol/mg DNA of AFB₁-N⁷-guanine and 0.42 ± 0.13 pmol/mg DNA of AFM₁-N⁷-guanine were detected. Rats excreted 1.00 ± 0.21 pmol AFB₁-N⁷-guanine/mg creatinine and 0.29 ± 0.10 pmol AFM₁-N⁷-guanine/mg creatinine as compared to 0.60 ± 0.12 pmol AFB₁-N⁷-guanine/mg creatinine and 0.69 ± 0.16 pmol AFM₁-N⁷-guanine/mg creatinine excreted by the tree shrew. Furthermore, tree shrew urine contained 40 times more of the hydroxylated metabolite, AFM₁, than was excreted by rats. In vitro experiments confirmed this difference in oxidative metabolism. Hepatic microsomes isolated from tree shrews failed to produce aflatoxin Q₁ or aflatoxin P₁ but formed a significantly greater amount of AFM₁ than rat microsomes. Bioassays indicated that the tree shrew was considerably more resistant than the rat to AFB₁ hepatocarcinogenesis, which may reflect the significant differences in metabolic profiles of the two species.