Molecular Pharmacology, Vol 18, 296-303, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Genetic Expression of Aflatoxin B₁ Metabolism: Effects of 3-Methylcholanthrene and 2,3,7 ,8,-Tetrachlorodibenzo-p-dioxin on the Metabolism of Aflatoxins B₁ and B₂ by Various Inbred Strains of Mice

¹ Department of Experimental Therapeutics and Grace Cancer Drug Center, Roswell Park Memorial Institute, New York State Department of Health, Buffalo, New York 14263

Hepatic microsomal metabolism of aflatoxin B₁ to aflatoxins M₁ (aflatoxin B₁-4-hydroxylase), Q₁ (aflatoxin B₁-9-hydroxylase) and to B₁-2,3-oxide (aflatoxin B₁-2,3-oxygenase) and of aflatoxin B₂ to aflatoxin M₂ (aflatoxin B₂-4-hydroxylase) was studied in inbred strains of mice pretreated with phenobarbital (PB), 3-methylcholanthrene (MC), or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In all strains tested (C57BL/6 Jacobs, BALB/cCr, CBA/J, C57L/J, DBA/2HaD, AKR/Sn, RF/J, and 129/J), PB induced aflatoxin B₁-2,3-oxygenase (2.4- to 6.4-fold) and, with the exception of C57L/J, aflatoxin B₁-9-hydroxylase (1.8- to 4.6-fold), whereas PB either had no effect or slightly induced aflatoxin in B₁-4-hydroxylase. In comparison to this, MC either had no effect or caused depression of aflatoxin B₁-2,3-oxygenase and of aflatoxin B₁-9-hydroxylase activities. On the other hand, MC induced aflatoxin B₁-4-hydroxylase (2.5- to 3.6-fold) and also aflatoxin B₂-4-hydroxylase (3.1- to 4.8-fold) only in Ah responsive strains and not in Ah nonresponsive strains. Dose-response studies with TCDD indicated that both aflatoxin B₁-4-hydroxylase and aflatoxin B₂-4-hydroxylase are induced by TCDD in the Ah nonresponsive strain (DBA/2) provided a dose 9-fold higher than that used for the Ah responsive strain (C57BL/6) is administered. The ED₅₀ value for aflatoxin B₁-4-hydroxylase and aflatoxin B₂-4-hydroxylase induction in DBA/2 strain (1.3 µg/kg) was accordingly 9-fold higher than that in C57BL/6 (0.14 µg/kg). In contrast, TCDD dose-response curves for aflatoxin B₁-2,3-oxygenase and aflatoxin-9-hydroxylase were essentially similar in both strains, i.e., DBA/2 and C57BL/6. At the lowest dose, both activities increased slightly but as the dose increased, the activities either were inhibited or remained unaffected. The data indicate that (a) several cytochrome P-450s of the mixed function oxygenase participate in the metabolism of aflatoxin B₁ via various pathways, (b) these pathways of aflatoxin B₁ metabolism are under different regulatory controls, (c) the regulation of aflatoxin B₁-4-/B₂-4-hydroxylase and aryl hydrocarbon hydroxylase induction is controlled by the same or closely linked genetic factors at the regulatory gene level and not at the structural gene level, and (d) the regulation in (c) is most likely mediated by an induction-specific receptor which apparently has lower affinity for the inducer in the nonresponsive strain DBA/2.

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ACKNOWLEDGMENTS The author wishes to acknowledge the technical assistance provided by Ms. Lurine Hauser and also wishes to thank Ms. Karen Schrader and Ms. Sandi Randazzo for the assistance provided in the preparation of this manuscript.