The effects of mixed function oxidase inducers on the metabolism and mutagenicity of aflatoxin B₁

Abstract

The metabolism of aflatoxin B₁ to aflatoxins M₁ and Q₁ by rat liver microsomes from animals pretreated with polychlorinated or polybrominated biphenyl congeners depended on the structure of the halogenated biphenyl inducers. Microsomes from rats treated with phenobarbital (PB) or halogenated biphenyls that exhibit PB-type activity preferentially enhanced the conversion of aflatoxin B₁ to aflatoxin Q₁. In contrast, microsomes from rats treated with 3-methylcholanthrene (MC) or halogenated biphenyls that exhibit MC-type induction activity increased the metabolism of aflatoxin B₁ to aflatoxin M₁. The coadministration of PB and MC produced microsomes that exhibited both types of induction activity (mixed-type) in catalyzing the oxidative metabolism of diverse xenobiotic agents. However, PB-plus-MC-induced hepatic microsomes from immature male Wistar rats preferentially increased the metabolism of aflatoxin B₁ to aflatoxin M₁ but did not enhance the conversion of aflatoxin B₁ to aflatoxin Q₁. Comparable results were observed with microsomes from rats pretreated with halogenated biphenyls classified as mixed-type inducers. A method to quantitatively determine aflatoxin B₁ (AFB₁) and metabolites employing a reverse phase high performance liquid chromatography (HPLC) protocol was developed. Standards of aflatoxins B₁, M₁, Q₁, P₁, aflatoxicol and AFB₁-dihydrodiol were dissolved in methanol and chromatographed on an octadecylsilane column. Elution was achieved with a linear gradient of 25 to 100% methanol in water over 30 min with UV detection (365 nm). All compounds were resolved at a flow rate of 1 ml/min. Retention times and peak areas were highly reproducible and all compounds gave a linear response. The in vitro effects of various compounds which induce the pregnenolone-16a-carbonitrile (PCN)-inducible form of cytochrome P-450 (P-450p) on the hepatic microsomal metabolism of AFB₁ were also investigated. Treatment of immature male rats with PCN resulted in a 6 fold increase in the formation of aflatoxin Q₁ (AFQ₁). Treatment of mature female rats with PCN resulted in a 16 fold increase in the formation of AFQ₁. The formation of AFQ₁ from AFB₁, was selectively stimulated when liver microsomes from triacetyloleandomycin(TAO)-treated rats were treated with potassium ferricyanide, which dissociates the P-450p-TAO complex. The cytochrome P-450p inducer, dexamethasone, also increased the formation of AFQ₁ as well as enhancing the mutagenicity of AFB₁ to Salmonella typhimurim TA98 and TA100.