Glutathione S-transferase-catalyzed conjugation of bioactivated aflatoxin B1 in rabbit lung and liver.
Aflatoxin B1 (AFB1) requires bioactivation to AFB1-8,9-epoxide for carcinogenicity, and glutathione S-transferase (GST)-catalyzed conjugation of activated AFB1 with glutathione (GSH) is a critical determinant of susceptibility to the mycotoxin. Incubations containing [3H]AFB1, rabbit liver microsomes, an NADPH-generating system, 1 mM GSH, and GST-containing lung or liver cytosol were performed to assess the abilities of lung and liver GSTs to conjugate AFB1-8,9-epoxide. [3H]AFB1-GSH was isolated by isocratic reverse-phase high-performance liquid chromatography (HPLC) and quantitated by liquid scintillation spectroscopy. Maximal [3H]AFB1-GSH formation rates were significantly lower for lung than for liver (0.3 +/- 0.1 and 1.7 +/- 0.4 nmol/mg/hr, respectively). Immunoprecipitation of rabbit pulmonary cytosolic GSTs with anti-alpha or anti-mu GST antisera decreased [3H]AFB1-GSH production by approximately 45 and 51%, respectively, indicating that alpha-class and mu-class GSTs are of similar importance in catalyzing this reaction in the lung. Because mu-class GSTs comprise only a small proportion of total lung GST content, these enzymes have high specific activity toward AFB1-8,9-epoxide. In contrast, the pi-class GST appeared to play a negligible role. Using a rat liver microsomal system to generate both AFB1 exo- and endoepoxide isomers, and analysis based on chiral HPLC, we found that rabbit liver cytosolic GSTs catalyzed formation of both AFB1 exo- and endo-epoxide-GSH conjugates, whereas pulmonary cytosolic GSTs catalyzed formation of only the exo stereoisomer at detectable levels. Despite a preference for conjugating the more mutagenic AFB1 exo-epoxide isomer, the relatively low capacity for GST-catalyzed detoxification of bioactivated AFB1 in lung may be an important factor in the susceptibility of the lung to AFB1 toxicity.
Department of Pharmacology and Toxicology, Queen's University, Kingston, Ontario, Canada.
R K
RK
Stewart
C J
CJ
Serabjit-Singh
T E
TE
Massey
D016604:Aflatoxin B1 / Q000378:metabolism* / Q000493:pharmacokinetics*; D000818:Animals; D001711:Biotransformation / Q000187:drug effects; D002384:Catalysis; D002851:Chromatography, High Pressure Liquid; D004852:Epoxy Compounds / Q000378:metabolism; D005982:Glutathione Transferase / Q000378:metabolism* / Q000502:physiology*; D015151:Immunoblotting; D008099:Liver / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism*; D008168:Lung / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism*; D008297:Male; D011233:Precipitin Tests; D011817:Rabbits; D051381:Rats
D004852:Epoxy Compounds; D016604:Aflatoxin B1; D005982:Glutathione Transferase