GLUTATHIONE S-TRANSFERASE-CATALYZED CONJUGATION OF BIOACTIVATED AFLATOXIN B-1 IN RABBIT LUNG AND LIVER

Aflatoxin B-1 (AFB(1)) requires bioactivation to AFB(1)-8,9-epoxide fo r carcinogenicity, and glutathione S-transferase (GST)-catalyzed conju gation of activated AFB(1) with glutathione (GSH) is a critical determ inant of susceptibility to the mycotoxin. Incubations containing [H-3] AFB(1), 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 AFB(1)-8,9-epoxide. [H- 3]AFB(1)-GSH was isolated by isocratic reverse-phase high-performance liquid chromatography (HPLC) and quantitated by liquid scintillation s pectroscopy, Maximal [H-3]AFB(1)-GSH formation rates were significantl y lower for lung than for liver (0.3 +/- 0.1 and 1.7 +/- 0.4 nmol/mg/h r, respectively). Immunoprecipitation of rabbit pulmonary cytosolic GS Ts with anti-alpha or anti-mu GST antisera decreased [H-3]AFB(1)-GSH p roduction by approximately 45 and 51%, respectively, indicating that a lpha-class and mu-class GSTs are of similar importance in catalyzing t his 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 AFB(1)-8,9-epoxide. In contrast, the pi-class GST app eared to play a negligible role. Using a rat liver microsomal system t o generate both AFB(1) exo- and endo-epoxide isomers, and analysis bas ed on chiral HPLC, we found that rabbit liver cytosolic GSTs catalyzed formation of both AFB(1) exo- and endo-epoxide-GSH conjugates, wherea s pulmonary cytosolic GSTs catalyzed formation of only the exo stereoi somer at detectable levels. Despite a preference for conjugating the m ore mutagenic AFB(1) exo-epoxide isomer, the relatively low capacity f or GST-catalyzed detoxification of bioactivated AFB(1) in lung may be an important factor in the susceptibility of the lung to AFB(1) toxici ty. (C) 1996 Academic Press, Inc.