Metabolic activation of 4-ipomeanol by avian tissue microsomes

Abstract

The formation of highly reactive metabolites from 4-ipomeanol, a selective lung toxin in rodents and other mammals and a potent hepatotoxin in birds, was studied in tissues from roosters and Japanese quail in vitro. Consistent with previous in vivo studies on the target organ selectivity for covalent binding and toxicity of 4-ipomeanol metabolites in birds, the rates of reactive metabolite formation were very high in bird liver microsomes compared to avian pulmonary or renal microsomes where this activity was relatively very low or absent. The formation of reactive 4-ipomeanol metabolites in quail lung and liver microsomes was enzyme-mediated and dependent upon cytochrome P-450 monooxygenases. The covalent binding of 4-ipomeanol metabolites to microsomal protein was strongly inhibited by boiling the microsomes, by deleting the NADPH generating system, or by incubation in a CO:O 2 atmosphere. Incubation of quail liver or lung microsomes, NADPH, and reduced glutathione resulted in low levels of covalent binding and in two water-soluble glutathione conjugates which had identical high-pressure liquid chromatographic characteristics to those produced in similar incubations with rat lung or liver microsomes. The K m for 4-ipomeanol covalent binding was nearly fourfold lower in quail liver as compared with quail lung microsomes while the V max was eight times higher in liver than lung; these kinetic results were in sharp contrast to those obtained previously in similar studies with rat microsome preparations, where the pulmonary system was much more active. These results indicate the potential value of in vitro investigations in evaluating biochemical mechanisms underlying target tissue selectivity of toxic damage by xenobiotics acting via a highly reactive metabolite(s) produced in situ.