Microsomal Formation of a Pyrrolic Alcohol Glutathione Conjugate of Clivorine: Firm Evidence for the Formation of a Pyrrolic Metabolite of an Otonecine-Type Pyrrolizidine Alkaloid

The formation of the pyrrolic alcohol glutathione (GSH) conjugates of two different types of pyrrolizidine alkaloids (PAs), i.e. clivorine (an otonecine-type PA) and retrorsine (a retronecine-type PA), was investigated with rat microsomes in the presence of GSH. Two GSH conjugates identified as metabolites of retrorsine were the pyrrolic alcohol conjugated with one [7-GSH-dehydroretronecine (DHP)] or two (7,9-diGSH-DHP) molecules of GSH. diGSH-DHP, the less abundant of the two conjugates, had not been previously identified as a metabolite of PAs. In the case of clivorine, 7-GSH-DHP was identified. This is the first unequivocal identification of a pyrrolic metabolite of an otonecine-type PA. Consequently, this study provides the strongest evidence obtained to date to support the hypothesis, suggested >25 years ago, that the mechanism of hepatotoxicity induced by otonecine-type PAs involves key metabolic steps similar to those for retronecine-type PAs.