Abstract

The metabolism of the hepatotoxic otonecine-type pyrrolizidine alkaloid (PA), clivorine, was investigated using rat liver microsomes. The metabolites dehydroretronecine (DHR), 7-glutathionyldehydroretronecine (7-GSH-DHR), 7,9-diglutathionyldehydroretronecine (7,9-diGSH-DHR), and clivoric acid were identified using chromatographic and mass spectrometric analyses. NMR characterizations were also performed on the isolated clivoric acid and the synthetic 7-GSH-DHR and 7,9-diGSH-DHR. The results indicated that the two glutathione (GSH) conjugates were formed by reaction of the unstable toxic pyrrolic ester with GSH added in the microsomal incubation system, whereas DHR was generated from hydrolysis of the unstable pyrrolic ester, and that clivoric acid was produced from all these further conversions of the unstable pyrrolic ester. Furthermore, tissue-bound pyrroles were also determined to be present after microsomal incubation of clivorine. Clivoric acid has not been previously identified, and DHR and 7,9-diGSH-DHR were found, for the first time, as metabolites of an otonecine-type PA, while 7-GSH-DHR was previously reported by us to be a microsomal metabolite of clivorine. The in vitro metabolic pathway of clivorine was delineated to be the initial formation of the unstable pyrrolic ester, which then may undergo hydrolysis, GSH conjugations, or covalent binding with hepatic tissues that may lead to hepatotoxicity. The present definitive identification of four pyrrolic ester-related metabolites of clivorine and indirect determination of bound pyrroles provide the strongest evidence to date to support the hypothesis that the formation of an unstable pyrrolic ester plays a key role in otonecine-type PA-induced hepatotoxicity.