9‑Glutathionyl-6,7-dihydro-1-hydroxymethyl‑5H‑pyrrolizine Is the Major Pyrrolic Glutathione Conjugate of Retronecine-Type Pyrrolizidine Alkaloids in Liver Microsomes and in Rats

Retronecine-, otonecine-, and heliotridine-type pyrrolizidine alkaloids (PAs) are all reported to be hepatotoxic. These PAs are suggested to be metabolized to the corresponding electrophilic dehydropyrrolizidine alkaloids (dehydro-PAs) and subsequently conjugated with macromolecules, such as glutathione (GSH). In the present study, a total of five glutathione conjugates, named M1–M5, were detected in rat and human liver microsomal incubations with three retrornecine-type PAs (isoline, retrorsine, or monocrotaline) in the presence of glutathione, and were chemically synthesized. M1 and M3 were unambiguously identified as a pair of epimers of 7-glutathionyl-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine (7-GSH-DHP), and M4 and M5 were epimers of 7,9-diglutathionyl-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine (7,9-diGSH-DHP). M2, an extremely unstable conjugate, was proposed to be 9-glutathionyl-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine (9-GSH-DHP). It was the most abundant among the five GSH conjugates, and the finding corrects the mistake that 7-GSH-DHP is the predominant GSH conjugate derived from dehydro-PAs. Similar patterns in glutathione conjugate profile were observed in the bile of rats treated with the PAs. This is the first study to describe 9-GSH-DHP as a major pyrrolic GSH conjugate of retronecine-type PAs, providing insight into the interactions of dehydro-PAs with biomolecules.