Pyrrolizidine alkaloids (PAs) are
phytochemicals present in more than 6000 plant species worldwide; about half of the PAs are hepatotoxic, genotoxic, and carcinogenic. Because of their wide exposure and carcinogenicity, the International Programme on Chemical Safety (IPCS) concluded that PAs are a threat to human health and safety. We recently determined that PA-induced liver
tumor initiation is mediated by a set of four (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5 H-pyrrolizine (DHP)-
DNA adducts and proposed that these DHP-
DNA adducts are
biomarkers of PA exposure and liver
tumor initiation. To validate the generality of this metabolic activation pathway and DHP-
DNA adducts as
biomarkers, it is significant to identify reactive metabolites associated with this metabolic activation pathway. Segall et al. ( Segall et al. ( 1984 )
Drug Metab. Dispos. 12 , 68 - 71 ) previously reported that
1-formyl-7-hydroxy-6,7-dihydro-5 H-pyrrolizine (1-CHO-DHP) is generated from the metabolism of
senecionine by mouse liver microsomes. In the present study, we examined the metabolism of seven hepatocarcinogenic PAs (
senecionine,
intermedine,
retrorsine,
riddelliine, DHR,
heliotrine, and
senkirkine) and one noncarcinogenic PA (
platyphylline) by human, rat, and mouse liver microsomes.
1-CHO-DHP was identified as a common metabolite from the metabolism of these hepatotoxic PAs, but not from
platyphylline. Incubation of
1-CHO-DHP with HepG2 and A549 cells produced the same set of DHP-
DNA adducts, which were identified by both LC/MS MRM mode and selected ion monitoring analyses through comparison to synthetic standards. In the incubation medium of
1-CHO-DHP treated HepG2 cells, both DHP and
7-cysteine-DHP were formed, which were capable of binding to cellular
DNA to produce DHP-
DNA adducts. These results suggest that
1-CHO-DHP is a proximate
DNA metabolite of genotoxic and carcinogenic PAs.