Species-specific metabolism of triphenyl phosphate and its mono-hydroxylated product by human and rat CYP2E1 and the carp ortholog.

Organophosphorus flame retardants (PFRs) are a class of flame retardants and environmental pollutants with various biological effects. Recentstudies have evidenced activation of some PFRs by human CYP enzymes (including CYP2E1) for genotoxic effects. However, the activity of CYPs in fish species toward PFR metabolism remains unclear. This study was aimed on comparing the metabolism of triphenyl phosphate (TPHP) and 4-OH-TPHP in human, rat, and common carp, and the involvement of human CYP2E1 and its orthologs in the metabolism, by using fomepizole (4-MP, CYP2E1 inhibitor) as a modulator, in silico molecular docking and dynamics analyses. The rate of TPHP metabolism was apparently faster with human and rat, microsomes than with fish microsomes, the major metabolites were phosphodiester and hydroxylated phosphate, with 30-80 % of TPHP forming unidentified metabolites in the system of each species. 4-OH-TPHP was readily metabolized by both human and rat microsomes, whereas it was hardly metabolized in carp assays. Meanwhile, with 4-MP the transformation of TPHP to 4-OH-TPHP was enhanced in the human/rat systems while suppressed in the carp system. Moreover, the formation of unidentified metabolites in human and rat systems was mostly inhibited by 4-MP. Through molecular dynamics analysis TPHP and its primary metabolites showed high affinity for human and rat CYP2E1, as well as the carp ortholog (CYP2G1-like enzyme), however, the 4-OH-TPHP bond to the latter was too far from the heme to permit a biochemical reaction. This study suggests that the metabolism/activation of TPHP might be favored in mammals rather than carp, a fish species.