In Vitro Metabolism of the Flame Retardant Triphenyl Phosphate in Chicken Embryonic Hepatocytes and the Importance of the Hydroxylation Pathway

We report for the first time either in vitro or in vivo the phase I hydroxylation and phase II conjugation metabolic pathways of an organophosphate flame retardant, triphenyl phosphate (TPHP), in addition to diphenyl phosphate (DPHP) metabolite formation. Using a chicken embryonic hepatocyte (CEH) assay, TPHP was phase I metabolized to p- and m-hydroxy-TPHP metabolites, which were largely present in the assay medium and cells as phase II conjugates with glucuronic acid. After treatment with β-glucuronidase, deconjugated p-OH-TPHP was present in both the medium and cells at increasing concentrations of 0.073 ± 0.003, 1.95 ± 0.03, and 2.10 ± 0.09 nmol/well at CEH incubation time points of 0, 12, and 36 h, respectively. Similarly, after β-glucuronidase treatment, there were increasing m-OH-TPHP concentrations of 0.0050 ± 0.0005, 0.18 ± 0.01, and 0.18 ± 0.01 nmol/well. p-OH-TPHP at 36 h accounted for 60% of the initial TPHP treatment concentration, which was 3.5- or 12-fold greater than that of the DPHP or m-OH-TPHP metabolites, respectively. Overall, in TPHP-exposed organisms, this study demonstrates the importance of phase I and II metabolic processes in the biological fate of TPHP.