Kinetics of tris (1-chloro-2-propyl) phosphate (TCIPP) metabolism in human liver microsomes and serum

Abstract

Tris(1-chloro-2-propyl) phosphate (TCIPP) is an emerging contaminant which is ubiquitous in the indoor and outdoor environment. Moreover, its presence in human body fluids and biota has been evidenced. Since no quantitative data exist on the biotransformation or stability of TCIPP in the human body, we performed an in vitro incubation of TCIPP with human liver microsomes (HLM) and human serum (HS). Two metabolites, namely bis(2-chloro-isopropyl) phosphate (BCIPP) and bis(1-chloro-2-propyl) 1-hydroxy-2-propyl phosphate (BCIPHIPP), were quantified in a kinetic study using HLM or HS (only BCIPP, the hydrolysis product) and LC-MS. The Michaelis-Menten model fitted best the NADPH-dependent formation of BCIPHIPP and BCIPP in HLM, with respective V(MAX) of 154 ± 4 and 1470 ± 110 pmol/min/mg protein and respective apparent K(m) of 80.2 ± 4.4 and 96.1 ± 14.5 μM. Hydrolases, which are naturally present in HLM, were also involved in the production of BCIPP. A HS paraoxonase assay could not detect any BCIPP formation above 38.6 ± 10.8 pmol/min/μL serum. Our data indicate that BCIPP is the major metabolite of TCIPP formed in the liver. To our knowledge, this is the first quantitative assessment of the stability of TCIPP in tissues of humans or any other species. Further research is needed to confirm whether these biotransformation reactions are associated with a decrease or increase in toxicity.

Biotransformation of three phosphate flame retardants and plasticizers in primary human hepatocytes: untargeted metabolite screening and quantitative assessment

“…For BCIPHIPP, the truncated increase in formation at 100 μM TCIPP suggests that the enzymatic capacity for BCIPHIPP production was reaching saturation, which is in line with its observed K m of 80 μM in human liver microsomes (Van den Eede et al, 2015c).…”

“…Remarkably, BCIPP formation in hepatocytes was slightly lower compared to extrapolated human liver microsome data (Table S3), while BCIPHIPP did not show this trend. Possibly, the observed hydrolysis by buffer components in that study influenced the estimated non-linear regression parameters (Van den Eede et al, 2015c).…”

Regional comparison of organophosphate flame retardant (PFR) urinary metabolites and tetrabromobenzoic acid (TBBA) in mother-toddler pairs from California and New Jersey

“…The low detection frequency of BCIPP may be due to the formation of other non-dealkylated TCIPP metabolites, namely BCIPHIPP, as suggested by an in vitro study 14 . However, further work by the same research group showed that BCIPHIPP formation was 10-fold lower than BCIPP in human liver microsomes 15 . Thus, the low detection frequency of BCIPP may be due to the higher detection limits.…”

“…In vitro studies show that PFRs are readily metabolized to their diester metabolites as well as various phase II conjugate metabolites 14, 15 . Predominately detected diester metabolites include diphenyl phosphate (DPHP) and bis(1,3-dichloro-2propyl) phosphate (BDCIPP).…”

“…Predominately detected diester metabolites include diphenyl phosphate (DPHP) and bis(1,3-dichloro-2propyl) phosphate (BDCIPP). As well, recent work has identified conjugates of bis(1-chloro-2-propyl) 1-hydroxy-2-propyl phosphate (BCIPHIPP) as significant metabolites of tris(1-chloro-2-propyl) phosphate (TCIPP) 14, 15 . Further, studies from our group have shown that EH-TBB is metabolized to 2,3,4,5-tetrabromobenzoic acid (TBBA) 16 .…”