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5184790 
Journal Article 
Abstract 
Hydrolysis of triphenyl phosphate and 2-ethylhexyl diphenyl phosphate by human serum enzymes 
Van Den Eede, N; Gomez, AMB; Maho, W; Neels, H; Covaci, A 
2015 
1 
Toxicology Letters
ISSN: 0378-4274
EISSN: 1879-3169 
238 
2 
S121-S121 
English 
WOS:000370693801225 
Question: Phosphate flame retardants (PFRs) are abundant chemicals in house dust but little is known about their biological levels and their relationship with house dust concentrations. These relationships provide insight into major exposure pathways and potential health risks. The correlation between triphenyl phosphate (TPHP) in dust and its metabolite diphenyl phosphate (DPHP) in human urine has not been successfully proved yet. A possible explanation is that DPHP is not a specific biomarker of TPHP, but also of other PFRs, such as 2-ethylhexyl diphenyl phosphate (EHDPP). Since serum enzymes are capable of hydrolyzing organophosphate structures, such as paraoxon (PXN), we compared their activity towards TPHP and EHDPP in production of DPHP.

Methods: We incubated 20 and 50 μM for TPHP and EHDPP each separately with diluted human serum (1:50, v/v) in TRIS buffer (2 mM CaCl2). The assay was based on optimal conditions for paraoxonases (Furlong et al., 1983). 20 μM PXN was used as positive control (metabolite 4-nitrophenol, 4-NP), while negative controls were prepared without serum. Reactions were started by addition of parent compound and quenched by mixing with acetonitrile (1:1, v/v) and internal standard solution. DPHP and 4-NP formation was measured by liquid chromatography-tandem mass spectrometry.

Results: A distinct positive linear correlation was observed for DPHP formation with increasing time or serum concentration. Chemical hydrolysis was measured and subtracted from enzymatic hydrolysis. Maximum DPHP formation was 21 and 9.2 pmol/min/μl serum from EHDPP and TPHP, respectively, compared to 88 pmol 4-NP/min/μl serum.

Conclusion: TPHP and EHDPP were hydrolyzed in serum and this should be taken into account for the determination of biotransformation rates in humans for PBPK modeling. DPHP was more quickly produced from EHDPP than from TPHP, which should be taken into consideration for interpreting DPHP levels in human urine. The possibility of similar PFRs contributing to DPHP formation needs to be investigated for a complete assessment of DPHP as a biomarker of exposure.