Toxicity Evaluation of Organophosphate Flame Retardants in terms of Cholinesterase Inhibition

摘要

Organophosphate flame retardants (OPF) have been widely used as substitute for polybrominated flame retardants. OPF have been recognized as low level harmful substances based on LD50. However, it is concerned that OPF could cause neurotoxic effects such as cholinesterase inhibition because of the similar chemical structures to organophosphate pesticides. In this study, cholinesterase inhibition of various OPF was measured to assess the neurotoxicity of OPF. The cholinesterase inhibition was measured by the spectrophotometiric method developed by Ellman et al.(1961). Decomposition reaction of butyrylthiocholine chloride (BTCh) by butyrylcholinesterase (BChE) was used for the assessment. Effect of the addition of OPF, namely, tris(2-chloroisopropyl)phosphate (TCIPP), tris(2-chloroethyl) phosphate (TCEP), tris (1,3-dichloro- 2propyl) phosphate (TDCPP), trisphenyl phosphate (TPP), which are commonly used as building materials, on the decomposition rate of BTCh was investigated. BTCh and the OPF were dissolved in phosphate buffer solution (0.1 M, pH 7.4), and DTNB was added as a color reagent. Then, the decomposition reaction was initiated by adding BChE. The dissociation rate was determined by the measuring the absorbance change at 412 nm wavelength. Cholinesterase inhibition was observed for all the OPF used in this study, which indicates these OPF could have neurotoxic effects. The observed order of inhibition was: TCEP ＞ DDVP ＞ TPP ＞ TDCPP ＞ TCIPP. This order is in accordance with the order of toxicity based on LD50, except TDCPP. Although LD50 of TCEP (1230 mg/kg) is much higher than that of DDVP (250 mg/kg), the degree of inhibition of TCEP is almost similar to that of DDVP. This result suggests that the neurotoxicity of the OPF could be underestimated when assessed in terms of LD50. The assessment based on the cholinesterase inhibition is essential for the evaluation of toxicity of OPF, and useful to select OPF.