RESUMO
We investigated the inhibitory effects of 13 organophosphate esters (OPEs) and hydrolytic metabolites on the carboxylesterase activity of rat liver microsomes in vitro in order to examine whether there might be a potential impact on human health, and to elucidate the structure activity relationship. Among the test compounds, 2-ethylhexyl diphenyl phosphate (EDPhP) was the most potent inhibitor of carboxylesterase activity, as measured in terms of 4-nitrophenol acetate hydrolase activity, followed by tri-m-cresyl phosphate (TmCP), cresyl diphenyl phosphate (CDPhP) and triphenyl phosphate (TPhP). The IC50 values were as follows: EDPhP (IC50: 0.03 µM) > TmCP (0.4 µM) > CDPhP (0.8 µM) > TPhP (14 µM) > tris(1,3-dichloro-2-propyl) phosphate (17 µM) > tris(2-ethylhexyl) phosphate (77 µM) > tri-n-propyl phosphate (84 µM) > tris(2-chloroethyl) phosphate (104 µM) > tris(2-butoxyethyl) phosphate (124 µM) > tri-n-butyl phosphate (230 µM). The IC50 value of EDPhP was three orders of magnitude lower than that of bis(4-nitrophenyl) phosphate, which is widely used as an inhibitor of carboxylesterase. Trimethyl phosphate, triethyl phosphate and tris(2-chloroisopropyl) phosphate slightly inhibited the carboxylesterase activity; their IC50 values were above 300 µM. Lineweaver-Burk plots indicated that the inhibition by several OPEs was non-competitive. Diphenyl and monophenyl phosphates, which are metabolites of TPhP, showed weaker inhibitory effects than that of TPhP.
