Inhibitory effects of organophosphate esters on carboxylesterase activity of rat liver microsomes

Area of Science:

• Environmental Chemistry
• Toxicology
• Biochemistry

Background:

• Organophosphate esters (OPEs) are widely used as flame retardants and plasticizers.
• Concerns exist regarding their potential health impacts due to environmental persistence and bioaccumulation.
• Carboxylesterases are crucial enzymes involved in detoxification and metabolism of various xenobiotics.

Purpose of the Study:

• To investigate the in vitro inhibitory effects of 13 OPEs and their metabolites on rat liver carboxylesterase activity.
• To elucidate the structure-activity relationships (SAR) of OPEs concerning carboxylesterase inhibition.
• To assess the potential human health risks associated with OPE exposure through enzyme inhibition.

Main Methods:

• In vitro enzyme inhibition assays using rat liver microsomes.
• Measurement of 4-nitrophenol acetate hydrolase activity to quantify carboxylesterase inhibition.
• Determination of IC50 values for various OPEs and their metabolites.
• Lineweaver-Burk plot analysis to determine inhibition kinetics.

Main Results:

• 2-ethylhexyl diphenyl phosphate (EDPhP) exhibited the most potent inhibition (IC50: 0.03 μM), significantly stronger than bis(4-nitrophenyl) phosphate.
• Tri-m-cresyl phosphate (TmCP), cresyl diphenyl phosphate (CDPhP), and triphenyl phosphate (TPhP) also showed considerable inhibitory effects.
• Inhibition kinetics for several OPEs were determined to be non-competitive.
• Metabolites like diphenyl and monophenyl phosphates displayed weaker inhibition than their parent compound, TPhP.

Conclusions:

• Certain OPEs, particularly EDPhP, are potent inhibitors of carboxylesterase activity.
• The study highlights potential toxicological risks of OPEs, necessitating further investigation into human health impacts.
• Understanding the SAR of OPEs is crucial for designing safer alternatives and assessing environmental risks.