Binding of the organophosphates parathion and paraoxon to bovine and human serum albumin

Binding of parathion and paraoxon to bovine serum albumin (BSA) and human serum albumin (HSA) was studied by using equilibrium dialysis. The concentration of unbound organophosphate was determined from its anticholinesterase activity.Binding of parathion to BSA was shown to be reversible. The organophosphates interact with only one type of binding sites in BSA and HSA. The affinity constants at pH 7.2 and 4° C for the interaction of BSA or HSA and parathion were found to be 2.7×10⁶ and 1.5×10⁶ M^–1, respectively. The affinity constants for the interaction of the serum albumins and paraoxon were considerably lower, 6.0×10³ and 1.6×10⁴ M^–1, respectively. Lowering the pH from 7.2 to 4.8 did not significantly affect the binding parameters. The great difference of affinity of the serum albumins to parathion and paraoxon is discussed with respect to the fate of parathion in the body.