Cooperative binding of the organophosphate paraoxon to the (Na+ + K+)-ATPase.

Paraoxon, the main active metabolite of the organophosphorus insecticide parathion, exerted a dose-dependent inhibitory effect on the activity of pig kidney (Na+ + K+)-ATPase contained in microsomal fraction and purified from it. Substrate kinetics studies revealed the existence of two active sites with high and low affinity to ATP. The Dixon analysis of the mode of the inhibition indicated its noncompetitive character. The purified enzyme was more affected than enzyme contained in the microsomal fraction. The inhibition constant Ki ranged from 73 to 245 microM depending on the type of preparation. The Hill coefficient (n) fulfilled the relationship 1 < n < 3. These properties of the interaction suggest the cooperative binding of paraoxon to the enzyme. An indirect mechanism of the interaction was proposed: paraoxon could inhibit the activity of the (Na+ + K+)-ATPase by excluding the enzyme protein from its normal lipid milieu.