Inhibition of [Na a. K]-ATPase by organophosphorus insecticides

The organophosphorus insecticide bromfenvinios and its methylated homologue methylbrom- fenvinfos inhibited the activity of pig kidney (Na++K+)-ATPase contained in the microsomal fraction and purified from it. The effect was dose-dependent Subrate kinetic studies of the enzyme revealed the existence of two active sites with high and low affinity to ATP respectively. The Dixon analysis of the mode of inhibition indicated its noncompetitive character. The inhibition was more pronouced for bromfenvinfos than for methylbromfenvinfos and the purified enzyme was more affected than the enzyme contained in the microsomal fraction. The Hill plot of inhibition indicated a multisite binding of both insecticides exhibiting cooperativity in binding. The Hill coefficient (n) fulfilled the relationship 1 < n < 3. These properties of the interaction suggest an allosteric nature of the inhibitory action of the insecticides. An indirect mechanism of the interaction was proposed: methylparathion could inhibit the activity of the (Na+ + K+)-ATPase by excluding the enzyme protein from its normal lipid milieu.