Aims: 

The toxic effects of organophosphorus compounds are primarily based on inhibition of acetylcholinesterase enzyme which participates in the transfer of impulse in central synapses of the cholinergic nervous system through a chemical mediator, acetylcholine. At the same time, organophosphates and their metabolites are extruded from the organism by the kidney through the secretory pathway of the organic anions. This secretion is active and depends on the activity of tubular ATPases, mainly Na⁺/K⁺-ATPase. The aim of this work was to investigate the influence of organophosphate chlorpyrifos (O,O-diethyl-O-3,5,6-trichlor-2-pyridinyl-thiophoshate) on ATP hydrolysis catalyzed by Na⁺/K⁺-ATPase.

Methods: 

The Na⁺/K⁺-ATPase activity was assayed in the absence (control) and presence of inhibitor (within the range 1 × 10^-8 - 1 × 10^-3mol/l) in standard medium containing 50mM Tris-HCl (pH 7.4), 100mM NaCl, 20mM KCl, 5mM MgCl₂, 2mM ATP and 25mg enzyme in a final volume of 200ml. Incubation mixtures were preincubated for 10min at 37°C in the presence of organophosphate chlorpyrifos or distilled water (control). The inorganic orthophosphate (P_i) liberated from the hydrolysis of ATP, was measured using modified spectrophotometric procedure.

Results: 

The increasing concentrations of chlorpyrifos induced inhibition of enzymatic activity in a concentration-dependent manner. Dependence of the Na⁺/K⁺ATPase activity, expressed as the percent of control value, on the inhibitor concentration fitted a sigmoidal function. The half-maximum inhibitory concentration (IC₅₀ values): (1.3 ± 0.1) × 10^-4mol/l, was obtained from the inhibition curve.

Conclusions: 

Chlorpyrifos, beside specific inhibition of acetylcholinesterase, alters activity of Na⁺/K⁺-ATPase, plasma membrane enzyme essential for functioning and homeostasis of almost all animal cells.