Inhibitory effect of acephate (N-acetyl O, S-dimethyl thiophosphoramide) on serum cholinesterase--effect of acephate on cholinesterase.

The Lineweaver-Burk plot of the activity of human serum cholinesterase against the concentration of butyrylthiocholineiodide was shown by two intersecting lines. The Hill plot of cholinesterase activity was linear over the entire range of the substrate concentration. The n value, an interaction coefficient, was less than 1.0 (about 0.8). These results suggest that cholinesterase has multiple substrate binding sites. Acephate, one of the organophosphorous insecticides, inhibited the activity of cholinesterase. Acephate at concentration under 1.25 mM (about 230 ppm in serum) did not inhibit the activity of cholinesterase. The minimum concentration of acephate inhibition of cholinesterase activity was at 2.5 mM. An equilibrium constant(K) can be used as an indicator of inhibitory effect on cholinesterase. The serum cholinesterase activity of workers who were exposed to acephate is not affected when the concentration of acephate in serum is less than 200 ppm. This result suggests that the activity of serum cholinesterase is not an accurate indicator of the exposure of the low toxic insecticides, e.g. acephate. The inhibitory effect of acephate on cholinesterase decreased after the incubation with S-9 mixture. This result suggests that a part of acephate is metabolized to inactive substances in the liver.