Cholinesterase inhibition in vitro by OO-diethyl O-p-nitrophenyl thiophosphate (parathion, E 605).

00 Diethyl 0 -p -nitrophenyl thiophosphate (I), generally known as parathion or E 605, is an insecticide which is of value against a variety of crop pests. Its mammalian toxicity is usually ascribed to its power of inhibiting cholinesterase, and this is also possibly its mechanism of insecticidal action. The in vitro inhibition of the enzyme was first studied by DuBois, Doull, Salerno & Coon (1949), who found that with their experimental conditions a concentration of 1-2 x 1O-6"M was necessary to produce 50% inhibition of the enzyme; they therefore regarded it as a strong in vitro inhibitor. Later, Aldridge (1950) showed that a concentration of the same order was required to inhibit red-cell cholinesterase. Hestrin (1949) has described a chemical method for the estimation of acetylcholine, which has been used recently by Wilson & Bergman (1950) to determine cholinesterase activity. We have employed a modification of this method to assess the inhibition produced by samples of parathion and analogous compounds from different sources, and the wide variation in the results obtained has been related to the purity of the samples. of this solution were diluted to 100 ml. with phosphate saline and brought to 370 in a constant temperature bath. Alkaline hydroxylamine reagent. A 20% (w/v) solution of hydroxylamine hydrochloride was prepared and stored in the refrigerator; it was discarded after 2 weeks. When the reagentwas requiredthehydroxylamine hydrochloride solution was mixed with an equal volume of 20 % (w/v) NaOH. Diluted HCI. Conc. HCl diluted with an equal volume of distilled water. FeCl3 reagent. 10% (w/v) Hydrated FeCl3 in 01 N-HCI. All the above solutions, with the exception of the acetylcholine solution, were prepared with analytical quality reagents.