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Abstract
A method has been developed to determine the dissociation constant Kd and the phosphorylation constant k 2 of an irreversible organophosphorus inhibitor of cholinesterase. The method consists in recording the progress curve of the enzymatic hydrolysis of a substrate in the presence of the inhibitor. The use of a very low substrate concentration limits the competitive action of the substrate and renders the method more sensitive to inhibition, particularly to the binding step. The method is more general than the early method of Hart and O'Brien1 because it is not limited by the extent of substrate hydrolysis. The method has been used to study the anticholinesteratic activity of the anticancer drug triethylene-thiophosphoramide (thio-TEPA). The results are discussed with respect to the chemical reactivity of this compound.