Site-specificity of butyrylcholinesterase alkylation with N,N-dimethyl-2-phenylaziridinium ion

Abstract The complete kinetic analysis of butyrylcholinesterase alkylation with N , N -dimethyl-2-phenylaziridinium has been made in the presence of three alkylammonium ions and three alkylboronic acids. Alkylammonium ions protect the enzyme against labelling and the protection efficiency is in good agreement with their potency to inhibit the butyrylcholinesterase-catalyzed acetylthiocholine hydrolysis. The alkylboronic acids do not protect the enzyme against the labelling reaction, pointing to the fact that the functional groups of the esteratic site are not involved in the alkylation process. At the same time, the reactivity of the alkylated enzyme in reaction with [ 3 H]diisopropylfluorophosphate is 25 000-fold lower than the reactivity of the native enzyme. Therefore, the alkylation reaction seems to result in some essential alterations in the structure of the whole active center, affecting the reactivity of the enzyme in reaction with non-ionic substrates and quasisubstrates.