Photosuicide inactivation of acetylcholinesterase by nitrosamine derivatives

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Abstract
Methyl(acetoxymethyl)nitrosamine and methyl(butyroxymethyl)nitrosamine [carcinogens], are, respectively, substrate (Km = 10-2 M) and competitive inhibitor (Ki = 2 .times. 10-3 M) of electric eel acetylcholinesterase (EC 3.1.1.7). Irradiation of an incubation mixture of this enzyme with either nitrosamine leads to an irreversible loss of enzyme activity. The inactivation rates are dependent on photolysis wavelength, light intensity and inhibitor concentration. Experiments where acetylcholinesterase was radioactively labeled by [14C]-methyl(acetoxymethyl)nitrosamine show that the incorporation of 1 mol of radioactive label per active site is sufficient to cause complete enzyme inactivation irrespective of the reaction conditions used. Methyl(acetoxymethyl)nitrosamine shows no affinity for horse serum butyrylcholinesterase (EC 3.1.1.8) while methyl(butyroxymethyl)nitrosamine is a competitive inhibitor (Ki = 2 .times. 10-3 M), but no irreversible inhibition is induced by the action of light. A suicide type of inhibition is possibly responsible for the inactivation of acetylcholinesterase, based on photoactivation of nitrosamines only when associated with an acidic hydrogen of the active site.