Microcalorimetric determination of binding sites of acetylcholinesterase.

Acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7), phosphorylated with dichlorvos, showed relatively more reactivity toward the substrate indophenyl acetate than the enzyme that was carbamylated with carbaryl. When the anionic subsite of the phosphorylated or carbamylated enzyme was alkylated with an aziridinium ion, the reaction velocity toward indophenyl acetate increased in the phosphorylated enzyme but not in the carbamylated enzyme. The organophosphate binding site--which appears to be different from that of carbamate or indophenyl acetate, but probably the same as that of acetylcholine -- is apparently alkylated in such a way that the modified (phosphorylated) enzyme is better-fit for the binding (and hydrolysis) of indophenyl acetate. The modified conformation presumably results in the release of the phosphoryl group from the esteratic subsite.