Kinetics of inhibition of acetylcholinesterase in the presence of acetonitrile.

The hydrolysis of acetylthiocholine by acetylcholinesterase from Electrophorus electricus was investigated in the presence of the inhibitors tacrine, gallamine and compound 1. The interaction of the enzyme with the substrate and the inhibitors was characterized by the parameters K(I), alpha', b or beta, K(m) and V(max), which were determined directly and simultaneously from nonlinear Michaelis-Menten plots. Tacrine was shown to act as a mixed-type inhibitor with a strong noncompetitive component (alpha' approximately 1) and to completely block deacylation of the acyl-enzyme. In contrast, acetylcholinesterase inhibition by gallamine followed the 'steric blockade hypothesis', i.e. only substrate association to as well as substrate/product dissociation from the active site were reduced in the presence of the inhibitor. The relative efficiency of the acetylcholinesterase-gallamine complex for the catalysis of substrate conversion was determined to be 1.7-25% of that of the free enzyme. Substrate hydrolysis and the inhibition of acetylcholinesterase were also investigated in the presence of 6% acetonitrile, and a competitive pseudo-inhibition was observed for acetonitrile (K(I) = 0.25 m). The interaction of acetylcholinesterase with acetonitrile and tacrine or gallamine resulted in a seven- to 10-fold increase in the K(I) values, whereas the principal mode of inhibition was not affected by the organic solvent. The determination of the inhibitory parameters of compound 1 in the presence of acetonitrile revealed that the substance acts as a hyperbolic mixed-type inhibitor of acetylcholinesterase. The complex formed by the enzyme and the inhibitor still catalysed product formation with 8.7-9.6% relative efficiency.