Abstract

The mechanism of interaction of acetylcholine (ACh) and horse serum cholinesterase was investigated by relaxation time measurements on the high resolution proton magnetic resonance spectrum of ACh. Upon addition of cholinesterase, there is a transient increase in the relaxation rate of the methyl protons of the quaternary ammonium group and of the acetate group of ACh, lasting only a few minutes. After an initial sharp rise, the line width reaches a maximum and then decays exponentially. The time-dependent broadening of the ACh peaks in the presence of horse serum cholinesterase is correlated with the binding to the enzyme. Both the maximal line width and the time of maximum broadening depend on the ACh to enzyme ratio and on the temperature. The higher the enzyme concentration, the faster the reaction and the shorter the time to maximum line broadening. The higher the substrate concentration, the longer the time to maximum line broadening. A rise in temperature from 6° to 45° increases the rate of hydrolysis of ACh as well as the initial rate of interaction between ACh and cholinesterase. No such transient maximum line broadening is seen in the presence of cholinesterase inhibitors, such as eserine, neostigmine, and edrophonium.