Slow detection reaction can mimic initial inhibition of an enzymic reaction.

An analysis of sigmoid-shaped progress curves in the reaction between Electric Eel acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7, AChE) and its substrate acetylthiocholine in low concentrations at pH 7 is presented. In order to be able to explain an initial apparent inhibition of the enzyme-substrate reaction, the rate of detection reaction had to be taken into account. The theoretical curves obtained by the fitting of differential equations for the reaction mechanism to the data of six progress curves simultaneously, exactly reproduce the course of the experimental curves. The measurements performed with various concentrations of detection reagent confirm the proposed cause of sigmoidity.