Cholinesterase (CHE) catalyzes the hydrolysis of butyrylthiocholine substrate forming butyrate and thiocholine. The latter reduces 5,5´- mercaptobis-2-nitrobenzoic acid (DMNB) to 5-mercapto-2- nitrobenzoate (5-MNBA), a colored compound.
The reaction is monitored kinetically at 405 nm by the rate of formation of the yellow color produced, proportional to the activity of CHE in the sample.