A kinetic study of irreversible enzyme inhibition by an inhibitor that is rendered unstable by enzymic catalysis. The inhibition of polyphenol oxidase by L-cysteine.

A kinetic study of the irreversible inhibition of an enzyme by an inhibitor that is depleted in the medium by its reaction with the product of enzymic analysis was made. The model is illustrated by the study of the inhibition of catecholase activity of polyphenol oxidase by L-cysteine. The inhibition is characterized by an initial lag period followed by a concomitant decrease in enzymic activity expressed when the steady state is reached, both kinetic parameters being modulated by enzyme, substrate and inhibitor concentrations. There is no analytical solution to the non-linear differential-equation system that describes the kinetics of the reaction, and so computer simulations of this dynamic behaviour are presented. The results obtained show that the system here studied presents kinetic co-operativity for a target enzyme that follows the simple Michaelis-Menten mechanism in its action on the substrate.