Modelling the effect of superatmospheric oxygen concentrations on in vitro mushroom PPO activity

Abstract

The kinetics of polyphenol oxidase (PPO, EC 1.14.18.1) with respect to oxygen concentrations from 5 to 100% using chlorogenic acid (CGA) as substrate was examined. In vitro mushroom PPO activity was determined by measuring the consumption of oxygen during the oxidation reaction. A differential Michaelis-Menten model was fitted to the obtained total depletion curves. The product concentration as well as the concentration of oxygen had a clear inhibitory effect on the reaction rate. However, the inhibitory effect of oxygen was more evident at low product concentration. A linear mixed inhibition model that considered both the product (oxidised CGA) and oxygen as inhibitors was developed. A model with the product as a competitive inhibitor and oxygen as an uncompetitive inhibitor was the most appropriate to explain the reaction kinetics. The values of the inhibition constants calculated from the model were 0.0032 mmol L-1 for Km (Michaelis-Menten constant related to oxygen), 0.023 mmol L-1 for Kmc (constant for competitive inhibition due to the product), 1.630 mmol L-1 for K mu (constant for uncompetitive inhibition due to oxygen) and 1.77 × 10-4 mmol L-1 s-1 for Vmax (maximum reaction rate). The results indicate that superatmospheric oxygen concentrations could be effective in preventing enzymatic browning by PPO. © 2006 Society of Chemical Industry.