Maillard reaction products (MRPs) prepared from aqueous equimolar glucose or fructose with glutathione (GSH) model solutions
(0.25 M), when heated at 90 C for 15–39 h, were previously recognized as strong apple PPO inhibitors. This paper reports the
inhibition mode of the purified o-diphenoloxidase activity from apple, using 4-methylcatechol as the substrate. Assuming a reversible
inhibition, Lineweaver-Burk plotting showed that both MRPs were mixed-type inhibitors, glucose/GSH being the most effi-
cient model system, with Ki values ranging between 11.6 and 1.1 ml MRPs, according to the heating treatment and the sugar tested.
Enzyme inactivation took place during pre-incubation (0–180 min) of PPO with various MRPs at 0 C, and was only partly
reversed by exhaustive dialysis or gel filtration. Activity initially lost was also partly restored when cupric ions (CuSO4) were added
to the reaction medium, suggesting a chelating effect of copper ion at the active site of PPO, as already observed with cysteinederived
MRPs. MRPs derived from the tripeptide GSH were more potent inhibitors of apple PPO than those prepared from
cysteine