Effect of glutathione and Maillard reaction products prepared from glucose or fructose with glutathione on polyphenoloxidase from apple—II. Kinetic study and mechanism of inhibition
Sophie Brun-Mérimée, Catherine Billaud, Loïc Louarme and Jacques Nicolas
Food Chemistry Volume 84, Issue 2 , February 2004, Pages 235-241
2004
บทคัดย่อ
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 efficient model system, with Ki values ranging between 11.6 and 1.1 l 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 cysteine-derived MRPs. MRPs derived from the tripeptide GSH were more potent inhibitors of apple PPO than those prepared from cysteine.