Inhibitory effect of unheated and heated d-glucose, d-fructose and l-cysteine solutions and Maillard reaction product model systems on polyphenoloxidase from apple. II. Kinetic study and mechanism of inhibition

A previous study showed that Maillard reaction products (MRP) derived from equimolar glucose or fructose and l-cysteine model solutions (1 M) were potent inhibitors of polyphenoloxidase (PPO) from apple. In the present paper, a kinetic study was undertaken of the behaviour of a Michaelis–Menten PPO-catalyzed reaction in the presence of MRP heated at 90 °C for various times (2–39 h), using 4-methylcatechol as the substrate. Results obtained revealed that both MRP systems were mixed-type inhibitors, the glucose/cysteine model solution being the most effective one. Activity of preincubated PPO at 0 °C for different times with either heated l-cysteine or MRP was only slightly restored via exhaustive dialysis. Recovery of enzyme activity was more marked when cupric ions were added to the reaction medium. Possible mechanisms of inhibition of PPO activity by MRP are discussed, in relation to chelation of cupric ions.