A KINETIC-MODEL FOR THE REACTION OF TRYPTOPHAN WITH GLUCOSE AND MANNOSE - THE ROLE OF DIGLYCATION IN THE MAILLARD REACTION

The kinetics of the reaction of tryptophan with excess mannose and glucose were studied at two temperatures (110-degrees-C and 140-degrees-C) under identical conditions. The rate of disappearance of tryptophan and the rate of accumulation of the Amadori rearrangement product (ARP) formed were followed by RP-HPLC, over a period of 6 h. Since both sugars produce the same ARP, any changes in the kinetic parameters of the reaction can be attributed to the differences in the sugars, such as mutarotation rates and contents of open-chain forms. Kinetic analysis indicated that the first-order rate constant for the disappearance of tryptophan in the mannose system was 1.3 times that of glucose at 110-degrees-C, and 1.8 times that at 140-degrees-C. However, the rate of accumulation of ARP was generally much lower in the mannose system than in the glucose system. The differences in the rates of tryptophan disappearance can be attributed to the higher percentage of the acyclic form in mannose and its higher rate of mutarotation; the differences in the rate of ARP accumulation can be explained by the ARP formed in the mannose/tryptophan system reacting further with mannose to produce the diglycated Amadori product (diARP), whereas in the glucose/tryptophan system glucose reacts with the ARP at a much slower rate. It seems that the diglycated amino acid provides a more efficient pathway for browning.