METAL-ION COMPLEXATION BY PRODUCTS OF THE MAILLARD REACTION

Potentiometric proton-liberation experiments were used to study the in teraction between metal ions and a crude Maillard reaction product (MR P) of glucose-glutamate or a pure product of the Maillard reaction, th e Amadori compound fructosyl glycine. The present study is the first t o report values for stability constants for complex formation involvin g products of the Maillard reaction and metal ions. Proton displacemen t suggested binding of Zn2+, Cu2+, Mg2+ and Ca2+ by the glucose-glutam ate MRP. The strength of binding occurred in the order Mg2+ > Cu2+ = C a2+ > Zn2+. The order of binding affinity differs from that expected f or a mononuclear binary system as predicted by the Irving-Williams ser ies which might suggest the presence of two or more distinct ligands i n the MRP.Proton displacement suggested the formation of complexes of fructosyl glycine with zinc but not with calcium. The pK(a) value of f ructosyl glycine was notably higher than that of glycine (3.25 vs 2.23 , respectively), whereas the pK(b) value was reduced (8.93 vs 9.73, re spectively). Such changes in protonation may help to explain the lower affinities for zinc exhibited by fructosyl glycine compared with glyc ine (glycine: log K-1, 5.40; log K-2, 4.47; log K-3, 2.73. Fructosyl g lycine: log K-1, 4.27, log K-2, 3.83; log K-3, 1.92). The results of t he present study may explain some of the effects of Maillard reaction products on mineral homoeostasis in vivo. However, it is likely that t he biokinetics and metabolism of such compounds also play a major role in mediating observed effects. Copyright (C) 1996 Elsevier Science Lt d.