The role of β-hydroxyamino acids in the Maillard reaction—transamination route to Amadori products

Abstract

Analyses of degradation products of β-hydroxyamino acids—l-serine and l-threonine have indicated that the formation of 2-ketoacids and 2-amino alcohols such as pyruvic acid and ethanolamine constitutes a major degradation pathway. Simple mechanisms of decarboxylations and dehydrations can explain their formation in these systems as confirmed by labeling studies. Interestingly, the amino carbonyl interaction between the resulting pyruvic acid and the amino ethanol can lead to the formation of an Amadori product identical in structure to that formed from alanine and glycolaldehyde, indicating the existence of another route to this important Maillard intermediate. In this study, using variously [13C]-labeled pyruvic acids, amino ethanol and alanine, detailed investigation of this interaction was carried out. The initial imine formed during this reaction undergoes an isomerization through a 1,3-prototropic shift followed by enolization and formation of an Amadori product. Aternatively, the isomerized imine can undergo hydrolysis to produce an amino acid and an α-hydroxyaldehyde. The results of these labeling studies have also confirmed the formation of pyruvic acid in the reaction mixture of alanine/glycolaldehyde and the presence of alanine and glycolaldehyde in pyruvic acid/ethanolamine mixture, consistent with the proposed mechanism. It can be concluded therefore that in addition to sugar/amino acid mixtures, 2-ketoacid and α-amino alcohol interaction can also lead to the formation of Amadori product and initiation of Maillard reaction through transamination process.