Novel Forms of Tryptophan Glycoconjugates: Chemical Versus Enzymatic Glycosylation

Abstract

Until recently, investigations on the glycosylation of amino acids during the Maillard reaction, on the formation of advanced glycation endproducts (AGE) and the postranslational modification of proteins mainly concentrated on the amino acids lysine, arginine, asparagine, serine and threonine whereas the significance of tryptophan remained largely unnoticed. While studying occurrence and relevance of novel tryptophan metabolites in biological systems, our attention on tryptophan glycosylation was attracted by reports on mannosylated tryptophan residues in proteins [1-6]. In this contribution, we describe the characterization and structure elucidation of novel tryptophan glycoconjugates resulting from the chemical condensation of tryptophan and aldohexoses and demonstrate their occurrence in food samples [7]. In addition, we report on the identification of tryptophan-N- and -C-glycoconjugates, namely N1-(b-D-glucopyranosyl-4C1)-L-tryptophan and 2-(a -manno-pyranosyl-1C4)-L-tryptophan, formed enzymatically as novel tryptophan metabolites in plants [8] and man [7].