Tryptophan glycoconjugates in food and human urine

Abstract

Evaluating the formation of tryptophan glycoconjugates other than well-established Amadori rearrangement products, HPLC-tandem MS (MS/MS) analysis of human urine collected from several healthy individuals proved the presence of one distinct tryptophan C-glycosyl compound [Horiuchi, Yonekawa, Iwahara, Kanno, Kurihara and Fujise (1994) J. Biochem. (Tokyo) 115, 362-366]. After isolation, unambiguous identification of this novel tryptophan metabolite as 2-(α-mannopyranosyl)-L-tryptophan was achieved by tandem MS combined with NMR spectroscopy including homonuclear COSY, heteronuclear multiple-bond connectivity and 1H-detected heteronuclear multiple-quantum coherence experiments. Remarkably, a thorough evaluation of vicinal proton-proton coupling constants in different solvents and nuclear Overhauser effect experiments demonstrate the predominant axial orientation of the hydroxymethyl group of the hexopyranosyl residue. Likewise this spatial arrangement indicates that the respective α-anomeric C-mannosylhexopyranose is preferentially adopting a 1C4 conformation in acidic methanol. Whereas only one distinct tryptophan mannoconjugate could be observed in human urine, HPLC-MS/MS analysis of food samples for the first time led to the identification of numerous N1-(β-D-hexopyranosyl)-L-tryptophan, 2-(β-D-hexopyranosyl)-L-tryptophan and 1-(1,2,3,4,5-p e n t a h y d r o x y p e n t - 1 - yl) - 1, 2, 3, 4 - t e t r a h y d r o - β - c a r b o l i n e - 3 -carboxylic acid derivatives derived from the condensation of tryptophan with aldohexoses. Taking into consideration the significant differences between profiles and configurations of tryptophan glycoconjugates originating from dietary sources and human urine, C-2 mannosylation of tryptophan residues [de Beer, Vliegenthart, Loeffler and Hofsteenge (1995) Biochemistry 34, 11785-11789] represents a novel enzymic pathway in tryptophan metabolism in humans.