NEW-TYPE OF LINKAGE BETWEEN A CARBOHYDRATE AND A PROTEIN - C-GLYCOSYLATION OF A SPECIFIC TRYPTOPHAN RESIDUE IN HUMAN RNASE U-S

We report a new type of linkage between a carbohydrate and a protein, involving the rarely modified side chain of a tryptophan residue. An a ldohexopyranosyl residue was found to be linked via a C-C bond to the indole ring of the tryptophan residue at position 7 of human RNase U-s . Mass spectrometric analysis of peptides containing this residue show ed a molecular mass 162 Da higher than that expected for tryptophan. T he fragmentation pattern of the modified amino acid side chain was rem iniscent of that of aromatic C-glycosides, suggesting a direct attachm ent of a hexose residue to a C-position of the tryptophan indole moiet y. H-1 and C-13 NMR spectroscopic data confirmed this inference and un equivocally demonstrated the substituent to be an aldohexopyranosyl re sidue, C-glycosidically linked to the C2 atom of the indole. This mode of attachment differs from the ones known so far, in which carbohydra tes are Linked to an amino acid side chain by N- or O-glycosidic bonds .