Serine and threonine glycosidic linkages in bovine submaxillary mucin

Gottschalk et. al. (1960, 1961 a, 1961 b, 1963 a, 1963 b) provided evidence for ovine submaxillary mucin that the aspartic and glutamic acids of the peptide core are linked to oligosaccharide side chains mainly by ester bonds. Hashimoto and Pigman (1962, 1963) suggested, after comparisons of the amino acid compositions of mucins from several different animals and of the glycopeptides formed by enzymatic degradation of bovine submaxillary mucin that a glycosidic linkage through the hydroxyl groups of serine and threonine is more probable for this mucin. The presence of such a serine linkage was suggested for chondroitin sulfate by Anderson, Hoffman and Meyer (1968) because a disappearance of serine occurred after treatment with alkali; this was ascribed to a beta-elimination reaction from an O-substituted serine with the creation of a double bond and a loss of serine. In the present work, the reductive cleavage of bovine submaxillary mucin by alkaline sodium borohydride is reported. The premise was that if the hydroxyl groups of serine or threonine are involved in glycosidic linkage, reduction of the double bonds by alkaline borohydride may occur after the beta elimination reaction and that a conversion to alanine or to α-aminobutyric acid would result. In this communication it is reported that treatment of bovine submaxillary mucin with sodium borohydride caused an extensive conversion of serine to alanine and a small conversion of threonine to α-aminobutyric acid.