Studies on glycoproteins XII. About glycosidic-ester linkages in submaxillary gland glycoproteins prepared from sheep

OSM prepared from sheep is known to contain O-glycosidic linkages involving the reducing group of N-acetylgalactosamine and the hydroxyl groups of serine and threonine. It is the aim of this paper to establish whether or not in addition to this linkage another type of alkali-labile linkage, in particular a glycosidic-ester linkage, is present in OSM.

In the first set of experiments it was found that the release of 35.2% of N-acetylgalactosamine, when OSM was kept at pH 8.0 and 42° for 120 h, was matched by the loss of an equimolar amount of serine and threonine without a significant change in the other amino acids. In a second set of experiments it was shown that on treating OSM with 0.1 N NaOH at 4° the decrease of bound hexosamine with time was linear. Finally, glycopeptides prepared by trypsin (EC 3.4.4.4) or pronase action on OSM were treated with LiBH₄ in tetrahydrofuran. No significant difference in the dicarboxylic amino acids before and after such treatment was observed. Only traces of homoserine and α-amino-δ-hydroxy-n-valeric acid were detectable after LiBH₄ reduction.

Since the LiBH₄ treatment of trypsinized OSM prepared from Australian sheep resulted in the loss of 35% of the dicarboxylic acid content, the suggestion is made that the fine structure of the ovine submaxillary glycoproteins may vary with the age of the animals. In Australia, lambs of an average age of 4 months were used for the work; in Europe only sheep of three years or more are slaughtered. Similar differences with age have been described for the fine structure of the chondroitin sulphates composing human hyaline cartilage.