Isolation and characterization of the major oligosaccharide of human platelet membrane glycoprotein GPIb.

Treatment of intact human platelets with chymotrypsin released a glycopolypeptide that was shown to be derived from the major membrane glycoprotein, GPIb. The glycopolypeptide contained 59% carbohydrate on a molar basis and was rich in serine, threonine and proline. Almost all the carbohydrate could be released from the glycopolypeptide by treatment with alkali in the presence of NaBH4. The major component (comprising 80% of the released sugar) was purified and shown to be a hexasaccharide containing sialic acid, galactose, N-acetylglucosamine and N-acetylgalactosaminitol in the molar ratios 2:2:1:1. Two possible structures for this hexasaccharide are proposed on the basis of the known biosynthetic pathways of mucus-type glycoproteins. Our data is consistent with the occurrence of an O-glycosidically linked oligosaccharide on one amino acid in four of the glycopolypeptide. These results suggest that glycoprotein Ib can best be described as a membrane-bound mucus-type glycoproteins. Our data are consistent with the occurrence of an O- in the process by which platelets adhere to the exposed subendothelium of damaged blood-vessel walls. The possible role of the glycopolypeptide portion of GPIb in this process was investigated. Neither the major oligosaccharide nor the glycopolypeptide itself inhibited ristocetin-induced platelet agglutination at the concentrations tested. It is suggested that the carbohydrate moieties of GPIb molecules at the cell surface interact to form a barrier to macromolecules. Such a barrier could play a major role in modulating platelet function.