STRUCTURAL STUDY OF THE SUGAR CHAINS OF CD36 PURIFIED FROM BOVINE MAMMARY EPITHELIAL-CELLS - OCCURRENCE OF NOVEL HYBRID-TYPE SUGAR CHAINS CONTAINING THE NEU5AC-ALPHA-2-]6GALNAC-BETA-1-]4GLCNAC AND THE MAN-ALPHA-1-]2MAN-ALPHA-1-]3MAN-ALPHA-1-]6MAN GROUPS

CD36 is a glycoprotein included in the bovine milk fat globule membrane derived from mammary secretory epithelial cells during lactation. Asparagine-linked sugar chains were quantitatively released from CD36 as oligosaccharides by hydrazinolysis. These sugar chains were converted to radioactive oligosaccharides by reduction with (NaBH4)-H-3 and separated into neutral and acidic fractions by paper electrophoresis. Most of the acidic oligosaccharides were converted to neutral ones by sialidase digestion, indicating that they are sialyl derivatives. The neutral and sialidase-treated acidic oligosaccharides were fractionated by Bio-Gel P-4 column chromatography in combination with serial chromatography on immobilized lectin columns including a Wistaria floribunda agglutinin (WFA)-agarose column. WFA is known to bind oligosaccharides terminating with either an alpha- or beta-N-acetylgalactosamine residue. Structural studies of oligosaccharides in each fraction by sequential exoglycosidase digestion as well as methylation analysis revealed that CD36 contains high mannose-type, hybrid-type, and bi-, tri-, and tetraantennary complex-type sugar chains. A portion of the hybrid-type and the complex-type sugar chains which bound to a WFA-agarose column (28% of all oligosaccharides) contained the GalNAcbeta1-->4GlcNAc group(s) instead of the Galbeta1-->4GlcNAc group(s) in their outer chain moieties. Like oligosaccharides found in human luteinizing hormone [Weisshaar, G., Hiyama, J., Renwick, A. G., & Nimtz, M. (1991) Eur. J. Biochem. 195, 257-268], some of the GalNAcbeta1-->4GlcNAc groups found in the CD36 oligosaccharides were sialylated as the Neu5Acalpha2-->6GalNAc group. Furthermore, most of the hybrid-type sugar chains of CD36 with the Gal/GalNAcbeta1-->4GlcNAcbeta1-->2 outer chain on their Manalpha1-->3 arm contained an unusual Manalpha1-->2Manalpha1-->3 group on their Manalpha1-->6 arm.