Periodate oxidation and Smith degradation of desialized human erythrocyte glycoproteins lead to an almost complete destruction of galactose and some loss of galactosamine and glucosamine. Neutral sugar and hexosamine content of native MN substances are only slightly influenced by periodate oxidation. Destruction of galactose is accompanied by an uncovering of A-like receptor sites detectable by agglutinins from plants and snails, which are directed against terminal N-acetylgalactosamine. From the specificity of these antibody-like substances, it is deduced that the amino sugar functioning as receptor is α-glycosidically linked. Destruction of the receptor sites by subsequent alkaline borohydride treatment suggests that the N-acetylgalactosamine is unmasked by degradation of the alkali-labile tetrasaccharide of human erythrocyte glycoproteins. Periodate-oxidized and Smith-degraded NANA-free erythrocyte and ovarian cyst glycoproteins treated by acetolysis are potent inhibitors of anti-Tn from man, animals, and plants, suggesting that Tn-antigen is represented by terminal, α-glycosidic, peptide-bound N-acetylgalactosamine. It is concluded that Tn-antigen is probably a cryptic determinant of the alkali-labile tetrasaccharide, which has been isolated from human erythrocyte glycopeptides.

The nature of the receptor sites for several agglutinins is characterized by hemagglutination inhibition assays. The inhibitory activity of human erythrocyte glycoproteins, from which sialic acid, sialic acid and galactose or alkali-labile oligosaccharides have been removed, is compared to the inhibitory effect of compounds with known structure. It is shown that the lectin from Arachis hypogea and anti-T bind to alkali-labile galactosyl-residues. Agglutinins from Bauhiniapurpurea and variegata (non- or N-speciflc), Maclura aurantiaca, Iberis amara, sempervirem, umbellata hybrida and umbellata nana (M- or nonspecific), Moluccella laevis (A- plus N-specific), Helix pomatia, Helix aspersa, Helix lucorum and Caucasotachea atrolabiata interact with alkali-labile V-acetylgalactosamine. The results obtained with the anti-A agglutinins from various snails suggest that human erythrocyte glycoproteins contain, besides the alkali-labile tetrasaccharide, a peptide-linked sialyl-N-acetyl-galactosaminyl-residue. The investigations do not allow a precise definition of the receptor sites for the lectins having M- or N-specificity.

It is shown that M-active glycoproteins react de novo with anti-N lectins from Vicia graminea and Bauhinia purpurea after blocking of free amino and carboxyl groups. The significance of this phenomenon is briefly discussed.