Blood group MN-dependent difference in degree of galactosylation of O-glycans of glycophorin A is restricted to the GalNAc residues located on amino acid residues 2-4 of the glycophorin polypeptide chain.

Glycophorin A (GPA) of human erythrocytes contains a minor number of unsubstituted GalNAc residues (Tn receptors) which are recognized by Moluccella laevis lectin (MLL). The lectin reacts better with blood group N- than M-type of GPA which suggests a higher number of Tn receptors in GPA-N than in GPA-M. To find out whether this difference is restricted to a defined domain of GPA, the N-terminal tryptic glycopeptides of GPA-M and GPA-N (a.a. residues 1-39) and their fragments obtained by degradation with CNBr (a.a. residues 1-8 and 9-39) were analyzed. The untreated and desialylated glycopeptides were tested as inhibitors of MLL in ELISA, and the content of GalNAc-ol was determined in the products of beta-elimination of the asialoglycopeptides by gas-liquid chromatography/mass spectrometry. The asialoglycopeptides 1-39 and 1-8 derived from GPA-N showed about 2 and 4 times higher content of non-galactosylated GalNAc residues, respectively, and higher reactivity with MLL than their counterparts derived from GPA-M, while asialoglycopeptides 9-39 of GPA-M and GPA-N did not show such differences. These results demonstrate that higher expression of non-galactosylated GalNAc in GPA-N than in GPA-M is confined to GalNAc residues located in the amino-terminal portion of GPA polypeptide chain, between the blood group M- and N-specific amino acid residues 1 and 5.