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

Abstract

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 β-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.