Structure of the Ss blood group antigens. I. Isolation of Ss-active glycopeptides and differentiation of the antigens by modification of methionine.

The Ss blood group antigen determinants were found to be associated with the N-terminal tryptic and chymotryptic glycopeptides (residues 1--35 or 1--32) of the Ss sialoglycoprotein from human erythrocyte membranes. The N-terminal portion (residues 1--26) of these peptides is largely identical with that of the MN sialoglycoprotein. Therefore, and since the Ss activity of tryptic glycopeptides was higher than that of chymotryptic fragments, it is concluded that the structural difference between the S and s antigens is located on the C-terminal part (residues 27--32) of these peptides. Chemical modification of sialoglycoproteins by various methods suggests that Glu residue(s) (positions 29 or 28, 31) and possibly alpha-GalNAc-Thr (residue 25) are recognized by anti-S and -s. Carboxymethylation, performic acid, hydrogen peroxide and cyanogen bromide treatment destroy the S antigen, but have no effect on the s receptor. This suggests that the S antigen is determined by a methionyl-residue.