Desialosylation of mammalian and non mammalian red blood cells: Effect on in vivo survival in relation to the number of unmasked T-sites on VCN treated erythrocytes and serum T-agglutinin titers

Specific 13C reductive methylation of glycophorin A. Possible relation of the N-terminal amino acid and the lysine residues to MN blood group specificities

Heterozygous and homozygous glycophorin A were partially and fully reductively methylated with 13C-enriched formaldehyde in the presence of sodium cyanoborohydride. Total reductive methylation modified the five lysine residues (to produce N epsilon,N-[13C]dimethyl lysine) and the N-terminal amino acid residues (N alpha,N-[13C]dimethyl serine and leucine) of glycophorins AM and AN, respectively. 13C-NMR spectra of these species indicated that the 13C-enriched methyl carbons of the five lysyl derivatives all occur at 44.1 ppm downfield from Me4Si. Titration results indicate that the pK alpha of these methylated lysines is greater than 10. The chemical shift equivalent methyl resonances of the 13C-enriched methylated N-terminal Leu derivative were found to occur at 42.8 ppm downfield from Me4Si and exhibited a normal pH titration behavior (pK alpha approximately 7.4). The methyl resonances of the N alpha,N-[13C]dimethyl Ser derivative, on the other hand, were found to exhibit chemical shift nonequivalence, indicating rotational constraints about the C alpha-N bond. The linewidths of the two methyl resonances were also found to be considerably different; this phenomenon could be eliminated by running spectra of the sample (pH approximately 5.0) at elevated temperatures (75 degrees C). This result suggested that for the N alpha,N-[13C]dimethyl Ser derivative of glycophorin AM, hindered rotation must occur about one of the N alpha-13CH3 bonds. This structural difference at the N-terminal residue of glycophorins AM and AN may be related to the MN blood group determinants displayed by these related glycoproteins.

Magnetic resonance study of glycophorin A-containing 13C-enriched methionines

Methionine-81 and/or -8 of the transmembrane sialoglycoprotein, glycophorin A, have been specifically alkylated with 13CH3I to produce the sulfonium ion derivatives [S-[13C]methylmethionine-8]glycophorin A and [S-[13C]methylmethionine-8 and -81]glycophorin A. 13C NMR spectra of these species show that the resonances of the methyl groups of the modified glycophorins occur at 26.1 ppm downfield from Me4Si. A spin-lattice relaxation time of 0.4 s was observed for the 13C-enriched methyl resonances of the sulfonium ion derivatives of Met-8 and -81, which corresponds to an effective correlation time of less than 2 X 10 - 10 s. Demethylation of the 2 glycophorin A sulfonium ion species with 2-mercaptoethanol produces native glycophorin A which now has the epsilon-carbon of the methionine residue(s) 45% isotopically enriched. The epsilon-carbon of Met-8 was found to occur at 15.7 ppm downfield from Me4Si whereas the epsilon-carbon of Met-81 exhibited an unusual chemical shift of 2.0 ppm downfield from Me4Si. The spin-lattice relaxation time of both resonances was found to be approximately 0.3 s.