Structural studies on rabbit transferrin: isolation and characterization of the glycopeptides

Abstract

The structure of rabbit transferrin was investigated with regard to number, size, and composition of the heteropolysaccharide units and their relative location on the polypeptide chain. The composition and molecular weight of the Pronase glycopeptides revealed that rabbit transferrin contains two heteropolysaccharide units, each composed of 2 sialic acid residues, 2 galactose residues, 3 mannose residues, and 4-N-acetylglucosamine residues. The composition and molecular weight of the tryptic glycopeptides further substantiated the existence of two identical heteropolysaccharide units and revealed that both units have identical amino acid residues in the immediate vicinity of the carbohydrate attachment sites to the polypeptide chain, suggesting a sequence homology surrounding the two glycosylation sites. Characterization of the cyanogen bromide fragments from rabbit transferrin indicated that both heteropolysaccharide units are located within a single polypeptide fragment representing approximately one-third of the molecule.

Structure of the tryptic glycopeptide isolated from rabbit transferrin

“…The results are consistent with the structure proposed in Figure 3. Since the stoichiometry of this glycopeptide is 2 mol/mol of protein (Strickland & Hudson, 1978), rabbit transferrin contains two structurally identical glycopeptide segments, both of which are located within a polypeptide region representing approximately one-third of the transferrin molecule (Strickland & Hudson, 1978).…”

“…Transferrin. The tryptic glycopeptides from S-aminoethylated rabbit transferrin were isolated as previously described (Strickland & Hudson, 1978).…”

“…Analytical Procedures. Neutral sugars, amino sugars, amino acids, and sialic acid were quantitated as previously described (Strickland & Hudson, 1978). Ar-Acetylglucosamine released as the result of /3-Ar-acetylhexosaminidase hydrolysis was also quantitated by the Elson-Morgan reaction (Levvy & McAllen, 1959).…”

“…Rabbit transferrin is a single-chain glycoprotein (Hudson et al, 1973) that contains two identical heteropolysaccharide units which are both attached to the chain within the same region, representing approximately one-third of the molecule (Strickland & Hudson, 1978). These units can be isolated in the form of a single tryptic glycopeptide with a stoichiometry of 2 mol of glycopeptide/mol of transferrin, indicating that both units have identical amino acid residues in the immediate vicinity of the carbohydrate-peptide attachment sites.…”

Chemistry and Metabolism of the Transferrins

“…As early as in 1968, it was shown that serum and milk transferrins differed from each other by one sialic acid residue, the milk transferrin containing less sialic acid (Baker et at., 1968). Strickland and Hudson (1978) determined that again there are two identical oligosaccharide chains for each rabbit serotransferrin molecule, and glycopeptides they isolated following tryptic digestion of the protein had molecular weights of near 3000. The structure of the carbohydrate moiety was identical to that of human serotransferrin as determined by Spik et al (1975) (Strickland et al, 1979).…”