Differences among five main forms of serum transferrin.
Five forms of human serum transferrin with different isoelectric points were purified by isoelectric focusing in agarose gels. They constitute a set of main transferrin forms in serum of healthy individuals. Their relative proportions are changed in serum from alcoholic individuals, where the more basic of these forms are increased. Tryptic peptides of all forms were compared by high performance liquid chromatography (HPLC) patterns (fingerprint analysis). Two major differences were detected between corresponding tryptic peptides of the five transferrin forms. Some minor differences were also noticed. The corresponding peptides were purified from the main form and analyzed for total composition and amino acid sequence. The peptides with the major differences between the forms were found to be the two with the glycosylation attachment at positions 413 and 611. A minor peptide difference between the transferrin forms contains a product that is explained by incomplete stoichiometry in cleavage of the peptide bond at positions 217-218. Successive loss of sialic acid is compatible with the presence of different transferrin forms, and our results indicate that the main differences between the serum transferrin forms from healthy and alcoholic individuals are in successive changes of the carbohydrate chains attached at positions 413 and 611.