Some observations on the carbohydrate composition of purified transferrin

Five subfractions were isolated on DEAE-cellulose from samples of a commercially available human transferrin preparation and their carbohydrate composition was analysed. Hexosamine, galactose and total hexose were determined in four subfractions and sialic acid in all five. The data obtained indicate that the excess number of electrophoretic bands observed in transferrin from this source is due to the loss of carbohydrates which only affects sialic acid and none of the other sugar types. The availability of the penultimate galactose residues as the terminal residues in the subfractions deficient in sialic acid was also confirmed by a biological test utilizing the rat liver. The reason for the gradual loss of sialic acid from transferrin is unknown. Freezing-thawing and lyophilization did not detectably affect the sialic acid content of purified transferrin. However, free sialic acid did appear in some preparations on storage. It is concluded that similar changes in the carbohydrate composition of other plasma glycoproteins before, during, or after purification can be expected to exert an adverse effect on their usefulness as metabolic tracers.