Studies on the Chemical and Enzymatic Modification of Glycoproteins

A general method is described for the radioactive labeling of proteins containing terminal sialic acid in the carbohydrate moieties. Quantitative conversion of these residues to a radioactive 7-carbon analogue of sialic acid was achieved by sequential periodate oxidation and tritiated borohydride reduction. Evidence is presented to establish the identity of the radioactive derivative as 5-acetamido-3, 5-dideoxy-l-arabino-2-heptulosonic acid and to indicate it as the sole site of tritium incorporation in the carbohydrate chain. Upon injection into rats, labeled preparations of ceruloplasmin and orosomucoid exhibited a normal half-life in the plasma and proved suitable for metabolic studies. The data indicate that the 7-carbon sialic acid analogue mimics sialic acid with respect to hydrolysis by neuraminidase and in regulating survival of glycoproteins in the circulation.