The role of sialic acid in determining the survival of glycoproteins in the circulation.

Evidence is presented to indicate a generalized role for the terminal sialic acid residues of circulating glycoproteins. Upon injection into rats, all of the desialylated plasma proteins tested, with the exception of transfer& were promptly removed from the circulation and were recovered from the liver. The materials examined included orosomucoid, fetuin, ceruloplasmin, haptoglobin, arz-macroglobulin, thyroglobulin, lactoferrin, and the two gonadotropic hormones, human chorionic gonadotropin and follicle-stimulating hormone. Competitive inhibition of hepatic uptake was demonstrated by the injection of tracer amounts of 64Cu-ceruloplasmin together with substantive amounts of the above-mentioned desialylated proteins or the glycopeptides derived from them. Uptake was not inhibited by the fully sialylated protein or their glycopeptides.