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On the Mechanism of Erythropoietin-induced Differentiation

XIII. THE ROLE OF SIALIC ACID IN ERYTHROPOIETIN ACTION

From the ¹ From The Franklin McLean Memorial Research Institute and the Department of Biochemistry, University of Chicago, Chicago, Illinois 60637

Erythropoietin, a glycoprotein that induces normal erythrocyte development, has 16 to 18 sialic acid residues per mole. Desialation results in complete loss of biological activity when it is assayed in vivo. When the assay is done in vitro asialoerythropoietin has full activity, or when assayed at low levels of hormones is about three times more active than the native hormone. The loss of activity can be explained by the hepatic removal of asialoglycoproteins from the circulation (Morell, A. G., Irvine, R. A., Sternlieb, I., Scheinberg, I. H., and Ashwell, G. (1968) J. Biol. Chem. 243, 155). Asialoorosomucoid or stachyose injected into assay animals act as competitors and permit 25 to 35% of the original activity of erythropoietin to be seen in vivo after desialation. Treatment of asialoerythropoietin with galactose oxidase results in recovery of 45% of the original activity, confirming an earlier finding (Lukowsky, W. A., and Painter, R. H. (1972) Can. J. Biochem. 50, 909). Asialoerythropoietin is more sensitive to heat denaturation and trypsin action than the native hormone. Our data indicate that terminal sialic acid is required for chemical and biological stability but not for action on target cells of the bone marrow. The asialohormone has enhanced activity towards the target cells.

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