Relationship of sialic acid to function and in vivo survival of human factor VIII/von Willebrand factor protein.

Abstract

Studies of the functional properties and intravascular survival of normal and desialylated human Factor VIIIlvon Willebrand factor protein have been performed. The purified normal protein, possessing both procoagulant activity and ristecetin-induced platelet-aggregating activity, contains 154 = 15 nmol of sialic acid/mg of protein and 28 f 3 mol of Sk&k acid/m01 of 200,000 molecular weight subunit. Incubation of the normal protein with purified neuraminidase results in a decrease in ristocetin-induced platelet-aggregating activity. This decrease in activity is related to the extent of desialylation and when completely desialylated, only 35% of the ristocetin-induced platelet-aggregating activity of this molecule iB retained. In contrast, the procoagulant activity remains constant. Normal and asialo-Factor VIII/van Willebrand factor were radioiodinated using mild conditions, infused in normal rabbits and the intravascular survival times measured. The normal protein had an apparent circulatory half-life of 240 min compared to 5 min for the asialoprotein. Furthermore, the circulatory clearance of asialo 1251-Factor VIII/van Willebrand factor was accompanied by the quantitative appearance of radiolabel in the liver. Blockade of the reticuloendothelial system had no effect on the clearance rate of the asialoprotein. However, simultaneous infusion of asialo-at-acid glycoprotein, a protein known to bind to hepatic asialoglycoprotein receptors, dramatically extended the intravascular BUrViVd time of asialo 1251Factor VIIIlvon Willebrand factor. The results in this study demonstrate that desialylation of Factor VIII/van Willebrand factor does not affect procoagulant activity, decreases ristocetin-induced platelet-aggregating activity, and as observed with other plasma glycoproteins, facilitates its rapid hepatic clearance. These results have particular significance to recent proposals regarding the molecular basis of von Willebrand’s disease.