Functional Properties and in Vivo Survival of Normal and Asialo Human Factor VIII/VON Willebrand Protein

Abstract

Recent evidence suggests that the molecular defect in von Willebrand’s disease resides in the carbohydrate moiety of Factor VIII/von Willebrand factor (FVIII/vWF). In light of this, we have examined and compared certain properties of normal and asialo FVIII/vWF. Purified human FVIII/ vWF was desialylated using a protease-free neuraminidase. At various incubation times, sialic acid released, ristocetin-induced platelet aggregating activity(RPA) and procoagulant activity (PCA) were measured. RPA decreased with increasing amounts of sialic acid released. When completely desialylated (>95%), only 35±10% of the initial RPA was retained. In contrast, PCA remained constant with up to 80-85% sialic acid removed. Normal and asialo FVIII/vWF displayed similar immunological cross-reactivity to normal FVIII/vWF rabbit anti-sera. However, immuno-electrophoretic results clearly indicated a reduced anodic mobility for asialo FVIII/vWF relative to normal. Normal and asialo FVIII/vWF were then radiolabeled with 125I, infused in rabbits and the circulatory survival times measured. Asialo FVIII/vWF was cleared from circulation at a rate 50-fold greater than that observed for normal FVIII/vWF. This clearance was accompanied by quantitative appearance of radioactivity in the liver. In addition, simultaneous infusion of human asialo α1-acid glycoprotein, a protein known to bind to hepatic asialoglycoprotein receptors, competitively inhibited asialo FVIII/vWF clearance. These results indicate that desialylation of FVIII/vWF does not alter PCA, decreases RPA and, as observed with other plasma asialoglycoproteins, facilitates its rapid hepatic clearance. Understanding these properties of asialo FVIII/vWF represents an important initial step towards defining a possible carbohydrate defect as a cause for von Willebrand’s disease.