Quantitation of Binding of Ricinus communis Agglutinin I to Von Willebrand Factor (VWF): Investigation of Relationship with Plasma Clearance of VWF in Type 1 Von Willebrand Disease.

Type 1 von Willebrand disease (VWD) is a heterogeneous bleeding disorder manifest by a partial quantitative deficiency of the sialoglycoprotein von Willebrand factor (VWF). Glycosylation of VWF results in the addition of 12 N-linked and 10 O-linked oligosaccharide chains and modification of these structures has been shown to result in increased plasma clearance of VWF. Exposed galactose residues are required for the binding of VWF to the hepatic asialoglycoprotein receptor. The lectin Ricinus communis agglutinin I (RCA I) has a high affinity for the terminal β-linked galactose residues on VWF. Recent findings have reported increased binding of RCA-I to VWF in a proportion of patients with a partial quantitative deficiency of VWF. We have evaluated the binding of RCA-I to VWF in patients with type 1 VWD and normal controls and investigated its relationship with the plasma clearance of VWF antigen (VWF:Ag) in patients with type 1 VWD. An ELISA was developed to assess binding of biotinylated RCA-I to VWF in 30 patients with type 1 VWD and 17 normal controls. Plasma samples were analysed at baseline in both groups and following desmopressin (DDAVP) infusion in VWD group. VWF:Ag levels were assessed in parallel and RCA-I binding to VWF was expressed as a ratio of RCA-I/VWF:Ag. In the VWD patients VWF:Ag levels were quantitated over 6 hours following the administration of intravenous DDAVP. The half-life of VWF (VWF:Ag t_1/2) was calculated by determining the first-order rate constant for the elimination phase of VWF:Ag.¹ Median VWF:Ag levels in the VWD patients and normal controls were 36 IU/dL (range, 4–50), and 103 IU/dL (64–197) respectively. At baseline, RCA-I lectin was found to bind to VWF with a median RCA-I/VWF:Ag ratio of 0.98 (0.48–1.74) in VWD group as compared to 0.85 (0.69–1.28) in the controls. This represents a significant difference in RCA-I/VWF:Ag ratios between the two groups, p<0.05. Quantitation of RCA-I binding to VWF secreted post DDAVP in VWD group is pending. Median VWF:Ag t_1/2 in the type 1 VWD patients was 4.1 hours (0.92–8.7) representing significantly increased plasma clearance of VWF:Ag in these patients.² There was significant correlation between baseline RCA/VWF:Ag ratio and VWF:Ag t_1/2 in the type 1 VWD patients, r=0.48, p<0.01. This represents significant inverse correlation between RCA-I/VWF binding and plasma clearance of VWF:Ag. There was no correlation between baseline RCA/VWF:Ag ratio and VWF:Ag in either the patient or control groups, p>0.1. The finding of increased binding of RCA-I to VWF in type 1 VWD patients at baseline is suggestive of variation in glycosylation in a proportion of these patients. This could be causative of or contributory towards increased VWF clearance, be otherwise related to the type 1 VWD disease phenotype or may represent normal variability. The inverse correlation found between baseline RCA-I/VWF binding and plasma clearance of VWF:Ag may reflect differences between VWF secreted post-DDAVP and baseline VWF: investigation is ongoing.