PLATELET VON WILLEBRAND FACTOR IN NORMALS AND PATIENTS WITH VON WILLEBRAND'S DISEASE

Abstract

Platelet von Willebrand factor was compared to plasma von Willebrand factor in normal subjects and in patients with various forms of von Willebrand's disease. von Willebrand factor activity, antigen level and multimeric structure of the von Willebrand factor were examined. In normals, platelet von Willebrand factor had similar activity to plasma von Willebrand factor but had lower antigen levels resulting in a higher ratio of von Willebrand factor activity to antigen. Platelet von Willebrand factor contained larger multimers than plasma von Willebrand factor and demonstrated a different multimeric organization. Patients with von Willebrand's disease exhibited considerable heterogeneity in the activity, antigen content and multimeric structure of their platelet von Willebrand factor depending upon the subtype of the disease. Of particular interest was the observation that nine patients with type I von Willebrand's disease and dominant inheritance who had low plasma von Willebrand factor activity and antigen levels but normal to increased platelet von Willebrand factor activity and antigen levels had normal bleeding times. Type I patients with low plasma and platelet von Willebrand factor activity and antigen levels had prolonged bleeding times. This suggested that platelet von Willebrand factor could play an important role in primary hemostasis at the site of vascular injury. Patients with recessive inheritance had no detectable plasma or platelet von Willebrand factor activity or antigen. Four patients with type ha von Willebrand's disease had relatively normal von Willebrand factor antigen levels with decreased von Willebrand factor activity levels and ratios of activity:antigen in both their plasma and platelets. In addition, the multimeric structure of von Willebrand factor intheir platelets lacked the largest and intermediate sized multimers. When blood was drawn into anticoagulant with protease inhibitors added to it, the abnormal multimeric structure of the plasma von Willebrand factor was partially corrected in 3 of 4 patients studied. Type IIb patients studied had decreased ratios of plasma von Willebrand factor activity to antigen. All of them had normal platelet von Willebrand factor antigen levels. Four of six patients had reduced von Willebrand factor activity and thus reduced ratios of activity to antigen. These patients had prolonged bleeding times despite normal or high platelet von Willebrand factor antigen levels. Plasma von Willebrand factor multimeric structure showed varying degrees of abnormality in that the largest and/or intermediate multimers were absent, but the platelet von Willebrand factor multimeric structures were normal. Unlike the plasma and platelet von Willebrand factor of type ha patients, the addition of protease inhibitors did not correct the abnormal plasma multimeric pattern of the type IIb patients. In 11 normal subjects, platelets were separated into subpopulations by buoyant density. von Willebrand factor activity, antigen level, multimeric structure and protein content of each fraction was examined. The fraction of platelets with the highest buoyant density (fraction 4) tended to have higher activity, antigen level and protein content; the platelets with the least buoyant density (fraction 1) had lower values of the same paramenters. Since the fraction 3 and fraction 4 subpopulations tended to contain larger, younger platelets in it, this suggested a progressive decrease in von Willebrand factor activity and antigen as platelets aged. Platelet and plasma von Willebrand factor subjected to digestion by various enzymes in an effort to reproduce the degraded multimeric pattern seen in type ha and type In patients yielded the following results.Carboxypeptidase A had no effect on platelet or plasma von Willebrand factor. When elastase or non-specific protease was incubated with normal von Willebrand factor, the von Willebrand factor was almost completely digested within 4 hours. However, the resulting multimeric structure did not resemble that of type ha or type IIb patients. Erythrocyte or platelet calciumdependent protease incubated with normal plasma von Willebrand factor resulted in degradation and a pattern similar to that of type ha patients. The addition of EDTA or inhibitors directed against the calcium-dependent protease inhibited this degradation.