By using a combination of a heterologous antiserum to GPIb/
glycocalicin and a radiolabeled
monoclonal antibody to GPIb/
glycocalicin, we were able to develop a sensitive and specific radioimmunoelectrophoretic assay that can distinguish small amounts of
glycocalicin from GPIb. Normal plasmas were found to contain
glycocalicin, even in samples treated with
protease inhibitors and centrifuged extensively to remove platelets and platelet fragments. Confirmation that the plasma
antigen had a relative molecular weight similar or identical to
glycocalicin was obtained from studies employing gel chromatography and affinity chromatography. An immunoradiometric assay was developed to quantify plasma
glycocalicin, and normal plasma was found to contain approximately 1-3 micrograms/ml. The plasma of a patient with severe
thrombocytopenia due to
aplastic anemia had less than 12.5% of the normal level of
glycocalicin, whereas two patients with
thrombocytopenia due to diseases of increased platelet destruction (
idiopathic thrombocytopenic purpura and
hemolytic-uremic syndrome) had normal levels. Thus, there appears to be ongoing catabolism of platelet GPIb in vivo, and we postulate that the plasma level of
glycocalicin reflects a complex function of factors, including platelet count, platelet turnover, and the site of platelet destruction.