Human platelet membrane receptor for bovine von Willebrand factor (platelet aggregating factor): An integral membrane glycoprotein

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Abstract
The platelet membrane receptor for bovine von Willebrand factor, platelet aggregating factor, has been reported to be a property of a soluble glycoprotein, glycocalicin, that is loosely attached to the platelet surface and represents one of the major glycoproteins of the platelet glycocalyx. In this study fractions from human platelets containing glycocalicin had no bovine von Willebrand factor receptor activity. Only fractions containing platelet membranes had receptor activity. By using a nonionic detergent, Brij 99, active receptor could be solubilized from the membrane. Some quantitation of the intact or solubilized receptor activity was possible because the aggregation curves produced by mixtures of various dilutions of membranes and a constant concentration of standard normal bovine plasma were linear when plotted against the logarithm of the concentration of receptor. The dose-response curve obtained with Brij 99-solubilized membranes was not parallel to that obtained with intact membranes. Lectin-specificity studies of the bovine von Willebrand factor receptor, soluble in Brij 99, demonstrated binding to a wheat germ agglutinin-Sepharose 4B affinity gel but little or no binding to similar affinity gels of concanavalin A or Lens culinaris lectin. By using wheat germ agglutinin-Sepharose 4B as a lectin affinity column, partial purification of the receptor was possible. Stability studies of the receptor in intact membranes showed essentially no loss of activity for at least 6 days when membranes were stored at 4.degree. C in buffers containing 1 mM EDTA. One freezing and thawing cycle resulted in minimal loss of initial activity but the receptor activity of the thawed material was less stable over time than was fresh material. Repeated freezing and thawing destroyed the activity and, once lost, it could not be recovered, even with detergents.