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

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. The studies reported here, however, demonstrate that fractions from human platelets containing glycocalicin have no bovine von Willebrand factor receptor activity. Instead, only fractions containing platelet membranes have receptor activity. By using a nonionic detergent, Brij 99, active receptor can be solubilized from the membrane. Some quantitation of the intact or solubilized receptor activity is possible because the aggregation curves produced by mixtures of various dilutions of membranes and a constant concentration of standard normal bovine plasma are linear when plotted against the logarithm of the concentration of receptor. The dose-response curve obtained with Brij 99-solubilized membranes is not parallel to that obtained with intact membranes. Lectin-specificity studies of the bovine von Willebrand factor receptor, soluble in Brij 99, demonstrate 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 is possible. Stability studies of the receptor in intact membranes show essentially no loss of activity for at least 6 days when membranes are stored at 4 degrees C in buffers containing 1 mM EDTA. One freezing and thawing cycle results in minimal loss of initial activity but the receptor activity of the thawed material is less stable over time than is fresh material. Repeated freezing and thawing destroys the activity and, once lost, it can not be recovered, even with detergents.