Binding of selenoprotein P to heparin: Characterization with surface plasmon resonance

The binding of selenoprotein P to glycosaminoglycans using heparin as a mod el compound was studied by surface plasmon resonance. It was found that hep arin contains two binding sites for selenoprotein P, a high-affinity, low-c apacity site (K-d similar to 1 nM) and a low-affinity, high-capacity site ( K-d similar to 140 nM). Binding at both sites is sensitive to pH and ionic strength, and the high-affinity site is abolished by histidine carbethoxyla tion with diethylpyrocarbonate. The pH and salt dependence of binding sugge sts electrostatic interactions with heparin. The concentrations of selenopr otein P in plasma (similar to 50 nM) are sufficiently high to facilitate bi nding of selenoprotein P to proteoglycans on the vascular endothelium, and this may contribute to the formation of a protective barrier against oxidan ts such as peroxynitrite or hydroperoxides.