title: Stabilization mechanism of prostacyclin by human serum: an approach by binding kinetics using a stable prostaglandin I2 analogue, iloprost.
abstract: We used a gel filtration method and a stable prostaglandin I2 (prostacyclin) analogue, iloprost, to study the kinetics of prostaglandin I2 binding by human serum proteins. Binding equilibrium experiments conducted at physiological prostaglandin I2 concentration (nM) yielded a KD of 10(-9) and a capacity of approx. 50 nM for the serum binding protein(s). Kinetic measurements gave a dissociation rate constant of 10(-3) s-1. When binding equilibrium was established at various ligand concentrations ranging from nM to microM, a result indicating an unsaturable binding was obtained utilizing this method. On the other hand, saturation was achieved with a ligand concentration as high as 50-100 microM by another binding method. A KD of 7 X 10(-5) and a capacity of approx. 600 microM was obtained. This apparent discrepancy was resolved by performing parallel experiments using purified human serum albumin samples and serum. It is concluded that the large quantity of serum albumin, approx. 600 microM, in serum may compensate for its low KD (approx. 10(-5] for prostaglandin I2, thus simulating a binding protein with a KD of 10(-9) and a limited capacity. These data offer direct information regarding how prostaglandin I2 is stabilized by serum and is transported to the platelet prostaglandin I2 receptors. There is a strong implication that serum albumin is the major if not the only protein responsible for binding of prostaglandin I2.
mesh_terms: D001798:Blood Proteins / Q000302:isolation & purification / Q000378:metabolism*; D011464:Epoprostenol / Q000097:blood*; D006801:Humans; D016285:Iloprost; D007700:Kinetics; D011485:Protein Binding
chemical_list: D001798:Blood Proteins; D011464:Epoprostenol; D016285:Iloprost