Interaction of prothrombin and its fragments with monolayers containing phosphatidylserine. 1. Binding of prothrombin and its fragment I to phosphatidylserine-containing monolayers.

The adsorption isotherms of prothrombin and its fragment I on phosphatidylserine monolayers and on mixed monolayers of phosphatidylcholine and phosphatidylserine were determined by measuring surface radioactivity emanating from the tritium-labeled absorbed proteins at 0.1 N NaCl and between 0 and 10 mM Ca2+. The proteins were absorbed from very dilute solutions, about 10 times more than in previous investigations on bilayer vesicles. The binding constants as obtained from the Scatchard plots were between 3 X 10(6) and 3 X 10(8) mol/L, depending on the experimental conditions. These values are between 2 and 50 times larger, respectively, than the binding constants obtained on bilayer vesicles. Prothrombin absorbs appreciably also in the absence of Ca2+. The significance of these results is discussed.