Phospholipid-binding properties of bovine factor V and factor Va

Abstract

Factor V and factor Va binding to single bilayer phospholipid vesicles was investigated by light-scattering intensity measurements. This technique allowed measurement of free and phospholipid-bound protein concentrations from which equilibrium constants could be obtained. As controls, the Ca2+-dependent phospholipid binding of prothrombin and factor X were also studied. Average values obtained for Kd and lipid to protein ratio at saturation, mol/mol (n), for prothrombin (Kd = 2.3 .times. 10-6 M, n = 104) and factor X (Kd = 2.5 .times. 10-6 M, n = 46) binding to vesicles containing 25% Folch fraction III and 75% phosphatidylcholine in the presence of 2 mM Ca2+ were in agreement with those reported in the literature. Average factor V and factor Va values for the Kd and lipid to protein ratio at saturation (mol/mol) were Kd = 7.2 .times. 10-8 M and n = 270 for factor V and Kd = 4.4 .times. 10-7 M and n = 76 for factor Va. In contrast to prothrombin and factor X, factor V and factor Va demonstrated Ca2+-independent lipid binding. The number of factor V and factor Va molecules bound per vesicle was dependent on the phosphatidylserine content of the vesicle and the ionic strength of the buffer.