Blood coagulation factors X and V adsorbed to phospholipid monolayers and induced surface pressure changes. These proteins also adsorbed to the air-water interface and formed protein surface films. Plots of surface pressure change (Δπ) vs. initial monolayer surface pressure (π0) appeared biphasic for factor X and factor V, indicating two distinct adsorption processes. Surface pressure changes in monolayers spread below the collapse pressure of the respective proteins were characteristic of protein adsorption to the air-water interface while those observed above the protein collapse pressures were consistent with specific protein-acidic phospholipid interactions. Phosphatidylserine-dependent surface pressure changes were very small for both proteins. Factor X induced surface pressure changes required the presence of calcium while factor V induced changes occurred in the presence or absence of calcium. Protein-monolayer binding characteristics were comparable to those obtained by using bilayer vesicles of similar composition and indicated the absence of significant membrane surface curvature effects. The maximum surface concentration corresponded to one bound factor X molecule per 1400 Å2. Comparison of surface pressure changes induced by factor V with those induced by myelin basic protein suggested that the membrane-binding processes of the two proteins involve similar but small degrees of acyl chain perturbation. Thrombin digestion of factor V had no effect on surface pressure change and the isolated 80 000-dalton peptide of factor Va also showed approximately similar surface pressure effects. The vitamin K dependent proteins caused a smaller surface pressure change per bound protein molecule. The results indicate that the prothrombinase proteins associate primarily, if not exclusively, with the head groups of the phospholipids.
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