Factor Va-factor Xa interaction. Effects of phospholipid vesicles of varying composition

Abstract

The interaction between [human] factor Xa and factor Va was investigated both in solution and in the presence of phospholipid vesicles with varying contents of phosphatidylserine. The binding parameters were inferred from the kinetics of prothrombin activation. Factor Xa and factor Va form in solution an equimolar complex with a Kd of 3.3 .times. 10-9 M. Phospholipid veiscles promote the fomration of the factor Xa-Va complex. The Kd of complex formation is dependent on both the phospholipid concentration and the composition of the phospholipid vesicle. For the interaction betwewen factor Xa and factor Va in the presence of phospholipid vesicles containing 40 mol % dioleoylphosphatidylserine (DOPS) and 60 mol % dioleoylphosphatidylcholine (DOPC), the Kd increases linearly with increasing phospholipid concentration. In the presence of 10 .mu.M phospholipid (DOPS/DOPC, 40/60 mol/mol), Kd was 3 .times. 10-11 M. When the mole percentage of DOPS in the phospholipid vesicles is lowered from 20-5 mol %, there is a gradual increase of the Kd. In the presence of 10 .mu.M phospholipid vesicles containing 5 mol % DOPS and 95 mol % DOPC, Kd was 2.8 .times. 10-10 M. The Kd measured in the presence of phospholipid vesicles containing 5 mol % DOPS and 95 mol % DOPC is independent of the phospholipid concentration. Two models are discussed that can quantitatively explain the effect of phospholipid vesicles on the complex formation between factor Xa and factor Va. Studies on the effect of the polypeptides with MW 80,000 and MW 94,000 of which factor Va is composed on the Kd of the factor Xa-Va complex suggest that factor Xa binding to factor Va requires a Ca2+-mediated interaction between the 2 polypeptides.