The interaction of bovine factor IX, its activation intermediate, factor IX α, and its activation products, factor IXaα and factor IXaβ, with acidic phospholipid vesicles of various compositions

Abstract

The interactions of bovine factor IX, its activation intermediate, Factor IX.alpha., and its activation products, Factor IXa.alpha. and Factor IXa.beta., with phospholipid vesicles, of mean radius of approx. 30 nm, containing various amounts of phosphatidylserine (PS) and phosphatidylcholine (PC), have been examined. For Factor IX, Factor IX.alpha., Factor IXa.alpha. and Factor IXa.beta., the dissociation constants, at saturating levels of Ca2+, are independent of the PS concentration in the vesicle after levels of 20-30% (w/w) have been reached, and attain minimum values of approx. 1.7, 1.7, 0.7 and 1.0 .mu.M, respectively, with vesicles containing 50% PS. The amount of protein bound per vesicle particle is independent of the PS content, above 20% PS, for Factor IX and Factor IXa.beta., with values of approx. 995-1197 and 1128-1566 molecules/vesicle, respectively. With Factor IX.alpha., a dependence on the amount of protein bound with the content of PS is seen, which ranges from 338 to 619 molecules/vesicle with membranes containing 30-50% PS. For Factor IXa.alpha., no regularity is noted and a range of 583-1083 molecules of protein/vesicle is observed with the systems employed. Examination of the radii of the proteins on the vesicle demonstrates that Factors IX.alpha. and IXa.alpha. occupy considerably more of the surface than do Factors IX and IXa.beta., suggesting that a reason for the decreased number of binding sites for the former two proteins on the vesicle may be related to their greater surface spatial requirements.