The binding of factor Va to phospholipid vesicles

The analysis of free sulfhydryl groups in factor Va using dithiobis-(nitrobenzoic acid) (DTNB) indicated the presence of one accessible thiol in each of the two subunits of the cofactor. Intact factor Va contained one readily accessible sulfhydryl group under native conditions and approximately two such groups after denaturation. A comparison of the rate of modification of the accessible thiol in factor Va under native conditions to those observed with the isolated subunits indicated that the thiol present in component D of the cofactor was readily accessible to reaction with DTNB. Factor Va was reacted with the sulfhydryl-directed fluorophore N-(1-pyrene)maleimide, resulting in the concomitant loss of the accessible thiol with no detectable alteration in the activity of the cofactor. This fluorescent derivative of factor Va (Pyr-Va) was used to examine the binding of factor Va to phospholipid vesicles by fluorescence polarization. Fluorescence polarization of the pyrene moiety increased saturably when Pyr-Va was titrated with increasing concentrations of vesicles composed of phosphatidylcholine and phosphatidylserine (PS). Systematic analysis of the binding of Pyr-Va to PCPS (75% phosphatidylcholine, 25% PS) indicated that the binding interaction was characterized by a dissociation constant of 2.7 x 10(-9) M with 42 mol of PCPS bound per mol of Va at saturation. The data obtained by varying the PS content of the vesicles are consistent with the interpretation that the Va-combining site on the vesicle surface is composed of a discrete number of PS molecules. The binding of Pyr-Va to PCPS was independent of added calcium ion and could be reversed by the addition of unlabeled Va or isolated component E but not by component D. Analysis of the displacement curves indicated that native factor Va or isolated component E and Pyr-Va mutually excluded each other on the vesicle surface with identical affinities. Competition experiments conducted using component E digested by factor Xa or the isolated derivative peptides indicated that the cleavage of component E by factor Xa had no effect on the PCPS binding properties of this subunit. Further, the data obtained with the isolated peptides suggest that the lipid-binding domain of component E is present in the amino-terminal region of this subunit.