Regulation of factor VIIIa by human activated protein C and protein S: inactivation of cofactor in the intrinsic factor Xase

Factor Villa is a trimer of Al, A2, and A3-C1-C2 subunits. Inactivation of the cofactor by human activated protein C (APC) results from preferential c leavage at Arg336 within the Al subunit, followed by cleavage at Arg562 bis ecting the A2 subunit, In the presence of human protein S, the rate of Ape- dependent factor VIIIa inactivation increased several-fold and correlated w ith an increased rate of cleavage at Arg562, (Active site-modified) factor IXa, blocked cleavage at the A2 site. However, APC-catalyzed inactivation o f factor Villa proceeded at a similar rate independent of factor IXa, consi stent with the location of the preferential cleavage site within the Al sub unit, Addition of protein S failed to increase the rate of cleavage at the A2 site when factor IXa was present. In the presence of factor X, cofactor inactivation was inhibited, due to a reduced rate of cleavage at Arg336, Ho wever, inclusion of protein S restored near original rates of factor Villa inactivation and cleavage at the At site, thus overcoming the factor X-depe ndent protective effect, these results suggest that in the human system, pr otein S stimulates APC-catalyzed factor Villa inactivation by facilitating cleavage of A2 subunit (an effect retarded in the presence of factor IXa), as well as abrogating protective interactions of the cofactor with factor X , (C) 2000 by The American Society of Hematology.