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 cleavage at Arg336 within the Al subunit, followed by cleavage at Arg562 bisecting the A2 subunit, In the presence of human protein S, the rate of Ape-dependent factor VIIIa inactivation increased several-fold and correlated with an increased rate of cleavage at Arg562, (Active site-modified) factor IXa, blocked cleavage at the A2 site. However, APC-catalyzed inactivation of factor Villa proceeded at a similar rate independent of factor IXa, consistent with the location of the preferential cleavage site within the Al subunit, 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, However, inclusion of protein S restored near original rates of factor Villa inactivation and cleavage at the At site, thus overcoming the factor X-dependent protective effect, these results suggest that in the human system, protein 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.