The inactivation of factor Va by the natural anticoagulant, activated protein C (APC) is subject to a number of regulatory mechanisms. This report examines the efficacy of APC in plasma and evaluates the role of the APC cofactor protein S in this milieu. The ability of protein S to enhance the anticoagulant effects of activated protein C was demonstrated using a factor Xa recalcification time of Al(OH)3 adsorbed plasma. At a set concentration of APC, increasing concentrations of protein S resulted in a linear and saturable potentiation of the activity of APC. This result was not reflected in a purified component assay, where the extent of factor Va inactivation by APC was only marginally augmented by protein S. The efficacy of the cofactor was not affected by variations in the concentration of factor Va. Similarly, increasing the protein S:APC molar ratio to 200:1 resulted in only a trivial enhancement of APC activity. To directly examine the proteolysis of factor Va by APC in plasma, a novel assay system containing Al(OH)3 adsorbed, factor V deficient plasma supplemented with purified human factor Va was developed. The addition of APC in varying concentrations to this system consistently yielded factor Va inactivation rates inferior to those seen in a purified component assay. This finding is consistent with the presence of APC inhibitory activity in plasma. When protein S was added to the plasma system, factor Va inactivation by APC was restored to a similar level to that observed in the purified system. A dose-dependent reduction in APC activity was observed when increasing concentrations of Al(OH)3 adsorbed factor V deficient plasma was added to the purified system. This diminution of APC activity was mirrored by a progressive increase in protein S activity when the cofactor was included in the assay. This finding suggests that protein S activity is primarily mediated through interference with the inhibition of APC by an as yet uncharacterized plasma factor.
|Number of pages||5|
|Journal||Blood Coagulation and Fibrinolysis|
|Publication status||Published - 1 Jan 1992|