Activated Protein C was found to inhibit Factor Xa initiated clotting of plasma. Activated Protein C did not inhibit prothrombin activation by Factor Xa and Ca2+ or Factor Xa, Ca2+ and lipid. However, activated Protein C did inhibit prothrombin activation by Factor Xa, Ca2+ and Factor Va or Factor Xa, Ca2+, lipid, and Factor Va. Excess Factor Va could reverse the inhibition of prothrombin activation. Incubation of Factor V with activated Protein C had little effect on Factor V activity either in the presence or absence of phospholipid. Preincubation of Factor V with activated Protein C had no effect upon the degree to which Factor V could be activated. When Factor V was activated with thrombin in the presence of activated Protein C, a rapid decline in Factor Va activity was observed. When activated Protein C was incubated with purified Factor Va in the absence of thrombin, a similar rapid decay in Factor Va activity was observed. Activated Protein C catalyzed decay of Factor Va activity was not obligately dependent on the presence of lipid. However, lipid enhanced the rate of inactivation. Analysis of sodium dodecyl sulfate gels of either Factor V or Va treated with activated Protein C indicated that activated Protein C degraded a slow migrating band of the Factor V doublet and that it also was able to degrade both the heavy and light chains of Factor Va. The results indicated that activated Protein C can inhibit Factor Xa initiated clotting by degrading Factor Va. Factor Va could be protected from activated Protein C inactivation by the presence of Factor Xa, suggesting that activated Protein C binds to Factor Va or near the Factor Xa binding site. The specificity of activated Protein C for Factor Va and the ability of Factor Xa to stabilize Factor Va may both play important functions in the regulation of blood coagulation. © 1979.
