Characterization of rat factors X and Xa: demonstration of factor Xa in rat plasma

We have found that rat plasma corrected the non-activated PT of human normal or factor-X deficient plasma, and the factor Xa-like activity being constantly detected in every 1 ml of blood collected via the cannulated carotid artery of rats. The present study was undertaken to characterize the factor Xa-like activity in rat plasma by preparing rat factor X and a monoclonal antibody against it. Factor X was purified from a BaCl2 eluate of rat plasma by chromatographies on columns of DEAE-Sepharose CL-6B and Sulfate Cellulofine or on a column of Affi-Gel 10 conjugated with a monoclonal antibody against rat factor X. Factor Xa-like activity in rat plasma was eliminated by the treatment of rat plasma with a monoclonal antibody which recognized the heavy chain portions of rat factors X and Xa. A kinetical study demonstrated that rat factor Xa was strongly inhibited by rat antithrombin III, with a Ki of 2.2 x 10(-11) M, in the presence of heparin. However, in the absence of heparin, the second order rate constant for the inhibition of rat factor Xa by rat antithrombin III was 2.6 x 10(4) M-1.min-1, which was one forty-third that for the inhibition of human factor Xa by human antithrombin III. Furthermore, rat factor Xa was resistant to the inhibition by rat alpha-1-antitrypsin and alpha-2-macroglobulin.(ABSTRACT TRUNCATED AT 250 WORDS)