Titration of activated bovine Factor X.

p-Nitrophenyl-p’-guanidinobenzoate is a suitable substrate for the titration of purified bovine Factor X which has been activated by a coagulant protein from Russell’s viper venom. Reaction of Factor Xa with p-nitrophenyl-P’-guanidinobenzoate occurs at the site which is responsible for the clotting activity of Factor Xa. Activation of bovine Factor X by the Russell’s viper venom coagulant protein yields a single active site per 53,000 daltons of zymogen.