J. Biol. Chem., Vol. 261, Issue 19, 8695-8702, 07, 1986
Analysis of the generation and inhibition of activated coagulation factor X in pure systems and in human plasma
J Jesty
The overall generation and inhibition of human factor Xa have been studied
in pure systems and plasma to determine the kinetic characteristics of
inhibition during factor Xa generation. Generation curves were measured
amidolytically in a pure system containing factor X and antithrombin, which
was activated with the factor X-activating enzyme of Russell's viper venom
(RVV-X). The measured change in factor Xa level with time was fitted to a
3-parameter 2-exponential model to determine apparent first-order rates of
inhibition. With antithrombin at 4.5 microM, the inhibition rate constant
thus obtained was very close to the known rate of inhibition of exogenous
enzyme. Factor Xa generation curves were also analyzed in plasma; however,
to reduce interference in the assay of thrombin, congenitally prothrombin-
deficient plasma was used containing 0.5 microM D-Phe-Pro-Arg-
chloromethylketone. In plasma, factor Xa generated in the presence of
phospholipid and Ca2+ ions by RVV-X, factor IXa, or tissue factor was
inhibited more slowly than exogenous enzyme. The reduction was particularly
severe with tissue factor activation, where the rate was 0.04-0.06 min-1.
This protection by tissue factor was also observed in pure systems and
apparently required factor VII.