title: Optimization of a mechanized amidolytic factor-X assay; influence of reaction conditions and reagents.
abstract: We describe the effect of pH and NaCl concentration on the activation of factor X in plasma by Russell's viper venom and on the amidolytic activity of the activated enzyme towards factor Xa-sensitive chromogenic substrates. An increasing NaCl concentration results in a decrease in the activation rate of factor X by Russell's viper venom, whereas in this step no pH effect is observed. Increasing NaCl concentrations decrease the Km and Vmax for both the factor Xa-sensitive chromogenic substrates, S 2222 and S 2337. Km values were lowest between pH 7.8 and 8.6; Vmax increased with increasing pH. Comparison of NaCl and KCl in the activation step as well as in the amidolytic step shows that the observed effect is specific for Na+, not just an effect of ionic strength. No difference was detected between activation by the crude venom or its purified factor X-activating enzyme. Also, values with the substrates S 2222 or S 2337 were the same in the mechanized amidolytic factor X assay.
mesh_terms: D000925:Anticoagulants / Q000627:therapeutic use; D003124:Colorimetry / Q000379:methods; D005170:Factor X / Q000032:analysis*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D007700:Kinetics; D012016:Reference Values; D014757:Viper Venoms
chemical_list: D000925:Anticoagulants; D007202:Indicators and Reagents; D014757:Viper Venoms; D005170:Factor X