Activation of bovine factor X in the presence of calcium, magnesium, barium or manganese ion
Yue, R.H.; Gertler, M.M.
Thrombosis and Haemostasis 40(2): 358-367
1978
ISSN/ISBN: 0340-6245 PMID: 734635 Document Number: 132411
The binding of divalent metal ions to bovine factor X, factor Xa and the coagulant protein in Russell's viper venom was studied by the technique of fluorescence quenching. Titration of factor X with Ca2+, Mg2+ or Ba2+ revealed that these metal ions can bind to factor X. A tightly binding site(s) was observed with Kd of 79 and 98 .mu.M for Ca2+ and Mg2+, respectively. A loosely binding site(s) was evident with Kd of 0.55, 0.50 and 0.35 mM for Ca2+, Mg2+ and Ba2+, respectively. The quenching phenomenon was also observed when Mn2+ was used as titrant but factor X precipitated out when the concentration of Mn2+ was 10 mM. The binding of Ca2+, Mg2+, Ba2+ or Mn2+ to bovine factor Xa or to the purified coagulant fraction of Russell's viper venom was very weak in each case. In the absence of Ca2+, the coagulation fraction of Russell's viper venom could not activate bovine factor X. Activation of factor X was achieved when Ca2+ was replaced by Mg2+, Ba2+ or Mn2+. When the concentration of these ions were 5 mM, the efficiency of factor Xa generation was estimated to be: Ca2+ > Mg2+ > Ba2+ > Mn2+. Higher concentration of Mg2+, Ba2+ or Mn2+ retarded the activation process. Ca2+, Mg2+, Ba2+ or Mn2+ had little or no influence on the esterase activity of factor Xa or purified Russell's viper venom. Complexation of divalent metal ion with factor X is apparently prerequisite in the activation process. The binding of Mg2+, Ba2+ or Mn2+ to these loosely binding sites might have altered the geometrical configuration and the electrostatic environment on factor X. It is more difficult to form the binary complex and a slower generation of factor Xa results. Therefore, divalent metal ion serves as a dual role in the activation of factor X to factor Xa depending upon the ionic concentration.