Inhibition by calcium ions of thrombin

Amidolytic activity of thrombin for Boc-Val-Pro-Arg-MCA was inhibited by CaCl2 at relatively high concentrations. Kinetic analysis showed neither competitive nor noncompetitive inhibition by CaCl2 of thrombin. The nonspecific interaction of thrombin with calcium ions prolonged fibrinogen-clotting and neutralization by antithrombin III (AT III) in the absence of heparin. However, calcium ions had no effects on rate of thrombin inactivation in dilute solution and affinity of the enzyme for Sepharose 6B. The inverse effects of calcium ions on the thrombin activities in vitro and on generation of the enzyme in plasma ex vivo depended mostly on the ion concentration.