Thrombin was inactivated by antithrombin-III and the rate of inactivation was accelerated by heparin either in 0.04 M sodium phosphate buffer or in 0.04 M Tris-HCl buffer, at pH 7.4. Calcium chloride, at a final concentration of 1 mM, influenced slightly thrombin inactivation by both antithrombin and antithrombin plus heparin in Tris-HCl buffer, whereas in phosphate buffer the sensitivity of enzyme to antithrombin and heparin decreased. Thrombin showed also increased stability against the inactivating effect of heat at 54°C in sodium phosphate buffer containing calcium chloride. These findings suggest that thrombin bound to calcium phosphate is extremely stable against inactivation by antithrombin and heparin, while its clotting activity does not change. This nature of enzyme may play a role in predisposition for thrombosis during arteriosclerosis.
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