Molar antithrombin concentration in normal human plasma.

Crude, commercial thrombin preparations and purified bovine thrombin were incubated with normal human reference plasma and the amount of thrombin inactivated was calculated. 1 ml of human plasma inactivated 140-193 NIH U of the various crude thrombin preparations. In the presence of heparin, a lower thrombin-inactivating capacity of plasma was confirmed using crude thrombin, but this phenomenon was less pronounced with the purified thrombin preparation. The molar concentration of the purified bovine thrombin was determined by active site titration. Comparing with protein concentration (A280), this preparation was 92% pure. 1 ml of human plasma inactivated 2.57 mumol of thrombin in the absence of heparin, and 2.50 mumol with heparin. Assuming 1:1 stoichiometry in the thrombin-antithrombin reaction, these results suggest that the concentration of antithrombin in the pooled reference plasma is approximately 2.57 mumol/l or 0.15 g/l.