Summary
A relatively simple method has been developed to evaluate the total amount of antithrombin III in plasma. Since purified and stable bovine thrombin was readily prepared in our laboratory, it was selected to be used in our assay system. Purified α2-macroglobulin was found to inactivate thrombin following a pseudo first-order rate of reaction. However, purified antithrombin III inactivated thrombin following a bimolecular type of reaction. The second-order rate constant for antithrombin III and thrombin interaction was found to be 94 1/M/sec. under these conditions. A least-square straight line was drawn between 15 and 90 minute intervals and the intercept yielded the total amount of antithrombin III expressed NIH thrombin units. This assay system and the graphical analysis yielded the total amount of antithrombin III in plasma quantitatively. The results are reproducible.