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X concentrate of antithroinbin was made from bovine plasma. When examined with an analytical ultracentrifuge it was found to contain a main component and a sinall ail~ount of more rapidly sediinentating material. For the main component S j , , was 4.01 S. The antithroinbin concentrate destroyed the activity of purified autoprothrombin C, and most likely antithrombin and antiautoprothrombin C are one and the same substance. The antithrombin preparation was limited in its capacity to destroy autoprothrombin C activity, and equilibriunl conditions were reached in 3 to 4 hours. At equilibrium the quantity of autoprothrombin C destroyed \\;as proportional to the antithrombin added to an "excess" of autoprothroinbin C. 'The last traces of the latter activity could only be neutralized with difficulty. 1;nder optimum conditions 1 ml of plasma can neutralize the autoprothrombin C that inight generate froill 27 ml of plasma. About 0.83 ml of oxalated bovine plasma can neutralize the activity in 1 mg of purified autoprothrombin C.

Methods were developed for the isolation of autoprothrombin C, which is the second enzyme obtained from purified bovine prothrombin. In the presence of lipids and standardized conditions 0.35 μg of the purified autoprothrombin C were sufficient for clotting recalcified plasma in 15 seconds. Some physicochemical properties are as follows: S20, w is 2.27S, the diffusion constant 8.4 × 10−7 cm2/second, and the partial specific volume 0.695. The molecular weight from physicochemical measurements was 21,500. All amino acids were found, but only 1 molecule of methionine. On the basis of amino acid composition the molecular weight was found to be 27,000, giving an average of 24,200 for our two determinations. Prothrombin contains sufficient of each amino acid residue to supply autoprothrombin C and thrombin. Autoprothrombin II, however, has only sufficient amino acids for either autoprothrombin C or thrombin, but not both. The purified autoprothrombin C contained 7% carbohydrate (orcinol) and 3.8% hexosamine. It was stable at pH 7.2 for more than a week at room temperature, and longer in subzero glycerol solution. Autoprothrombin C was used to obtain a single precipitin band in agar diffusion plates with antibody to prothrombin. The band also identified with a single plasma antibody.

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