Summary
Fraction III + IV obtained by a low temperature — low ionic strength fractionation
procedure was found to contain 97 percent of the antithrombin activity of whole plasma.
The mean antithrombin activity in this fraction from 88 normal plasma specimens was
172.6 ± S. D. 19.8 units per ml. Further purification experiments resulted in fraction
III representing 96 per cent of the antithrombin activity in fraction III + IV. There
was no evidence for fibrinogen, prothrombin and antihemophilic globulin contamination
in fraction III. Over 90 per cent of the plasma gamma globulins was present in this
fraction.
The kinetics of the thrombin-antithrombin reaction were investigated. Under the specific
conditions employed, the reaction was found to follow a first order course. The heat
of activation was determined to be 11,300 calories. A standard antithrombin unit is
defined. A quantitative test procedure which is accurate at both high and low concentrations
of the circulating anticoagulant is described and its application on fractions, heat
defibrinated plasma and serum is discussed.