Quantitative aspects of antithrombin and heparin in plasma.

An examination is made of the characteristics of antithrombin and heparin in plasma as described in the literature. Antithrombin action follows the equation -dT/dt = k1T(A0 – T0 + T – k2t) + k2 where T, A and t refer to thrombin, antithrombin and time in minutes, respectively. One unit of antithrombin is considered to inactivate one NIH unit of thrombin. Subscripts refer to initial concentrations. The rate constant k1, = 0.0066 ± 0.0007 refers to an apparent bimolecular reaction between thrombin and antithrombin and the rate constant k2 = 0.008 ± 0.0014 to an apparent zero order thrombin decay. A comparison of the effect observed when heparin is added to plasma with those obtained on an isolated heparin cofactor preparation (Arch. Biochem. & Biophys. 58: 431, 1955) suggests that an independent heparin cofactor does not exist in normal plasma. The action of heparin in plasma may be accounted for quantitatively by two effects. First, an increase by a factor of approximately 20 in the rate at which antithrom...