Quantitative Aspects of Antithrombin and Heparin in Plasma

¹ From the Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts

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 antithrombin inactivates thrombin and second, a decrease in the capacity of antithrombin to inactivate thrombin by a factor of 1.9 or 2.2, depending on whether or not a small (15%) portion of the antithrombin normally present is insensitive to heparin. It is suggested that isolated ‘heparin cofactors’ originate from normal antithrombin. Saline dilution of plasma produces a profound change in antithrombin capacity as well as in rate of thrombin inactivation. The minimum capacity (35% of normal when referred to 1 ml original plasma) occurs at a 3-fold plasma dilution, about 90% of the original capacity reappearing when a 20-fold dilution is used.