Summary
The inhibitory capacity of antithrombin III (AT III) was measured by a quantitative
method independent of the velocity of inhibition. When AT III was in excess of thrombin
in plasma or in purified system the capacity of inhibitor decreased quantitatively
in proportion to the amount of thrombin neutralized. Heparin present in reaction together
with thrombin invariably induced a more extensive utilization of inhibitor than thrombin
alone. The extent of this additional loss of inhibitory capacity was to a limited
degree related to the concentration of heparin. Heparin itself was neutralized in
thrombin-AT III reaction losing its anticoagulant property in proportion to the amount
of thrombin bound by inhibitor. This quantitative neutralization of heparin occurred
not only when the anticoagulant participated in thrombin-AT III binding but also when
heparin was added to a medium containing a preformed thrombin-AT III complex. These
results suggest that acceleration of binding and increased utilization of binding
capacity are the two regular effects of heparin on thrombin-involving reactions of
AT III. Both of these effects may be abolished by quantitative binding of heparin
to thrombin-AT III complex.
