Summary
The plasma of individuals, hetero- or homozygous for α1-antitrypsin deficiency, contains greatly decreased amounts of antithrombin activity
as assayed against factor Xa. However, heparin stimulation of the residual antithrombin
activity is observed, which is comparable to that of normal plasma. Antithrombins
isolated from both normal and α1-antitrypsin deficient plasma by a simplified procedure are indistinguishable in both
properties and yields. The microheterogeneity observed on isoelectric focusing of
both preparations can be eliminated by treatment with neuraminidase. Neither purified
human antithrombin nor α1-antitrypsin, when assayed against bovine trypsin, is stimulated by heparin. These
results clearly establish the unique natures of antithrombin and α1-antitrypsin and show that about 75% of the antithrombin activity measured in normal
plasma is due to α1-antitrypsin. Estimates of anti thrombin III activity in normal plasma by assays dependent
on enzymatic activity can probably be obtained only in the presence of heparin.