antithrombin III

an α₂-globulin of the serpin family synthesized in the liver and found in plasma and various extravascular sites, which inactivates thrombin in a time-dependent irreversible reaction. It also inhibits certain other proteinases with serine active sites, including coagulation factors Xa, XIIa, XIa, and IXa, and kallikrein. Inherited deficiency of the protein, a rare autosomal dominant disorder, is associated with recurrent deep vein thrombosis and pulmonary emboli; the complications are prevented and, in conjunction with heparin, treated with a preparation of antithrombin III from pooled human plasma, administered intravenously.