Antithrombin III

Antithrombin III (AT III) is a small protein molecule that plays a critical role in the regulation of blood coagulation. It is a type of serpin (serine protease inhibitor) that inhibits several enzymes in the coagulation system, most notably thrombin and Factor Xa. The inhibition of these enzymes is crucial for preventing the formation of inappropriate blood clots (thrombosis) and ensuring the fluidity of the blood within the circulatory system.

Function

Antithrombin III acts as a major anticoagulant in the body by binding to and inactivating several enzymes of the coagulation cascade. Its primary targets are thrombin and Factor Xa. Thrombin is a key enzyme that converts fibrinogen into fibrin, the strands that weave together to form a blood clot. Factor Xa plays a pivotal role earlier in the coagulation cascade, catalyzing the conversion of prothrombin to thrombin. By inhibiting these enzymes, AT III helps to regulate blood clot formation and prevent the development of potentially harmful clots.

Genetics

The gene for antithrombin III is located on chromosome 1 (1q23-q25.1). Mutations in this gene can lead to antithrombin III deficiency, which is associated with an increased risk of venous thrombosis. There are two types of antithrombin deficiency: Type I, characterized by a quantitative deficiency of the protein, and Type II, characterized by a qualitative deficiency (the protein is produced but is dysfunctional).

Clinical Significance

Antithrombin III deficiency is a rare, inherited disorder that significantly increases the risk of developing abnormal blood clots, such as deep vein thrombosis (DVT) and pulmonary embolism (PE). This condition can be acquired or hereditary, with the hereditary form being more prevalent. Treatment for antithrombin III deficiency typically involves anticoagulant medications to reduce the risk of clotting. In some cases, antithrombin III concentrates may be administered to increase the levels of this protein in the blood, especially during high-risk situations like surgery or childbirth.

Diagnosis

The diagnosis of antithrombin III deficiency involves a series of blood tests to measure the activity and level of antithrombin in the blood. These tests include the antithrombin activity test, which assesses the ability of antithrombin to inhibit thrombin, and the antithrombin antigen test, which measures the amount of antithrombin protein present.

Treatment

Treatment for antithrombin III deficiency is primarily focused on preventing thrombotic events. This may involve the use of anticoagulant drugs such as heparin or warfarin, which help to reduce the blood's tendency to clot. In acute situations or when anticoagulants are contraindicated, antithrombin III concentrates may be administered to temporarily increase the levels of antithrombin in the blood.

See Also

• Coagulation
• Thrombosis
• Protein C
• Protein S

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