Antithrombin is a small molecule that inactivates several enzymes of the coagulation system. Its affinity for these molecules (i.e. its effectivity) is enhanced by heparin.

Function

Antithrombin is a serpin (serine protease inhibitor) that inactivates a number of enzymes from the coagulation system, namely Factor X, Factor IX and Factor II (thrombin).

Role in disease

Hereditary disorders of antithrombin are very rare and generally come to light when a patient suffers recurrent venous thrombosis and pulmonary embolism.

In renal failure, especially nephrotic syndrome, antithrombin is lost in the urine, leading to a higher activity of Factor II and Factor X and in increased tendency to thrombosis.

Genetics

The gene for antithrombin is located on the first chromosome, locus 1q23_q25.1.

Nomenclature

Antithrombin is officially called antithrombin III and is a member of a larger family of antithrombins (numbered I, II etc. to VI). All are serpins. Only AT III (and possibly AT I) is medically significant, with AT III generally referred to as antithrombin.

See further

• Patient's pages on antithrombin (http://www-admin.med.uiuc.edu/hematology/PtAntithrombin.htm)
• OMIM 107300 (http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=107300) (antithrombin deficiency)