thrombin

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thrombin:

see blood clottingblood clotting,
process by which the blood coagulates to form solid masses, or clots. In minor injuries, small oval bodies called platelets, or thrombocytes, tend to collect and form plugs in blood vessel openings.
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Thrombin

 

an enzyme of the hydrolase class, a major element in the blood-clotting system of man and animals. It is present in blood in the form of the inactive precursor prothrombin and is activated by prothrombinase, or active thromboplastin. Chemically, thrombin is a glycoprotein with a molecular weight of about 40,000; it contains about 5 percent carbohydrate.

Thrombin was isolated in crystalline form in 1972 by the American physiologist W. Seegers and his associates. It is similar in primary and tertiary structure to the serine proteolytic enzymes such as trypsin. A thrombin molecule consists of two polypeptide chains linked by a disulfide bond. The A chain of bovine thrombin contains 49 amino acid radicals, and the B chain, 265 radicals. The active center of the enzyme, as well as the carbohydrate constituent, are located in the B chain. Thrombin exists in several active forms, which differ in the structure of the B chain.

The main function of thrombin is to convert fibrinogen into fibrin. Thrombin hydrolyzes four arginyl-glycine bonds in the fibrinogen molecule. Four peptides then split off to form a fibrin monomer, which becomes polymerized into the fibrin clot that is the basis of a thrombus. The reactions of limited proteolysis with the participation of thrombin are accompanied by the activation of Factor XIII (fibrin stabilizing factor) and of Factors V and VIII, which take part in the reactions of the internal blood-clotting mechanism. Thrombin aids in platelet aggregation and in clot retraction. A relative excess of thrombin in the body has been shown to automatically initiate the activity of the anticoagu-lating system. This is accompanied by the introduction of heparin and the activator of plasminogen into the bloodstream, substances that help maintain the blood’s liquid state.

Thrombin is inactivated by diisopropyl fluorophosphate, which blocks the hydroxyl group of serine that enters the active center, and by other inhibitors characteristic of the serine proteinase group. Thrombin is inactivated in blood by the plasma antithrombins α2-macroglobulin, antithrombin III, and/or heparin. A specific nonplasma thrombin inhibitor is the polypeptide hirudin, found in the buccal glands of leeches.

Thrombin is used to arrest capillary bleeding by the application of fibrin foam soaked in thrombin to the affected surface.

REFERENCES

Magnusson, S. “Thrombin and Prothrombin.” In The Enzymes, 3rd ed., vol. 3. New York–London, 1971.
“A New Thrombin: Purification, Aminoacid Composition, and Crystallization.” Thrombosis Research, 1972, vol. 1, p. 533.

I. P. BASKOVA

thrombin

[′thräm·bən]
(biochemistry)
An enzyme elaborated from prothrombin in shed blood which induces clotting by converting fibrinogen to fibrin.
References in periodicals archive ?
Factor V Leiden and prothrombin G20210A mutations in young adults with cryptogenic ischemic stroke.
Type and location of venous thromboembolism in carriers of FV Leiden or prothrombin G20210A mutation versus patients with no mutation.
Since both his mother and his maternal grandfather with PGM and TVE, thrombophilia screen was performed, and he was also found to have one copy of the prothrombin G20210A mutation.
Although prothrombin G20210A mutation is found in only 1% to 2% of the general population, its prevalence among those with a history of thromboembolic events is estimated at 5% to 19%.
4,10,20,49-5l) Compared to risks among noncarriers who are not using COCs, the risk of VTE among COC users with the factor V Leiden mutation or the prothrombin G20210A mutation are increased substantially.
A total of 401 patients who had either active thrombosis or recent thrombotic episodes were tested for FVL and prothrombin G20210A mutations between January 1998 and September 1998.
The prevalence of Factor V Leiden and prothrombin G20210A mutations in an Albanian population Total n % Frequency FV 1691A 225 4 1.
The odds ratios (ORs) for VTE associated with values below the 10th percentiles were derived from 2 x 2 tables (univariate OR) or from a logistic regression model that included VTE as the dependent variable and age, sex, hormone treatment, the FV Arg506Gln mutation, and the prothrombin G20210A mutation as independent covariables.
Association of the prothrombin G20210A mutation with factor V Leiden in a Midwestern American population.
In this report, we describe the novel application of the Factor II Invader[R] assay for the genotyping of patient specimens for the prothrombin G20210A mutation.

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