fibrinogen (redirected from fibrinogen deficiency)

fibrinogen

a soluble protein, a globulin, in blood plasma, converted to fibrin by the action of the enzyme thrombin when blood clots

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fibrinogen [fī′brin·ə·jən]

(biochemistry)

A plasma protein synthesized by the parenchymal cells of the liver; the precursor of fibrin. Also known as factor I.

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Fibrinogen

The major clot-forming substrate in the blood plasma of vertebrates. Though fibrinogen represents a small fraction of plasma proteins (normal human plasma has a fibrinogen content of 2–4 mg/ml of a total of 70 mg protein/ml), its conversion to fibrin causes a gelation which blocks the flow of blood. Upon injury, sufficient amounts of the clotting enzyme, thrombin, are generated in about 5 min clotting time to produce a gel. Although clotting in the circulation (thrombosis) can be extremely dangerous, clotting is an essential and normal response for preventing the loss of blood. Individuals born with the hereditary absence of fibrinogen (afibrinogenemia) suffer from severe bleeding, which can be counteracted by transfusing normal plasma or purified fibrinogen.

Fibrinogen is synthesized by the hepatocytes in the liver, and the synthetic rate can be stimulated by hormones. Significant amounts of carbohydrates become attached to the protein before it is secreted into the circulation; alterations in its carbohydrate composition as found in some liver diseases can give rise to abnormal fibrinogens with defective clotting properties.

Clotting is regulated by two enzymes, thrombin and factor XIII_a (fibrinoligase, activated fibrin-stabilizing factor, transglutaminase). Thrombin exerts a dual control by regulating the rate of fibrin formation as well as producing factor XIII_a. In the plasma milieu, the fibrin molecules readily aggregate into a clot. In order to obtain a clot structure of a strength sufficient to stem bleeding, however, it is necessary for the thrombin-modified factor XIII to be activated to XIII_a. Factor XIII_a acts as a transamidating enzyme which strengthens the fibrin clot by creating cross-links between the molecules. Without such cross-links, a clot structure would be like a brick wall without mortar. Individuals with the hereditary absence of factor XIII often suffer from severe bleeding, even though their clotting times are in the normal range. See Blood, Immunoglobulin