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An inactive plasma protein precursor of thrombin. Also known as factor II; thrombinogen.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(also factor II), in man and animals, a protein present in the blood plasma that is the primary component of the blood coagulation system.

Chemically, prothrombin is a glycoprotein composed of approximately 12 percent carbohydrates; the protein part of the molecule is in the form of one polypeptide chain. The molecular weight of prothrombin is approximately 70,000. Prothrombin is the precursor of the enzyme thrombin, which stimulates the formation of thrombi.

In an organism, prothrombin is activated into thrombin by prothrombinase, which is made up of phospholipid, Ca2+ ions, and coagulation factors V (accelerator globulin) and X (thrombokinase, autoprothrombin C). Factor X is a proteolytic enzyme and the active source of prothrombinase. Upon its conversion to thrombin, the prothrombin molecule loses approximately one-half of its former weight and three-quarters of its carbohydrate content.

The biosynthesis of prothrombin occurs in the cells of the liver and is regulated by vitamin K, which is produced by intestinal flora. In instances of vitamin K deficiency, the level of prothrombin in the blood drops below the normal level of 10 mg percent, often resulting in a tendency to bleed, which can manifest itself in early childhood hemorrhaging, obstructive jaundice, and certain diseases of the liver. It is believed that vitamin K deficiency causes the biosynthesis of anomalous molecules of prothrombin and reduces the ability of prothrombin to convert into thrombin. In medical practice, the prothrombin level is measured to determine the characteristics of an individual’s blood coagulation system (prothrombin index).


Magnusson, S. “Thrombin and Prothrombin.” In The Enzymes, 3rd ed., vol. 3. New York-London, 1971.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.