Analytical Review: Conversion Factors and Accelerators in the Formation of Thrombin

Prothrombin and fibrinogen represent the fundamental materials in the formation of the solid clot. During the process of blood coagulation, prothrombin is converted to thrombin, and this enzyme, in turn, clots fibrinogen.

The formation of thrombin is the essential step in the clotting of blood. Besides prothrombin, at least five other agents participate in the reaction. They may be divided into: a) conversion factors, agents actively participating and indispensable for the formation of thrombin, at least for physiologic needs; and b) accelerators, which increase the speed of the conversion of prothrombin to thrombin and the yield of this enzyme once the process has been initiated. The first group includes thromboplastin, calcium and a plasma prothrombin conversion factor (PPCF); the second, a serum accelerator and a platelet accelerator. Platelets may also supply a factor which accelerates the formation of fibrin from fibrinogen.

Thrombin, once formed, is able to accelerate its own production (autocatalysis), thus determining an "explosive" formation of fibrin. This mechanism is of great importance in assuring efficient hemostasis. Adsorption of the thrombin on the fibrin clot and neutralization of the enzyme by the natural plasma antithrombin when it is slowly released during the retraction of the clot are two known mechanisms by which the organism protects itself from any undue extension of the intravascular thrombus.