296 ANIMAL BIOCHEMISTRY 



blood. They are also important in maintaining proper water balance 

 between blood and tissues. Serum albumin, because of its lower 

 molecular weight, is especially effective in this respect. When food 

 proteins are lacking, serum proteins appear to serve as a protein 

 reserve from which essential amino acids can be drawn for use by 

 other body tissues. Blood plasma, preserved by lyophilization, can be 

 reconstituted by the addition of sterile water and used to replenish 

 blood in the treatment of shock, burns, and injury. Prompt adminis- 

 tration of plasma saved many lives during World War II. 



6/ood clotting. Although the fibrinogen content of blood is much 

 smaller than that of the other plasma proteins (0.2 to 0.4 per cent 

 compared with 4 to 5 per cent albumin and 2 to 3 per cent globulin), 

 it plays a most important role in the organism. When a blood vessel 

 is damaged, or when blood is drawn from a vessel, clotting or coagu- 

 lation occurs within a very short time. Fundamentally, the formation 

 of this clot is caused by the creation of stringy, insoluble fibrin from 

 the soluble fibrinogen of the plasma. The stringy, jelly-like fibrin 

 enmeshes the corpuscles, forming the clot and preventing excessive 

 hemorrhage from small wounds. The importance of this process to 

 the well-being of the animal is demonstrated by its absence in so-called 

 bleeders, individuals who lack the factors necessary for normal blood- 

 clot formation. This hereditary condition is called hemophilia. Clot 

 formation is the final step in a complicated series of reactions which 

 is not completely defined. In normal in vivo blood, fibrinogen remains 

 in solution unless contamination with juices from damaged cells 

 occurs. Coagulation may also be prevented by precipitation of calcium 

 ions as the insoluble calcium oxalate. 



Production of fibrin is the immediate cause of clotting. The soluble 

 fibrinogen is hydrolyzed into smaller, insoluble fibrin protein and 

 several small peptides, a reaction catalyzed by thrombin, a proteolytic 

 enzyme. The enzyme thrombin is not present in normal blood, rather, 

 it exists as prothrombin, an inactive precursor which can be converted 

 into the active form through the action of thromboplastin. This 

 reaction is also thought to be a proteolytic hydrolysis, yielding the 

 thrombin plus a small peptide. Thromboplastin is apparently released 

 from disintegrating platelets or other sources such as damaged brain 

 tissue. 



Blood contains an accelerator for the clotting process, known as 

 accelerator globin or AC-globin. If one sets up a model system contain- 

 ing prothrombin, calcium, thromboplastin, and fibrinogen, the velocity 

 of clotting is slow, but addition of a small amount of serum globulin 

 accelerates the process by stimulating the conversion of prothrombin 



