CATALYSIS AND ENZYME ACTION 73 



Thrombin is not present in circulating blood, but is found 

 as thrombogen, which can be converted into thrombin. In 

 order to convert thrombogen into thrombin, two substances 

 are required, namely calcium salts and thrombokinase. The 

 calcium is required in blood coagulation for the formation 

 of thrombin, not for the actual clotting as in milk. 



Thrombokinase is obtained from tissue cells, white blood 

 corpuscles and platelets, and it is not until these are damaged 

 that coagulation occurs. This is why coagulation does not 

 occur in the blood vessels, but only after the blood is shed, 

 when it comes in contact with tissues, or the contained 

 corpuscles break down.* 



In the actual plugging of blood vessels adhesion of blood 

 cells to the edges of the injured vessel helps to stop the bleeding, 

 but that is a process which is outside of the scope of this 

 book. 



ZYMOIDS 



If the enzymes consist of two parts, one which combines 

 with the substrate and the other that produces the reaction, 

 it is possible to stop the reaction by altering the second part 

 and leaving the combining part unchanged. Zymoids are 

 enzymes that have been altered so that not only are they 

 inactive, but they prevent the action of fresh enzyme, f It 

 has been suggested that the zymoids combine with the 

 substrate, thus preventing the enzyme from attacking the 

 substrate, yet do not act upon the substrate itself : in fact 

 they play the role of a dog in the manger. 



Biologically, enzymes are important, as they enable cells 

 to perform reactions which without the presence of enzymes 

 require agencies which destroy all living structures. Tempera- 

 ture and acidity affect the rate of enzyme action, as do also 

 the presence or absence of co-enzymes or anti-enzymes. 



GENERAL REFERENCES 



F. BATTELLI and L. STERN, Ergeb. d. Physiol., 1912, vol. 12, pp. 96-268. 

 W. M. BAYLISS, The Nature of Enzyme Action, 1911. Longmans, Green & Co. 



G. BREDIG : Ergeb. d. Physiol., 1902, vol. i, part i, pp. 134-212. 



B. MOORE : Recent Advances in Physiology and Biochemistry. 1906, pp. 1-195. 



Edited by L. HOI. Arnold. 



P. MORAWITZ : Ergeb. d. Physiol., 1905, vol. 4, p. 307. 

 H. M. VERNON : Intracellular Enzymes, 1908. John Murray. 



* For some recent observations see C. A. Mills, Journ. Biol. Chem., 

 ij2i, vol. 46, pp. 135 and 167. 



f W. M. Bayliss, Arch. d. Sciences liologique, 1904, vol. n, suppl., 

 p. 271. 



