FERMENTS, OR ENZYMES 83 



other ways in which the enzyme does not suffer destruction, as is shown 

 by the fact that it resumes its original activities on removal of the 

 products. 



Enzymes, both intracellular and extracellular, are very sensitive to- 

 wards the inorganic composition of the medium in which they are act- 

 ing. For the intracellular enzymes this is what we should expect when 

 we bear in mind the profound influence of inorganic salts on the heart 

 beat and on cell growth and division. This influence of salts and of 

 reaction (acidity, etc.) on the life of the cell is so pronounced as to lead 

 some observers to believe that abnormal cell multiplication in the body, 

 as in the case of tumor formation, is due to changes in the inorganic 

 composition of the tissue fluids. Extracellular enzymes are also very 

 susceptible to the influence of inorganic salts but more especially so 

 towards the reaction of the solution. In terms of modern chemistry 

 we may say that the concentration of H- and OH' ions has a profound 

 influence on the activities of enzymes. Most of the enzymes of the an- 

 imal body perform their action normally in the presence of a slight ex- 

 cess of OH' ions, that is, in faintly alkaline reaction. Indeed the only 

 exception of importance to this is the pepsin of gastric juice, which nor- 

 mally acts in an acid medium. An excess of either OH' or H- ions 

 inhibits the activity of the enzyme and usually destroys it permanently. 

 The activities of enzymes are also influenced by light, many of them 

 being destroyed by sunlight; cells such as microorganisms are similarly 

 affected. 



Before being secreted the digestive enzymes exist in the cells which 

 produce them as inactive precursors called zymogens. The granules seen 

 in resting gland cells are of this nature. The activation of the zymogen, 

 or its conversion into the enzyme, occurs after it has left the cell, and 

 this has been considered as another safeguard to digestion of the cell. 

 Sometimes the activation does not occur until the zymogen has travelled 

 some distance along the gland duct, as in the case of the proteolytic 

 enzyme of pancreatic juice. Till it reaches the intestine, this exists as 

 trypsinogen (the zymogen), but it is here acted on by another enzyme- 

 like body produced by the intestinal epithelium and called enterokinase. 



PHYSICOCHEMICAL REFERENCES 



(Monographs and Original Papers) 



^Bayliss, W. M.: Principles of General Physiology, Longmans, Green & Co., 1915. 

 2 Philip, J. C.: Physical Chemistry, Its Bearing on Biology and Medicine, Arnold, 



ed. 2, 1914. 

 3 McClendon, J. S.: Physical Chemistry of Vital Phenomena, Princeton University 



Press, 1917. 

 Starling, E. H.: Principles of Human Physiology, ed. 2, 1915, Lea and Febiger. 



