CHAPTER XII. 

 PHYSIOLOGICAL ACTIVITIES (Continued). 



PRODUCTION OF ENZYMES. 



Most of the physiological activities of bacteria which 

 have been discussed are due to the action of these peculiar 

 substances, so that a knowledge of their properties is essen- 

 tial. This knowledge cannot as yet be exact because no 

 enzyme has, up to the present, been obtained in a "pure 

 state," though it must be admitted that there are no certain 

 criteria which will enable this "pure state" to be recognized. ' 

 It was formerly thought that they were protein in nature, 

 but very "pure" and active enzymes have been prepared 

 which did not give the characteristic protein reactions, so 

 this idea must be abandoned. That they are large mole- 

 culed colloidal substances closely related to the proteins in 

 many respects must still be maintained. There are certain 

 characteristics which belong to enzymes, though no one of 

 them exclusively. These may be enumerated as follows: 



1. Enzymes are dead organic chemical substances. 



Dead is used in the sense of non-living, never having 

 lived, not in the sense of "ceased to be alive." 



2. They are always produced by liiiing cells: 

 Sometimes as active enzymes, sometimes as pro-enzymes 



or zymogens which are converted into enzymes outside the 

 cell by acids, other inorganic substances or other enzymes. 



3. They produce very great chemical changes without 

 themselves being appreciably affected. 



Enzymes will not continue to act indefinitely, but are 

 used up in the process (combination with products?) . The 

 amount of change is so great in proportion to the amount of 

 enzyme that the above statement is justified in the rela- 

 tive sense- Thus a milk-curdling enzyme has been prepared 



