198 CHEMISTRY OF PLANT LIFE 



taken in all cases when dialysis is employed, to see that a possible 

 coenzyme is not separated from an otherwise active preparation. 



An entirely different type of phenomenon is that exhibited by 

 " antienzymes." These are found in the various intestinal worms 

 which live in the digestive tracts of animals; and prevent the diges- 

 tive action of the enzymes of the stomach and intestines upon these 

 worms. Probably similar " antienzymes " are located in the mu- 

 cous linings of the intestinal tract itself, and serve to prevent the 

 auto-digestion of these organs by the active enzymes with which 

 they are almost continually in contact. 



The difference between an antienzyme, which protects material 

 which would otherwise be subject to the attack of an enzyme, and 

 an inhibitor, which renders the enzyme itself inactive, is apparent. 



So far as is known, however, no such substances as antienzymes 

 are present in plant tissues; although the question as to why the 

 proteoclastic enzymes which are elaborated by a given mass of 

 protoplasm do not attack the protoplasm itself, might well be 

 raised. 



ZYMOGENS 



It is apparent that, since enzymes are produced by protoplasm 

 for the special needs of any given moment or stage of development, 

 there must be a preliminary stage, or condition, in which they do 

 not exert their characteristic catalytic effect. When in this 

 stage, the compound is known as " proenzyme," or " zymogen." 

 In this stage, it is inactive, but can be made to exhibit its catalytic 

 effect, usually by bringing it into contact with a suitable activator. 

 When once so activated, however, it cannot be returned again to 

 the inactive state. 



This phenomenon has been studied in connection with the 

 zymogens of the digestive proteases, pepsin and trypsin. Tryp- 

 sixiogen may be rendered active by contact with either calcium 

 salts or with another substance (apparently itself an enzyme) 

 known as enterokinase, which is secreted in the intestinal tract. 



Similarly, proenzymes have been reported as occurring in 

 numerous plant tissues. These proenzymes are believed to be 

 present in the plant cells in the form of definite characteristic 

 granules, which may be observed under the microscope, and which 

 disappear when the enzyme becomes active. Thus, " proinu- 



