THE PLANT OXIDASES 227 



tyrosinase is similar to that of other oxidases. Bach 

 (1909) has suggested that it may be quite different, and 

 have effect only on substances which contain a labile atom 

 of hydrogen. 



INHIBITORS OB PARALYZORS OF OXIDASE ACTION. 



It has already been mentioned that many substances 

 lessen or stop the activity of oxidases, as, for example, the 

 majority of acids when in anything more than a very dilute 

 condition. Such a result may be due to a reversible pro- 

 cess, the original activity being restored by removal of the 

 harmful substance, or it may be occasioned by the com- 

 plete destruction of the peroxidase, peroxide, or of both. 

 If only the peroxide is put out of action, it can easily be 

 replaced by hydrogen peroxide, and so the presence of the 

 uninjured peroxidase may be detected. 



In the living cell, however, the activity of oxidases is 

 regulated and co-ordinated with the functions of the proto- 

 plasm in general. This is, in some cases at least, brought 

 about by the production of inhibitors. The nature and 

 mode of action of these substances is not fully understood 

 at present, but it has been shown by Keeble and Armstrong 

 (1912, 3) that the effect of the inhibitor is to paralyze the 

 peroxidase, for when it is removed by suitable means the 

 enzyme resumes its activity. Thus treatment of certain 

 white flowers, which showed no oxidase reactions in the 

 epidermis, with 0-4 per cent, hydrogen cyanide for twenty- 

 four hours, followed by thorough washing with water, 

 resulted in the benzidine being acted upon strongly in the 

 epidermis after the addition of peroxide. These workers 

 also found that a saturated solution of carbon dioxide 

 served to remove inhibitor, but was not so effective. 



It has been stated above that the peroxidase is paralyzed 

 by the inhibitor. It is, however, quite possible that its 



