218 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



It has been stated by Woods (1902) that the peroxidase 

 from tobacco leaves may be formed afresh within two 

 hours when a boiled solution is allowed to stand. This he 

 attributes to the presence of thermostable zymogens, 

 which yield a new supply of enzyme. It is remarkable that 

 no other workers have as yet been able to find this zymogen, 

 though organic catalysts of oxidation which are moderately 

 thermostable have been shown to exist. 



CLASSIFICATION OF OXIDASES. 



How far the oxidases are specific in their action seems 

 to be in doubt. Five different oxidases at least have been 

 described as occurring, or rather five different classes of 

 oxidases viz., laccases, tyrosinases, alcoholases, purine 

 oxidases, and aldehydases. The laccases or phenolases act 

 on many phenols, and are very widely distributed in plants, 

 The tyrosinases act on tyrosin, or polypeptides from which 

 tyrosin can be split off, to produce a body which further 

 reacts with amino-acids, yielding dark-coloured pigments 

 termed " melanins," as shown by Abderhalden and Gug- 

 genheim (1907, 1908). An example of the alcoholases is 

 furnished by the enzyme found in certain bacteria, which 

 converts ethyl alcohol into acetic acid. The purine 

 oxidases have been extracted so far from animal tissues 

 only, and the existence of specific aldehydases is still 

 hypothetical. 



Researches in plant physiology deal almost entirely with 

 the two classes of enzyme at the head of the list; there is 

 at present no proof that the laccase or tyrosinase from one 

 species is identical with that from another, though they 

 may produce certain colour reactions in common. The 

 part played by inhibitors in bringing about apparent 

 specific action by oxidases will be treated of later on. 



