OXIDASES 149 



various subjects of experiment are narrow and the figure of 

 acid sensitiveness is almost invariable in a particular genus. 



Two kinds of oxidases are generally recognized in the plant 

 world, namely, the direct acting oxidases which turn blue 

 an alcoholic solution of guaiacum without the addition of 

 hydrogen peroxide, and the indirect acting peroxidases which 

 are unable to produce a change in colour until hydrogen per- 

 oxide has been added. 



The direct acting oxidases are supposed to consist of two 

 parts : — 



(a) An autoxidisable thermostable substrate which is 



generally considered to be a derivative of catechol 



OH 

 " v ,OH 



(b) The enzyme proper or peroxidase, which is thermolabile. 



The function of this enzyme (b) is to liberate active oxygen 

 by decomposing the peroxide formed according to (a) in the 

 direct acting oxidase, or by decomposing hydrogen peroxide 

 when added to the indirect oxidase system. 



It is to be remembered that the distinction between the 

 direct and indirect oxidase depends upon the reagent guaiacum ; 

 it may be that the indirect oxidases differ only in strength 

 from the direct oxidases, or, alternatively as has been suggested 

 by Szent Gyorgyi, that the indirect acting enzymes differ 

 from the direct acting in the nature of their substrate which 

 may not be of a catechol nature (see Vol. I., p. 501). 



The mechanism of the " direct " and " indirect " oxidases 

 has been questioned by Pugh and Raper,* but the views of 

 these authors are disputed by Onslow and Robinson. f 



CATALASE. 



Catalase has the power of setting free gaseous oxygen from 

 hydrogen peroxide ; although of common occurrence, it is 

 not universally present in living cells and is often associated 

 with oxidase. For this reason many observations have been 



* Pugh and Raper: " Biochem. J.," 1927. 2I » x 37 - 

 | Onslow and Robinson : id., 1928, 22, 1327. 



