98 RESPIRATION 



material containing so-called oxidases but also by material 

 which at present only gives the indirect action. If this 

 substance were known, the distinction between peroxidase 

 and oxidase would no longer be necessary, as indeed it would 

 seem even now to be, inasmuch as this distinction is entirely 

 dependent upon the external reagent, guaiacum, employed 

 and disappears when another reagent is used. Moreover, 

 if in order to activate a peroxidase it is necessary to add 

 hydrogen peroxide, since the plant itself does not supply 

 the necessary peroxide, unanswerable questions arise regarding 

 the employment of these enzymes in those plants which con- 

 tain only peroxidases. 



It would, in fact, appear to be more likely that the difference 

 between oxidase and peroxidase is one of degree rather than 

 of kind, and that the so-called peroxidase is really only a 

 slightly less powerful oxidizing enzyme than the so-called 

 oxidase ; the lack in power may be due to a lower concentra- 

 tion, as has already been suggested by Ewart,* or to some 

 slight difference in the molecular complex, rather than to a 

 different mechanism involving the necessity for the presence 

 of a third substance. 



An alternative explanation of the working of oxidizing 

 enzymes has been formulated by Wieland and has been con- 

 sidered in previous pages. 



Catalase, which has the property of setting free gaseous 

 oxygen from hydrogen peroxide, although of common occur- 

 rence, is not universally present in living cells. 



Zymase is the enzyme particularly associated with yeast 

 and is responsible for the alcoholic fermentation of sugar. It 

 has been described as occurring in higher plants such as the 

 beetroot, potato, lupin, and others. f 



Carboxylase has the property of removing carbon dioxide 

 from carboxyl groups, thereby converting pyruvic acid, for 

 example, into carbon dioxide and acetic aldehyde ; J it occurs 

 in yeast, where it is associated with zymase, and, according to 

 Bodnar, in the beetroot and potato tuber. 



* Ewart : " British Ass. Rep.," 1915. 



t Palladia and Kostytschev: " Ber. deut. bot. Gesells.," 1906, 24, 273; 

 Stoklasa and Chocensky : Id., 1907, 25, 122. 



t See Vol. I., p. 383- 



Bodnar: " Biochem. Zeitsch.," 1916, 73, 193; see also Zaleski : "Ber. 

 deut. bot. Gesells.," 1913, 31, 349. 



