COUPLED OXIDATION OF ALCOHOL 445 



Reactions 1 and -i proceed as normally, except that alcohol is bound to 

 the catalase. After reaction 2 the symplex is in a form in which no catalatic 

 reaction can occur before its XH> group has been reoxidized by hydrogen 

 peroxide. In the absence of alcohol this reaction occurs practically simul- 

 taneously with the addition of the hydrogen peroxide to the hematin iron 

 and catalatic reaction ensues. Alcohol bound in the complex is assumed to 



C2H50H I * 1 



Fig. 7. Mode of action of catalase as alcohol peroxidase. 



delay the reoxidation of the XH. group by hydrogen peroxide, so that the 

 enzyme now behaves as peroxidase — reactions 4 and .5. This occurs, how- 

 ever, only when the concentration of hydrogen peroxide is very low and that 

 of catalase, exceptionally high. Under other conditions the XH2 group is 

 oxidized by hydrogen peroxide even in the presence of alcohol before the 

 peroxidative reaction can proceed. Since reactions 1 and 2 only initiate the 

 cycle of reactions 3 to b, the oxygen development due to reaction 2 is not 

 measurable. 



A corollary to this action of catalase as peroxidase may be the fact observed 

 by Keilin and Mann {l')02) that in the presence of a large excess of hydrogen 

 peroxide, when the peroxidative action of peroxidase is inhibited, peroxidase 

 destroys hydrogen peroxide catalatically. Peroxidase may contain a XHj 

 group in its protein which, after dehydrogenation by an excessive amount of 

 hydrogen peroxide, may turn the enzj'me into a catalase. 



