BIOLOGICAL FUNCTION OF CATALASE 417 



it should be noted that the frequently used cerous hydroxide method 

 gives misleading results if ascorbic acid is present {1697). Peroxidase 

 is widespread in plants and in the presence of this enzyme, whose 

 action is not inhibited by catalase (c/. Section 3), the protective 

 action of catalase can only be of minor importance. An interesting 

 biological role of the competition between peroxidase and catalase 

 for hydrogen peroxide has, however, been reported by Hurst {1371). 

 According to his observations peroxidase plays a role in the hardening 

 of the insect cuticle and this is counteracted and controlled by 

 catalase. 



In strictly anaerobic bacteria, hydrogen peroxide is probably not 

 formed inside the cell. The experiments of McLeod and Gordon 

 {1822-1825), which appear to prove the contrary, have to be inter- 

 preted in a different manner {cf. Chapter X). Some facultative 

 anaerobes, e.g., Pneumococcus, lactic acid bacteria, and certain strep- 

 tococci, do not contain catalase but form hydrogen peroxide. In 

 the case of the lactic acid bacteria and Alcaligenes faecalis there is 

 evidence of the deleterious effect of hydrogen peroxide on the organ- 

 ism, while in Pneumococcus the peroxide may be used up in a direct 

 reaction with substrate (pyruvic acid). 



2.7.3. Catalase as a Peroxidative Enzyme. Keilin and Hartree 

 {1500) have recently come to doubt the theory of the biological role 

 of catalase as a safety valve against accumulation of hydrogen 

 peroxide. Hydrogen peroxide, they believe, is not formed in vivo as 

 frequently as is often assumed; if it is formed, catalase is frequently 

 found to be unable to exert the postulated protective function, and 

 consequently a peroxidative rather than a catalatic function is postu- 

 lated as the role of catalase in the cell. Similar ideas had been 

 expressed in 1927 by L. Stern {2662). 



Keilin's objections against the protection theory are based on the following 

 observations: 



(a) Keilin and Hartree {l^SG) have shown that, in tlie presence of catalase. 

 ethyl alcohol and its homologues, as well as ethanolamine, can he oxidized 

 by hydrogen peroxide formed by such systems as xanthine oxidase, glucose 

 oxidase, and a-amino acid oxidase. In this "coupled oxidation of alcohol" 

 catalase behaves as a peroxidase. Later Marsh and Carlson [1873) and 

 Keilin and Hartree (1500) demonstrated that alcohol is also oxidized if 

 hydrogen peroxide itself is added very gradually in small concentrations to 

 a solution containing catalase in a concentration much higher than that 

 needed for catalatic activitv. The mechanism of this reaction will be dis- 



