442 IX. HEMATIN ENZYMES, II 



Although they show that in the presence of azide and hydrogen 

 peroxide a reduction of the hematin iron occurs, Keihn's experiments 

 do not provide convincing evidence that the same holds for uninhibited 

 catalase. 



4.2.3. Theories of Stern and Sumner. Two theories have been 

 suggested in order to explain catalatic action without recourse to 

 valency change. Stern (£647) has assumed that hydrogen peroxide 

 unites with two molecules of hematin and is transformed into two 



/ 

 OH radicals: 



TJf XT /" 



Fe3+ • 0—0- Fe3+ -^ 2 Fe3+ + 2 OH 



For thermodynamic as well as for stereochemical reasons this theory 

 has little to recommend it. 



The theory of Sumner (2700) is apparently simple and straight- 

 forward. He formulates the reactions as follows: 



(Fe3+0H) + H.O2 -^ (Fe3+00H)+ H.O (1) 



(Fe'+OOH) + H2O2 -^ (Fe'+OH) + HoO + Oo (2) 



The combination of hydrogen peroxide with the hematin iron of the 

 enzyme is made very likely by the reaction of other hematin com- 

 pounds, such as hemi'globin or peroxidase, with hydrogen peroxide, 

 as well as by the formation of a compound of catalase with ethyl 

 hydrogen peroxide. Theorell has shown that at physiological pH 

 the hematin iron of catalase is in the form (FeOH) rather than (Fe"^) 

 and that the former is the active form (cf. Section 2.2.); he has thus 

 lent support to equation 1 of Sumner (above). There is no evidence 

 yet for a reaction of the type assumed in equation 2. 



The main weakness of the theory, however, is its failure to account 

 for catalase specificity, which is undoubtedly based on the protein 

 part of the enzyme. The catalatic activity of peroxidase is negligibly 

 small if compared with that of catalase, and so is that of hemoglobin 

 hydroxide; the latter does not appear to combine with hydrogen 

 peroxide, however. 



4.2.4. Attempts at a New Theory. In putting forward a new 

 theory of catalase action we are aware of the fact that it contains an 

 unproven assumption, an assumption, however, which is not entirely 

 unsupported by facts and which is amenable to experimental 

 verification. 



