388 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



It is therefore necessary to replace these theories by a theory which 

 allows monovalent reactions to occur, but does not postulate the 

 formation of free radicals or radical chains. The fundamentals of 

 such a theory can be found in the work of Michaelis on autoxidation 

 of metal-cysteine complexes {IdSJj) and in the emphasis of Szent- 

 Gy6r.gyi on the biological importance of intracomplex reactions {2726). 

 In addition to the long-known fact that enzymes unite with their 

 substrates, there is now evidence for the formation of even more 

 complicated complexes such as oxygen -cytochrome oxidase -cyto- 

 chrome c, or hydrogen peroxide -peroxidase -ascorbic acid. Mono- 

 valent electron transfers can be assumed to occur in such complexes, 

 but there is no evidence and no need to assume that the radical thus 



/ 

 formed {e.g., HO2 from oxygen) leaves the complex before it has 

 reacted a second time. It is obvious that if the reaction mechanism 

 consists of such intracomplex rearrangements, the formation of radi- 

 cals as intermediates within the complex, while quite likely, cannot 

 be proved and need not be considered any further. The formation 

 of radical chains is thus excluded, since no free radicals occur. 



It must be emphasized that this criticism of the radical chain theory 

 applies only to the explanation of the biological function of the hematin 

 enzymes, and does not minimize its value for illuminating the mechanism of 

 reactions of inorganic and organic molecules with oxygen or hydrogen 

 peroxide outside tlie cells. It is quite possible that radical chains also play 

 a role in some unphysiologic reactions catalyzed by hematin enzymes under 

 certain conditions outside their normal environment. 



6.3. Hemoglobin as a Cytochrome Oxidase Model 

 6.3.1. Introduction. Before attempting an explanation of the 

 mode of action of cytochrome oxidase on the basis of the conception 

 developed in the preceding section, we will study certain reactions 

 of hemoglobin which are of interest in this regard. In Chapter VI 

 and in the first section of this chapter the differences between hemo- 

 globin and the respiratory enzymes has been stressed, but it has been 

 mentioned that they are important but not absolute. In this section 

 we discuss such reactions of hemoglobin in which the similarity is 

 more important than the difference, i.e., mainly the autoxidation of 

 hemoglobin and the activation of oxygen by modified hemoglobin. 



While the autoxidation of hemoglobin is slow, it can under certain 

 conditions play a catalytic role (c/. Chapter XI, Section 4.). Like 

 the respiratory ferment, hemoglobin combines with oxygen and carbon 



