BIOLOGICAL FUNCTION OF THE CYTOCHROME SYSTEM 373 



There are thus at present some indications for the existence of a 

 separate Pasteur enzyme, distinct from the respiratory ferment, but 

 there is no conclusive evidence. 



5. BIOLOGICAL FUNCTION 

 OF THE CYTOCHROME SYSTEM 



5.1. Introduction 



There can be httle doubt that the catalysis by hematin enzymes 

 (the cytochrome system) provides the major pathway of respiration 

 in the cells of higher animals; the same probably holds for lower 

 animals, plants, and aerobic microorganisms. Respiration catalyzed 

 by other enzyme systems, such as copper-containing enzymes or 

 flavoproteins, is known to occur. To discuss to what extent the 

 respiration in various organisms or tissues can be ascribed to the 

 hematin enzymes and to what extent to such other enzyme systems 

 is beyond the scope of this book. In some organisms the whole of 

 the respiration is catalyzed by the cytochrome system. The oxygen 

 consumption of baker's yeast, for instance, is fully accounted for by 

 the rate of alternate reduction and oxidation of cytochrome c in the 

 intact cell {1073). Even though flavoproteins are autoxidizable, in 

 the yeast at least they are oxidized through the cytochrome system 

 at the low oxygen pressure in the cell {2767). 



The great difficulties in the study of the biological role of the 

 hematin enzymes should be realized. While Warburg initially 

 believed that only one respiratory ferment (in the earlier wider mean- 

 ing) could reasonably be expected in all the different species, it is now 

 certain that a variety of cytochrome systems exist, different species 

 containing a variety of respiratory ferments (in the present narrower 

 sense of cytochrome oxidase) as well as of cytochromes. The species 

 specificity of the oxidase is proved by the differences found in the 

 photochemical absorption spectra of yeast, Acetobacfer pasteurianum, 

 and rat heart, the few species so far studied. We have seen above 

 that different organisms contain different cytochrome systems. It 

 now appears very likely that the biochemistry of the hematin enzymes, 

 when more completely known, will reveal species differences in the 

 chemistry of their protein components and in some instances, of their 

 prosthetic groups. This would be analogous to the species differences 

 found for the oxygen-carrying pigments hemoglobin, erythrocruorins, 

 and chlorocruorin {cf. Chapter VII), with the additional complication 



