CYTOCHROME SYSTEM 343 



3. THE CYTOCHROME SYSTEM 



3.1. Introduction 



Fischer established the wide distribution of hematin compounds in 

 the cells of plants, animals, and microorganisms (S79). For the devel- 

 opment of our knowledge on the biological role of these substances in 

 cell respiration, however, chemical methods do not suffice. Rapid 

 progress could only be made when other methods were applied to the 

 problem which are far more sensitive, which do not damage the 

 unstable cell compounds, and which interfere little with the life of 

 the cell. Two such methods were used, both of general applicability 

 at least in principle. 



Spectroscopic investigation with refined methods led Keilin to a 

 rediscovery of hemochrome-like substances in living cells which he 

 called cytochromes. His method is based on the known spectroscopic 

 properties of the simpler synthetic hemochromes, discussed in Chapter 

 V, and particularly on the fact that the absorption bands of the 

 hemochromes are sharper and more intense than those of the hem?'- 

 chromes. Thus it becomes possible to study reduction and oxidation 

 of the cytochromes in the living cell. 



In the second type of method use is made of the fact that the rate 

 of a catalyzed reaction is in general proportional to the concentration 

 of the catalyst. Warburg observed that cell respiration is inhibited 

 by cyanide and carbon monoxide, substances which are known to 

 combine with iron compounds. In Chapter V we have discussed the 

 combination of these substances with hematin compounds. Warburg 

 developed an ingenious method by which the absorption spectrum of 

 the carbon monoxide compound of the respiratory ferment can be 

 measured photochemically by studying the inhibitory effect of carbon 

 monoxide on cell respiration in the dark and the diminution of this 

 effect by irradiation (cf. Chapter II). 



Both methods have complemented each other. We realize now 

 that the study by inhibitors alone could not have thrown much light 

 on the structure and role of the cytochromes, while the extremely 

 small concentration of the respiratory ferment in most cells probably 

 overtaxes the sensitivity of direct spectroscopic observation, except 

 in the case of yeast and other microorganisms. Some recent develop- 

 ments give hope that it may become possible in the future to correlate 

 spectra observed directly with those obtained by the photochemical 

 method, but at present this cannot yet be done satisfactorily. 



