340 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



predominantly in this direction. If, however, a reaction occurs by 

 which Oi is remove'd far more rapidly than O2, the equihbrium may 

 be shifted toward the left, and the system with the lower potential 

 will oxidize that with the higher one (cf. 2079, p. 91). 



Inhibition of Hematin Enzymes. Developing from the early con- 

 ception of Warburg of an iron-catalyzed respiration, the study of 

 hematin catalysis by inhibitors such as cyanide and carbon monoxide 

 has played a fundamentally important role in the development of 

 our knowledge, and is still of considerable importance. Nevertheless 

 it should be realized that an ideal inhibitor, which would be specific 

 for all hematin-catalyzed reactions, and for them alone, does not 

 exist. We have discussed the chemical basis of the reactions of 

 hematin compounds with cyanide and carbon monoxide in Chapter 

 V, and have seen that cyanide reacts with ferric hematin as well as 

 with ferrous heme compounds, though not with all the latter (e.g., 

 little with hemoglobin; cf. Chapter VI); carbon monoxide, on the 

 other hand, reacts only with the ferrous heme compounds. It will be 

 seen in this chapter that the cytochromes do not react with carbon 

 monoxide, but since the oxidase (which with them forms the cyto- 

 chrome system) does so in its ferrous form, the failure of cytochrome 

 to react is of little practical importance. Those hematin enzymes, 

 however, which are ferric compounds and do not change their valency 

 during the catalyzed reaction are not inhibited by carbon monoxide. 

 These include peroxidase and probably catalase. 



Cyanide inhibits a variety of nonhematin enzymes, e.g., the copper- 

 containing enzymes. Carbon monoxide is a more specific inhibitor, 

 but is still not entirely so, since some copper catalysts (e.g., poly- 

 phenoloxidase) are also inhibited by carbon monoxide. The carbon 

 monoxide compound of the polyphenoloxidase is, however, not disso- 

 ciated by light, and the light sensitivity of carbon monoxide inhibition 

 is so far the best indirect evidence for the hematin nature of an 

 enzyme. Some other iron systems such as ferrocysteine {512) and 

 ferroglutathione (1591) are known, which are also able to form light- 

 sensitive carbon monoxide compounds, but so far no evidence has 

 been found that such compounds occur in the cell. In Chapter II we 

 have discussed how, by combining carbon monoxide inhibition with 

 irradiation in specific ranges of the spectrum, the heme nature of a 

 catalyst can be established beyond doubt (cf. also this chapter. Sec- 

 tion 3.6.1.). Finally, in some cases the conditions under which light 

 sensitivity of the inhibition has been measured may not have been 



