398 



Vlir. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



this form (containing X, not XH2) the ferrous oxygen compound of 

 the oxidase is relatively stable. Only after X has been reduced to 

 H2X by means of substrate, in a reaction probably mediated by 



Fe^^O. 



XH. Fe 



2+ 



2 + 

 Fe O2 



Fe 



3+ 



[6] 



+ 3 HA 



Fe 



3 + 



[1] 



X 



+ 0j 



Fe 



3+ 



+ 2 OH 



Fe 



2+ 



[5] 



X 



Fe 



3 + 



Fe 



(21 



X 



+ HA 



Fe 



3+ 



HA 



-H 



Fe 



34- 



X 



:4i 



Fe 



+ A 



[31 

 Fig. 5. Mode of action of cytochrome oxidase. 



cytochrome c, and the iron of the latter has been reduced to ferrous 

 (step 6), is the stage set once more for the autoxidation and the cycle 

 begins once again. In this way the oxygen is transformed to water 

 without the formation of free reactive radicals or hydrogen peroxide. 

 Reactions 1 and 6 probably involve several monovalent steps, but 

 the radicals formed in them do not leave the complex before the 

 reaction proceeds. It is of historical interest to note that this theory 

 uses a combination of Warburg's and Wieland's theories of respiration 

 even at the level of the first "oxygen-activating" enzyme. 



The school of Shibata and Tamiya {2733) holds that cytochrome c is 

 oxidized by two enzymes acting in succession, a ferrous oxygen-transporting 

 enzyme, which reacts with oxygen and carbon monoxide, and a cytochrome 

 oxidase, the ferrous form of which is oxidized to ferric by the oxygen-trans- 

 porting enzyme. The ferric form of the second enzyme is assumed to combine 

 with cyanide {2733, cf. also 2656, p. 7) : 



O2 — > oxygen-transporting enzyme 



Fe^ ;=i Fe^+Oj 

 CO inhibition 



cytochrome oxidase 



Fe2+ ;:^ Fe3+ 

 CN~ inhibition 



cytochrome c 



A large part of the evidence brought forward to support this theory has 

 not been confirmed by Keilin and Hartree {1491, cf. also 2677). It is now 



