158 A SYMPOSIUM ON RESPIRATORY ENZYMES 



the new component, a^. Since these positions correspond to those of 

 the Warburg enzyme, a^ may be identical with this enzyme. On the 

 other hand, the alpha-band of the compound itself is claimed to be 

 at 6000 A., whereas Warburg and co-workers (41, 42, 43) believe 

 their compound to have a band at 5890 A. This compound (also 

 called cytochrome a^) likewise reacts with oxygen, and formation of 

 its cyanide compound prevents reoxidation of the other cytochromes. 

 Furthermore, its carbon monoxide compound has an alpha-band at 

 5920 A. 



Cytochrome a^, in either the reduced or the oxidized form, com- 

 bines with potassium cyanide. The cyanide complex of the reduced 

 form is readily autoxidizable, whereas that of the ferric form is not 

 easily reduced. Ferric cytochrome a^ also reacts with hydrogen 

 sulfide, sodium azide, and hydroxylamine. It is thermolabile and 

 easily destroyed by organic solvents, acids, or alkalies. Cytochrome 

 flg is reduced along with the other cytochromes by e.g., succinate. 

 It is also autoxidizable. It is therefore believed that a^ is also identi- 

 cal with cytochrome oxidase. 



The authors have themselves offered certain objections to the 

 above conclusion: 



1. It was impossible to demonstrate the reduction of cytochrome 

 flg by added reduced cytochrome c, but technically these experi- 

 ments were not satisfactory. In this connection it is interesting that 

 Ball (26) noticed that the portion of the 6000-6050 A. band at- 

 tributed to ^3 by Keilin has a higher potential than any of the 

 other cytochromes, an expected but not an essential condition for 

 oxidase function. 



2. The carbon monoxide compound of ferro -cytochrome flg did 

 not appear to be sensitive to light anaerobically. The effect of light, 

 however, may become apparent only in the presence of oxygen, 

 which oxidizes the a^ component and thereby prevents its reaction 

 with carbon monoxide. Such an explanation of the light effect could 

 also explain its property of relieving inhibition of carbon monoxide. 



3. Finally, it has been found possible in the presence of carbon 

 monoxide to oxidize cytochromes a, b, and c by air while the spec- 

 trum of the carbon monoxide complex of reduced a^ remains visible. 

 It is therefore difficult to explain the oxidation of a, h, and c through 

 the flg component. 



In general, it may be said that the Keilin and Hartree paper by 

 no means clarifies the whole problem of the identity of cytochrome 

 oxidase and oxygen-transferring enzyme or of either one with the 



