370 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



piratory ferment of Warburg and of cytochrome ai. It is thermo- 

 labile and easily destroyed by organic solvents, acid, and alkali. It 

 also reacts with azide and hydroxylamine. The absorption band of 

 ferrocytochrome a.3 is given as about 600 m/x. This would make it 

 different from cytochrome ai, the band position of which is 589 m/x, 

 though not necessarily from the respiratory ferment, the band position 

 of which in the reduced form, uncombined with carbon monoxide, is 

 not known. Fujita and Kodama (962) have pointed out that the 

 absorption band of cytochrome a falls off much more steeply toward 

 the red than toward the green; we can therefore assume that a com- 

 paratively weak band of ferrocytochrome as, b'ing at 589 mfx, remains 

 merged with the cytochrome a band, causing only a slight shift 

 toward 600 mn, while the band of the carbon monoxide compound 

 of cytochrome as, being stronger, appears as a separate band. There 

 appears no need then to assume that cytochrome as and cytochrome 

 ai are really different. 



There are, however, difficulties remaining which caused KeiHn and Hartree 

 at first to reject the idea of identity between cytochrome oxidase and cyto- 

 chrome as. Oxidation of cytochromes a, h, and c was. observed while cyto- 

 chrome as still remained combined with carbon monoxide; it is probable, how- 

 ever, that a small proportion of uncombined cytochrome aj may have been 

 present, sufficient to catalyze the oxidation. A reduction of ferricytochrome 

 as by ferrocytochrome c has not yet been demonstrated, nor a light dissocia- 

 tion of the carbon monoxide compound of cytochrome as. In their later 

 paper, however, Keilin and Hartree no longer consider these difficulties 

 sufficiently great to reject the assumption that cytochrome as is identical with 

 cytochrome oxidase. 



There remains the difference in the position of the Soret band at 450 m/i 

 found by Melnick {1909) for the photochemical absorption spectrum of rat 

 heart tissue and the 43''2-m;u band found by Keilin for cytochrome as. 

 Melnick assumes that this may be due to an error of Keilin and Hartree in 

 attributing the bands at 450 and 43'-2 mpi to carbon monoxide cytochrome a 

 and carbon monoxide cytochrome as, respectively. Stern {26oG) and Melnick 

 made the suggestion therefore of attributing the band at 450 m/x to carbon 

 monoxide cytochrome as, that at 43'-2 m^t to carbon monoxide cytochrome a. 

 This is, however, in contradiction to Keilin's and Straub's experiments, and 

 the cause ol the divergency is not clear. 



3.6.6. Cytochrome ao. The absorption band of cytochrome a.2 at 

 628-632 m/i was observed in bacteria by Japanese workers {2735,2736, 

 31Jf9; cf. also 962). It became of greater importance after Negelein 

 and Gerischer {2021^,2025) had demonstrated that the cytochrome az 



