RESPIRATORY FERMENT AND RELATED CYTOCHROMES 369 



carbon monoxide the bands of cytochrome a did not appear to be 

 homogeneous, and somewhat simikr observations were made by Ball 

 {123) in the same year. The experiments of Keilin and Hartree 

 {lJf92,lJ^93) are more decisive. The band of cytochrome a is split in 

 two by carbon monoxide; the main band remains unaltered, but 

 another slight peak of absorption at 593 m/x appears. Simultaneously 

 the absorption band at 450 m/z is decreased in strength, that of the 

 432-mM band is greatly increased. The diagram in Figure 4 gives 

 the relations of the bands as found by Keilin. It is assumed that 

 cytochrome a, which does not react with carbon monoxide, is accom- 

 panied by cytochrome as, which has similar absorption bands but 

 reacts with carbon monoxide. To the carbon monoxide compound of 

 the latter, the bands at 590-593 mju and 432 m^t are ascribed. In all 

 cells studied the relative concentrations of cytochromes a and as 

 were found to be the same {e.g., in the flight muscles of insects), and 

 it is assumed that they are interconvertible. A similar position of 

 the absorption bands in the presence and absence of carbon monoxide 

 was later observed by Straub {2684-) in solutions of cytochrome 

 oxidase from heart, obtained by the cholate method; the band of 

 ferrocytochrome as was found, however, at 443 m/u instead of at 

 450 m^i, the same position as that of the carbon monoxide compound. 



Cyanide added to a completely reduced preparation causes a partial 

 shift of the band of cytochrome a, similar to that caused by carbon 

 monoxide. There is thus evidence of the formation of a cyanide 

 compound of ferrocytochrome as. On oxygenation in the presence 

 of cyanide, this weak band disappears, while the main band at 600 m/x 

 remains slightly weaker than before the reaction with cyanide. 

 Evidently cyanide ferrocytochrome as is oxidized to cyanide ferri- 

 cytochrome as, while the nonautoxidizable cytochrome a remains 

 reduced. Under these conditions the y band at 450 mn almost dis- 

 appears. From these observations it can be concluded that the 

 greater part of the a band of a + as (at 600 m/x) is due to ferrocyto- 

 chrome a, while the greater part of the y band (at 450 m/x) is due to 

 ferrocytochrome as. Cytochrome as has thus a strong Soret band, 

 like other heme compounds, while cytochrome a is abnormal in having 

 a relatively weak Soret band. 



The cytochrome as occurring in heart muscle cytochrome oxidase 

 preparations thus combines with carbon monoxide and cyanide, is 

 autoxidizable, and as carbon monoxide compound has an absorption 

 band similar to that of the carbon monoxide compounds of the res- 



