332 



Discussion 



largely ionic, the models are largely covalent. This is consistent with the interpreta- 

 tion of the Soret band shift and to a considerable degree with the changes in intensity 

 of the a-band (see Williams, Chem. Rev. 56, 299, 1956). 



The same analysis leads us to suggest that cytochrome a^ is more largely in the low- 

 spin form. 



The interpretation of the spectra of the ferric porphyrin a complexes, both band 

 positions and intensities, needs no elaboration and is as in the text of my paper (this 

 volume, p. 41). 

 Chance: It is highly desirable to emphasize the properties of the CO compound of 

 cytochrome a^, especially in consideration of the models studied by Lemberg and his 

 collaborators. Our data are based upon very precise photochemical action spectra 

 for the relief of CO-inhibited respiration; a compilation of two results from Castor's 

 studies (Castor and Chance, J. biol. Chem. Ill, 453, 1955) is given in Fig. 2 and 

 Table 1. 



585 595 



'550 



= 10 



I — I — i — i—r 

 400 



I I I 



450 



500 



I I I — I — r 

 550 



600 



650 



-Kim) 



Fig. 2. The absorption bands of the carbon monoxide compounds of 



cytochrome a^-CO in ascites tumour cell and baker's yeast obtained by 



photochemical action spectra. 



The first point to be noted is the great similarity in the position of the peaks for 

 yeast cells and ascites cells, even though the haem may well be bound to different 

 proteins and is probably at a lower pH in yeast than the ascites cells. The second point, 

 in addition to the well recognized properties of the a and y bands, is that there is a 

 clear /S band; the absence of the P band in the CO haemochrome and the multiplicity 



