Composition of Cytochrome c Oxidase 



293 



in Fig. 2 (see also Sekuzu et al., 1959). The existence of two components, 

 one which reacts with nitric oxide and one which does not, is shown even 

 more distinctly here. The difference spectrum has maxima for the nitric 

 oxide compound of the reduced enzyme at 426 m/i, 545 m/< and 597 m/^. 



.100 - \ 



T 



Reduced 



CO 7 Reduced 



Difference 



*.zoo 



*.I00 



-.100 



400 450 500 550 600 

 Wave Lcnqih (m/i) 



650 



Fig. 1. Effect of carbon monoxide on the spectrum of reduced cytochrome c 

 oxidase. The ordinate on the right is for the curve of the difference spectrum. 



The effect of carbon monoxide, which prompted Keilin and Hartree (1939) 

 to suggest the presence of two cytochromes a, and the more recently discovered 

 effect of nitric oxide must be reinterpreted in the light of the newer knowledge 

 that the enzyme contains copper. It is our suggestion that, if the site of 

 oxygen-binding is the copper, then it must also be the site of carbon 

 monoxide- and nitric oxide-binding. However, it must be carefully noted 



