CYTOCHROMES 151 



narrow slit, have, however, found evidence of a ferro-cytochrome- 

 carbon monoxide compound throughout the entire pH range. The 

 change in spectrum was reversible; that is, the carbon monoxide 

 could be removed by nitrogen. The carbon monoxide compound 

 was light-sensitive. Keilin and Hartree (10) attribute this finding to 

 the presence of denatured cytochrome or other hematin compounds. 

 At neutral pH they were able to liberate and measure manometri- 

 cally only 10 per cent of the theoretical amount of carbon monoxide 

 that should combine with reduced cytochrome. 



Potter (16) concluded that cyanide also, contrary to popular 

 belief, forms a complex with ferri-cytochrome c. This conclusion 

 was based not only on the fact that a change in the spectrum was 

 detected but also upon studies on the enzymatic reduction of cyto- 

 chrome c. Since the spectral shift is small, as with carbon monoxide. 

 Potter questions whether one can safely conclude from simple spec- 

 troscopic observation that a given inhibitor has not reacted with 

 cytochrome. 



Considerable gains have been made in determining the structure 

 of the prosthetic group of cytochrome c and how this might explain 

 the peculiar stability of the heme-protein linkage. Hill and Keilin 

 (17) obtained a porphyrin by hydrochloric acid and sulfur dioxide 

 treatment of cytochrome c which, unlike most porphyrins, was solu- 

 ble in water. Zeile and Piutti (18), in extensive synthetic work, were 

 able to introduce various nitrogen bases into the unsaturated side 

 chains of protoporphyrin and obtain porphyrins whose solubility 

 was similar to that obtained from cytochrome c. When iron was 

 introduced into some of these compounds, they showed the charac- 

 teristic cytochrome c absorption band at 5500 A. Later Zeile and 

 Renter (7) isolated hematoporphyrin from a hydrobromic-acetic 

 acid degradation of cytochrome c. Theorell (19) isolated a sulfur- 

 containing porphyrin and postulated that the vinyl, groups of the 

 hemin are linked to amino acids of the protein by thio-ether bonds. 

 Upon demonstrating later that such a porphyrin could arise by con- 

 densation of hemato -porphyrin with cysteine during the course of 

 the cytochrome hydrolysis, Theorell (20) explained that either nitro- 

 gen or oxygen as well as sulfur might form the connecting link. 

 Zeile and Meyer (21) offer support to the sulfur-bridge theory in 

 obtaining the sulfur-containing porphyrin under conditions of 

 hydrolysis in which a condensation of porphyrin with free cysteine 

 would be very unlikely. A tentative structure of cytochrome c is 

 illustrated in Figure 1. 



