CYTOCHROME C 351 



that porphyrin c has the structure of a protoporphyrin to each of 

 whose vinyl groups one molecule of L-cysteine has been added (Fig. 1) . 

 Theorell later came to doubt this structure, when he found that proto- 

 porphyrin under the conditions of the hydrolysis was able to add two mole- 

 cules of cysteine and was transformed to porphyrin c {2770). Zeile and 

 Meyer {3 162, 3 167, 31 OS) have shown, however, that porphyrin c can be 

 obtained by hydrolysis of cytoclirome c under conditions in which its syn- 

 thesis from protoporphyrin and cysteine does not occur. The optical activity 

 of the product from cytochrome c differs from that of the protoporphyrin 

 cysteine adduct. It is therefore likely that the cysteine porphyrin of cyto- 

 chrome c is not an artifact, although the observation of Theorell that, on 

 short hydrolysis, compounds with less than two atoms of sulfur per atom of 

 iron could be obtained is still difficult to explain. 



In cytochrome c itself, the cysteine forms part of the protein in 

 which it is bound by peptide linkages. It is evident from this that 

 cytochrome c does not contain a separable prosthetic group, but an 

 active hematin center combined by thioether linkage to the protein. 

 The firm linkage between iron porphyrin and protein explains the 

 great stability to acids and alkalis. It is even possible to remove the 

 iron without breaking this linkage; this is achieved by treatment 

 with 0.1 N hydrochloric acid in the presence of substances which 

 keep the iron in the ferrous state, such as hydrogen activated by 

 platinum (3162). 



The hemochrome type of spectrum of cytochrome c must be 

 accounted for by linkages between the iron atom of the heme and 

 nitrogenous groups. These linkages are more easily broken than the 

 thioether linkages attaching the heme to the protein, their rupture 

 occurring below pH .3 and above pH 11. At pH to 1, cytochrome c 

 is transformed to an "acid hematin." 



Theorell showed that the nitrogenous groups concerned in these 

 linkages were part of the same protein as was involved in the thioether 

 linkages to the vinyl side chains. Were they contained in bases of 

 small molecular weight, these bases should be removed by dialysis 

 of cytochrome c in acid solution; it is found, however, that if the 

 dialyzed acid solution is neutralized, cytochrome c is re-formed. 

 Were they parts of protein molecules, other than those attached 

 firmly to the hematin side chains, the ultracentrifuge should separate 

 two components in acid solution and indicate a decrease of molecular 

 weight of the cytochrome; this however, was not found. The protein 

 is thus bound to the hematin center by four linkages, two stable 

 thioether linkages to the side chains and two less stable hemochrome 



