360 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



ations in the position of the absorption bands in different cells would 

 then be a result of the presence of different protoheme dehydrogenase 

 compounds, or of a mixture of them in different proportion. If these 

 cytochromes b combine in E. coli with cytochrome a2, the latter acting 

 as oxidase, the different sensitivities to carbon monoxide poisoning 

 of succinate, lactate, and formate oxidation (Cook, Haldane, and 

 Mapson, 485) would be explained in terms of different carbon monox- 

 ide affinities of the symplexes cytochrome aa- cytochrome b- suc- 

 cinic dehydrogenase, cytochrome a2-cytochrome b-lactic dehydro- 

 genase, and cytochrome a2-cytochrome b-formic dehydrogenase. 

 A similar explanation has already been suggested by Keilin {11^8 If). 

 So far this has been demonstrated as probable only for yeast and 

 E. coli. In other cells, cytochromes b independent of dehydrogenases 

 may exist. 



3.5. Cytochrome a 



Keilin had described cytochrome a as a compound which was not 

 autoxidizable and did not combine with carbon monoxide or cyanide. 

 Later Ball {123) found that the absorption band at 600-605 ran did 

 not behave homogeneously on oxidation with ferricyanide; the major 

 part of the absorption disappeared, but a weaker band at 595 m^ 

 remained. Keilin and Hartree {1492,14-93) found that the band at 

 600-605 m^t was due to two compounds, one of which (again called 

 cytochrome a) was not autoxidizable and did not combine with carbon 

 monoxide, while the other, called cytochrome as, was autoxidizable 

 and combined with carbon monoxide. 



Using the observation of Keilin and Hartree {11^93) that 2% sodium 

 cholate clears the suspensions of cytochrome oxidase preparations, 

 Straub (2684) and Yakushiji and Okunuki {31^3, cf. 2655) have ob- 

 tained solutions of the cytochromes a from heart tissue, using extrac- 

 tion with sodium cholate and secondary phosphate, followed by frac- 

 tional precipitation with ammonium sulfate. Such solutions still 

 show the Tyndall phenomenon. They contain both cytochromes a 

 and as {cf. Sections 3.6.4. and 3.6.5.). 



Both cytochromes a and as yield the same pyridine hemochrome, 

 and this probably also holds for the cytochrome a of bacteria. Negelein 

 {2021) obtained a porphyrin from pigeon muscle which gave a hemo- 

 chrome with two absorption bands at 582 and 532 \wn. This "crypto- 

 porphyrin" was later {2022) claimed by Negelein to be an artifact, 



