CYTOCHROME a 361 



arising from autoxidation of protoporphyrin in hydrochloric acid. It 

 may perhaps contain one acetyl and one vinyl group as side chains. 

 Later again Negelein {2023) isolated a different hematin from pigeon 

 breast muscle and heart. The hematin solution in acetone-hydro- 

 chloric acid had an absorption band at 675 mju- After further treat- 

 ment it was finally separated from protohematin, and its pyridine 

 hemochrome, which was obtained crystalline, showed only one absorp- 

 tion band (587 m/x) in the visible part of the spectrum. Like the 

 Spirographis hematin the new hematin was converted by methanol 

 treatment to a hemochrome with an absorption band at 558 m^u; from 

 this hemochrome it could be re-formed by treatment with acid acetone. 

 Roche and Benevent {2307) have repeated these experiments with 

 somewhat different results. According to the conditions, they 

 obtained either a hematin the hemochrome of which showed two 

 absorption bands (582, 530 m/x), or the hematin of Negelein, the 

 hemochrome of which shows only the one band at 587 mju. They 

 assume that the latter is an artifact, being formed from the former 

 by secondary oxidation. On standing of the hemin in aqueous 

 acetone, the 530-m)u band of the hemochrome decreased. If a two- 

 banded hemochrome exists, it is certainly different from the "crypto- 

 hemochrome" Negelein obtained from protoporphyrin (l698a). Spiro- 

 graphis pyridine hemochrome has a very weak second absorption 

 band 538 m^t. In view of the importance of these hematins in relation 

 to cytochrome a and the respiratory ferment, a reinvestigation, with 

 special precautions to avoid autoxidative processes, is urgently 

 required. 



From spectroscopic observations on the cell, Ball (123) found the 

 oxidation-reduction potential of cytochrome a to be + 0.29 (pH 7.4, 

 20°). This value is less certain than those for the cytochromes c and b. 



In the cells of certain microorganisms, the cytochrome a band is 

 absent and is replaced by bands described as those of cytochromes ai 

 and ao (cf. Sections 3.6.3. and 3.6.6.). Because of this replacement 

 Keilin {1J^79,1480) considered these substances to be cytochromes, not 

 respiratory ferments, but since he later discovered that his earlier 

 cytochrome a was a mixture of nonautoxidizable cytochrome a with 

 the respiratory ferment cytochrome as, this argument has lost its 

 validity. The biological significance of the cytochromes ai, a2, and 

 SLs is certainly different from that of the other cytochromes and these 

 substances will therefore be discussed in greater detail in the next 

 section dealing with the respiratory ferment. 



