CYTOCHROMES 153 



yeast cytochrome c, but found a much lower value of Eg' — +0.127 

 V, between pH 4.6 and 7.1. Finally, Wurmser and Filitti-Wurmser 

 (24) in France and Stotz, Sidwell and Hogness (25) in Chicago, 

 measured the potential of pure cytochrome c isolated from heart 

 muscle. The former measured the equilibrium potential in mixtures 

 of reduced and oxidized cytochrome c, the proportion being deter- 

 mined spectrophotometrically. They obtained the value Eo' = 

 + 0.254 V. between pH 5.0 and 8.0. Stotz et al. used a purely 

 spectrophotometric method and obtained a value of +0.262 v. in 

 the same pH range. The spectrophotometric method consisted in 

 measuring accurately the amounts of oxidized and reduced indicator 

 and cytochrome in equilibrium with each other. The potential of the 

 indicator being known, the potential of the cytochrome could be 

 readily calculated. The results recorded by the two groups were 

 reached independently and represent good agreement. They are 

 both in essential agreement with the potential of +0.27 v. reported 

 by Ball (26), who was able to estimate the potentials of the three 

 cytochromes as they existed in a heart muscle extract. At a physio- 

 logical pH, therefore, cytochrome c has about the same potential as 

 the hydroquinone-quinone system. 



Cytochrome h.—in the reduced state this cytochrome component 

 possesses an alpha-absorption band at 5640 A. and a beta-band 

 at 5300 A. It appears to be more closely bound to the insoluble 

 material in tissue extracts than is cytochrome c. Nevertheless Ya- 

 kushiji and Mori (27) claim to have isolated cytochrome Z? in a 

 soluble form. It seems doubtful from their method of preparation 

 whether the product obtained could be an undenatured cytochrome 

 h. In some of the original extracts the reduced band is not at 5640 A., 

 but as purification proceeds this band is shifted to the normal posi- 

 tion of reduced cytochrome h. They believe that the hemin portion 

 of their product is ordinary protohemin. Since several other proteins 

 combined with protohemin to form spectroscopically and catalyti- 

 cally similar hemochromogens, it is difficult to believe that these 

 workers actually obtained cytochrome h. 



Judged from its behavior in tissue extracts, cytochrome h appears 

 to be a thermolabile hemin-protein complex. Unlike cytochrome c, 

 the h component is autoxidizable. Since this component reaches 

 equilibrium with other reversible systems in a heart muscle extract. 

 Ball (26) was able to estimate its potential as —0.04 v., the lowest of 

 the cytochrome components. It does not combine with carbon 

 monoxide or other respiratory inhibitors (10). 



