348 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



the activity per atom of iron remaining unaltered. Since cytochrome 

 c is a rather strong base with an isoelectric point of 10.05 (2782), it 

 may be combined in the cell with a protein or peptide of more acidic 

 properties. It remains uncertain whether the cytochrome c with 

 0.34% iron still contains this original peptide, or a part of it, or 

 whether the peptide has been introduced during the preparation. 

 The constant iron content and the fact that the peptide group cannot 

 be removed at a pH below the isoelectric point make it improbable 

 that it is a mere admixture. 



Properties. Cytochrome c is stable to dilute acids, even mineral 

 acids, 0.1 iV alkali, and boiling. At physiological pH it is not autoxi- 

 dizable and does not react with carbon monoxide, hydrogen sulfide, 

 azide,* or hydroxylamine {U79,U88,U93,2782). The slight reversible 

 spectral shift by carbon monoxide observed by Altschul and Hogness 

 (45) was probably caused by impurities. Stotz, Altschul, and 

 Hogness (2677) showed that in the complete system, only cytochrome 

 oxidase, not cytochrome c, reacts with carbon monoxide. At pH 

 above 11.5 and below 4, however, cytochrome c is modified to give 

 autoxidizable compounds which react with carbon monoxide. In 

 contradistinction to Keilin, Potter (2176) found a small effect of 

 cyanide on the absorption spectrum of ferricytochrome e and an 

 inhibition of the reduction of the latter by the succinic dehydrogenase 

 system, though not under all conditions (2179). This has recently 

 been confirmed by Horecker and Kornberg (1347), who have clearly 

 demonstrated the formation of a dissociable cyanide compound. The 

 absorption band at 692.5 m/x of ferricytochrome c is abolished by its 

 combination with cyanide. The reaction is, however, much slower 

 than that of the oxidase with cyanide, and, at least under normal 

 conditions, plays no part in the cyanide inhibition of respiration 

 (cf. 2677). Ferricytochrome c also combines with nitric oxide (14-88) 

 to form a compound with two equally strong absorption bands (563 

 and 527 m/i)- Ferricyanide and cupric ions oxidize ferrocytochrome c 

 to ferricytochrome, while hydrogen activated by platinum or pal- 

 ladium, dithionite, cysteine, p-phenylenediamine, ascorbic acid, 

 catechol, pyrogallol, and succinic dehydrogenase reduce ferricyto- 

 chrome to ferrocytochrome. 



Molecular weight. Cytochrome c contains only one hematin group 

 per molecule. Zeile and Renter (3171) found a molecular weight of 



* An easily dissociable azide compound of ferricytocliroine c has recently been found 

 by Horecker and Stannard (l-J^Ta). 



