REGULATORY SUBSTANCES 519 



Then came the work of KeiUn. He showed that cytochrome 

 exists in reversibly oxidized and reduced forms and acts as an 

 intermediate carrier of hydrogen. He then tried inhibitors and 

 found that narcotics, e.g., ethyl urethane added to yeast (which 

 Warburg supposed to block the surface of the oxidizing ferment), 

 prevent the reduction of cytochrome and do not inhibit oxida- 

 tion of the pigment, while cyanide causes complete and irre- 

 versible reduction of cytochrome to such an extent as to prevent 

 oxidation even in the presence of oxygen. Keilin also showed 

 that the mechanisms that reduce cytochrome are mainly dehy- 

 drases and their substrates; the oxidizing mechanism is indo- 

 phenol oxidase. 



Thus, it seems that cytochrome is the respiratory intermediary, 

 a role that has been attributed to numerous substances, notably 

 glutathione when it was first isolated by Hopkins. Hopkins and 

 Elliott, however, showed that while glutathione can function as a 

 respiratory enzyme, it accounts for but 7 per cent of the oxygen 

 intake of tissues. Warburg's first respiratory enzyme, an oxygen 

 carrier, the active group of which is hemin, has now been supple- 

 mented, if not replaced, by a second oxidation ferment which he 

 and Christian isolated (from yeast and lactic-acid bacteria) ; it is 

 free from hemin bodies. Warburg is thus forced to retreat from 

 his original position that there is only one enzyme involved in 

 respiration. The new respiratory ferment, a yellow-red oxida- 

 tion pigment, consists of a carrier of high molecular weight 

 (50,000 to 70,000, as determined by Svedberg and Eriksson) and 

 coloring matter which is a fiavin related to ovo- and lactoflavin. 

 It is given the formula C13H13N4O2. 



Recently, a whole series of pigments (from yeast, liver, heart, 

 and urine) have been described and claimed to be concerned in 

 respiration. 



Briefly summarizing, we find that the flavins (now identified as 

 a component of vitamin B2) are probably directly concerned in 

 respiration, but it is the combination of phosphoflavin with a 

 specific protein — a flavoprotein (the new yellow pigment of 

 Warburg and Christian) — which is the important respiratory 

 catalyst. The strongly reducing qualities of vitamin C (ascorbic 

 acid) also make it active in respiration. The enzymes peroxidase 

 (in plants) and catylase (in all anerobic organisms) appear also 

 to be significant in respiration. 



