i 4 6 RESPIRATION 



excessive, aerobic respiration ends, since the respiratory 

 chromogen is transformed into the stable pigment. 



Palladin thinks that most pigments take part in respiration, 

 but the essential character of such bodies, that of reversible 

 oxidation and reduction with great ease, is not possessed by 

 them ; only by drastic chemical operations is the reversibility 

 effected. But three pigments which meet with physiological 

 requirements have since been discovered, namely chlorogenic 

 acid, cytochrome and hermidin. Oparin * isolated from sun- 

 flower seeds the depside chlorogenic acid which acts like a 

 respiratory pigment ; by oxidation it loses four atoms of 

 hydrogen and is converted into a green pigment. 



Oparin considers that aerobic respiration essentially con- 

 sists of two phases : the respiratory chromogen is oxidized 

 to a pigment by atmospheric oxygen in the presence of a 

 phenolic oxidase ; and that the pigment is reduced to the 

 chromogen by the hydrogen contained in the water molecule, 

 the oxygen being taken up by the living cell. From Oparin's 

 experiments it would appear that chlorogenic acid is more 

 particularly active in the oxidation of natural amino acids and 

 other compounds (see p. 167). 



Cytochrome is oxidized by a thermolabile indophenol 

 oxidase and is reduced by a dehydrase, it appears to act as 

 a carrier between the two activating mechanisms of oxidase 

 and dehydrase respectively.^ 



But this, together with hermidin, has already been com- 

 mented on. I 



GLUTATHIONE. 



Glutathione is the name given to a substance first isolated 

 by Hopkins § from yeast. It is a thermostable substance, 

 and consequently non-enzymic, whose function would appear 

 to facilitate the oxidation of hydrogen by alternately com- 

 bining with and yielding up this element, in this way acting 

 as an hydrogen transporter. The constitution of glutathione 



* Oparin : " Biochem. Zeit.," 1921, 124, 90 ; 1927, 182, 155. 



t Keilin : " Proc. Roy. Soc," B, 1929, 104, 206. 



J Vol. I.. 305, 343. 



§ Hopkins : " Biochem. Journ.," 1921, 15, 286. 



