68 RESPIRATION 



substances as possible intermediate metabolites which when 

 left in contact with methylene blue in the absence of oxygen 

 decolorize this substance and therefore act as hydrogen 

 donators. His technique consists in placing the material 

 under examination in a test tube with freshly washed frog's 

 muscle, to supply the dehydrase, and methylene blue, filling 

 the tube with boiled water, and leaving the whole in a ther- 

 mostat and examining at intervals. If the methylene blue is 

 decolorized, the substance in question is a hydrogen donator 

 and consequently a possible intermediate metabolite. 



A highly important contribution to the mechanism of 

 oxidation in the living cell is furnished by the discovery and 

 isolation by Hopkins * of a dipeptide composed of cystein and 

 glutamic acid to which he gives the name of glutathione. 

 This substance can act alternatively as an hydrogen acceptor 

 or as an hydrogen donator, according as it exists in the 

 oxidized or the reduced form. This will be intelligible from 

 an examination of the accompanying formulae : 



CH . SH 



From these formulae the glutamic acid residues have been 

 omitted for the sake of simplicity; the mode of action of 

 the dipeptide as hydrogen donator in the first instance and 

 as hydrogen acceptor in the second is obvious ; in other words, 

 this substance may act alternately as an oxidizable reducing 

 substance and as a reducible oxidizing agent. Glutathione is 

 not uncommon in plant tissues although the coloration by 

 which it is recognized usually is far less intense in vegetable 

 than in animal tissue. Its presence in yeast may be de- 

 monstrated by grinding the cells in a mortar with a little sand 

 and some saturated solution of ammonium sulphate. On 

 pouring off and adding to the supernatant liquid a few drops of 



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



