PHYSIOLOGICAL 389 



of the first enzyme, Schardinger's xanthine-oxidase, is to transport 

 hydrogen from the xanthine to some other substance; the action 

 of the second enzyme, peroxidase, is to transport oxygen from 

 hydrogen peroxide to some other substance. One sets free hydrogen 

 in an active state ; the other sets free oxygen in an active state. 



It is highly probable that this is the sort of thing that goes on 

 in the living cell with its oxidising enzymes. It is probable that 

 catalase is present, acting as a chemical safety-valve, destroying 

 the hydrogen peroxide if it should be formed in dangerous amount. 



All this is difficult; but the essential point is that there are 

 cellular enzymes which transport hydrogen and others which trans- 

 port oxygen, and that oxidations will proceed most smoothly and 

 rapidly when hydrogen peroxide is formed by the one and destroyed 

 by the other. The whole system or cycle may be regarded as an 

 arrangement for changing molecular oxygen, as in the air, into the 

 active state. Here, then, seems part of the solution of the problem of 

 cell-combustion. 



But there are a number of substances, said to be auto xidi sable, 

 which readily combine with ordinary oxygen from the air, i.e. 

 molecular oxygen, and this without the aid of enzymes or great 

 heat. One of these, very widely distributed in living organisms, was 

 isolated by Gowland Hopkins in 1921, and is called "glutathione". 

 It is a compound of three amino-acids (cystein, glycine and glutamic 

 acid) and contains sulphur. It exists in two forms, the oxidised and 

 the reduced, and it can change from one to the other while in the 

 cell and under natural conditions. In the reduced form the sulphur 

 is united to hydrogen, so that the formula may be written 



G-S-H 



It is oxidised by losing hydrogen, so that two molecules join 

 together 



G-S-S-G 



and two molecules of active hydrogen are liberated. These probably 

 go to combine with oxygen, forming hydrogen peroxide (H2O2), 

 which in turn is attacked by peroxidase and yields active oxygen 

 to aid in the burning or combustion of energy-yielding substances 

 in the cell. But at the same time the oxidised glutathione can also 

 further this burning by taking away hydrogen from the materials of 

 the cell and returning to the reduced form. 



Here again is a system or cycle in which oxygen from the air is 

 made available for the oxidation of food-materials, and this is at 

 the same time hastened by the removal of hydrogen. In the con- 

 tinuous cycle of change between oxidised and reduced glutathione, 

 supplying oxygen by removing hydrogen, there is no need for the 

 help of enzymes ; but it is likely that the reactions are assisted by a 



