142 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



latter presumably is contained in the nondialysable residue 

 and is capable of digestion by the proteolytic enzyme also 

 present therein. Harden and Young's further researches 

 have shown that phosphoric acid is at any rate a necessary 

 constituent of this dialysable substance. On addition of a 

 phosphate to unfiiltered yeast juice a great increase of fer- 

 mentation is obtained. On the other hand, this efiect is not 

 produced if the phosphate is added to the residue or to the 

 filtrate separately, and consequently the phosphate, though 

 apparently necessary to the reaction, is not the initially active 

 agent. Moreover, the phosphate does not affect living yeast. 

 It appears that the phosphate actually takes part in the 

 fermentation reaction, and that for every molecule of sugar 

 which is broken down into alcohol and carbon dioxide, a 

 molecule of a complex hexose phosphate, a compound of 

 a sugar molecule with two of phosphate, is simultaneously 

 formed. This compound has actually been isolated. The 

 ordinary fermentation involves the phosphate present in 

 ordinary yeast juice, either as hexose phosphate or free 

 phosphate, and this phosphate passes repeatedly through 

 the cycle of changes represented in the following equations : — 



(1) 2C,H,,0, + 2R,HP0, 



= 2C0, + 2C^,0 + CeHioO,(PO,R,), + 2R,0 



(2) C,H,oO,(PO,R,), + 2H,0 = 0,Hj,0, + 2R,HP0, 



The hexose phosphate as is shown in equation (2) is 

 hydrolysed with the production of free phosphate, which 

 again undergoes reaction (1), partly with the sugar formed 

 at the same time, and partly with fresh sugar from the 

 solution. The rate at which the second of these reactions 

 occurs determines the rate of fermentation observed when 

 glucose is fermented by yeast juice, which is therefore a 

 measxire of the rate at which phosphate is being formed in 

 the juice. 



