1911. ] The Alcoholic Ferment of Yeast-juice. 461 
The facts that much more than an equivalent of carbon dioxide is 
evolved at the accelerated rate and that the arsenate is found in the free 
state at every stage of the fermentation render it necessary to suppose that 
any arsenic compound which is formed is of a very unstable nature, and 
only exists at any moment in a very low concentration. 
On the other hand, it is also possible that the arsenate accelerates the 
_ fermentation by increasing the rate at which free phosphate is produced 
from the hexosephosphate. This substance is hydrolysed by the enzyme 
hexosephosphatase according to the equation : 
(2) CegHi04( POsNas)2 + 2H,O = CgH120¢ + 2Na2H PO,, 
and it is the rate of this reaction which is the controlling factor of the rate 
of fermentation of sugar by yeast-juice in the absence of added phosphate.* 
Acceleration of this change would therefore result in a constant increased 
supply of phosphate, and would enable reaction (1) to proceed at a 
correspondingly greater rate. In other words, the effect of the arsenate 
would be to produce a condition similar to that existing when free 
phosphate is added gradually to yeast-juice so that its concentration is 
maintained at the optimum, and the reaction proceeds at its maximum 
velocity as long as this condition is maintained.f 
It. also follows from the fact that the rate of fermentation is dependent 
upon the rate of production of free phosphate that the acceleration produced 
by the addition of arsenate cannot be due to an increase in the activity of the 
zymase alone. In a mixture of fresh yeast-juice and sugar this enzyme is not 
producing its maximum effect, as is shown by the fact that the addition of 
phosphate causes a large increase in the rate of fermentation. In other words 
there is an excess of enzyme already present in the mixture. An increase 
in the activity of the zymase, therefore, without an increase in the supply of 
phosphate, could not be expected to cause any acceleration in the rate of 
fermentation. It is possible, however, that the activity both of the hexose- 
phosphatase and of the zymase might be simultaneously increased by the 
addition of arsenate. In this case it would be possible to obtain a higher 
maximum by the addition of arsenate in presence of the right amount of 
phosphate, than could be obtained by adding phosphate alone. 
1. Proof that Arsenate cannot replace Phosphate in the Fermentation of 
Sugars by Yeast-juice—tThe first of these alternative explanations, according 
to which arsenate is supposed to take the place of phosphate and to react 
with the sugar to form an unstable hexose-arsenate, can be investigated in 
* Harden and Young, ‘Roy. Soc. Proc.,’ B, 1908, vol. 80, p. 299. 
+ Harden and Young, ‘Roy. Soc. Proc.,’ B, 1908, vol. 80, p. 309. 
VOL. LXXXIIL—B. 2M 
