IV. Effect of Arsenates on the Fermentation of Sugars by Yeast-Juice and Zymin.

The close analogy which exists between the chemical functions of phosphorus and arsenic lends some interest to the examination of the action of sodium arsenate upon a mixture of yeast-juice and sugar, and experiments reveal the fact that arsenates produce a very considerable acceleration in the rate of fermentation of such a mixture [Harden and Young, [1906, 3]; [1911, 1]]. The phenomena observed, however, differ markedly from those which accompany the action of phosphate.

The acceleration produced is of the same order of magnitude as that obtained with phosphate, but it is maintained without alteration for a considerable period, so that there is no equivalence between the amount of arsenate added and the extra amount of fermentation effected. Further, no organic arsenic compound corresponding in composition with the hexosephosphates appears to be formed.

Increase of concentration of arsenate produces a rapid inhibition of [p076] fermentation, probably due to some secondary effect on the fermenting complex, possibly to be interpreted as the formation of compounds incapable of combining with sugar and hence unable to carry on the process of fermentation. An optimum concentration of arsenate therefore exists just as of phosphate, at which the maximum rate is observed, and this optimum concentration and the corresponding rate vary with different samples of juice and are less for glucose than for fructose. The rate of fermentation by zymin is relatively less increased than that by yeast-juice.

Owing to the fact that the rate is permanently maintained the addition of a suitable amount of arsenate increases the total fermentation produced to a much greater extent than phosphate.

The nature of these effects may be gathered from the result of a few typical experiments. In one case the rate of fermentation of glucose by yeast-juice was raised by the presence of 0·03 molar arsenate from 2 to 23 c.c. per five minutes, and the total evolved in ninety-five minutes from 51 to 459 c.c. The accelerating effect on 20 c.c. of juice, of as little as 0·005 c.c. of 0·3 molar arsenate, containing 0·11 mgrm. of arsenic, can be distinctly observed, but the maximum effect is usually produced by about 1 to 3 c.c., the concentration being therefore 0·015 to 0·045 molar. Greater concentrations than this produce a less degree of acceleration accompanied by a shorter duration of fermentation, as shown by the following numbers which refer to 20 c.c. of yeast-juice in a total volume of 40 c.c. containing 10 per cent. of glucose:—

C.cs. of 0·3 Molar Arsenate in 40 c.c.Molar Concentration of Arsenate.Maximum Rate of Fermentation.
00 3·5
0·0050·0000375 6·3
0·010·000075 8
0·020·0001514·2
0·040·000319·9
0·10·0007529·7
0·20·001535
0·50·0037534·9
1·00·007529·5
2·00·01523·2
5·00·037514·5
10·00·075 8·7
15·00·1125 5·3
200·15 3·2

The contrast between glucose and fructose in their relations to [p077] arsenate are well exhibited in the following table, in which the rates of fermentation produced by arsenate in presence of excess of glucose and fructose respectively are given:—

Concentration of Arsenate.Rate.
Glucose.Fructose.
0·0075 molar 12·1 26·6
0·0225 (opt. for glucose) 13·4 —
0·0525 (opt. for fructose) — 45·8
0·1125 5·1 39

Here the optimum concentration for fructose is more than twice that for glucose, whilst the maximum rate of fermentation obtainable with fructose is between three and four times the maximum given by glucose.