A similar uncertainty attaches to the determination of the molecular weight from the freezing-point depression and conductivity of the acid potassium salt [Euler and Fodor, [1911]]. Euler however concludes [p051] that both a hexosediphosphoric acid and a triosemonophosphoric acid are formed, but has not prepared any derivatives of the latter.
As regards the constitution of the hexosephosphoric ester several suggestions have been made by Young, but no decisive evidence at present exists. The identity of the products from glucose, mannose, and fructose may be explained by regarding the acid as a derivative of the enolic form common to these three sugars (p. [97]), or by supposing that portions of two sugar molecules may be concerned in its production. The formation and composition of the hydrazone and osazone are of great importance as they indicate that in all probability one of the phosphoric acid residues is united with the carbon atom adjacent to the carbonyl group of the hexose. They moreover render it certain that the original phosphoric ester is a hexosediphosphoric ester and not a triosemonophosphoric ester.
Hexosediphosphoric acid has not as yet been discovered in the animal body. The action of a number of enzymes upon it has been examined [Euler, [1912, 2]; Euler and Funke, [1912]; Harding, [1912]; Plimmer, [1913]] with the following results.
The lipase of castor oil seeds, a glycerol extract of the intestinal mucous membrane of the rabbit and pig, and an aqueous extract of bran have a slow hydrolytic action, whereas pepsin and trypsin are without effect. Feeding experiments with rabbits and dogs indicate that the ester is capable of hydrolysis in the animal body, a large proportion of the phosphorus being excreted as inorganic phosphate. The ester is also decomposed by Bacillus coli communis.
It is remarkable that the hexosephosphate is not fermented nor hydrolysed by living yeast, a fact observed by Iwanoff, Harden and Young, and Euler, although, according to the experiments of Paine [[1911]], the yeast cell is at all events partially permeable to the sodium salt.
The Equation of Alcoholic Fermentation.
An equation can readily be constructed for the reaction in which hexosephosphate is formed, the data available being the formula of the product and the relation between the phosphate added and the carbon dioxide and alcohol produced:—
(1) 2 C6H2O6 + 2 PO4HR3 =
2 CO2 + 2 C2H6O + 2 H2O + C6H10O4(PO4R2)2.
According to this, two molecules of sugar are concerned in the change, the carbon dioxide and alcohol being equal in weight to one [p052] half of the sugar used, and the hexosephosphate and water representing the other half.
Additional confirmation of this equation is afforded by the determination of the ratio between sugar used and carbon dioxide evolved when a known weight of sugar together with an excess of phosphate is added to yeast-juice at 25°. The phenomena then observed are precisely similar to those which occur when a phosphate is added to a fermenting mixture of yeast-juice and excess of sugar as described above. The rate of fermentation rapidly rises and then gradually falls until a rate is attained approximately equal to that of the autofermentation of the juice in presence of phosphate. At this point it is found that the extra amount of carbon dioxide evolved, beyond that which would have been given off in the absence of added sugar, bears the ratio expressed in equation (1) to the sugar added [Harden and Young, [1910, 2]]. The results of four estimations made in this way were (a) 0·2 grams of glucose gave 26·5 and 27·9 c.c. of carbon dioxide at N.T.P.; (b) 0·2 grams of fructose gave 27·9 and 28·9 c.c. The carbon dioxide calculated from the sugar added in each of the four cases is 26·96 c.c.