The spontaneous production of ketonic aldehydes from amino-acids and from hydroxy-acids in aqueous solution, which has been demonstrated by Dakin and Dudley [1913], points however to the possibility that the ketonic acid may be a secondary product derived from the corresponding ketonic aldehyde [see also Dakin, 1908; Neuberg, 1908, 1909]. This itself may either arise directly from the amino-acid or from a previously formed hydroxy-acid, the latter alternative being, however, improbable in view of the small yield of alcohol obtained from hydroxy-acids by the action of yeast in the experiments of Neubauer and Fromherz. R·CH(NH{2})·COOH → R·CH(OH)·COOH R·CH(NH{2})·COOH ⇌ R·CO·CHO R·CO·CHO ⇌ R·CH(OH)·COOH 2R·CO·CHO + O{2} → 2R·CO·COOH (2) Whatever be the exact mode by which the ketonic acid is formed, it appears most probable that a compound of this nature forms the starting-point for the next stage in the production of the alcohols. The researches of Neuberg, which have already been discussed on p. 81, have revealed a mechanism in yeast--the enzyme carboxylase--by which these α-ketonic acids are rapidly broken up into an aldehyde and carbon dioxide: R·CO·COOH = R·CHO + CO{2} and it can scarcely be doubted that this is the actual course of the reaction. (3) The final conversion of the aldehyde into the corresponding alcohol is also a change which it has been proved can be effected by yeast [Neuberg and Rosenthal, 1913] probably by the aid of the reductase which is one of the weapons in its armoury of enzymes. Yeast is capable of producing many vigorous reducing actions and rapidly reduces methylene blue and sodium selenite. It is in all probability due to a reaction of this kind that the iso-amylaldehyde and isovaleraldehyde were reduced to the alcohols in Neuberg and Steenbock's experiments [1913, 1914], and that considerable quantities of ethyl alcohol are formed in the sugar free fermentation of pyruvic acid [Neuberg and Kerb, 1913, 1] (see later p. 110 for a discussion of this question). A further possibility exists that in some cases the aldehyde may [p094] be simultaneously oxidised and reduced or the molecule of one aldehyde reduced and that of another oxidised with production of the corresponding acid and alcohol by an "aldehydo-mutase," similar to that which has been observed by Parnas [1910] in many animal tissues. Finally the aldehyde may simply be converted into the corresponding acid by oxidation as appears to take place in the formation of succinic acid. The intermediate production of an aldehyde would thus be consistent both with the production of alcohols and acids from amino-acids. Fusel oil would be formed by the reduction of the aldehydes arising from the simple monobasic amino-acids, succinic acid would be produced by oxidation of the aldehyde derived from the dibasic glutamic acid.