The Pyruvic Acid Theory.
The third stage of Lebedeff's theory postulates the intermediate formation of pyruvic acid. This idea immediately suggested itself when it became known that yeast was capable of rapidly decomposing a-ketonic acids with evolution of carbon dioxide [see Neubauer and Fromherz, [1911], p. 350; Neuberg and Kerb, [1912, 4]; Kostytscheff, [1912, 2]].
This scheme has been differently elaborated by different workers. According to Kostytscheff it involves (1) the production of pyruvic acid from the hexoses, a process accompanied by loss of hydrogen; (2) the decomposition of pyruvic acid into acetaldehyde and carbon dioxide; and (3) the reduction of the acetaldehyde to ethyl alcohol.
(1) C6H12O6 = 2 CH3·CO·COOH + 4[H].
(2) 2 CH3·CO·COOH = 2 CH3·CHO + 2 CO2.
(3) 2 CH3·CHO + 4 H = 2 CH3·CH2·OH.
1. As regards the production of pyruvic acid from the hexoses by yeast, the only direct evidence is afforded by the experiments of Fernbach and Schoen [[1913]] who have obtained a calcium salt having the qualitative properties of a pyruvate by carrying out alcoholic fermentation by yeast in presence of calcium carbonate, but have not yet definitely settled either the identity of the acid or its origin from sugar. Pyruvic acid is, however, very closely related to several substances which are intimately connected both chemically and biochemically with the hexoses. Thus lactic acid is its reduction product,
CH3·CO·COOH + 2 H → CH3·CH(OH)·COOH,
glyceraldehyde can readily be converted into it by oxidation to glyceric acid followed by abstraction of water (Erlenmeyer), [p110]
CH2(OH)·CH(OH)·CHO + O → CH2(OH)·CH(OH)·COOH
CH2(OH)·CH(OH)·COOH − H2O → CH3·CO·COOH,
and finally methylglyoxal CH3·CO·CHO is its aldehyde.
2. The decomposition of pyruvic acid into acetaldehyde and carbon dioxide has already been fully discussed (Chapter VI). The universality of the enzyme carboxylase in yeasts and the rapidity of its action on pyruvic acid form the strongest evidence at present available in favour of the pyruvic acid theory. Given the pyruvic acid, there is no doubt that yeast is provided with a mechanism capable of decomposing it at the same rate as an equivalent amount of sugar.