544 



METABOLISM, NUTRITION AND DIETETICS 



and water, from lactic acid has given rise to much discussion, and is 

 not yet clearly known. The following scheme, based on the researches 

 of Embden and others, and quoted from Abderhalden, illustrates one 

 suggestion as to the course of the series of transformations, although 

 it must be taken only as a diagram of the sequence of some of the 

 possible stages. A molecule of dextrose is represented as giving rise 

 to two molecules of glyceric aldehyde, each of which then yields a mole- 

 cule of lactic acid. Each molecule of lactic acid, losing two atoms of 

 hydrogen, becomes converted into a molecule of pyruvic acid, which 

 by the loss of the elements constituting a molecule of carbon dioxide 

 becomes acetaldehyde or acetic aldehyde, and this by oxidation acetic 

 acid, which is then oxidized to carbon dioxide and water. Thus 



S H 



H C OH 



HO C H 

 H C OH 

 H C OH 

 CH 2 OH 



({dextrose. 



COOH 



CO -CO, 



H C OH 

 CH 2 OH 



\ 



H C OH 

 CH 2 OH 



2 molecules 

 gjyceric aldehyde. 



COOH 



H C OH -H 2 

 CH, 



d-lactic acid. 



c /-o 



U 



+O 



CH 3 



Pyruvic acid. 



CH 3 



Acetaldehyde. 



COOH 



Acetic acid. 



2CO a 



-> + 



2H a O 



Carbon dioxide 

 and water. 



It has been shown that acetaldehyde and carbon dioxide are formed 

 from pyruvic acid by the action of a ferment contained in yeast, and 

 there is some evidence that a similar transformation may occur in the 

 liver. 



It is to be particularly remarked that according to this scheme 

 the whole of the dextrose molecule is still represented in the lactic 

 acid formed from it. Up to this stage no part of the molecule has 

 been burnt. Nearly the whole of the chemical energy i.e., all but 

 about 3 per cent, of it is still available. For a gramme of lactic 

 acid yields 3,661, and a gramme of dextrose 3,762, small calories on 

 complete combustion. The intermediate products of the decom- 

 position may therefore be transported from the place of origin and 

 utilized elsewhere with scarcely any loss of energy. Further, it is 

 indicated in the scheme that the degradation process is not merely 

 a series of cleavages and oxidations, but that these may be inter- 

 spersed with stages of reduction. It is also clearly suggested that 

 at certain points the metabolism may become recessive and syn- 



