METABOLISM 157 



219. Intermediary katabolism. Regarding the intermediary 

 katabolism of the carbohydrates, not very much is known. 

 It appears probable, however, that dextrose undergoes pre- 

 liminary cleavage with the formation of glyceric aldehyde, 

 pyruvic aldehyde (methyl glyoxal) and either lactic or pyruvic 

 acid, which is then further oxidized to acetic acid, carbon dioxid 

 and water. Many facts, including especially those derived 

 from a study of the fermentation of sugar, seem to point to the 

 possibility of such reactions. Lactic acid is also widely dis- 

 tributed in the body, although its presence is also susceptible 

 of explanation as arising in the katabolism of protein (233), 

 and, moreover, it has been shown that lactic acid may give rise 

 to glycogen or dextrose in the animal body. Accordingly, 

 these changes, like the mutual transformations of glycogen and 

 dextrose, may be conceived of as constituting a series of re- 

 versible reactions. 



(Dextrose) 



Glycogen ; CH 2 OH(CHOH) 4 CHO 



(Glyceric aldehyde) 



CH 2 OH(CHOH) 4 CHO ^ 2CH 2 OH - CHOH CHO 



(Pyruvic aldehyde) 



CH 2 OH - CHOH - CHO-H 2 O ^ CH 3 CO CHO 



(Lactic acid) 



CH 3 CO CHO + H 2 O : CH 3 CHOH COOH 



or 



(Pyruvic acid) 



CH - CO CHO + O ; CH 3 CO - COOH 



The conversion of dextrose into lactic acid is a nearly iso- 

 thermic process, the resulting lactic acid containing almost the 

 same amount of chemical energy as the dextrose. If, then, 

 these cleavages occur in the katabolism of carbohydrates they are 

 obviously preparatory to the actual oxidation in which the 

 principal portion of the energy is liberated. 



The pentose carbohydrates 



The foregoing paragraphs have treated of the metabolism of 

 the hexoses, which constitute the chief carbohydrate supply of 

 man, and of the carnivora so far as the latter consume carbo- 

 hydrates. The feed of herbivora, however, contains also consid- 

 erable amounts of various pentose carbohydrates which are in part 



