CARBOHYDRATES AND FATS. 917 



But laboratory investigations suggest that this conversion takes 

 place indirectly with the intermediate formation of glyceric 

 aldehyde or methylglyoxal, or both, according to the following 

 reactions : 



2H 2 = 2(CH 3 COCOH) 



Dextrose. Methylglyoxal. 



C 6 Hi 2 O 6 H 2 O = CHaCOGOH + C2H 3 (OH) 2 COH 



Dextrose. Methylglyoxal. Glyceric aldehyde. 



The methylglyoxal may then be converted to lactic acid. 

 CH 3 COCOH + H 2 O = CH 3 CHOHCOOH 



Methylglyoxal. Lactic acid. 



Many of the tissues of the body contain an enzyme (glyox- 

 ylase)* capable of giving this reaction. Regarding the further fate 

 of the lactic acid there is also much uncertainty. The usual sup- 

 position is that it undergoes oxidation to carbon dioxid and water, 

 but the steps in this reaction are unknown. According to one 

 hypothesis the conversion takes place through a series of reactions 

 involving reduction and cleavage with the final formation of 

 acetic aldehyde (CH 3 COH), which then by oxidation is converted 

 to acetic acid, formic acid, and then to carbon dioxid and water. 



Other authors find no experimental evidence that lactic acid 

 itself is readily oxidized, and assume that the oxidation takes place 

 in the methylglyoxal stage. Lactic acid by dehydration may be 

 changed to methylglyoxal, and this, in turn, to acetic aldehyde, 

 which then undergoes oxidation f. On the other hand, it must be 

 remembered that there is clear evidence that the body can convert 

 lactic acid back to dextrose probably through the intermediate 

 formation of methylglyoxal. The change from dextrose to lactic 

 acid is a reversible reaction which in the body can take place in 

 either direction according to conditions. Under normal circum- 

 stances the carbohydrate of our food is all oxidized in a few hours, 

 and we may assume, therefore, that usually the lactic acid and other 

 intermediary products follow the path of oxidation. 



At least one other interesting product formed from sugar 

 has been detected in the body, namely, glycuronic acid. This 

 substance has been obtained from the blood, liver, and urine, and 

 exists usually combined with various toxic or injurious substances, 

 such as the phenols or camphor. When these substances are 

 given to animals or, as in the case of the phenol, are formed in the 

 body, they are apparently conjugated with glycuronic acid and 



* Dakin and Dudley, "Journal of Biological Chemistry," 14, 155, 1913. 

 f Lusk, "The Science of Nutrition/' 3d ed., 1917, p. 260. 



