CARBOHYDRATES AND FATS. 907 



lation is adjusted so that the percentage of sugar in the blood is 

 kept astonishingly constant, between 0.1 and 0.2 per cent., not 

 only during the conditions of ordinary living, but under such an 

 abnormal condition as prolonged starvation. It is assumed that 

 this constancy of composition is effected mainly by an enzyme 

 formed in the liver cells, which converts the glycogen to dextrose 

 in proportion as the sugar of the blood is used up by the tissues, 

 and, as was stated in the chapter on Internal Secretions, this 

 process of glycogenolysis may be accelerated or retarded by the 

 action of hormones arising in the ductless glands. 



The Intermediary Metabolism of the Carbohydrate in the 

 Body. Some of the carbohydrate of the food may be used in con- 

 structive processes in the body, in the building up of nucleic acid, 

 for example, but for the most part it is supposed to be an energy 

 food and eventually is oxidized to carbon dioxid and water. This 

 oxidation is not direct or immediate, but probably takes place 

 through a number of steps. Our knowledge of these intervening 

 stages in the breaking down of the molecule is very incomplete. 

 It is generally believed that the initial stages are of the nature of 

 hydrolyses or cleavages in which some of the chemical energy is 

 liberated and utilized, while the final stages are oxidations in which 

 the greater part of the available energy is set free, probably in the 

 form of heat. Many suggestions have been made in regard to the 

 successive steps in the metabolism, but perhaps the only stage 

 that may be considered as demonstrated is the formation of lactic 

 acid. The molecule of dextrose may give rise to two molecules of 

 lactic acid, according to the formula 



C 6 H 12 6 = 2(CH0,) 



It seems probable that this change is not direct, but takes place 

 through an intervening production of such compounds as glyceric 

 aldehyde or methylglyoxal. According to the latter view, the 

 steps might be represented as follows: 



C 6 Hi 2 O 6 2H 2 O = 2(CH 3 COCOH) 



Dextrose. Methylglyoxal. 



CH 3 COCOH -f H 2 O = CH 3 CHOHCOOH 



Methylglyoxal. Lactic acid. 



It is known that from many of the tissues of the body an enzyme 

 can be obtained (glyoxalase)* which converts methylglyoxal to 

 lactic acid. Regarding the further history of the lactic acid there 

 is also much doubt. In the first place, lactic acid may be formed 



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



