THE CARBOHYDRATES AXD THEIR METABOLISM 267 



swings to left and right, that is, to glucose and downward. The particles 

 that go over to glucose are ultimately broken down again, so that in the 

 course of time the whole amount given is completely oxidized to carbon 

 dioxid and water. Because of the relatively high concentration in the 

 blood of the substance under discussion, the kidney may excrete some of it 

 and also those products which stand nearest to it (excretion of lactic acid 

 in the urine after pymvic acid administration). But if the same metabo- 

 lite is fed to a diabetic animal, the moment a particle is converted to 

 glucose it becomes trapped, because these animals have lost the power of 

 splitting the glucose molecule. The reaction becomes one-sided and ir- 

 reversible, and if the oxidative processes are not very great the substance 

 may be completely converted to glucose. 



Glucose < - METABOLITE 7-* Lower product 



It will now be readily seen that a number of three carbon compounds, 

 namely glycerie aldehyde, dioxyacetone, pymvic aldehyde, lactic acid and 

 pymvic acid, may be safely considered stages of glucose catabolism, and 

 that these substances in the animal body may undergo reactions whereby 

 one is converted into the others either by processes of oxidation, reduction, 

 hydra tion, dehydration or by rearrangement of the position of hydrogen in 

 the molecule. All of these steps are reversible. 



One of the later stages in the reaction is a process of decarboxilation 

 during which a three carbon compound is converted into a two carbon com- 

 pound with the loss of carbon dioxid. This is the first irreversible reaction 

 in the entire chain. 



CO CHO + C0 2 



COOH 

 Pyruvic Acid -- > Acetaldehyde 



That pymvic acid can be converted into acetaldehyde was demonstrated 

 in a series of experiments by Is r euberg and Karczaz (1911, 1912). They 

 found that all yeast cells possess that power and that the decarboxilation 

 is brought about by an enzyme, "carboxylase." 



Acetaldehyde is a very important intermediary stage of carbohydrate 

 catabolism. Just as lactic and pymvic acids link the carbohydrate metab- 

 olism with that of protein, so acetaldehyde links carbohydrate with fat me- 

 tabolism. As will be shown later acetaldehyde is in all probability the start- 

 ing point from which fat is built up in the body. Acetaldehyde in the 

 organism may undergo oxidation to acetic acid which on further oxidation 

 is converted to carbon dioxid and water. It may also be reduced to ethyl 

 alcohol, which is ultimately oxidized to carbon dioxid and water. 



