274. PRINCIPLES OF GENERAL PHYSIOLOGY 



Putting chemical formulae in place of names, we have : 

 CH 2 OH.CHOH.CHOH.CHOH.CHOH.CHO 



CH OH.CHOH.CHO 



CH 2 OH.CHOH.CHO^:CH 2 OH.CHOH.CH 2 OH 



CH 3 . CO . CHO :> CH 3 CH(NH 2 ).COOH 



44 



CH 3 .CHOH.COOH 



n 



CH 3 .CO.COOH 



I 



CH 3 .CO.CH,.COOH <- CH 3 .CHO ;> CH 3 .CH 2 .OH 



i 



(CH 3 .COOH) 



* 

 2CO 2 + 2H 2 O 



It will be noticed that most of these reactions are marked as being reversible ; 

 we shall find evidence of this by direct experiment in most cases and, as is pointed 

 out by Embden and Kraus (1912), the processes of hydrolysis, oxidation, and 

 synthesis are all intimately connected iu carbohydrate metabolism. Carbohydrate 

 food, when stored, takes the form of glycogen and this, hydrolysed, becomes 

 glucose as required by the organism. Here we have clearly a reversible reaction 

 and the fact that glucose is produced warrants our taking glucose as the starting 

 point of our investigation. 



We may now inquire what experimental evidence there is for the series of 

 changes represented as occurring in the organism. It will be noted that some of 

 the reactions in the numbered paragraphs include more than one step. They are 

 numbered for convenience of future reference and the letter R directs attention 

 to the fact that the reaction so marked is the synthetic aspect of the reaction 

 with the same number. 



1. Glucose to Lactic Acid. Embden and Kraus (1912) showed that the liver, 

 when poor in glycogen, produces lactic acid when blood containing glucose is 

 perfused through it. If the liver contains much g4ycogen, lactic acid is given off, 

 without the necessity of adding glucose. 



1. R. Lactic Acid to Glucose. The previous reaction reversed. In the 

 above experiments, if the liver was poor in glycogen and blood containing 

 lactic acid was perfused, lactic acid was found to disappear. Further, lactic 

 acid is converted to glucose in the dog, made diabetic by removal of the 

 pancreas (Embden and Oppenheimer, 1912, p. 196), (Mandel and Lusk, 1906). 



The various changes with which we are dealing are, in all probability, some of them 

 certainly, carried out by the agency of enzymes. The conditions in which enzymes favour the 

 synthetic side of reversible reactions will be discussed in the next chapter. 



2. Glyceric Aldehyde to Lactic Acid. Why is glyceric aldehyde 



(CH 2 OH.CHOH.CHO), 



instead of dihydroxy-acetone (CH 2 OH.CO.CH 2 OH), indicated as the intermediate 

 stage between glucose and lactic acid ? From the action of alkali on glucose (see 

 Dakin's monograph, 1912, p. 86) it is most probable that one or the other of these 

 is the correct one. Embden, Baldes and Schmitz (1912) showed that washed blood 

 corpuscles readily form lactic acid from the former, as they do from glucose, but 

 that from dihydroxy-acetone very little is formed, Jess in fact than from glucose, 

 so that it does not appear to be the normal process. It is remarkable that the 

 unnatural /-lactic acid is formed in larger proportion than the eMactic acid. 

 The liver, when poor in glycogen, has the same effect. 



It is probable that the /-lactic acid appeared in these experiments because the racemic 

 glyceric aldehyde was used and the rf-component is used by the liver to form glucose more 

 rapidly than is the /-component, with which the lactic acid reaction has to be content, so to 

 speak. On the other hand, there is evidence that di-hydroxy-acetone is more readily 



