NUTRITION 275 



fermented by yeast than is glyceric aldehyde, so that, in this case, it may be the intermediate 

 stage ; although lactic acid itself does not seem to be so (see Harden's monograph, 1911 

 pp. 90-94). 



2. R. Lactic Acid to Glyceric Aldehyde. I am not aware that this change has, 

 directly, been shown to occur. But, of course, if glyceric aldehyde is an inter- 

 mediate stage, it must do so, since lactic acid is converted to glucose, as we 

 have seen. 



3. Glucose to Glyceric Aldehyde. This reaction also has not actually been 

 shown to happen, but the same argument as above applies. 



3. R. Glyceric Aldehyde to Glucose. Embden, Baldesand Schmitz (1912, p. 127) 

 have brought evidence to show that the liver performs this reaction. 



4. R. Glycerol to Glucose. Confirmatory evidence of the importance of glyceric 

 aldehyde is afforded by the behaviour of glycerol. Liithje (1904) showed that the 

 diabetic animal can form glucose from glycerol, and Schmitz (1912) found that 

 glycerol, added to blood perfused through the liver, diminished ; although if the 

 liver were rich in glycogen, this did not occur. 



5. Lactic Acid from Glycerol. Oppenheimer showed that lactic acid is formed 

 from glycerol by perfusion through the glycogen-free liver. The obvious way from 

 glycerol to lactic acid is by glyceric aldehyde, as a stage of oxidation, so that the 

 way to glucose is also, no doubt, through the same substance. 



5. R. Glycerol from Lactic Acid Embden, Schmitz and Baldes (1912, p. 185) 

 showed that the liver, perfused with glyceric aldehyde, forms glycerol, so that this 

 reaction, again, is a reversible one. 



6. R. Alanine from Pyruvic Acid. As already mentioned, the formation of 

 alanine from pyruvic or lactic acid has been shown by Knoop (1910), and by 

 Embden and Schmitz (1910). 



6. Pyruvic Acid from Alanine. Neuberg and Langstein (1903) obtained lactic 

 acid from alanine, so that the reaction is reversible and undoubtedly passes through 

 the stage of pyruvic acid. We are, therefore, justified in placing pyruvic acid as 

 a stage further on than lactic acid in the oxidation of glucose. 



7. Pyruvic Acid to Lactic Acid. Paul Mayer (1912) found that sodium 

 pyruvate in excess, administered subcutaneously, gave rise to both glucose and 

 lactic acid in the urine. Embden and Oppenheimer (1913) found that large 

 amounts of lactic acid were produced by perfusion of the glycogen-free liver with 

 pyruvic acid. 



8. Pyruvic Acid to Glucose. See number 7 above. A. I. Ringer (1913) also 

 found that pyruvic acid yields glucose in the organism ; but in certain cases it 

 was found that the amount obtained was much less than when corresponding 

 amounts of lactic acid or alanine were given. Pyruvic acid, apparently, is not a 

 necessary intermediate stage in the conversion of alanine into glucose. What the 

 intermediate stage is will appear presently. Dakin and Janney (1913) state that 

 pyruvic acid is only converted to glucose when the conditions are such as to favour 

 its initial reduction to lactic acid, which is the necessary intermediate stage. 



9. Pyruvic Aldehyde to Lactic Acid. Pyruvic aldehyde is sometimes, not quite 

 correctly, called methyl-glyoxal, but it cannot chemically be regarded as derived 



from glyoxal (| ) by replacement of a hydrogen atom in an aldehyde group 



VCHO/ 



by methyl. Although Embden and Oppenheimer (1913) do not think that this 

 substance is an intermediate stage between glucose and lactic acid on account of 

 its not being optically active, recent work by Dakin and Dudley (1913, 1, 2, 3) 

 indicates that it has, to say the least, considerable importance. These observers 

 find that there is present in nearly all tissues, especially in the liver and muscles, an 

 enzyme, glyoxalase, which acts with great rapidity on "glyoxals" of various 

 composition, transforming them into lactic acid compounds. The presence of this 

 enzyme strongly suggests that pyruvic aldehyde is an intermediate stage between 

 glucose and lactic acid and it might well come in between glyceric aldehyde and 

 lactic acid in the scheme given above. The fact that it does not possess an 

 asymmetric carbon atom and that, on this account, there are not two optical 

 isomers, as in lactic acid and in glyceric aldehyde, is no serious objection to the 



