276 PRINCIPLES OF GENERAL PHYSIOLOGY 



view of its importance as an intermediate substance between them. Dakin, 

 indeed, thinks that the fact of its optical inactivity is favourable to the synthesis 

 of dextrose through glyceric aldehyde. Suppose that both optical isomers of lactic 

 acid be present, then, if converted first into pyruvic aldehyde, dextroglyceric 

 aldehyde may be formed from both, by an appropriate optically active catalyst, 

 and, from this, dextrose. In fact, ^-lactic acid, the unnatural form, readily yields 

 glucose in the animal organism, when rendered diabetic by phloridzin (Dakin and 

 Dudley, 1913, 2, p. 129). We shall see later that an optically active catalyst is 

 able to form, from optically inactive substances, a preponderance of one optical 

 isomer of an optically active product. When acting on pyruvic aldehyde, 

 glyoxalase yields a mixture of the two forms of lactic acid, but in unequal 

 proportion, and the authors think that two enzymes are concerned, since an 

 enzyme preparation, giving, when fresh, a preponderance of the Isevo-acid, after 

 standing, gave an excess of the dextro-acid, when acting on a new supply of the 

 substrate. Glyoxalase appears to have a wide distribution ; it has been found in 

 the oyster and in yeast. It is absent from the pancreas and a substance is present 

 in extracts of this gland which has the power of actually inhibiting the action of 

 glyoxalase (Dakin and Dudley, 1913, 3). These facts are significant in view of 

 the profound relation between the pancreas and carbohydrate metabolism. 



9. R. Lactic Acid to Pyruvic Aldehyde. Dakin and Dudley (1913, 1) 

 showed that lactic acid is readily converted into pyruvic aldehyde by digestion 

 with nitro-phenyl-hydrazine. Further, that in faintly acid solution both lactic 

 acid and alanine are decomposed with formation of pyruvic aldehyde : 

 CH 8 .CHOH.COOH^CH 3 .CO.CHO + H,O 

 CH 3 .CH.NH 2 .COOH->CH 3 .CO.CHO + NH 3 



By the action of glyoxalase, then, lactic acid can be obtained from alanine through 

 the intermediation of pyruvic aldehyde. With the exception of the direct con- 

 version of pyruvic aldehyde to alanine, all the reactions involving the interconversion 

 of glucose, pyruvic aldehyde, lactic acid, and alanine are shown to be reversible and 

 the authors named have obtained the analogous synthesis of glycine from glyoxal. 



Acetaldehyde. Neubauer (1909) showed that a-ketonic acids are changed 

 in the organism into the ordinary fatty acid with one less carbon atom ; so that 

 pyruvic acid will go into acetic acid. In this process it does not seem possible 

 that any intermediate stage other than that of acetaldehyde would be passed 

 through. 



According to Neuberg and Karczag (1911) yeast juice ferments pyruvic acid with the 

 production of carbon dioxide and acetaldehyde. Masuda (1912) found that the liver, perfused 

 with blood containing alcohol, forms aldehyde and Enibden and Baldes (1912) that the reverse 

 change from acetaldehyde to alcohol also takes place, even in the presence of oxygen. 



There is no evidence that ethyl alcohol is a direct stage in the oxidation of 

 glucose in the animal organism, but it appears that acetaldehyde may well be 

 a stage in the formation of alcohol from sugar in fermentation, although it does 

 not seem to be a necessary one. Alanine is fermented by yeast with formation 

 of alcohol, carbon dioxide, and ammonia and the most likely stages seem to 

 be pyruvic acid and acetaldehyde. 



Mention may be made of the fact that, under certain conditions, probably of deficient 

 oxidation, ethyl alcohol may be obtained by distillation of various tissues, especially nni-rlr. 

 The possibility of absorption from the alimentary canal seems to have been excluded in some 

 of these experiments, although it must be admitted that it does not appear to be an easy 

 matter to lie certain that it is so. 



Aceto acetic-acid is produced by the liver from pyruvic acid (Embden and 

 Oppenheimer, 1912). It must be formed by aldol condensation from acetaldehyde, 

 through jft-oxy-butyric acid. 



" Aldol con(l<>i*<itif>n," the reader may be reminded, is simply the union of two molecules 

 of an aldehyde, which may lie effected, for example, by the action of strong hydrochloric 

 acid, thus : 



CH :1 CHO + CH 3 CHO = CH :( CH(OH)CH. 2 CHO. 



Aceto-acetic acid is found in the urine in certain pathological states associated 

 with distui'bed carbohydrate metabolism and has been found by Masuda (1912) 



