CARBOHYDRATES 



191 



yields less glucose than does casein, from which this amino- 

 acid is absent. Of the other amino-acids, several, including 

 glycine, alanine, aspartic acid and glutamic acid, have 

 been proved to be sources of glucose. The chemical 

 changes involved are sometimes very comphcated. There 

 is good reason to beheve that in some cases methyl-glyoxal,- 

 CH3CO.CHO, is formed as an intermediate compound. 

 There exist in various tissues ferments, called glyoxylases, 

 which transform methyl-glyoxal into lactic acid, the re- 

 action being reversible. Methyl-glyoxal yields glucose in 

 the diabetic organism, glyceric aldehyde being probably 

 an intermediate compound, for this also is a source of 

 glucose under the same conditions. Taking alanine as an 

 example, it is probable that the change takes place in the 

 following stages : — 



CH2OH 



CH, 



CH, 



CH, 



13 V.Vii3 vyj^ig 



II I 



2 CHNHo->2 CH0H->2 CO 



CH2OH 



2 CHOH 



CHOH 



I 

 CHOH 



COOH 



COOH 



CHO CHO 



CHOH 



CHOH 



Alanine. 



Lactic acid. 



Methyl 

 glyoxal. 



Glyceric 

 aldehyde. 



CHO 



Glucose. 



The conversion of protein into sugar appears to take 

 place not only in the hver but in the tissues generally, for 

 it occurs after the hver has been short-circuited by an 

 Eck fistula. 



From Fats. — Either component of a fat, glycerine or the 

 fatty acid might conceivably form a source of carbohydrate. 

 Although the conversion of glycerine into glucose is not 

 difficult to perform in vitro, it has been consistently found 

 impossible to increase the excretion of glucose by admin- 



