380 THE METABOLISM OF THE CARBOHYDRATES 



body from proteid from another point of view, namely, that the 

 sugar is derived from some of the decomposition products of the 

 proteid molecule itself that is, from amido-acids, hexone bases, 

 aromatic bodies, &c. 



In considering the possibility of such a process we must admit 

 at the outset that a liberation of the amido group (NH 2 ) is possible 

 in the body. That such a process does actually obtain we have 

 already seen in connection with glucosamin and amido-purins. 



The amido-acids which we would naturally look to as possible 

 precursors of dextrose are those which contain either three or 

 six carbon atoms. Such amido-acids are leucin and alanin. 



Alanin is amido-propionic acid and has the formula 



CH 3 



CH - NH 2 



COOH 



By the action of nitrous acid T ( 1 ) on this, the NH 2 group is sub- 

 stituted by an OH group, the resulting body being lactic acid or 

 a-hydroxy-propionic acid : 



CH 3 



CHOH 



I 

 COOH 



Now lactic acid is closely allied to glyceric aldehyde 2 ( 2 ), which 

 by a process of condensation easily passes into dextrose, thus : 



CH OH CH,OH CH.,OH 



I I I . 

 CHOH +CHOH =(CHOH) 4 



I I I 

 CHO CHO CHO 



(Glyceric aldehyde) (Dextrose) 



1 ( x ) When nitrous acid is allowed to act on any organic substance of the 

 paraffin series containing an amido group, the amido group is replaced by 

 hydroxyl, and free nitrogen is evolved. 



2 ( 2 ) Glyceric aldehyde can be obtained from acrolei'n dibromide by treating 

 the latter with baryta water. 



CH 2 Br CH 2 OH 



CHBr + Ba(OH) 2 = CHOH + BaBr 2 



COH COH 



