Chemical Constituents of the Animal Body. 245 



(3) The Distribution of the Diamino Nitrogen between 

 Arginine, Histidine, and Lysine, and the Indirect Deter - 

 mination of these Hydrolysis Products. It has recently 

 been shown that the amount of the three diamino-acids 

 present in the phosphotungstic acid precipitate can be 

 determined. It may be recalled that when a substance 

 containing an amino group is treated with nitrous acid 

 nitrogen is evolved, and the amino group is replaced by 

 the hydroxyl group. Glycine, for example, will react with 

 nitrous acid according to the equation 



NH, CH 2 -CO-OH + HO-NO = N 2 + 2H 2 + OH-CH 2 CO OH 



One-half the nitrogen evolved will come from the amino 

 group, and the other half from the nitrous acid. This 

 reaction is characteristic of the amino group only. Now, 

 if reference is made to the formulae of the diamino-acids 

 (p. 222), it will be seen that all the lysine nitrogen is in 

 the form of amino-nitrogen, whereas only one-third is in 

 histidine. In arginine only one-fourth of the nitrogen is 

 evolved on treatment of this substance with nitrous acid. 

 If, now, the percentage of the total nitrogen contained in 

 the phosphotungstic acid precipitate is ascertained, and 

 the amount of this evolved when the diamino-acids there- 

 from are treated with nitrous acid, then, if D = the amount 

 of nitrogen which does not act as amino-nitrogen (that is, 

 difference between the total and amino-nitrogen), and if 

 Arg = arginine nitrogen, then the histidine nitrogen = 

 |{D f Arg). Now the arginine nitrogen can be deter- 

 mined directly, for arginine is the only one of the three 

 diamino-acids from which ammonia is evolved when the 

 substance is treated with potassium hydroxide. Under this 

 treatment arginine decomposes into a base called ornithine 

 and ammonia, according to the following equation : 



C 6 H 14 2 N 4 + 2H a O = C 6 H 12 2 N a (ornitliine) + 2NH, + C0 a 



