Chemical Constituents of the Animal Body. 241 



sequently shown by Hopkins and Cole that a positive 

 result is only obtained when the acetic acid contains gly- 

 oxylic acid, which it does if the former acid has been 

 exposed to air in the presence of sunlight. The latter 

 acid is now substituted for the former when carrying out 

 the test. A solution of this is prepared by treating 

 250 c.c. of a concentrated solution of oxalic acid with 

 10 grammes of magnesium powder. The solution is then 

 filtered, slightly acidified with acetic acid, and then diluted 

 to 1 litre. A few drops of this are added to the protein 

 solution, and then concentrated sulphuric acid is carefully 

 poured in ; if tryptophan is present a violet colour de- 

 velops. There are, in addition, various other reactions for 

 tryptophan. 



(5) A substance, discovered by Ruhemann, known as 

 triketohydrindene hydrate, yields, on boiling with neutral 

 solutions of proteins and amino-acids, a blue colour. This 

 is known now as the " ninhydrin " reaction, and has been 

 largely used in recent years for certain clinical tests. 

 Various other colour reactions for proteins have been de- 

 scribed, but these mentioned are, perhaps, more often 

 employed than any of the others. 



METHODS FOE THE IDENTIFICATION OF INDIVIDUAL 

 PROTEINS. 



As already stated, the proteins differ from one another 

 in the quality and quantity of their hydrolysis products. 

 The complete quantitative estimation of the monoamino- 

 acids is a complicated process requiring relatively large 

 amounts of material, and even when carried out with great 

 care and expenditure of time, it yields only approximately 

 correct results. The estimation of the diamino-acids by 

 the method indicated (p. 226) is simpler and more satis- 

 factory ; even this, however, is too cumbersome as a routine 



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