228 PEACTICAL PHYSIOLOGY 



examples. An amido acid is derived from an organic acid (containing 

 therefore the - COOH group) by the substitution of one of the hydrogen 

 atoms of a methyl (CH 3 ) group by the amido group (NH 2 ). 



Thus acetic acid has the formula CH 3 COOH. 



If now we displace one of the ' H's ' of the CH 3 group by NH 2 we 

 obtain NH 2 



' H 2<COOH 



which is amido acetic acid, also called glycin or glycocoll. 1 It is formed 

 during the digestion of gelatine, but not of native proteids. It also 

 exists in the bile, where it enters into the formation of one of the bile 

 salts (e.g. glycocholate of soda is glycin + cholalic acid). It likewise 

 occurs in combination with benzoic acid as hippuric acid in the urine of 

 herbivorous animals, and in traces in the urine of man. 



If we take the next acid of the acetic acid series, viz., propionic 

 acid CH 3 CH 2 COOH, we obtain amido propionic acid, or alanin, 



In the free state it is only produced from a few proteids, 2 and is 

 unimportant, but it is frequently combined with oxyphenyl, the 

 resulting compound being tyrosin. If in the formula of oxyphenyl 



^ 

 CH 



the H atom opposite to the x 

 OH group be replaced by QJJ QJJ 



CH amido propionic acid, we | || 



II obtain para-oxyphenylamido CH CH 



propionic acid, which is v(y 



^CH/ tyrosin. It therefore belongs | 



(Oxyphenyl.) to the aromatic group of CH 2 - 



organic bodies, and it will (Tyrosin.) 



be remembered that it is on account of its containing oxyphenyl 

 that it reacts red with Millon's reagent (see Proteids, p. 172). 



EXPERIMENT VI. Add Millon's reagent to some pancreatic digest ; 

 a white coagulum of proteids results. Filter. Boil the nitrate. It 

 turns red, because it contains tyrosin. 



1 This relationship to the fatty acids is demonstrated by the following reaction. 

 If acetic acid be treated with bromine gas, brom -acetic acid is formed, 



If now we treat this with ammonia we get : 



Brom-acetic acid. Glycin. 



2 By the hydrolysis of haemoglobin, however, alanin is a very abundant decom- 

 position product. 



