390 PRACTICAL PHYSIOLOGY 



produced, of which leucin and tyrosin are examples. An amino acid is 

 derived from an organic acid (containing therefore the - COOH group) 

 by the substitution of one of the hydrogen atoms attached to a carbon 

 atom of the chain (e.g. CH 2 - , = CH - ) by the amino group (NH 2 ). 



Thus acetic acid has the formula CH 3 COOH. 



If one of the "H's" of the CH 3 group be displaced by NH 2 , the 

 result is NH 2 . CH 2 . COOH, which is amino acetic acid, also called 

 glycin and glycocoll. 



Glycin is formed during the digestion of gelatine and globulin, but 

 not of albumin. 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 to a less 

 extent in the urine of man. 



The amino acid corresponding to the next acid of the acetic acid 

 series, viz. propionic acid CH 3 . CH 2 . COOH is amino- 

 propionic acid, or alanin, CH 3 . CHNH 2 . COOH. In T 

 the free state it is only produced from a few proteins, 1 

 and is unimportant, but it is frequently combined 

 with phenol, the resulting compound being tyrosin. 

 If in the formula of phenol the H atom in the para |' / 2 



position to the OH group be replaced by amino CH CH 



propionic acid, para-hydroxyphenylamino-propionic \ 



acid, or tyrosin results. It, therefore, belongs to the COOH. 



aromatic group of organic bodies, and because it Tyrosin. 

 contains hydroxyphenyl (the radicle of phenol) it reacts red with 

 Millon's reagent (see Proteins, p. 302). 



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

 a white coagulum of proteins results. Filter. Boil the filtrate. It 

 turns red, because it contains tyrosin. 



EXPERIMENT V. Examine the crystals of tyrosin supplied you 

 under the microscope, and note that they consist of fine needles 

 grouped into star-shaped masses (Fig. 236). 2 



There are no other important amino acids till we come to the 

 member of the series which contains six carbon atoms, viz., caproic 



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

 position product. 



2 A body very closely related to tyrosin in its chemical structure has recently 

 been described amongst the products of hydrolysis of proteins by acids. This is 

 phenylalanin, differing from tyrosin only in that it does not contain the phenolic 

 OH group. It has also been discovered in the products of the prolonged action 

 of pepsin on proteins. 



