86 PHYSIOLOGY 



not formed by the closing of an open chain of a compound belonging 

 to the amino-acids in the fatty series. Thus from an oxy-amino-valeri- 

 anic acid CH 2 OH.CH 2 .CH 2 .CH.NH 2 .COOH we can by dehydration make 

 the compound CH 2 CH 2 .CH 2 .CH.COOH, the formula of which will be seen 



NH 



to be identical with that given for proline. 



The third member of this group contains the iminazol ring : 



CH NH, 



!! > H 



CH - N^ 



and is known as HISTIDINE. Its structural formula is as follows : 



CH NH 



C - W 



CH 2 .CH.NH 2 .COOH 



i.e. it is iminazol oc-amino-propionic acid or iminazol alanine. Since it occurs 

 in the phosphotungstic precipitate from the products of acid disintegration 

 of proteins and contains six carbon atoms, it was formerly classified with 

 lysine and arginine as a hexone base. 



D. SULPHUR-CONTAINING AMINO-ACIDS 



Sulphur forms an integral part of the molecule of all classes of proteins 

 except protamines. In some substances allied to proteins, such as keratin, 

 it may occur to the extent of 3 per cent. On boiling proteins with caustic 

 potash or soda, a portion of the sulphur is split off to form a sulphide, which 

 gives a black precipitate on addition of copper salts. On this account it was 

 formerly thought that the sulphur must be present in two forms, the oxidised 

 and the unoxidised, in the protein molecule. Recent investigation has 

 shown, however, that practically the whole of the sulphur is present in the 

 form of CYSTINE, and that this body on boiling with alkaline solutions gives 

 up only a little more than half its content in sulphur. 



This substance, which has been known for many years as the chief con- 

 stituent of a rare form of urinary calculus and as occurring in the urine in 

 certain cases of disordered metabolism, is again a derivative of the three- 

 carbon propionic acid. On reduction it gives a body known as cysteine, 

 which is a-amino-thiopropionic acid. 



CH 2 SH 



CH.NH 2 



COOH 



