84 PHYSIOLOGY 



CH 2 CH.NH 2 COOH 



It is an almost constant constituent of proteins. 



TRYPTOPHANE was known long before it had been isolated, owing to, 

 the fact that with bromine water it gives a rose-red colour. It had long 

 been observed that this substance was to be obtained at a certain stage in the 

 digestion of proteins by pancreatic juice, but nothing was known about its 

 constitution until Hopkins succeeded in isolating it by precipitation with 

 mercuric sulphate dissolved in 5 per cent, sulphuric acid. Cystine is also 

 precipitated by this reagent, but comes down with a less concentration of the 

 salt than tryptophane, so that it is possible to separate the two substances 

 by a species of fractional precipitation. Tryptophane can be isolated by 

 decomposing the mercury salt with sulphuretted hydrogen, and is obtained 

 in a crystallised form. On distillation it gives an abundant yield of indol and 

 skatol, bodies also obtained during the putrefaction of proteins. Trypto- 

 phane itself is indol amino-propionic acid : 



,C.CH 2 CHNH 2 .C( 



CysJ 



CH 

 NH 



C. AMINO-ACIDS OF HETEROCYCLIC COMPOUNDS 



Three of the disintegration products of proteins can be grouped in this 

 class. Two of them contain the pyrrol ring, namely, proline and oxyproline. 



PROLINE, which was first isolated by Fischer, is a-pyrrolidin carboxylic 

 acid and has the formula 



CH 2 CH 2 



I I 

 CH 2 CH.COOH 



NH 



OXYPROLINE is the oxy-derivative of this body and has the formula 

 C 5 H 9 N0 3 , the exact position of the oxy-group having not yet been deter- 

 mined. Doubts have been expressed whether the pyrrol group is present 

 as such in the protein molecule, or whether proline, for example, is 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-valerianic acid 

 CH 2 OH.CH 2 .CH 2 .CH.NH 2 .COOH we can by dehydration make the com- 

 pound CH 2 CH a .CH 2 .CH.COOH, the formula of which will be seen to be 



NH 



identical with that given for proline. 



The third member of this group contains the iminazol ring : 



