THE PROTEINS 81 



LEUCINE, one of the oldest known members of the group of amino-acids, 

 is obtained in large quantities from the disintegration of nearly all the animal 

 proteins, of which in some cases it may form as much as 20 per cent. It 

 has the formula 



CH 3 CH 3 



x/ 



CH 



i 

 CH 2 



I 

 CH.NH 2 



I 

 COOH 



i.e. it is amino-isobutyl acetic acid. On evaporating a tryptic digest of 

 protein, impure leucine crystallises out in the form of imperfect crystals, 

 the so-called ' leucine cones.' 



Lately another isomer of leucine has been discovered, namely, a-amino -methyl 

 ethyl propionic acid. This is called isoleucine. 



(2) Mono-amino Derivatives of Dibasic Acids 

 Of these two are known, namely, aspartic and glutamic acids. 

 ASPARTIC ACID is a-amino-succinic acid : 



COOH 



I 

 CH.NH 2 



I 

 CH 2 J 



I 

 COOH 



and glutamic acid is the next homologue, namely, a-amino-glutaric acid : 



COOH 



I 

 CH.NH 2 



I 

 CH 2 



I 

 CH 2 



I 

 COOH 



Owing to the possession of two carboxyl groups these amino-acids have a 

 much more pronounced acid character than is the case with the other 

 members of the group which we have been studying. 



Aspartic acid was first found in the shoots of asparagus in the form of the amide, 

 asparagine : i 



COOH 



I 

 CHNH 2 



I 

 CH 2 



I 

 CONH 2 



