14 THE AMINO ACIDS 



animal and vegetable proteins and with the possible exception 

 of arginine is the most widely distributed amino acid found as 

 a protein cleavage product. Leucine has been found also in 

 the urine under pathological conditions. 

 Isoleucine. a-amino-/3-ethyl-propionic acid. 



NH 2 

 > 



This amino acid was not described as a protein constituent 

 until 1903 when it was isolated as a decomposition product of 

 fibrin and other proteins by F. Ehrlich. 



Norleucine. a-amino-normal-caproic acid. CHs.CEb. 

 CH2.CH2.CH.NH2.COOH. From the leucine fraction of 

 the decomposition of the proteins of nervous tissue this amino 

 acid has recently been isolated by Abderhalden and Weil. It 

 is probable that other proteins may yield it also. 



Phenylalanine -. /3-phenyl-a-amino-propionic acid. 



'-COOH 



Although it had been recognized for many years that a 

 substance having the composition of CgHnNC^ could be ob- 

 tained by cleavage of both animal and vegetable proteins, -it 

 was Fischer who first proved the presence of phenylalanine as 

 a protein derivative. In those proteins lacking tyrosine, as 

 gelatin, for example, the aromatic ring is supplied by phenyla- 

 lanine. 



Tyrosine. /3-para-oxyphenyl-a-amino-propionic acid. 



HO.CeH,CH,CH<^ H 



In 1846 Liebig isolated from a decomposition of cheese a 

 substance possessing the property of crystallizing in silky 

 needles. He named it tyrosine. Since then tyrosine has been 

 regarded as a protein cleavage product. It was not until 1882, 



