78 PHYSIOLOGICAL CHEMISTRY 



It closely resembles leucine in many of its properties, but is more soluble 

 in water. It is a difficult matter to identify valine in the presence of 

 leucine and isoleucine inasmuch as these amino acids crystallize together 

 in such a way that the combination persists even after repeated recrys- 

 tallizations. Valine is dextro-rotatory. 



Arginine, C6Hi 4 N 4 2 . Arginine is d-guanidine-a-amino-valeric acid 

 and possesses the following structural formula: 



H H H NH 2 



I I I I 

 NH C C C C COOH. 



I I I I I 

 NH=C H H H H 



NH 2 



It has been obtained from every protein so far subjected to decomposi- 

 tion. The arginine obtained from proteins is dextro-rotatory, and has 

 pronounced basic properties, reacts strongly alkaline to litmus, and forms 

 stable carbonates. Because of these facts, Kossel considers arginine to 

 be the nucleus of the protein molecule. It is obtained in widely different 

 amounts from different proteins, over 85 per cent of certain protamines 

 having been obtained in the form of this amino acid. It is claimed that 

 in the ordinary jnetabolic activities of the animal body arginine gives 

 rise to urea. While this claim is probably true, it should, at the same 

 time, be borne in mind that the greater part of the protein nitrogen is 

 eliminated as urea and that, therefore, but a very small part can arise 

 from arginine. 



Leucine, CeHi 3 NO 2 . Leucine is an abundant end-product of the 

 decomposition of protein material, and was one of the first of these 

 products to be discovered. It is a-amino-isobutyl-acetic aad,jtand 

 therefore has the following formula. 



CH 3 NH 2 



I I 



H C.CH 2 -C COOH. 



I I 



CH 3 H 



The leucine which results from protein decomposition is /-leucine. 

 Leucine is present normally in the pancreas, thymus, thyroid, spleen, 

 brain, liver, kidneys, and salivary glands. It has been found patholog- 

 ically in the urine (in acute yellow atrophy of the liver, in acute phos- 

 phorus poisoning, and in severe cases of typhoid fever and smallpox), 

 and in the liver, blood and pus. 



Pure leucine crystallizes in thin, white, hexagonal plates. Crystals 



