THE PROTEINS 17 



first demonstrated its structure in 1900 by obtaining cadav- 

 erine from it by putrefaction. 

 Arginine. a-amino-5-guanidine-valerianic acid. 



NH 2 

 HN = C NH.CH 2 .CHa.CH 2 .CH.NH2.COOH. 



Among the products of a decomposition of casein Drechsel 

 found a substance which he called lysatinine. Later, in 1894, 

 Hedin demonstrated that this product was in reality a mix- 

 ture of lysine and arginine. Arginine had been obtained pre- 

 viously by E. Schulze and Steiger from the seedlings of 

 various plants. Urea and ornithine are among its decomposi- 

 tion products. 



Histidine. /3-imidazole-a-amino-propionic acid. 



CH 



N NH 



.CH 2 .CH.NH 2 .COOH 



Histidine was discovered by Kossel in 1896 among the de- 

 composition products of the protamine of sturgeon testes. 

 From the fact that histidine, arginine, and lysine each contain 

 six carbon atoms Kossel called these three substances the 

 hexone bases, and they were regarded as a very important 

 portion of the protein molecule. It was not until 1904 when 

 the structure of histidine was shown by Pauly and Wind- 

 haus and Knopp that it was recognized to belong to a group 

 of compounds entirely different from that including arginine 

 and lysine. 



Proline. o-pyrrolidine-carboxylic acid. 



CH2 CH 2 



H 2 CH.COOH 



