420 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



of amino-acids which are called dmramo-acids ; that is, fatty acids in 

 which two hydrogen atoms are replaced by NH 2 groups. Of these we 

 may particularly mention lysine, ornithine, arginine, and histidine. 



Lysine is diamino-caproic acid. Caproic acid is C 5 H n . COOH. 

 .Mono-amino-caproic acid or leucine, we have already learnt, is 

 CJEL . NIL . COOH. Lysine or diamino-caproic acid is C JHL . 

 (NH 2 ) 2 .COOH. 



Ornithine is diamino-valeric acid, and the following formulae 

 will show its relationship to its parent fatty acid 



C 4 H 9 GOOH is valeric acid. 



C 4 H 7 (NU 2 ) 2 COOH is diamino-valeric acid or ornithine. 



Arginine is a somewhat more complex substance, which contains 

 the ornithine radical. It belongs to the same group of substances as 

 creatine, another important cleavage product of the protein molecule. 

 Creatine is methyl-guanidine acetic acid, and has the formula 



; N(CH 3 )CH 2 . COOH 



H 2 N 



On boiling it with baryta water, it takes up water (H 2 0) and splits at 

 the dotted line into urea (CO(NH 2 ) 2 ) and sarcosine, as shown below. 



H 2 N 



C O 



NH.CH Q .CH .COOH 



[Urea.] [Sarcosine or Methyl-glycine.] 



Arginine splits in a similar way, urea being split off on the left, 

 and ornithine instead of sarcosine on the right. Arginine is, there- 

 fore, a compound of ornithine with a urea group. 



Histidine, though not strictly speaking a diamino-acid, is a 

 diazine derivative (imidazole - amino - propionic acid), and so may 

 be included in the same group. 



These substances we have spoken of as acids, but they may also 

 play the part of bases, for the introduction of a second amino-group 

 into the fatty acid molecules confers upon them basic properties. 

 The three substances, 



Lysine . . C 6 H 14 N 2 O 2 



Arginine . . C 6 H 14 N 4 O 2 

 Histidine . . C 6 H 9 N 3 O 2 



are in fact often called the hexone bases, because each of them 

 contains 6 atoms of carbon, as the above empirical formulae show. 



