392 PRACTICAL PHYSIOLOGY 



di-basic acids. One of the simplest of these latter is succinic acid, 

 COOH.CH 2 .CH 2 .COOH. 



If an " H " atom attached to a carbon atom of the chain be replaced 

 by the amido group (NH 2 ), aspartic add results, COOH . CHNH 2 . CH 2 . 

 COOH. Besides being produced in the intestine by the action of 

 trypsin on protein, it also occurs plentifully in plants. 1 



Another important di-basic amino acid, which is also a common 

 decomposition product of protein, is glutaminic acid. It contains one 

 more C atom than aspartic acid, and has the formula: COOH . CHNH 2 . 

 CH 2 .CH 2 .COOH. 



All these amino acids retain to a certain extent their acid properties. 

 Thus they can combine with bases to form salts. On the other hand, 

 on account of the NH 2 group which they contain, they also show faint 

 basic properties, in that they can unite with metallic salts, forming 

 double compounds, which are very useful in preparing the pure amino 

 acid. Their ethereal salts shew marked basic properties. 



Besides these mono-amino acids, there are also produced bodies in 

 which more than one amino group exists. These have a distinctly 

 basic reaction, and combine with weak acids, such as phosphotungstic. 2 

 They also form double salts with silver nitrate. These two reactions 

 are taken advantage of in separating these bases from the mono-amido 

 acids. Since these bases contain six carbon atoms, they are called 

 hexone bases, and the most important are lysine (C 5 H 9 (NH 2 ) 2 COOH) 

 and arginine (C 6 H 14 N 4 2 ). 



Lysine is a-e-di-amino-caproic acid, being therefore somewhat related 

 to leucine. Its structural formula is : 



NH 2 . CH . CH 2 . CH 2 . CH 2 . CHNH 2 . COOH. 



Arginine, the most frequently occurring decomposition product of 

 protein, is chemically S-guanidin a-amino-valerianic acid. This is a 

 peculiarly interesting decomposition product of protein because on 

 hydrolysis it is split into urea and di-amino-valerianic acid or ornithin. 

 The urea comes from the guanidin radicle thus : 



H H H NH 2 



NH-C-C-C-C-COOH + H = 



NH= C H H H H 



a-amino-valerianic 

 acid. 



guanidin 

 radicle. 



1 If the OH group of the COOH radicle of aspartic acid be further replaced by 

 NH 2 we have asparagin. 



2 This complex acid has the formula H 3 PO 4 . 11WO 3 . 12H 2 0. 



