230 PEACTICAL PHYSIOLOGY 



scope, and note that they consist of round balls not unlike oil globules, 

 but having concentric markings (Fig. 148). 



Leucin and tyrosin were among the first-discovered decomposition 

 products of proteids, and, on account of the ease with which they are 

 isolated, they have been detected in nearly every organ and tissue of 

 the body, being probably produced, however, by the chemical agencies 

 employed in examining these, and not existing as such in the living 

 tissue. They also occur, along with free ammonia, in the urine of 

 patients suffering from severe disease of the liver. 



Not only are amido derivatives of mono-basic acids produced during 

 proteid decomposition, but we may also have similar derivatives of 

 dibasic acids. 1 One of the simplest of these latter is succinic acid. 

 CH 2 - COOH. ^ now we replace an ' H ' of 



| methyl radicle by the amido group i 



CH 2 - COOH. (NH 2 ) we obtain aspartic acid. CH 2 - COOH 



(Succinic acid.) Besides being produced in the intestine (Aspartic acid.) 

 by the action of trypsin on proteid, it also occurs plentifully in plants. 2 



If one of the ' H ' atoms of the other methyl group of this be re- 

 placed by CH 3 , we obtain another important di-basic amido acid, viz., 

 glutaminic acid, and this is also a common decomposition product. 



All these amido 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 acids, forming weak salts. 

 Most of them can also unite with metallic salts, forming double com- 

 pounds, which are very useful in preparing the pure amido acid. 



Besides these mon-amido acids, there are also produced bodies in 

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

 basic reaction, and combine with weak acids, such as phosphotungstic. 3 

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

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

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

 hexone bases, and the most important are lysin (diamido caproic acid, 

 C 5 H 9 (NH) 2 COOH), arginin (C 6 H M N 4 2 ), lysatin (C 6 H 13 N 3 2 ), and 

 histidin (C 6 H 9 N 3 2 ). 



By the action of trypsin on fibrin a body is produced belonging to 

 none of the three groups described above. This is cystin. It is 



1 Acids are mono- or di-basic according to whether they contain one, or two, 

 replaceable OH (hydroxyl) groups belonging to a COOH (carboxyl) radicle. 



2 If the OH group of the COOH radicle be further replaced by N.H 2 we have 

 asparagin. 



3 This complex acid has the formula 



