40 A TEXTBOOK OF PHYSIOLOGY 



In the process of hydrolysis, water at first enters into the molecule, 

 which then breaks into smaller molecules. The first products are 

 known as proteoses, the next as peptones, both of which groups 

 retain many protein characters. The peptones are further split 

 into groups of amino-acids known as polypeptides, and finally these 

 groups are broken down into the individual amino-acids organic 

 acids in which an atom of hydrogen has been replaced by an NH 2 

 group. 



The chief products formed as the result of protein hydrolysis may 

 be grouped as follows : 



GROUP I. Monamino-acids (containing one NH 2 group) : 



(a) Monocarboxylic (containing one COOH group), the chief 



of these being glycin, alanin, serin, valin, leucin. 

 (6) Dicarboxylic (containing two COOH groups), the chief 

 being aspartic acid and glutaminic acid. 



GROUP II. " Ringed " amino-acids, containing 



(a) The benzene ring, such as phenyl alanin, ty rosin. 

 (6) Heterocyclic rings, such as prolinc, oxyproline, tryptophan, 

 and histidin. 



GROUP III. Diamino-acids (containing two NH 2 groups). 



In this group are contained the two " hexone bases " lysin, arginin. 

 With these cystin, a sulphurized diamino-acid, may be classed. 

 Other constituents arc : 



(a) Pyrimidin bases, cytosin, thymin, and uracil. 

 (6) Purin bases, adenin and guanin. 



The chief of these bodies will now be considered in a little more 

 detail. 



Glycin is a monamino-acetic acid, CH 2 NH 2 COOH. Hippuric 

 acid is ite benzoyl compound C 6 H.CO.NH.CH 2 COOH. Glycin is 

 also compounded with cholalic acid, and forms sodium glycocholate, 

 one of the salts of the bile. 



Alanin is the amino-acid of the next acid atove acetic, namely, 

 propionic, C 2 H 5 COOH. Its formula, therefore, is C,H 4 NH 2 .COOH 

 (8-amino-propionic acid). 



Serin is oxy-alanin (oxj'-amino-propionic acid), CH,OH.CH. 

 NHCOOH. 



Valin is amino-valerianic acid, 



CH N 



C j^ / X CH ! CH.NH,COOH. 



Leucin, C.H 10 NH. ( .COOH, is the u-amino-acid of caproic acid, 

 CjHjjOQOH. It crystallizes readily as spherules, is widespread, and 

 present normally in the pancreas, thymus, thyroid, spleen, brain, liver, 

 kidneys, and salivary glands. 



