Till. METABOLISM OF PBOl I IN 



It will be observed thai the only difference I ids is 



dependenl upon a change in the group thai is attacl upper verti- 



cal valency bond of the central carbon atom, which th< 

 considered as the center of the entire molecule. The various amino acids 

 produced from protein differ from one another Bolely with regard 

 chemical nature of the group that is attached to this vertical vi 

 bond. Evidently, then, //" reactions that amino acids pot 

 must depend on the terminal groups containing the carboxyl and amino 

 radicles, whereas tin characteristic reaction of each of thi eighteen am 

 acids must depend upon the diffen in the radicles attached 



upper vertical bond. This may be represented thus: 



Any radicle 



I 

 XII ,-C-COOII 



I 



H 

 Any amino acid 



The end groups endow the amino acids with the power to combine with 

 both acids and bases. With acids they behave like substituted ammoi 

 to form salts, which can ionize into the amino acid, as the cation, and I 



acid group, as the anion. With bases tl arboxyl group reacts to form 



salts, which yield amino acid as the anion. A most important reaction con- 



- in th> condensation of aldehydes with thi amino group. This occ 

 particularly readily with formaldehyde, water. being eliminated in the 

 action, and the basic nature of the amino acid being thus destroy 

 Upon this reaction depends the method of Sorensen for determining the 

 amount of amino acid in a mixture see page 606 The titration g 

 formed by rendering the solution of amino acids neutral, then add 

 formaldehyde and titrating with standardized acid, using phenolphtha- 

 lein as the indicator, and thus finding to what degree the acidity of the 

 mixture has become increased as a result of adding the formaldelr 

 Since this in iii acidity must depend upon the mini' lino 



groups, it furnishes us with an ind estimate of the concentration of 



the amino acids. The reaction is illustrated by the equation: 



radicle II radi 



Ml C COOH M ' CH N-C-< OOH B 



II TI 



(amino aci 



Another reaction of amino acid of physiologic u wn 



•he carbamino reaction, consisting in a union i I d with 



calcium and carbonic acid. Finally, il is important to note that the annuo 



