THE METABOLISM OF PROTEIN 599 



It will be observed that the only difference between the two acids is 

 dependent upon a change in the group that is attached to the upper verti- 

 cal valency bond of the central carbon atom, which therefore must be 

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

 produced from protein differ from one another solely with regard to the 

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

 bond. Evidently, then, the reactions that amino acids possess in common 

 must depend on the terminal groups containing the carboxyl and amino 

 radicles, whereas the characteristic reaction of each of the eighteen amino 

 acids must depend upon the differences in the radicles attached to the 

 upper vertical bond. This may be represented thus: 



Any radicle 

 NH,-C-COOH 



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 ammonias 

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

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

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

 sists in the condensation of aldehydes with the amino group. This occurs 

 particularly readily with formaldehyde, water being eliminated in the re- 

 action, and the basic nature of the amino acid being thus destroyed. 

 Upon this reaction depends the method of Sorensen for determining the 

 amount of amino acid in a mixture (see page 606). The titration is per- 

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

 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 formaldehyde. 

 Since this increase in acidity must depend upon the number of amino 

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

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



radicle H radicle 



NH.-C-COOH -1-11-0 = = CH, = N-C-COOH + H 2 O 



i A 



(amino acid) (formaldehyde) 



Another reaction of amino acid of physiologic interest is that known 

 as the carbamino reaction, consisting in a union of the amino acid with 

 calcium and carbonic acid. Finally, it is important to note that the amino 



