220 TEXTBOOK OF PLAXT PHYSIOLOGY 



Reserve proteins usually are stored in seeds in the form of 

 special bodies, called "aleurone grains." Essentially, they are 

 dried-up vacuoles, very rich in soluble proteins. In many aleurone 

 grains, it is possible to see distinctly well-defined crystals of protein 

 substances, from which it is apparent that colloidal substances may 

 have the capacity to crystallize. In underground food-storage 

 organs, as in the tubers of the potato, reserve proteins are often 

 found in a crystalline form. 



Protein substances can be easily detected by the aid of color 

 reactions, of which the most important is the biuret test. In this 

 particular reaction, proteins give a blue-violet or red-violet color 

 upon the addition of a solution of sodium hydroxide and a few drops 

 of a weak solution of copper sulphate. Upon heating with strong 

 nitric acid, proteins give a bright yellow color (xanthoproteic reac- 

 tion), and upon boiling with mercuric nitrate in the presence of 

 nitrous acid, a brick-red color (Millon's reaction). 



It must be noted, however, that all these reactions, except the 

 biuret, are not tests for the whole protein molecule, but only 

 for some of its more characteristic integral parts. These parts, 

 however, may be studied best by way of the hydrolytic decompo- 

 sition of the protein molecule. For this purpose, the protein is 

 subjected to a prolonged boiling with mineral acids, thus break- 

 ing it down into the constituent amino acids. Almost always these 

 are a-amino acids, in which NH 2 is linked to the same carbon atom 

 to which is attached the COOH group. Otherwise, their carbon 

 atom configuration is quite varied, being not only of the aliphatic, 

 but also of the aromatic series. All of them, with the exception of 

 glycine (cc-amino-acetic acid), are optically active. 



The main amino acids found in proteins are the following: 



A. Aliphatic series: 



1. Monoamino acids: 



o-Glycine, CH 2 HN 2 -COOH 



d-Alanine, CH 3 -CHNH 2 -COOH 



/-Leucine, CH 3 \ 



^CH • CH 2 • CHNH 2 • COOH 

 CH 3 / 



/-Aspartic acid, COOH -CH 2 CHNH 2 - COOH 



(/-Glutamic acid, COOH CH 2 CH 2 CHNH 2 - COOH 



