210 PROTEIDS 



The protamines, which have been found chiefly in fish sperm, 

 consist very largely of arginine and other diamino-acids. The 

 histones yield about 30 per cent of diamino-acids. 



The albumins contain no glycine, but the globulins yield up to 3 

 or 4 per cent of this substance. 



The crystallised vegetable globulins, e.g., edestin, found in hemp 

 seed, cotton seed and sunflower seed, yield about the same quantity of 

 glycine as the animal globulins (serum globulin and fibrin), the glute- 

 lins, e.g., legumin, amandin, phaseolin and maize glutelin, much less 

 and the gliadins, practically none. The phospho-proteins, e.g., case- 

 inogen, also contain no glycine, but the scleroproteins, e.g., gelatin, 

 elastin, fibroin and keratin, are very rich in glycine, alanine and other 

 mono-amino acids. 



Of special interest is the occurrence of tryptophane in proteins, 

 because of its being apparently an indispensable constituent of the 

 food of animals. It appears to be present in most proteins, though its 

 amount has only seldom been determined ; but its absence from the 

 gliadin of maize zein is alleged to render that substance incapable 

 of forming the sole nitrogenous nutriment of animals. 



The proteids are extremely complex, therefore, in constitution and 

 any detailed account of them would be beyond the scope of this volume. 

 Indeed, the subject has an extensive literature of its own. As showing 

 the great complexity of these substances, the simplest formula which 

 will indicate the composition of globin, for example, has been cal- 

 culated to be C 726 H 1174 N 19 4S 3 214 . Some reference to certain import- 

 ant vegetable proteins will be made in subsequent chapters when 

 dealing with crops and with the feeding of animals. 



Proteids are optically active, being laevo-rotatory. They are pre- 

 cipitated by acids and by potassium ferrocyanide, by basic lead acetate, 

 by mercuric chloride, by copper sulphate, by saturation of the liquid 

 with ammonium sulphate or magnesium sulphate, or by alcohol. 



The following are general qualitative tests for proteids : 



1. Heated with strong nitric acid they yield a yellow colouration 

 which becomes orange on the addition of alkali (xantho-proteic re- 

 action). 



2. Millon's reagent (an acid solution of nitrate of mercury) gives 

 a precipitate which turns red on heating. 



3. With excess of strong sodium hydrate solution and a small 

 -quantity of copper sulphate a violet colour is obtained, becoming 

 deeper on warming. (Biuret reaction.) 



4. Adamkiewicz's reaction. The proteid is dissolved in glacial 

 acetic acid and strong sulphuric acid is added, when a violet colour 

 is produced. It has been shown : that the acetic acid only pro- 

 duces this reaction if it contains as an impurity, glyoxylic acid, 

 HCO.COOH, and that a better method of performing the test is to 

 reduce oxalic acid by means of a little sodium amalgam, thereby 

 forming glyoxylic acid, add a few drops of this solution to that of the 

 proteid, and then sulphuric acid. 



1 Hopkins and Cole, Proc. Boy. Soc., 1901, 21. 



