THE PROTEINS 109 



obtained from hemp seeds, cotton seeds, and sunflower seeds, zein 

 from maize, leyumin from beans. 



(5) GLIADINS, contained in cereals, and soluble in alcohol. 



(6) GLUTELINS, proteins also obtained from cereals and soluble 

 in weak alkalies. 



(7) DERIVATIVES OF PROTEINS. A. METAPROTEINS. These 

 may be regarded as compounds of the protein molecule or of part of 

 the molecule with acid or basic radicals. 



ACID ALBUMIN is formed by the action of warm dilute acids or of 

 strong acids in the cold on any of the preceding bodies. If a weak 

 alkali be added so as to nearly neutralise the solution of acid albumen, 

 this latter is precipitated. If the precipitate be suspended in water 

 and heated, it is coagulated and becomes insoluble in dilute acids or 

 alkalies. 



ALKALI ALBUMIN is formed by the action of strong caustic potash 

 on white of egg or on any other protein, or by adding alkali in excess 

 to a solution of acid albumen. It is precipitated on neutralisation of 

 its solution. 



In close association with this group may be included the proteins as they 

 occur in combination with the metallic salts, such as copper sulphate. On 

 splitting off the copper moiety from these compounds, the protein left is 

 practically free from ash, and behaves in many respects like an albuminate, 

 being insoluble in absolutely pure water, but easily dissolved by the addition of 

 a trace of free acid or alkali. 



A group of protein derivatives described by Hopkins is produced by the 

 action of the free halogens on protein solutions. We get in this way two 

 definite classes of compounds. One class, which contains the largest percentage 

 of halogen, is obtained by treating a protein solution with chlorine, bromine, 

 or iodine, dissolving up the resultant precipitate in alcohol and pouring the 

 alcoholic solution into ether, when the halogen compound is thrown down as 

 a fine white precipitate. By dissolving this precipitate in weak soda and 

 precipitating with acid, we obtain a series 'of compounds containing only about 

 one-third as much of the halogen as is contained in the first precipitate, 

 suggesting that the halogen forms both substitution and additive compounds 

 with the protein molecule. 



Albumins, globulins, and metaproteins are often associated 

 together as the coagulable proteins, since they may be thrown down 

 entirely from their solution on boiling in slightly acid medium in the 

 presence of neutral salts. 



B. HYDRATED PROTEINS. When proteins are subjected to 

 the action of superheated water or steam, or heated with acids, or 

 acted on at the body temperature by certain ferments, e.g. pepsin, 

 trypsin, or papain, they undergo a change which is attended by the 

 addition of a number of molecules of water to the protein molecule 

 (hydrolysis). This action, when carried to its end, results in the 

 production of the amino -acids which we have already dealt with. 



