96 PHYSIOLOGY 



Midway between these two groups may be placed the muscle protein, 

 myosin (or myosinogen), which, though soluble in pure water, resembles 

 the class of globulins in the ease with which it is precipitated by the addition 

 of neutral salts. 



In addition to the members of the globulins named above and derived 

 from the animal body, proteins allied to this class form an important con- 

 stituent of plants, and are found in large quantities in many seeds used as 

 articles of food. These are vegetable globulins. Prominent members of the 

 group are the edestins, which may be obtained from hemp seeds, cotton seeds, 

 and sunflower seeds, zein from maize, kgumin 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, or acid metaprotein, 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 nearly to neutralise the solution of acid metaprotein, 

 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, or alkaline metaprotein, 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 metaprotein. It is precipitated oh 

 nsutralisation of its solution. 



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

 hi 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 o^ 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 Ilio 

 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 com- 

 pounds 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 



