THE PROTEINS 47 



added to protein, and then a little strong sulphuric acid carefully; 

 a violet ring is developed at the junction of the fluids. 



(e) Proteins containing a carbohydrate moiety yield Molisch's 

 test. A purple colour over green is developed when a-naphthol and 

 sulphuric acid are added to the protein. 



(/) Proteins containing loosely combined sulphur yield with lead 

 acetate and caustic alkali a black precipitate of lead sulphide. 



(g) Many proteins are precipitated (" salted out ") from solution by 

 the addition of neutral salts, such as ammonium sulphate, magnesium 

 sulphate, sodium chloride, in varying concentrations. Such precipi- 

 tates are again soluble in the original solvents. 



(h) Proteins are coagulated that is, thrown down as a precipitate 

 no longer soluble in the original solvent by mechanical agitation, 

 addition of the salts of heavy metals, mineral acids and many other 

 acids, such as tannic, picric, etc. 



All proteins do not necessarily give all the above reactions. It is 

 perhaps somewhat difficult to define a protein. Most of them may 

 be defined as bodies of biological origin insoluble in alcohol and 

 giving the biuret test. (All these properties should be studied in 

 connection with the experiments given in Practical Physiology). 



SECTION III. 

 The Classification of Proteins. 



The following classification has been adopted; it is based partly 

 upon the results of chemical investigation, and partly upon such 

 physical properties as solubility, salting out, etc. It cannot be regarded 

 as complete. 



1. Protamines. 



2. Histones. 



3. Albumins. 



4. Globulins. 



5. Phosphoproteins. 



6. Scleroproteins. 



7. Compound proteins. 



The protamines are held to be the simplest proteins known. They 

 occur combined with nucleic acid in the spermatozoa of certain fishes 

 such as the salmon, sturgeon, mackerel, and herring. Sturin, from 

 the sturgeon, has the formula C 36 H 69 N ]9 7 ; salmin (salmon) and 

 clupein (herring) .have the formula C 30 H 5 .N 17 6 . They are difficult to 

 obtain in a state of purity. Upon hydrolysis they yield large amounts 

 of the hexone bases, arginin, lysin, and histidin, especially arginin. 



The histones occur mainly in combination. They are more com- 

 plex than the protamines, and are coagulable by heat, soluble in 

 dilute acid, and precipitated from watery solution by ammonia. The 

 best known is globin split off from the haemoglobin of the blood. 



