THE PROTEINS 95 



fishes, where they are in combination with nucleic acid. They are charac- 

 terised by the very large amount of bases contained in their molecule, 

 amounting to 85 per cent, of the total substance. It was formerly thought 

 by Kossel that the protamines contained only diamino-acids and bases, but 

 it has been shown later that a small proportion of mono-ammo-acids may 

 also be obtained from their disintegration (v. Table, p. 98). On account of 

 their constitution they possess strongly basic characters and form well- 

 marked salts, e.g. sulphates and chlorides, as well as double salts with 

 platinum chloride. They contain no sulphur and do not coagulate on 

 heating. 



(2) HISTONES. This class of proteins, like the protamines, only occurs in 

 combination with other groups, such for instance as nuclein and ha3matin. 

 They may be obtained from red blood-corpuscles, where they form the 

 globin part of the haemoglobin molecule, or from the leucocytes of the thymus 

 gland, or from the spermatozoa of fishes. The histones are precipitated 

 from their watery solutions by addition of ammonia, but are soluble in 

 excess of this reagent. In the presence of salts they are coagulated on boiling. 

 With cold nitric acid they give a precipitate which dissolves on warming, but 

 is thrown down again on cooling. The most characteristic feature of this 

 class of bodies is however the high proportion of diamino-acids and bases 

 contained in their molecule. 



(3) ALBUMINS. These are soluble in pure water and are precipitated 

 by complete saturation with ammonium sulphate, zinc sulphate, or sodio- 

 magnesium sulphate. 



EGG ALBUMEN forms the greater part of the white of egg. It gives 

 the ordinary protein tests, coagulates on heating at about 75 C., and 

 is precipitated from its solutions if shaken with a drop of dilute acetic 

 acid in excess of ether. It is laevo-rotatory, its specific rotatory power 

 being -35'5. 



SERUM ALBUMEN occurs in large quantities in the blood plasma, serum, 

 lymph, and tissue fluids of the body. It coagulates at 75 C., and is dis- 

 tinguished from egg albumen by its greater specific rotatory power, 56, 

 and by the fact tha*t it is not precipitated by ether and sulphuric acid. Some 

 vegetable proteins belong to this class, e.g. the leucosin of wheat. 



(4) GLOBULINS. These bodies are insoluble in pure water and require 

 the presence of a certain amount of neutral salt to dissolve them. They are 

 precipitated from their solutions by complete saturation with magnesium 

 sulphate or by half-saturation with ammonium sulphate. The chief members 

 of this class are : 



CRYSTALLIN, obtained from the crystalline lens by passing a stream of 

 carbon dioxide through an aqueous extract of this body. 



SERUM GLOBULIN or PARAGLOBULIN, a constituent of blood plasma and 

 blood serum. 



FIBRINOGEN, which occurs in blood plasma and is converted into fibrin 

 when the blood clots. 



PARAMYOSINOGEN, a normal constituent of muscle, 



