ADVANCED PHYSIOLOGICAL CHEMISTKY 429 



other proteids in that they do not yield mon-amido acids as decompo- 

 sition products, but only the hexone bases Arginin, Histidin, and 

 Lysin (see p. 170). They are very basic in nature, a solution of them 

 in water reacting alkaline to litmus. They accordingly combine readily 

 with acids to form salts, and it is as a salt with nucleic acid that 

 they exist in the spermatozoa of certain fishes. The protamin can be 

 displaced from its combination with nucleic acid by means of sulphuric 

 or picric acids, and the resulting salt is used for the isolation of 

 protamin. These artificial salts of protamin are precipitated by 

 phospho-tungstic acid, this being the precipitant used to separate 

 them from other bodies. 



If a solution of protamin in water be treated with an ammoniacal 

 solution of albumin, a precipitate is obtained which is said to be the 

 same as histon (see below). Protamines are considered by Kossel as 

 the nucleus of construction of all other proteids. Several varieties 

 have been isolated. 



II. Albuminoids. Collagen, the precursor of gelatin, forms the chief 

 constituent of white fibrous tissue and of the organic substance of bone. 

 It also exists in cartilage, where, however, it is mixed with several 

 other bodies (see p. 429). 



Preparation of Collagen. A piece of tendon is macerated overnight 

 in 1 per cent, caustic alkali to remove proteid and mucin, and then 

 washed with water till alkali free. The resulting mass is collagen. 

 Place a piece of this in a flask and boil it for ten minutes with water 

 which is rendered faintly acid with acetic acid. By this treatment, the 

 collagen is transformed into gelatin and, on cooling the solution, it 

 gelatinises. 



Gelatin. This is really the hydride of collagen, the boiling with 

 acidulated water in the above experiment having caused the collagen to 

 take up a molecule of water. Conversely, the gelatin can be recon- 

 verted into collagen by heating it to 130 C., whereby it loses water. 



Divide a solution of gelatin in lukewarm water into four portions, 

 to which apply the following tests : (1) the biuret reaction : a violet 

 colour is produced. (2) the xantho-proteic reaction: only a sHght 

 colouration is produced. (3) the Millon's test : only a slight reddening 

 of the precipitate occurs on boiling. (4) the glyoxylic test: absent 

 or very faint. 



The reason why the last two tests are not very distinct, is because 

 gelatin is an albuminoid, and consequently does not yield aromatic 

 bodies on decomposition, and both these tests depend on the presence 

 of aromatic bodies. Some varieties of gelatin give these reactions 

 more distinctly than others, and absolutely pure gelatin is said not 



