THE CHEMISTRY OF THE BODY. 9 



changes in its characters. The egg-albumin solution is neutral, and is 

 coagulated on heating if salts are present. The metaprotein solution 

 does not coagulate on heating, and gives a precipitate of metaprotein 

 on neutralisation, the precipitate being soluble in excess of either acid 

 or alkali. 



If a solution of egg-albumin is subjected to the action of superheated 

 steam, or is boiled for a long time with mineral acid, or is subjected to 

 the action of gastric juice or of pancreatic juice, the albumin takes up 

 water and the molecule is finally split up into small molecules, the 

 process being known as hydrolysis. The splitting up occurs in stages, 

 the molecules becoming progressively smaller. After the substance has 

 passed through a metaprotein stage, a series of hydrated proteins are 

 formed, the first formed products being called proteases and the later 

 ones peptones. The hydrated proteins are soluble in water and are not 

 coagulated on boiling. Proteoses are distinguished from peptones in 

 that they are precipitated if their solution is saturated with ammonium 

 sulphate, whereas peptones are not precipitated in this way. If the 

 hydrolysis is continued, the peptones are further split into substances 

 called polypeptides, which do not show protein characteristics, and which 

 consist of groupings of amino-acids. By still further hydrolysis these are 

 split into their constituent amino-acids. 



All proteins give certain colour reactions by which their presence 

 in solutions may be recognised. The most useful of these are the 

 following : 



(1) The Xanthoproteic reaction. Nitric acid is added to the solution 

 and it is boiled. A yellow colour is produced, which changes to orange 

 on cooling and adding ammonia. 



(2) Milton's reaction. A solution of mercuric and mercurous nitrates 

 is added to the protein solution. A precipitate is formed and becomes 

 red on heating. 



(3) Piotrowski's reaction. With dilute copper sulphate and excess of 

 caustic alkali, most proteins give a violet colour, but in the case of 

 proteose or peptone the colour is pink. 



(4) Hopkins' reaction. Glyoxylic acid is added to the protein 

 solution, and then strong sulphuric acid is poured down the side 

 of the tube so as to form a layer at the bottom. A violet colour is 

 produced at the junction of the two fluids. 



Millon's reaction depends upon the presence of tyrosine in the 

 protein molecule ; Hopkins' reaction depends upon the presence of 

 tryptophane. Gelatin, which does not contain either tyrosine or 

 tryptophane, gives neither of these reactions. 



The chief proteins found in the body are (1) protamines, (2) histones, 



