22 INTRODUCTORY — THE CONSTITUENTS OF MILK. 



The presence of protein may be considered as proved if re- 

 actions 1, 8, and 9 are given; many, though not necessarily 

 all, of the other reactions will also be obtained should proteins 

 be present. 



Products of Hydrolysis of Proteins. 



By the action of hydrolysing agents — acids, enzymes — proteins 

 are gradually split up into simpler compounds. These are 

 classified as : — 



(a) Meta-proteins. Products which have undergone but 

 little change and still have most of the protein characters ; the 

 product formed by heating lactalbumin in slightly acid solution 

 to its coagulating point comes under this class. The curd formed 

 by the action of rennet on casein will also come under tliis heading,, 

 and may be termed meta-casein (the usual name is para-casein). 



(6) Proteoses, which may be again subdivided into — 



1. Proto-proteoses. Insoluble in ammonium sulphate solution, 24 to 

 42 percent., saturated. Hetero-proteose contains phenyl-alanine, proline, 

 and glycine, but is free from tyrosine and tryptophane. Insoluble in 32 per 

 cent, alcohol. Hemi-proteose contains tyrosine and tryptophane. Soluble 

 in alcohol. 



2. Deutero-proteoses A. Insoluble in ammonium sulphate solution, 

 54 to 62 per cent., saturated. May be further fractionated by their solubility 

 in alcohol. 



3. Deutero-proteoses B. Insoluble in ammonium sulphate, 70 to 95 per 

 cent., saturated. Those soluble in alcohol contain no sulphur. Those 

 insoluble in alcohol contain sulphur. 



4. Deutero-proteoses C. Insoluble in saturated ammonium sulphate 

 solution and acid. Free from sulphur. 



(c) Peptones. Soluble in saturated ammonium sulphate solu- 

 tion and acid. Those insoluble in 96 per cent, alcohol contain 

 no tyrosine nor tryptophane. Those soluble contain both these 

 substances. 



(d) Polypeptides, subdivided into — 



1. Kyrins. Rich in basic substances — lysine and arginine. 



2. Peptides. Simple condensation products of amino-acids. DiUeto- 

 pipcrazines. 



(e) Amino-acids obtained on continued hydrolysis. 



This classification is not quite satisfactory, as the distinction 

 between the various classes is somewhat arbitrary, and there is 

 no sharp distinction between them. There is. however, a steady 

 fall in molecular complexity. ' 



During hydrolysis, the hydrolysts are not destroyed, or are 

 destroyed with extreme slowness, and appear to be 'able to act 

 on a relatively enormous quantity of the hydrolyte. The time 



