20 INTRODUCTORY — THE CONSTITUENTS OF MILK. 



doubt that the grouping— CIH—NH— CO— CH— is characteristic 

 of proteins ; Fischer also considers it proved that the di-keto- 

 piperazine group also occurs. 



Fischer has recently prepared a polypeptide containing 18 

 molecules of amino-acids ; it was Meucyl-triglycyl-Meucyl- 

 triglycyl-?-leucyl-octoglycyl-glycine, and gave many of the 

 protein reactions ; this compound was hydrolysecl by trypsin, 

 but not by pepsin. 



Classification of Proteins. — A joint committee of the Chemical 

 and Physiological Societies has proposed the following classi- 

 fication for proteins : — 



1. Protamines. These are characterised by being free from sulphur, 



and containing large amounts of arginine. Strongly basic. 



2. Histones. These are very basic substances, and are precipitated 



by ammonia. They contain little sulphur. 



3. Albumins. Soluble in water ; coagulated by heat. 



4. Globulins and their derivatives. Insoluble in water, but soluble in 



salt solutions. 



5. Sclero-proteins ; insoluble proteins which form the supporting 



structures or connective tissues of animals. 



6. Phospho-proteins ; derivatives of para-nucleic acid ; do not 



contain purine or pyrimidine derivatives ; are distinctly acid 

 substances. 



7. Conjugated proteins, subdivided into — 



(a) Nucleo - proteins ; derivatives of nucleic acid ; contain 



purme and pyrimidine derivatives. 



(b) Gluco - proteins ; containing a carbohydrate radicle. 



Mucins. 



(c) Chromo - proteins ; coloured protein substances as 



hicmoglobin. 



The proteins of milk are : — 



Class 3. Lactalbumin, . . . about 0-4 to 0-5 per cent. 



„ 4. Lacto-globulin, . . . traces. 



„ 76. Storch's mucoid protein, . traces. 



„ 6. Clasein, . . . about 3-0 per cent. 



General Reactions of Proteins.— The following are the general 

 reactions of proteins : — 



1. The Biuret Reaction. — Add to a solution an excess of 

 caustic alkali, and one drop of copper sulphate solution. Proteins 

 give a violet colour, and proteoses and other products of hydro- 

 lysis a reddish tint. 



This reaction is characteristic of proteins. 



2. The Xantho-protein Reaction. — On heating with strong 

 nitric acid proteins yield a yellow colour, darkened by alkalies. 

 This reaction is due to the presence of homocyclic ring compounds, 

 chiefly tryptophane and tyrosine, and to a less degree by phenyl- 

 alanine. 



