THE CHEMICAL BASIS OF THE BODY 755 



characterised by yielding certain purin bases on hydrolysis — viz., 

 xanthine, guanine, adenine, and bases of the pyramidine group, which 

 serves to distinguish it from the phospho-proteins. Gluco-protein 

 is protein united with a carbohydrate group, of which mucin is 

 typical. On decomposition mucin yields a reducing substance — 

 glucosamine — which is not a true sugar. 



Derived Proteins. — The hydrolysis of protein, as we have previously 

 seen, yields a large number of end-products. But before this stage 

 is reached, protein splits into intermediate bodies — i.e., meta-proieins, 

 proteoses, peptones, and polypeptids. Meta-protein represents the 

 old acid and alkali albumins. Proteoses are of several varieties, 

 proto-, hetero-, and deutero-, according to the stage of digestion. 

 Peptones have previously been studied. Proteoses are precipitated by 

 saturation with ammonium sulphate, but are not coagulated by heat. 

 Peptones are not precipitated by ammonium sulphate. The 

 polypeptids axe formed as the result of digestive activity, but the 

 majority are artificial products created in the laboratory during the 

 reconstruction of the protein molecule. 



The Vegetable Proteins constitute a large class, and group them- 

 selves under the same headings as the proteins of animal origin ; 

 but it is considered doubtful whether they are completely analogous. 

 In the body they yield, during digestion, the same decomposition 

 products as proteins of animal origin ; but the end-products are not 

 identical, glutamic acid being, as we have seen at p. 754, abundant 

 in vegetable but low in animal protein ; while the important 

 tryptophane radicle is absent from maize. The solubility of vege- 

 table is different from animal proteins. Gliadin (wheat and rye), 

 horde in (barley), and zein (maize), are soluble in 70 to 80 per cent, 

 alcohol, but insoluble in water or absolute alcohol. The term 

 ' gliadins ' has been proposed for the alcohol-soluble proteins of 

 cereals. These are not found outside the cereal family. Other 

 proteins found in cereals are readily dissolved by very dilute acids 

 and alkalies. To these the term glutelins has been applied. 



A very remarkable fact about protein substances is that, though 

 they constitute the mainspring of organic life, yet they number 

 amongst them, or amongst their decomposition products, some of the 

 most powerful poisons known. Snake poison is a protein ; even the 

 albumose formed during the peptic digestion of albumin is highly 

 poisonous if injected into the circulation. 



Previous reference has been made to the fact that some proteins 

 are capable of crystallisation. The crystallisation of haemoglobin 

 and some plant protein has long been known. Egg-albumin may 

 also be crystallised, while serum-albumin of the horse is remarkable 

 for the ease with which it may be obtained in a crystalline form 

 (Fig. 260). 



Tests for Proteins. — There are certain chemical tests which apply 

 to the entire group of protein bodies, and others which are distinctive 

 of members of the group. Their reactions are based on colour tests, 

 precipitation, or coagulation. 



Colour Tests — Xantho-protein Reaction. — Solutions of protein 

 heated with strong nitric acid turn yellow, and on the addition 

 of ammonia or caustic soda change to orange. This reaction is 

 considered to be due to the presence in the molecule of some group 

 of the aromatic series. 



Millon's Reaction. — Millon's reagent is a mixture of mercurous 

 and mercuric nitrates in the presence of nitric acid. When boiled 



