468 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



GLUCOPROTEINS. 



Various tissues of the animal body contain proteins which on ex- 

 amination appear to belong to yet another class of proteins. They 

 -are characterised by the formation of a reducing carbohydrate on 

 hydrolysis and are consequently classed together as the gluco- 

 proteins. Like other conjugated proteins the prosthetic or carbo- 

 hydrate group is easily split off by hydrolysis. The carbohydrate 

 radicle in most cases is glucosamine, or a polysaccharide composed of 

 glucosamine units, such as chitin or chondroitin. 



Three classes of glucoproteins can be distinguished : 



(1) The Mucins. These are soluble in water and dilute alkali but 

 -insoluble in excess of acetic acid and very dilute hydrochloric acid ; 

 they give their solutions a slimy or gummy appearance and are pre- 

 cipitated in sticky strands of material. 



(2) The Pseudomucins. These are soluble in water and dilute 

 alkali, but unlike mucins are soluble in acetic acid. Their solutions are 

 slimy and they are precipitated by alcohol in the form of strands. 



(3) The Mucoids. These are soluble in water, dilute acids and 

 alkalies ; their solutions are not slimy ; on evaporation they leave 

 brownish membranes and they are precipitated by alcohol as white 

 powders. Most of the proteins belonging to this group have not been 

 extensively studied and they deserve further investigation. 



Mucin of the Submaxillary Gland. 



Preparation. 



The minced gland is treated with water. The liquid is poured off and con- 

 centrated hydrochloric acid is added until -15 per cent, is present. The mucin, 

 which is precipitated, dissolves on stirring but is thrown down by adding 2-3 

 volumes of water. The liquid is poured or strained off ; the precipitate is 

 dissolved in '15 per cent, hydrochloric acid and thrown down by adding water. 

 This procedure is repeated. The precipitate is washed with water, alcohol 

 and ether and dried. 



Properties. 



Submaxillary mucin forms an almost colourless powder which has an acid 

 reaction. It is not soluble in water but dissolves on adding a trace of alkali, 

 giving a slimy solution. The solution is not coagulated by boiling, but is 

 precipitated by acetic acid. The precipitate is insoluble in excess of acetic 

 -acid and has the appearance of gelatinous strings which collect upon a 

 ^lass rod when the precipitate is stirred. The solution gives most of the 

 colour reactions for proteins ; it is precipitated by alcohol in the presence of 

 neutral salts and also by heavy metals. A solution containing salts is not 

 precipitated by small quantities of acetic acid nor by hydroferrocyanic acid, 

 t>ut it is precipitated by tannic acid. Mucin is digested by pepsin and 

 trypsin ; it is decomposed by dilute alkali giving gummy solutions and it is 

 converted into a proteose by boiling with water at 110-150 with separation 

 of a reducing carbohydrate. 



20 per cent, of reducing carbohydrate calculated as glucose, together with 

 acetic acid, is formed by hydrolysis on boiling for 3 hours with 3 per cent, 

 hydrochloric acid. 



