170 THE PROTEIN SUBSTANCES. 



MULLER obtained 35 per cent glucosamine from mucous-membrane 

 mucin and 23.5 per cent from the submaxillary mucin. 



On boiling mucin with dilute mineral acids, acid aibuminate and 

 bodies similar to proteoses are obtained, besides a reducing substance 

 which is not free glucosamine (STEUDEL 1 ). By the action of strong 

 acids upon mucins or mucoids OTORI 2 obtained several of the cleavage 

 products of the proteins, such as leucine, tyrosine, glycocoll, glutamic 

 acid, oxalic acid, guanidine, arginine, lysine, and humus substances, 

 and also carbohydrate cleavage products, such as levulinic acid. Cer- 

 tain mucins, as the submaxillary mucin, are easily changed by very 

 dilute alkalies, as lime-water, while others, such as tendon-mucin, are 

 not affected. If a strong caustic-alkali solution, such as a 5-per cent 

 KOH solution, is allowed to act on submaxillary mucin, we obtain alkali 

 aibuminate, bodies similar to proteoses and peptones and one or more 

 substances of an acid reaction which have strong reducing powers. 



On peptic digestion proteoses and peptone-like bodies, still con- 

 taining the carbohydrate group, are produced. On tryptic digestion 

 still simpler cleavage products are formed, namely, leucine, tyrosine, 

 and tryptophane (POSNER and GiES 3 ). The glucosamine, so far as we 

 know, is not split off by proteolytic enzymes, but only after strong 

 hydrolysis with acids. 



In one or another respect the various mucins act somewhat dissimilarly. 

 For example, the snail and sputum mucins are insoluble in dilute hydro- 

 chloric acid of 1-2 p. m., while the mucin of the submaxillary gland and 

 the navel-cord is soluble. The former become flaky with acetic acid, 

 while the submaxillary mucin is precipitated in more or less fibrous, 

 tough masses. Still all the mucins have certain reactions in common. 



In the dry state mucin forms a white or yellowish-gray powder. When 

 moist it forms, on the contrary, flakes or yellowish-white tough lumps 

 or masses. The mucins are acid in reaction. They give the color reac- 

 tions of the proteins. They are not soluble in water, but may give a 

 neutral solution with water with the aid of small amounts of alkali. Such 

 a solution does not coagulate on boiling, but acetic acid gives at the 

 normal temperature a precipitate which is nearly insoluble in an excess 

 of the precipitant. If 5-10 per cent NaCl be added to a mucin solution, 

 it can be carefully acidified with acetic acid without giving a pre- 

 cipitate. Such acidified solutions are copiously precipitated by tan- 

 nic acid; with potassium ferrocyanide they give no precipitate, but on 

 sufficient concentration they become thick or viscous. A neutral solu- 

 tion of alkali mucin is precipitated by alcohol in the presence of neutral 



1 Zeitschr. f. physiol. Chem., 34. 



2 Ibid., 42 and 43. 



* Amer. Journ. of Phyaiol., 11. 



