166 THE PROTEIN SUBSTANCES. 



alkalies, as lime-water, while others, such as tendon-mucin, are not 

 affected. If a strong caustic-alkali solution, such as 5-per cent KOH 

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

 minate, bodies similar to proteoses and peptones and one or more sub- 

 stances of an acid reaction and with strong reducing powers. 



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

 the carbohydrate group, are produced. On tryptic digestion still simpler 

 cleavage products are formed, namely, leucine, tyrosine, and trypto- 

 phane (POSNER and GIES x ) . The glucosamine, so far as we know, is 

 not split off by proteolytic enzymes, but only after strong hydrolysis with 

 acids, and this speaks against the assumption that the glucosamine group 

 exists as a glucoside-like combination in the mucin molecule (NEUBERG 

 and MiLCHNER 2 ). 



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 reactions 

 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, this 

 can now be carefully acidified with acetic acid without giving a precipitate. 

 Such acidified solutions are copiously precipitated by tannic acid; with 

 potassium ferrocyanide they give no precipitate, but on sufficient con- 

 centration they become thick or viscous. A neutral solution of alkali 

 mucin is precipitated by alcohol in the presence of neutral salts; it is 

 also precipitated by several metallic salts. If mucin is heated on the 

 water-bath with dilute hydrochloric acid of about 2-per cent, the liquid 

 gradually becomes a yellowish or dark brown, and reduces copper salts 

 in alkaline solutions. 



The mucin most readily obtained in large quantities is the submaxil- 

 lary Tnucin, which may be prepared in the following way: The filtered 

 watery extract of the gland, free from form-elements and as colorless as 

 possible, is treated with 25-per cent hydrochloric acid, so that the liquid 

 contains 1.5 p. m. HC1. On the addition of the acid the mucin is immedi- 

 ately precipitated, but dissolves on stirring. If this acid liquid is imme- 



1 Ainer. Journ. of Physiol., 11. 2 Berl. klin. Wochenschr., 1904. 



