x THE GLYCO-PROTEIDS : MUCIN 535 



1. Submaxillary Mucin 



Mucin forms a white, loose, hardly hygroscopic powder, and may 

 be preserved in this dry state for years without its properties becom- 

 ing altered. It is only with difficulty soluble in water and neutral 

 salt-solutions ; it is insoluble in acids, but on the addition of acetic 

 acid it gives rise to a tough, adhesive curd. It is, however, readily 

 soluble in very dilute alkalies, and then forms a neutral or in some 

 instances a slightly acid solution. Mucins are therefore pronounced 

 acids. A solution containing 0*228 per cent of mucin behaves like a 

 typical mucilaginous solution, being viscous, adhesive, and readily 

 pulled out into threads. From its solutions it is precipitated by 

 acids, specially by acetic acid, but the precipitate, instead of being 

 flocculent, as in the case of ordinary albumins, forms a tough, mucila- 

 ginous mass which, if stirred with a glass rod, winds round the latter. 



Muciri is either not soluble or only soluble to a slight extent in an 

 .excess of acetic acid, while it is readily dissolved by O'l to 0'2 per 

 cent HC1, but not so readily as are the nucleo- albumins and the 

 globulins. Mucin is precipitated by acids only if the solutions are 

 poor in salts ; it is not precipitated in the presence of sodium chloride 

 or other neutral salts. Mucin, like all other glyco-proteids, is not 

 coagulated on being boiled, and the addition of acetic acid to a boiling 

 solution of mucin does not give rise to a more abundant precipitate 

 than can be obtained by adding acetic acid to a cold solution, and if 

 to the boiling mucin-solution some sodium chloride be added, acetic 

 acid will again cause no precipitate. Brauer has made use of this 

 property in demonstrating coagulable albumin in addition to mucin in 

 pathological bile : he very slightly acidifies the bile, which already 

 contains salts, and then boils it : the coagulable albumin is thrown 

 down, while the unaltered mucin remains in solution. Mucin is not 

 precipitated by alcohol except a sufficient amount of neutral salts be 

 present ; in the absence of salts, alcohol only gives rise to a more or 

 less marked opalescence. Mucin is precipitated by nitric acid, and 

 also by copper sulphate, mercuric chloride, ferric chloride, and lead 

 acetate. Potassium bichromate and alum do not give rise to a preci- 

 pitate, but convert mucin into a slimy, swollen mass. The alkaloidal 

 reagents, tannin, mercury + potassium iodide, etc., in neutral solu- 

 tions do not precipitate, but they do throw down mucin which has 

 been rendered soluble by the addition of an excess of hydrochloric acid. 

 Potassium ferrocyanide does not precipitate, but, at the most, renders 

 the solution somewhat more viscous ; it resembles the neutral salts in 

 preventing the precipitation by means of acids. 



