Mucin

MUCIN, a compound of protein with the amino-sugar glucosamine (see); it belongs to the group of phosphorus-free glucoproteids (see). Mucin was found in mucous and synovial membranes, mucous-type salivary glands, the umbilical cord, in the secretion of snails, the skin of frogs, and in eggs. The best method for obtaining mucin in pure form from the submaxillary gland consists of dissolving it in very dilute hydrochloric acid and precipitating it by dilution with water (Hammarsten). M. solutions do not coagulate or precipitate with potassium ferrocyanide upon boiling, but give the other protein precipitation reactions. During peptic digestion, M. forms albumoses and peptone-like substances containing a carbohydrate group; during tryptic digestion it is broken down to amino acids. The biological significance of M. consists in the tenacity and slipperiness of their precipitates and solutions, which facilitates the swallowing of a food bolus, protects objects from compression (especially important in relation to the umbilical cord) and from mechanical injuries (for example, in eggs).