Mucus in health and disease.

Abstract

The main constituent of mucus is a high molecular weight glycoprotein. Perhaps therefore we should spend a little time discussing how this particular type of material fits into the classification of biopolymers generally. Glycoproteins are a subdivision of ‘Glycoconjugates’ which are defined as carbohydrates covalently linked to lipid, protein or other non-carbohydrate substances but excluding nucleic acid. We are particularly concerned at this meeting with carbohydrate covalently linked to protein and this, in mammals, gives rise to two kinds of glycoconjugates, namely the proteoglycans and the glycoproteins. Although these two substances are made of the same types of material, there is little overlap between them. The main differences are shown in Table 1 although it must be emphasized that each statement is, of necessity, a generalization to which there are exceptions. Firstly, the two substances tend to occur in different tissues. The proteoglycans occur in skeletal and supporting tissues whereas most glycoproteins are present in body fluids such as blood, seromucous secretions and so on. They also differ in their carbohydrate side chains (the carbohydrate component of proteoglycans is usually termed ‘glycosaminoglycan’).