Synopsis
The mucosubstances embrace two types of molecule, the proteoglycans and the glycoproteins. These polymers have in common the existence of a single polypeptide chain to which is attached one or more polysaccharide structures. It is, however, possible to distinguish the two groups by means of several important structural characteristics of their carbohydrate components.
The presence of large amounts of carbohydrate in the molecules of many mucosubstances puts special difficulties in the way of their fixation for microscopical examination. The staining reactions used for the proteoglycans and glycoproteins depend entirely on the chemical properties of their carbohydrate components; in the past, these reactions have lacked specificity, but new, improved methods are becoming available.
In their biosynthesis, the mucosubstances seem to follow an intracellular pathway peculiar to those proteins destined for extracellular secretion, and distinct from that of the cytoplasmic proteins. The membrane systems of the endoplasmic reticulum and Golgi apparatus are of particular importance in the process.
Little is known of the biological role of many of the mucosubstances, and of their carbohydrate components in particular. A theory is put forward, proposing an essential role for the proteoglycans of cartilage in the maintenance of the mechanical function of the tissue. Work now in progress is giving strong indications that a lysosomal proteinase, cathepsin D, plays an important part in the pathological degeneration of cartilage, through its action on the proteoglycans. Lysosomes seem well equipped in their complement of hydrolytic enzymes to mediate the catabolism of mucosubstances in general.
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Barrett, A.J. The biochemistry and function of mucosubstances. Histochem J 3, 213–221 (1971). https://doi.org/10.1007/BF01005220
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DOI: https://doi.org/10.1007/BF01005220