Age-related changes in the structure of the proteoglycan subunits from human articular cartilage.

The proteoglycan of articular cartilage provides this tissue with its elastic properties, and the structure of these molecules plays a major role in determining the resilience of the tissue to compression. In the present paper we have shown that the structure of the proteoglycan subunits in human articular cartilage changes considerably between the fetus and the mature adult. These changes occur gradually and are essentially complete by the end of growth. With increasing age the following changes were most pronounced: 1) a decrease in the proteoglycan content of the cartilage, 2) a decrease in the size of the proteoglycan subunit, 3) an increase in keratan sulfate relative to chondroitin sulfate, 4) an increase in 6-sulfation relative to 4-sulfation along the chondroitin sulfate chains, 5) an increase in protein relative to glycosaminoglycan, and 6) a decrease in serine and glycine and an increase in arginine content of the core protein. The susceptibility of the proteoglycan to proteolytic degradation by pepsin also showed age-related variations, but papain always produced the same degradation products. Irrespective of age, the majority of the proteoglycan subunits possessed the ability to interact with hyaluronic acid. Thus, although large structural changes take place in the glycosaminoglycan attachment region of the proteoglycan, the hyaluronic acid-binding region would appear to be relatively invariable. Decreases in the concentration, size, and charge of the proteoglycan would be expected to lower the elastic properties of the older cartilage.