Vitreous structure. VI: Age-related changes in the thermal stability and crosslinks of vitreous, articular cartilage and tendon collagens

Abstract

Thermal stability studies were performed on samples of fetal, young and adult bovine vitreous, articular cartilage and tendon collagens. The major age-related change observed was a decrease in the quantity of tendon collagen solubilized by heat alone, whereas very small quantities of vitreous and articular cartilage collagens were rendered soluble by heat alone at all ages. Vitreous collagen and tendon collagens had similar heat-denaturation profiles, although at all ages vitreous was slightly more stable than tendon. The denaturation of articular cartilage collagen occurred at significantly higher temperatures that that of vitreous and tendon collagens. The relative proportions of reducible crosslinks in fetal and adult vitreous, articular cartilage and tendon collagens were also determined. In all fetal collagens dihydrolysinonorleucine was the major reduced crosslink detected. Fetal articular cartilage and tendon collagen had about twice the amount of dihydroxylysinonorleucine as vitreous collagen but when the adult samples were compared vitreous collagen contained a 3.4-times higher content of dihydroxylysinonorleucine than articular cartilage and only trace amounts of this crosslink were detected in tendon collagen. The 3-hydroxypyridinium content of adult vitreous collagen was also determined and was found to be about half that of articular cartilage collagen. These results indicate that there is maturation of hydroxylysino-5-ketonorleucine to 3-hydroxypyridinium crosslinks in a similar manner to that proposed for cartilage collagen. The presence of dihydroxylysinonorleucine crosslinks in the adult vitreous collagen, however, indicates that either the maturation process is much slower or incomplete or that there is a much more active synthesis of collagen containing immature hydroxylysino-5-ketonorleucine crosslinks in the vitreous than in articular cartilage.