Homeostasis in extracellular tissues: insights from studies on collagen

Spectroscopic studies on pepsin-solubilized vitreous and cartilage collagens

Circular dichroism and the fluorescent probe, 2-p-toluidinyl-naphthalene-6-sulfonate, were used to compare the molecular properties of pepsin-solubilized vitreous collagen with cartilage and calfskin collagens. Type II vitreous and cartilage collagens have more hydrophobic regions along their molecular domain than does type I calfskin collagen. The rate of fibril growth is faster in type II collagens than in type I. The increased hydrophobicity of type II collagens is attributed to high carbohydrate content and compositional variations. Although the secondary structures of the three collagens do not differ significantly, differences in carbohydrate content, composition, and hydrophobic character may cause some variations in the tertiary structures. It is suggested that the tertiary structure plays an important role in the nature and rate of fibril growth. Differences between cartilage and vitreous collagen in fluorescence behavior, fibril growth, and melting temperature indicate that vitreous collagen may be a "special type II collagen."

Aging, and crosslinking in mammlian collagen

The amount and type of borohydride-reducible crosslinks in collagen have been examined as a function of animal age. In a variety of bovine, canine and human tissues the level of redicible crosslinks decreases with time and the ratios of individual compounds change. There is both tissue and species specificity in the extent of these changes. A decrease in the level of reducible crosslinks correlates with the cessation of growth. Loss of reducible crosslinks does not imply a small total number of crosslinks since physical changes with age imply the opposite. We conclude that reducible crosslinks are converted to a stable nonreducible state and the persistence of low levels of reducible crosslinks may be indicative of a low level of turnover in the tissue. Changes in ratios of reducible crosslinks are of doubtful functioal significance and may simply reflect variation in post-translational modification of lysine residues.