The chemistry of the collagen cross-links. Age-related changes in the reducible components of intact bovine collagen fibres.

The change in the amounts of the three major reducible cross-links was followed throughout the bovine-life span. The major reducible cross-link in embryonic skin is 6,7-dehydro-N(epsilon) -(2-hydroxy-5-amino-5-carboxypentyl)hydroxylysine, but this is gradually replaced in the latter stages of gestation or early postnatal growth period by two other Schiff bases, 6,7-dehydro-N(epsilon)-(5-amino-5-carboxypentyl)hydroxylysine and a component not yet identified, designated Fraction C. These latter two Schiff bases increase in amount during the rapid growth period to a maximum, after which they then slowly decrease until at maturity they are virtually absent. The proportion of these Schiff bases closely reflects the rate of growth, i.e. the amount of newly synthesized collagen present at any one time. Similarly, the three Schiff bases present in tendon and the one in cartilage slowly decrease during maturation. No evidence for the possible stabilization of these aldimine bonds during maturation by reduction in vivo was found by three different analytical techniques. Concurrently with the decrease in the proportion of the Schiff bases some new reducible components increased during maturation, but their characterization as N(epsilon)-glycosylamines demonstrated that they were not related to the lysine-derived aldehyde components. The significance of these components in the aging process cannot at present be assessed. As no evidence was obtained for any new reducible cross-links replacing the Schiff bases, it is probable that the latter are intermediate cross-links and that during maturation they are stabilized to some as yet unknown non-reducible cross-link as previously proposed (Bailey, 1968).