Intermediate labile intermolecular crosslinks in collagen fibres

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).

Receptors: localization and specificity of binding of serotonin in the central nervous system

Formation of a Schiff base between the ethylamine residue of serotonin and an appropriate carbonyl residue at the receptor site may be among the forces holding serotonin onto the receptor. Reduction of this imine may provide a means of permanently labeling receptors as a preliminary to their isolation.

The effect of semicarbazide on the nature and stability of collagen fibrils

Semicarbazide affected both the final width and stability of fibrils reconstituted from solutions of acid-soluble collagen. Fibril width was increased after semicarbazide treatment at pH2.6 and 4.3, whereas after treatment at pH8.9 it decreased. Fibril stability was decreased after semicarbazide treatment at all values of pH and temperature, indicating the inhibition of intermolecular cross-linking. A direct binding of semicarbazide to the alphabeta-unsaturated aldehyde groups in the intramolecular cross-link was demonstrated.

Chemistry of the collagen cross-links. Isolation and characterization of two intermediate intermolecular cross-links in collagen

This paper describes the isolation from reduced collagen of two new amino acids believed to be involved, in their non-reduced form, as intermolecular cross-links stabilizing the collagen fibre. The reduction of intact collagen fibrils with tritiated sodium borohydride was found to stabilize the aldehyde-mediated cross-links to acid hydrolysis and thus allowed their location and isolation from acid hydrolysates on an automatic amino acid analyser. Comparison of the radioactive elution patterns from the autoanalyser of collagen treated in various ways before reduction permitted a preliminary classification of the peaks into cross-link precursors, intramolecular and intermolecular cross-links. The techniques employed to isolate the purified components on a large scale and to identify them structurally are described in detail. Two labile intermolecular cross-links were isolated in their reduced forms, one of which was identified by high-resolution mass spectrometry as N(in)-(5-amino-5-carboxypentyl)hydroxylysine. The structure of this compound was confirmed by chemical synthesis. The cross-link precursor alpha-aminoadipic delta-semialdehyde was isolated in its reduced form, in-hydroxynorleucine, together with its acid degradation product in-chloronorleucine. A relatively stable intermolecular cross-link was isolated and partially characterized by mass spectrometry as an aldol resulting from the reaction of the delta-semialdehyde derived from lysine and hydroxylysine.

Reversible transformation of fibrous collagen to a soluble state in vivo

Metabolic pathways of pre-existing and newly synthesized collagen fractions from skin and induced connective tissue were studied in rats that had been labeled with (3)H-L-proline 6 and 20 weeks previously. The distribution of specific radioactivity was determined for soluble and insoluble collagens. Multiple extractions of neutral salt-soluble and citrate-soluble collagen demonstrated constant specific radioactivities which indicated the achievement of an isotopic steady state. Citrate-soluble collagen was four times more radioactive than neutral salt-soluble collagen, which indicated a large difference in biological age (months) between fractions. Different levels of specific radioactivity in collagen fractions from implanted sponge indicated that both pre-existing (citrate-soluble) and newly synthesized collagen (salt-soluble) contributed to the insoluble collagen of induced connective tissue in normal and hypophysectomized rats. The isotopic data indicated that pre-existing fibrous collagen could be solubilized in vivo and translocated from skin to sponge via citrate-soluble collagen.

Collagen has a discrete family of reactive hydroxylysyl and lysyl side-chain amino groups

Several cross-linking substances which derive from lysine have been isolated from hydrolyzates of collagen and elastin. This observation suggests that certain lysyl side chains exhibiting enhanced reactivity and unique configuration may participate in cross-linking. In native collagen there is a small family of lysyl and hydroxylysyl side chains which exhibit a high propensity of Schiff base formation.