The hydroxypyridinium crosslinks of skeletal collagens: their measurement, properties and a proposed pathway of formation

Cross-linking in type IV collagen

Type IV collagen could not be extracted from human placenta using 6M-urea containing 10mM-dithiothreitol, indicating that the type IV molecule is stabilized within the basement membrane by covalent cross-links. However, various forms of type IV collagen molecule were extractable by means of increasingly severe proteolytic conditions. Type IV collagen tetramers ('spiders') lacking only the C-terminal globular region (NC1) were further digested to the 'long-form' 7S fragment and an assortment of helical rod-like molecules derived from the 'leg' region. These molecules were separated by salt fractionation and examined by rotary-shadowing electron microscopy. Isolation of these fractions from placenta which had been reduced with NaB3H4 revealed that both the 7S (N-terminal) and C-terminal regions contained significant proportions of reducible lysine-derived cross-links. These cross-links were shown to be exclusively the stable oxo-imine hydroxylysino-5-oxonorleucine. The number of the cross-links per molecule was significantly lower than might be expected in order to fully stabilize the molecule. These results suggest that the keto-imine cross-links in type IV collagen have been stabilized in part by conversion into an unknown non-reducible form, although a sensitive immunoassay failed to show the presence of any pyridinoline. Comparison with the fibrous collagen from placenta suggested that the rate of this conversion is much greater in basement-membrane collagen.

Quantitation of hydroxypyridinium crosslinks in collagen by high-performance liquid chromatography

An HPLC method for quantifying the 3-hydroxypyridinium crosslinks of collagen is described. It can be applied to crude hydrolysates of all types of connective tissue. Mineralized tissues can be hydrolyzed directly and analyzed without interference from the mineral ions. The hydroxylysyl (HP) and lysyl (LP) forms of hydroxypyridinium residue were resolved on a reverse-phase C18 column using a gradient of acetonitrile in water and 0.01 M n-heptafluorobutyric acid as an ion-pairing agent. The crosslinking amino acids were accurately quantified down to 2 PM (1 ng) injected, by detecting their natural fluorescence with a spectrofluorometer. Tissues in which hydroxypyridinium crosslinks were plentiful included all forms of cartilage, bone, dentin, ligament, tendon, fascia, intervertebral disc, lung, gut, cervix, aorta, and vitreous humor. Among normal tissues, LP, the minor form of the crosslink, was present in significant amounts relative to HP only in bone and dentin. Both crosslinks were essentially absent from skin, cornea, rat tail tendon, and basement membranes.

Cross-linking of collagen. Isolation, structural characterization and glycosylation of pyridinoline

A method for the isolation and purification of pyridinoline from bone collagen was developed, with the use of sulphonated polystyrene resins. The analytical techniques were used to quantify pyridinoline, for which hydroxyallysine is a known precursor, in a wide range of tissues. The structure of pyridinoline proposed by Fujimoto, Moriguchi, Ishida & Hayashi [(1978) Biochem. Biophys. Res. Commun. 84, 52-57] was confirmed by 13C-n.m.r. spectroscopy and fast-atom-bombardment mass spectrometry. At concentrations greater than about 0.1 mM, pyridinoline exhibited altered fluorescence properties that were consistent with excimer formation. From alkali hydrolysates of several different tissues, a fluorescent compound was purified by gel filtration and ion-exchange chromatography and was shown to be galactosylpyridinoline. This derivative was very labile to acid treatment compared with the bifunctional cross-link analogues, and was completely converted into free pyridinoline by heating at 108 degrees C for 8 h in 0.1 M-HCl. Galactosylpyridinoline was also partially converted into free pyridinoline by prolonged alkali hydrolysis. This lability, which could also apply to other multifunctional cross-link derivatives, may explain the fact that no disaccharide derivatives of pyridinoline were isolated.

Cross-linking of collagen. Location of pyridinoline in bovine articular cartilage at two sites of the molecule

The location of pyridinoline in 18-month-old bovine articular cartilage was investigated by fractionation of CNBr-derived peptides by ion-exchange chromatography and gel filtration. Two peptides, PCP1 and PCP2, were isolated and were shown to contain stoichiometric amounts of pyridinoline. From its amino acid composition and sequence studies, peptide PCP1 was shown to comprise two C-terminal non-helical chains (CB14) linked through pyridinoline to the alpha 1(II)-CB12 portion of the helix. The CB14 chains appeared to be labile at their C-terminal ends, resulting in lower-than-expected amounts of homoserine, and only the N-terminal portion of the peptide was sequenced. Similar studies of peptide PCP2 showed that it contained two N-terminal non-helical chains (CB4) linked to the alpha 1(II)-CB9,7 portion of the helix. The isolated peptides therefore confirmed the function of pyridinoline in stabilizing the 4D stagger of adjacent molecules. The possibility that the cross-link could act both as an intra- and an inter-microfibrillar cross-link was considered. A mechanism of formation of pyridinoline was postulated that, together with other evidence, appears to support the view that, in cartilage, pyridinoline acts primarily as an intramicrofibrillar cross-link and does not contribute to increased stability during maturation through lateral aggregation and bonding of filaments.

Collagen crosslinks in human dentin: increasing content of hydroxypyridinium residues with age

The hydroxypyridinium crosslinks of collagen are believed to derive from reducible, divalent crosslinks. To study this concept further, both types of crosslink were quantified as a function of age in dentin, a tissue thought to have minimal collagen turnover. Human (5, 15, 28 and 56 years) and bovine (fetal and adult) root dentin was analyzed by a procedure that measures both hydroxypyridinium and reducible crosslinks on the amino acid analyzer. In human dentin, hydroxypyridinium crosslinks increased with age and became the predominant crosslinks as the two reducible residues, dehydrodihydroxylysinonorleucine and dehydrohydroxylysinonorleucine, diminished. Similarly in adult bovine dentin, hydroxypyridinium residues were sixfold more concentrated than in fetal bovine dentin. Borohydride treatment of tissue did not influence the measured content of hydroxypyridinium residues. The analyses also ruled out natural reduction as a stabilizing reaction for the divalent, reducible crosslinks. Though hydroxypyridinium residues became the major aldehyde-mediated crosslinks of adult dentin collagen, significant levels of reducible crosslinks remain throughout the tooth's adult life.

Nonmineralized and mineralized compartments of bone: the role of pyridinoline in nonmineralized collagen

Collagen tryptic peptides obtained from the nonmineralized and mineralized compartments of diaphyseal bone have different distributions of intermolecular crosslinks. Pyridinoline, a collagen crosslink thought to be associated with chronologically older bone, was detected in peptides from nonmineralized collagen but not from mineralized collagen. Mineralization may prevent collagen maturation; conversely, pyridinoline in nonmineralized collagen may decrease the intermolecular distances among collagen chains in fibrils and preclude mineralization.

Modulation by propranolol of the lysyl cross-links in aortic elastin and collagen of the aneurysm-prone turkey

dl-Propranolol (propranolol) fed to immature and mature aneurysm-prone turkeys (Broad-Breasted White, BBW) for 6 weeks significantly raised the tensile strength of tissue rings from the abdominal aorta. The drug-mediated increase in tensile strength values was dose-related and independent of its heart rate- and arterial pressure-lowering effects. Propranolol acts, in part, by (a) stimulating lysyl oxidase to produce greater amounts of reactive aldehydes for intermolecular cross-links, (b) enhancing the progression of chemically unstable to stable forms of intermolecular elastin cross-links (lysinonorleucine and the desmosines), and (c) reducing the density of the age-related intermolecular cross-linking of collagen (pyridinoline). These propranolol effects on the lysyl cross-links were demonstrated in both the immature and mature animals and suggest a heretofore unrecognized potential for this widely used cardiovascular drug.

Cross-linking and fluorescence of pyrene-labeled collagen

The pyrene-labeling of acid-soluble (type I) and acid-insoluble collagens from young and old rat tail tendon has been investigated. The pyrene excimer fluorescence is associated with stabilized pyrene labels bound to two adjacent aldehydes in monomeric young collagens. Polymeric young collagens, as well as monomeric and polymeric old collagens, tend to lose this specific arrangement. This is shown by salt and new chromatographic fractionation of monomeric and polymeric collagens. During denaturation, pyrene labels are released from saturated aldehydes in both alpha 1 and alpha 2 chains. This unstable pyrene-labeling is stabilized by NaBH4 reduction of the hydrazone bonds between aldehyde groups and pyrene-containing hydrazines. This stabilization reveals that alpha 1 contains more aldehyde groups than does alpha 2 in young collagen. Pepsin-solubilized, acid-insoluble collagens are partly cross-linked and, like acid-soluble collagens, exhibit the fluorescence of pyrene aggregates probably located at unidentified cross-links, different from unsaturated aldehyde-containing cross-links in acid-soluble collagens.

Collagen cross-linking compounds in human urine

We report the isolation and chemical characterization of collagen cross-linking compounds, 3-hydroxypyridinium and dihydroxylysinonorleucine, from human urine.