An investigation of pyridinoline, and putative collagen cross-link

Collagen cross-links: location of pyridinoline in type I collagen

Collagen from bone, dentine and tendon (type I), all of which contain the pyridinoline cross-link at varying levels, were each digested with CNBr. The resulting peptide mixtures were resolved by gel filtration on A1.5m agarose and assayed for pyridinoline. The polymeric cross-linked peptide complex, poly alpha 1CB6 [(1980) Biochem. J. 189, 111] isolated from each of these tissues did not contain pyridinoline. Only one peptide fraction contained the pyridinoline cross-link; that identified as alpha 2CB3,5. However, this peptide showed only a small increase in Mr in its cross-linked form (approx. 2000-5000) demonstrating that pyridinoline is not involved in the formation of polymeric structures like poly alpha 1CB6. These data, considered in the light of the recent finding that pyridinoline is present in type I collagens from different sources in widely varying amounts, cast doubt on its role in collagen maturation.

Ultraviolet light- and ozone-induced changes in pyridinoline, a trisubstituted 3-hydroxypyridinium crosslink of collagen

Pyridinoline photo-degraded with the formation of photoproducts absorbing diffusely around 260-290 nm (pH 7) and sharply at 232 nm (pH 1). Subsequent heating partially regenerated the original pyridinoline, also producing new products absorbing at 417/440 nm (pH 7) and 300/412 nm (pH 1). Pyridinoline (pH 7) and its new products (pH 7 and pH 1) also underwent ozone-induced degradation.

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.

Changes in pyridinoline content of elastin during ontogeny

It has been shown that, contrary to desmosine and isodesmosine, the content of pyridinoline increases in elastin preparations obtained from rat and bovine aortae and from bovine ligmentum nuchae throughout the whole lifespan. The molar proportion of this cross-linking amino acid is within the range of 0.6 - 2.5 leucine equivalents per 100 amino acid residues.