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

Soft-tissue "flaws" are associated with the material properties of the healing rabbit medial collateral ligament

This study evaluated microscopic flaws in the healing rabbit medial collateral ligament and their significance in terms of the material properties of this ligament during healing. A gap injury was created in the midsubstance of the medial collateral ligament in the right hindlimb of 15 skeletally mature (12 months old) New Zealand White rabbits. At postoperative intervals of 3, 6, or 14 weeks, histomorphometric analysis of the flaws was carried out in subgroups of animals. The medial collateral ligaments from four of the left hindlimbs (randomly selected) were used as uninjured contralateral controls. In one histologic section of each area of scar tissue and the analogous area in the controls, specified tissue flaws (blood vessels, fat cells, hypercellular areas, loose matrix, disorganized matrix, or a combination of these) were measured by four independent and blinded observers. The results showed that the mean total area of the flaws, as a percentage of the total section, and the mean area of the largest flaw decreased with healing time in each healing group but did not achieve control values by 14 weeks. Because it was not possible to test the healing medial collateral ligaments mechanically prior to measurement of the flaws (due to the destructive nature of failure testing), the data on the flaws were compared with the material strength and stiffness of a separate series of similarly injured and mechanically tested medial collateral ligaments (data published previously). A maximum likelihood statistical analysis showed a very strong functional association between the mean area of the largest flaw and the stress at failure (p < 0.004) and between the mean flaw area as a percentage of the total section area and the elastic modulus (p < 0.001). This study therefore demonstrates that it is possible to quantify material flaws in scar tissue in rabbit medial collateral ligaments, that these flaws become smaller with healing time as the scar remodels, and that flaws are functionally associated with the material properties of the ligament in this model (larger flaws with less tensile strength and more flaws with less stiffness).

Structural analysis of cross-linking domains in cartilage type XI collagen. Insights on polymeric assembly

The collagen framework of hyaline cartilage is based on copolymers of types II, IX, and XI collagens. Previous studies have established specific covalent interactions between types II and IX collagens. The present study examined cross-linking sites in type XI collagen to define better the full heteropolymeric assembly. Pepsinsolubilized type XI collagen was purified from fetal bovine cartilage. The cross-linking amino acids in the preparation were primarily divalent, borohydride-reducible structures; pyridinoline residues were essentially absent. Individual alpha 1(XI), alpha 2(XI), and alpha 3(XI) chains were resolved by high performance liquid chromatography. Telopeptides still attached by cross-links to helical sites were released by periodate oxidation and identified by microsequencing. Analysis of cross-linked peptides isolated from trypsin digest of each alpha-chain identified the attachment helical sites for the telopeptides. A high degree of interchain specificity was evident in the cross-linking between N-telopeptides and the COOH terminus of the triple-helix, consistent with a head-to-tail interaction of molecules staggered by 4D (D = 67 nm) periods. In addition, alpha 1(II) C-telopeptide was linked to the amino-terminal site of the alpha 1(XI) triple helix. In summary, the results show that type XI collagen molecules are primarily cross-linked to each other in cartilage, implying that a homopolymer is initially formed. Links to type II collagen are also indicated, consistent with an eventual cofibrillar assembly. Analysis of cartilage extracts showed that all three chains, alpha 1(XI), alpha 2(XI), and alpha 3(XI), had at least in part retained their N-propeptides in cartilage matrix and that the alpha 3 (XI) chain was the IIB splicing variant product of the COL2A1 gene. Of particular note was the finding that the N-telopeptide cross-linking site in both alpha 1(XI) and alpha 2(XI) is located amino-terminal to the putative N-propeptidase cleavage site. This structural feature provides a potential mechanism for the proteolytic depolymerization of type XI collagen by proteases that can cleave between the cross-link and the triple helix (e.g. stromelysin).

Regional and age variations in growing tendon

Gastrocnemius tendons of 10 White Leghorn chickens at 6, 8, and 12 weeks of age were divided into proximal, middle, and distal portions to assess regional variability in composition and growth. Body weight increases approximately 150% during the period examined, whereas the lateral gastrocnemius muscle and tendon increase approximately 193% and 227%, respectively. No significant changes in cellularity (DNA concentration) or hydroxypyridinium (OHP) crosslinks occur with increasing age. Hydroxyproline (HYP) concentration increases by 12 weeks of age, as hexuronate, glucosamine, and galactosamine decrease. Composition shows some regional variation: the distal region of the tendon has a lower HYP concentration and increased GAGs and OHP crosslinks compared to either the proximal or middle regions, which do not differ from each other. The mean collagen fibril diameter increases with age, but the oldest tendons also contain more small diameter fibrils (< 40 nm). There is a unimodal fibril distribution at all three ages, although this has broadened by 12 weeks. The data from this study suggest that rapid tendon growth occurs throughout the time period examined and that changes characteristic of mature tendon, such as increased OHP crosslink concentration, have not yet developed in hatchlings because of the large amount of new tissue being produced. Whereas all three regions of the tendon are similar in size, composition of the distal region differs from that of the proximal and middle regions, suggesting that this portion of the tendon should be avoided when sampling a tendon.

Thermal stability of cortical bone collagen in relation to age in normal individuals and in individuals with osteopetrosis

The thermal stability of cortical bone collagen was determined in iliac crest biopsies obtained from 41 normal individuals (21 women aged 20-96 years and 20 men aged 21-84 years) and 8 individuals with autosomal dominant osteopetrosis type I (4 women and 4 men aged 17-63 years). The cortical bone was decalcified and the bone matrix was cut into 80-microns-thick freeze sections parallel to the bone surface. Circular specimens punched out from the sections were used for determination of area shrinkage during gradual heating and shrinkage temperature, Ts (representing the temperature for 50% of the area shrinkage). In normal men, Ts was not found to decrease until the age of 60-65 years, but was markedly decreased in the elderly individuals. In normal women, Ts varied considerably throughout the age range tested, without relationship to age. In contrast to age-matched controls, Ts decreased with age in men with osteopetrosis, whereas Ts in affected women was neither related to age nor different from the highly variable values found in age-matched normal women. Previous findings in rats indicate that Ts decreases with the chronological age of the bone collagen. The present results agree with these findings, which imply that a reduction in turnover rate of bone results in an increasing age and a reduced Ts of the constituent collagen. Following this assumption, the turnover rate of bone seems to be more variable in women than in men and reduced in osteopetrotic men.(ABSTRACT TRUNCATED AT 250 WORDS)

Dentinogenesis

The formation of dentin, dentinogenesis, comprises a sophisticated interplay between several factors in the tissue, cellular as well as extracellular. Dentin may be regarded as a calcified connective tissue. In this respect, as well as in its mode of formation, it is closely related to bone. Using dentinogenesis as an experimental model to study biomineralization provides several practical advantages, and the results may be extrapolated to understand similar processes in other tissues, primarily bone. After describing dentin structure and composition, this review discusses items such as the morphology of dentinogenesis; the dentinogenically active odontoblast, transport, and concentrations of mineral ions; the constituents of the dentin organic matrix; and the presumed mechanisms involved in mineral formation.

Site comparisons of dentine collagen cross-links from extracted human teeth

Covalent intermolecular cross-links in collagen provide the dentine matrix with stability and tensile strength. The density of collagen cross-links varies depending on the site within the same tissue. This variation is probably due to factors such as the different amounts of stress and different turnover rates at the respective sites. The aim was to quantify the collagen cross-links in different tooth groups as an adaptation to functional requirements. For this purpose, the types and content of major cross-links in dentine collagen of human teeth from three different sites were measured and compared: incisors, premolar-canines and molars. After removal of cementum and pulp, 23 extracted teeth at different ages (27-69 yr) were individually pulverized and demineralized with EDTA. Collagen was reduced with standardized NaB3H4, hydrolysed, and subjected to amino acid and cross-link analyses. Each cross-link was quantified on the basis of mole per mole of collagen. The results indicated: (1) all teeth contained labile, reducible (dehydro-dihydroxylysinonorleucine and dehydro-hydroxylysinonorleucine) and stable, non-reducible (pyridinoline and its lysyl analogue) cross-links, and (2) the content of both reducible and non-reducible cross-links was least in incisors and greatest in molars. This suggests that dentine collagen matrix maybe functionally adaptive.

Increased levels of urinary collagen crosslinks in females with rheumatoid arthritis

Bone loss is a feature of RA, but the exact mechanisms involved are not clear. The collagen crosslinks deoxypyridinoline (DPYR) and pyridinoline (PYR) are specific indices of 'mature' collagen breakdown and reflect increased bone turnover. The aims of the study were to examine crosslink levels in RA and their association with disease activity and the effect of steroids. Urinary crosslinks corrected for creatinine were measured on morning fasting samples by HPLC in 70 postmenopausal women with rheumatoid arthritis (RA) aged 45-65 and compared with 169 postmenopausal healthy age-matched controls from the population. Mean levels of PYR were significantly higher in RA cases than in controls (52.4 versus 37.5 nmols/mmolCr) although mean levels of DPYR did not differ significantly. A weak correlation was found with ESR and PYR (r = 0.35) but not with other markers of disease activity. Thirteen of the RA cases were current steroid users and their levels of DPYR and PYR even with low doses, were significantly elevated above those of non-users, ex-users and controls. The finding of raised urinary PYR but not the bone specific DPYR in nonsteroid using RA cases suggests that the increased collagen breakdown does not primarily come from bone but from other sources such as cartilage and synovium. The large increases in collagen excretion in low dose steroid users, may reflect the higher risk of osteoporosis in this group.

Collagen II from articular cartilage and annulus fibrosus. Structural and functional implication of tissue specific posttranslational modifications of collagen molecules

Collagen II was isolated and characterized from hyaline cartilage (articular cartilage) and fibro-cartilage (annulus fibrosus). Collagen II from the latter tissue has a substantially higher degree of hydroxylation and glycosylation than that isolated from articular cartilage. The higher degree of posttranslational modification was associated with a slower electrophoretic mobility, a greater resistance to mammalian collagenase digestion and a higher thermal stability. An increase of glycosylation accelerates the initial steps in fibril formation of collagen molecules but slows down the following lateral growth. The newly formed aggregates of collagen II from annulus fibrosus consisted of fibrils with a smaller diameter.

Assay of pyridinium crosslinks in serum using narrow-bore ion-paired reversed-phase high-performance liquid chromatography

The pyridinium crosslinks are important biomarkers of mature hard tissue collagen degradation. This paper describes an isocratic ion-paired reversed-phase high-performance chromatographic assay using narrow-bore columns and high-sensitivity fluorescence detection to enable for the first time the determination of pyridinium crosslinks in both serum and synovial fluid samples. Extracted freeze-dried acid hydrolysates were re-suspended in 20 mM pentafluoropropionic acid (PFPA). Separations were carried out using an Exsil 100 5-microns ODS2 column (100 mm x 2.1 mm I.D.) eluted with 10 mM PFPA in water at 0.15 ml/min and detected using a Jasco 821-FP detector (xenon lamp: excitation 290 nm, emission 400 nm). Fluorescent response was linear from 269 to 8620 fmol for pyridinoline (Pyr) and 85 to 2710 fmol for deoxypyridinoline (dPyr). The limits of detection were 28 and 57 fmol, respectively. The coefficient of variation for extraction and analysis of normal serum was 7.96% for Pyr and 6.30% for dPyr (n = 6). The mean +/- S.D. concentration of Pyr in normal serum was 3.26 +/- 0.83 nM.

Biochemical changes in the collagen of human osteoporotic bone matrix

Although it is known that collagen imparts mechanical strength to bone no detailed biochemical analysis has been made of osteoporotic bone collagen. We report for the first time significant changes in the properties of the collagen. Analysis of collagen types revealed little change in the proportion of Type III collagen, but in some cases there was a significant loss of the Type VI. However, the major differences were observed in the post-translational modifications, namely, in the stabilizing cross-links and the hydroxylation of the collagen. These changes indicated a higher turnover in the head region compared to the neck region of the femoral head and are consistent with the susceptibility of the neck region to fracture. Clearly, the collagen is altered in osteoporosis and these changes may play a role in the pathogenesis of the disease.

The post-translational chemistry and molecular packing of mineralizing tendon collagens

Using cross-linking stereochemistry as indicators, the molecular environment of two collagens in the turkey leg Achilles tendon were compared. The tendon from one year old turkeys was dissected into nonmineralized, fully mineralized and transitionally mineralized portions. Amino acid composition and cyanogen bromide peptide mapping of these portions indicated that the collagens were essentially type I throughout. The fully mineralized compartment had a lysine hydroxylation level similar to turkey or mammalian bone collagen. The non- and transitionally mineralized collagens had a significantly higher lysine hydroxylation, typical of tendon or ligament. However, unlike mammalian tendon, the collagen cross-links were essentially derived from the carboxy-terminal ends of the molecules. The predominant cross-link in this portion was pyridinoline having a high content of 0.95 +/- 0.09 res/mole of collagen. The cross-links in the fully mineralized collagen were also essentially derived from carboxy-terminal aldehyde. However, here significant amounts of the lysyl analog of pyridinoline and lysine-involved bifunctional cross-links were present. The molecular loci of pyridinoline in nonmineralized collagen and the lysyl analog of pyridinoline in mineralized collagen were found to be identical. The total trifunctional cross-link level in the mineralized collagen, 0.55 +/- 0.05 res/mole of collagen, was virtually identical to that observed in old mammalian bone and dentin, and in long term in vitro incubation studies of predentin. We have tentatively concluded that the post-translational chemistry and molecular environments are different in these two turkey tendon fibrils. However, a relative paucity of amino-terminal based cross-links is a feature they have in common. The possible involvement of the amino-terminal telopeptides in collagen mineralization is discussed.

Temporomandibular joint dysfunction. Connective tissue variations in skin biopsy and mitral valve function

Ten women with temporomandibular joint dysfunction and general joint hypermobility (score, 4 to 8) and 10 symptom-free female volunteers without systemic laxity (score, 0 to 2) were selected for the study. A biopsy of connective tissue from arm skin found that the total collagen concentrations were lower and the proteoglycan values were higher in the hypermobile TMJ patients than in the control subjects. The mitral region of the heart was inspected by echocardiography. Eight patients and four controls had slightly abnormal echocardiographic findings. Two patients fulfilled the criteria for mitral valve prolapse. The patients had significantly more musculoskeletal complaints than did the controls. The study suggests an association between joint hypermobility, abnormal skin connective tissue composition, mitral valve malfunction, and musculoskeletal disorders in young women with TMJ dysfunction, especially internal derangement.

Quantitative analysis of collagen and elastin cross-links using a single-column system

The separation of both the immature and mature cross-links present in collagen together with the stable cross-links of elastin has been achieved on a single ion-exchange column. This technique avoids the current necessity for two different systems, ion-exchange and high-performance liquid chromatography-fluorescence techniques. The value of the method is illustrated by the comparison of the cross-link contents of aging bovine skin, from foetal to old age.

Preliminary results of the use of urinary excretion of pyridinium crosslinks for monitoring metastatic bone disease

The collagen crosslinks, pyridinoline and deoxypyridinoline, are recently described markers of the rate of bone resorption. The urinary excretion of these compounds, expressed as a ratio to urinary creatinine, has been measured using ion-pair reversed phase high-performance liquid chromatography in 20 patients receiving oral pamidronate for bone metastases from breast cancer. Before treatment the ratio of pyridinoline and deoxypyridinoline to creatinine in urine (UPCR and UdPCR respectively) were each above normal in 16/20 (80%) patients. Urinary calcium excretion (UCCR) was elevated in 15/20 (75%). There was a strong correlation between UPCR and UdPCR, but neither of the crosslink measurements correlated well with UCCR. Urinary excretion of all three indices of bone resorption fell significantly during pamidronate treatment. The median values after 4 weeks treatment were 63% of baseline for UPCR, 45% for UdPCR and 26% for UCCR. From this preliminary study urinary pyridinoline and deoxypyridinoline excretion appear to be promising markers of bone resorption in advanced malignancy. Their role in response assessment and the advantages over UCCR measurements merit further study.

High-performance liquid chromatographic determination of 3-hydroxypyridinium derivatives as new markers of bone resorption

Hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP) are intermolecular cross-linking amino acids of collagen and their urinary excretion reflects bone resorption. An isocratic high-performance liquid chromatographic assay using a reversed-phase column with a prefractionation step and fluorescence detection was developed. The accuracy and reproducibility were assessed by loading experiments and by double analysis of urinary samples. The recoveries after various loads were above 90% for HP and between 87 and 94% for LP, with an intra-assay relative standard deviation (R.S.D.) of 5% for HP and 8% for LP. The inter-assay R.S.D.s were 8% for HP and 12.4% for LP. The fasting and 24-h urinary excretions of HP and LP were measured in 40 healthy subjects (mean age 35 years) of both sexes. There was no difference between males and females. Mean adult normal values were 33.6 +/- 8.1 pmol/mumol creatinine for HP and 7.0 +/- 2.5 pmol/mumol creatinine for LP in morning fasting urine and 2-.9 +/- 7.0 pmol/mumol creatinine for HP and 5.8 +/- 1.9 pmol/mumol creatinine for LP after 24-h urinary collection. HP and LP excretions were significantly higher in morning fasting urine than in 24-h collections, in agreement with the physiological circadian rythm of bone resorption. This simplified and optimized procedure is a good method for the determination of pyridinolines and should be useful for the evaluation of bone resorption.

Cross-linking connectivity in bone collagen fibrils: the COOH-terminal locus of free aldehyde

Quantitative analyses of the chemical state of the 16c residue of the alpha 1 chain of bone collagen were performed on samples from fetal (4-6-month embryo) and mature (2-3 year old) bovine animals. All of this residue could be accounted for in terms of three chemical states, in relative amounts which depended upon the age of the animal. Most of the residue was incorporated into either bifunctional or trifunctional cross-links. Some of it, however, was present as free aldehyde, and the content increased with maturation. This was established by isolating and characterizing the aldehyde-containing peptides generated by tryptic digestion of NaB3H4-reduced mature bone collagen. We have concluded that the connectivity of COOH-terminal cross-linking in bone collagen fibrils changes with maturation in the following way: at first, each 16c residue in each of the two alpha 1 chains of the collagen molecule is incorporated into a sheet-like pattern of intermolecular iminium cross-links, which stabilizes the young, nonmineralized fibril as a whole. In time, some of these labile cross-links maturate into pyridinoline while others dissociate back to their precursor form. The latter is likely due to changes in the molecular packing brought about by the mineralization of the collagen fibrils. The resultant reduction in cross-linking connectivity may provide a mechanism for enhancing certain mechanical characteristics of the skeleton of a mature animal.

The collagens of articular cartilage

Articular cartilage contains at least five genetically distinct types of collagen. Types II, IX, and XI are cartilage-specific and are cross-linked together in a copolymeric network that forms the extracellular framework of the tissue. Fibrils of type II collagen provide the basic architecture. Type XI, a quantitatively minor fibril-forming collagen, is probably copolymerized with type II collagen in the matrix. Type IX collagen accounts for approximately 1% of the collagenous protein in adult articular cartilage and its molecules exist in the tissue covalently linked to the surface of type II collagen fibrils. Its suspected functions include regulating fibril diameters and mediating fibril-fibril and fibril-proteoglycan interactions. Stromelysin, a matrix metalloproteinase, was recently shown to degrade type IX collagen. This action may cause the collagen network swelling seen in articular cartilage in early experimental osteoarthritis, (OA). Collagen type X is restricted to the underlying calcified zone of articular cartilage, a zone that exhibits active remodeling in joints with OA. Degradation products of the various cartilage collagens show promise as molecular markers of joint disease.

Cortical bone maturation in young hypophysectomized rats

Cortical bone growth and maturation were studied in young growing rats (62 days) that were hypophysectomized. After 10 days, femur bones were obtained for morphological and biochemical analysis of the middiaphysis. Hypophysectomy significantly decreased (42%) normal body growth demonstrated by reductions in body weight. Bone morphology data indicate that hypophysectomy significantly inhibits the circumferential growth of the middiaphysis as well as region-specific thickness compared with the control group. The effects of hypophysectomy on cortical bone mineral composition and density were significantly greater in the hypophysectomized rats. Bone density and total mineral density were significantly greater, 17% and 24%, respectively, in the hypophysectomized group compared with control animals. Bone collagen maturation, measured by the amount of the mature collagen cross-link hydroxylysylpyridinoline, in the hypophysectomized animals was significantly greater (130%). Also, collagen concentration and content per unit volume were significantly greater, 24% and 70%, respectively, in the hypophysectomized group. Our findings suggest that "normal" growth and maturation of cortical bone in rapidly growing animals is dependent upon the modulation of pituitary hormones. These data also suggest that an increased rate of mature collagen cross-link formation may be modified by the turnover rate of collagen and other matrix constituents. In addition, hormonal mechanisms associated with a suppression of bone growth may enhance the probability of mature collagen cross-link formation.

Age-related thermal stability and susceptibility to proteolysis of rat bone collagen

The shrinkage temperature (Ts) and the pepsin-solubilizability of collagen fibrils in bone matrix obtained from decalcified femur diaphysis from 2-, 5-, 15- and 25-month-old rats were found to decrease with age. Digestion with human fibroblast collagenase dissolved less than half of the collagen, whereas sequential treatment by pepsin followed by collagenase resulted in its complete dissolution. This result shows that collagenase and a telopeptide-cleaving enzyme, when acting in an appropriate sequence, have a great potential for the degradation of bone collagen. The 'melting' profile of the pepsin-solubilized collagen showed a biphasic transition with transition peak at 35.9 degrees C and 40.8 degrees C. With increasing age an increasing proportion of the collagen 'melted' in the transition peak at 35.9 degrees C (pre-transition), and the 'melting' temperature (Tm) of the collagen decreased in parallel with Ts in relation to age. Both Ts and Tm decreased by 3 degrees C in the age span investigated. The age-related change in Ts could therefore be accounted for by the decrease in molecular stability. The collagenase-cleavage products of the bone collagen obtained by the sequential treatment with pepsin and collagenase showed only one peak transition (at 35.1 degrees C), and the Tm for the products was independent of age. The results indicate that the pre-transition for the pepsin-solubilized collagen is due to an age-related decrease in thermal stability may have implications for the mechanical strength and turnover of the bone collagen. In contrast with bone collagen, soft-tissue collagen showed neither the age-dependency of thermal stability nor the characteristic biphasic 'melting' profile.

Changes with age in the urinary excretion of lysyl- and hydroxylysylpyridinoline, two new markers of bone collagen turnover

Two intermolecular cross-linking amino acids, hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP), are promising markers in urine of collagen resorption because their levels in urine should reflect only collagen resorption and, unlike hydroxyproline, should not be influenced by degradation of either newly synthesized collagen molecules or noncollagenous proteins. Changes with age in the urinary excretion of HP and LP were studied in 24 h collections of urine from a group of 22 male and 27 female healthy subjects aged from 2 to 70 years. The pyridinolines were quantitated, utilizing their natural fluorescence, after resolution by reversed-phase HPLC. Levels of both pyridinolines were higher in children than in adults, but in adults no evidence of age or sex variations were observed except in the 20-30 year age group. Mean values of HP/Cr and LP/Cr in 37 adults (21-70 years) were 27.2 +/- 1.9 and 8.8 +/- 0.8 mumols/mol, respectively; in the 12 children (2-15 years) the mean values were 14.4 and 12.4 times higher than the respective adult values. Making certain assumptions, the mean amount of bone resorbed in normal adults was tentatively estimated at 1.9 g per 24 h. The finding that differences between children and adults in these relatively specific markers were greater than with hydroxyproline suggests that hydroxyproline values may considerably underestimate the actual amount of bone turnover occurring in growing children or overestimate the adult turnover rate.

Rapid assay for hard tissue collagen cross-links using isocratic ion-pair reversed-phase liquid chromatography

Following a detailed study, a rapid and sensitive assay for the naturally fluorescent collagen cross-links pyridinoline and deoxypyridinoline has been developed using ion-pair reversed-phase high-performance liquid chromatography in the presence of 1-octanesulphonic acid (OSA). Pyridinoline and deoxypyridinoline were separated on an Exsil 100 ODS, 5-microns column (100 mm X 4.6 mm I.D.) using 25 mM sodium formate, 5 mM OSA and 1 mM ethylenediaminetetraacetic acid adjusted to pH 3.25, containing 20% (v/v) methanol. The mobile phase flow-rate was 1.5 ml/min. Compounds were detected by their natural fluorescence (xenon lamp; excitation wavelength 290 nm, emission wavelength 400 nm). Peak areas were linear to 25 pmol injected for pyridinoline and 20 pmol injected for deoxypyridinoline (r = 0.99). Intra-assay coefficients of variation for urinary extracts were 7.65 and 9.07% (n = 10), respectively. Limit of detection (signal-to-noise ratio = 5) was 200 fmol injected. Quantification of the cross-links in acid hydrolysates and human urine samples was possible in under 15 min.

Simultaneous determination of the reducible and nonreducible cross-links of connective tissue. Analysis of mineralized and nonmineralized bone collagen

Secondary amine cross-links occur in collagen and elastin from a number of tissue sources. Quantification of these cross-links by amino acid analysis is complicated by the problem of separating cross-links, which are often minor components, from the more common amino acids and also because relatively large amounts of a cross-link are required to determine a color factor. A specific radioactive labeling method has been developed and used to quantify cross-links in bone collagen. Primary amines such as lysine and hydroxylysine are first guanidinated with 3,5-dimethylpyrazole-1-carboxamidine nitrate (DMPC). Secondary amines, which are unreactive with DMPC, are then quantitatively cyanoethylated with [14C]acrylonitrile. This procedure can be used to detect any secondary amine cross-link, with higher sensitivity than ninhydrin analysis, in peptide form as well as in acid hydrolysates. It is applied here in conjunction with [3H]NaBH4 reduction to simultaneously quantify Schiff base cross-links and amounts of in vivo reduction of Schiff bases in mineralized versus nonmineralized bovine bone.

Cartilage macromolecules and the calcification of cartilage matrix

The calcification of cartilage matrix in endochondral bone formation occurs in an extracellular matrix composed of fibrils of type II collagen with which type X collagen is closely associated. Also present within this matrix are the large proteoglycans containing chondroitin sulfate which aggregate with hyaluronic acid. In addition, the matrix contains matrix vesicles containing alkaline phosphatase. There is probably a concentration of calcium as a result of its binding to the many chondroitin sulfate chains. At the time of calcification, these proteoglycans become focally concentrated in sites where mineral is deposited. This would result in an even greater focal concentration of calcium. Release of inorganic phosphate, as a result of the activity of alkaline phosphatase, can lead to the displacement of proteoglycan bound calcium and its precipitation. The C-propeptide of type II collagen becomes concentrated in the mineralizing sites, prior to which it is mainly associated with type II collagen fibrils and is present in dilated cisternae of the enlarged hypertrophic chondrocytes. The synthesis of type II collagen and the C-propeptide, together with alkaline phosphatase, are regulated by the vitamin D metabolites 24,25(OH)2 cholecalciferol and 1,25 (OH)2 cholecalciferol. At the time of calcification, type X collagen remains associated with type II collagen fibrils. It may play a role in preventing the initial calcification of these fibrils focusing mineral formation in focal interfibrillar sites. This process of calcification is clearly very complex, and involves different interacting matrix molecules and is carefully regulated at the cellular level.

Dentin matrix proteins: composition and possible functions in calcification

Dentin may be regarded as a mineralized connective tissue. In its composition as well as its mode of formation, dentin exhibits several similarities with bone, but also definite differences. The dentin organic phase, the matrix, determines its morphology and is believed to be instrumental in the formation of the mineral phase. A fibrous web of collagen type I dominates the organic matrix. Also, minor amounts of other collagen types may be present. The noncollagenous proteins (NCPs), which constitute about 10% of the matrix, fall into several categories: phosphoproteins, Gla-proteins of the osteocalcin type as well as matrix Gla-protein, proteoglycans, different acidic glycoproteins, and serum proteins. Some of these NCPs have unique chemical compositions that give them specific properties. Dentinogenesis occurs by two simultaneous processes: the formation of a collagenous web in predentin, which is followed by the formation of the inorganic phase at the mineralization front. The composition of the predentin organic matrix differs from that of dentin, as some NCP components are secreted extracellularly just in advance of the mineralization front. In addition, some constituents of predentin seem to be metabolized. The NCPs may be important to several processes during dentinogenesis. Much evidence indicates that noncollagenous components in the matrix are instrumental in mineral formation. New data show that polyanionic NCPs, such as phosphoprotein and proteoglycans, when immobilized on a solid support, induce apatite formation under physiological conditions. These data indicate that polyanionic NCPs may function as mineral nucleators in vivo. They may also act as size and rate regulators for crystallization and promote calcium ion diffusion in the tissue. In addition, NCPs may regulate collagen fibrillogenesis.

Quantitative assessment of collagen crosslinks in dissected predentin and dentin

Dentinogenesis offers a unique system for the study of changes in collagen structure occurring simultaneously with mineralization. Bovine dentin was found to contain about one reducible crosslink per collagen molecule; rat dentin contained twice this amount. In contrast, bovine dentin contained twice as much pyridinium crosslink as did rat dentin collagen. These results indicate that the collagen in rat teeth is less mature and again emphasize the difference in composition between the organic matrices of rat and bovine dentin. In dissected bovine predentin, the unmineralized precursor of dentin, the content of reducible crosslinks was almost double that of dentin. Only minute amounts of non-reducible crosslinks were found in predentin, whereas both pyridinoline and deoxy-pyridinoline were present in collagen from mineralized dentin. The observed differences in crosslinking between predentin and dentin of the same teeth may indicate some alterations within the area of mineralization.

Collagen cross-linking in human bone and articular cartilage. Age-related changes in the content of mature hydroxypyridinium residues

The concentration in collagen of hydroxypyridinium cross-linking amino acids was measured in samples of bone and cartilage from human subjects aged from 1 month to 80 years. Cortical and cancellous bone samples were dissected and analysed separately. In both bone and cartilage, the content of this mature form of cross-link reached a maximum by 10-15 years of age (the amount in cartilage being 5-10 times that in bone), then stayed essentially in the same range throughout adult life. In bone the ratio of the two chemical variants of the mature cross-link, hydroxylysylpyridinoline to lysylpyridinoline, was constant throughout adult life at 3.5:1, whereas in cartilage it was always greater than 10:1. The ratio of hydroxypyridinium cross-links to borohydride-reducible keto-amine cross-links also changed with age. The reducible cross-links in bone collagen decreased steeply in content between birth and 25 years, but persisted in significant amounts throughout adult life. Reducible cross-links had virtually disappeared from cartilage by 10-15 years of age, being replaced by hydroxypyridinium residues, their maturation products. Cancellous and cortical bone collagens showed similar trends with age in their content of mature cross-links, though for each subject the concentration in cancellous bone was always lower than in cortical bone, presumably reflecting the higher turnover rate and hence the more immature state of cancellous bone.

Fine powdering exposes the mineral-protected collagen of bone to protease digestion

The method of heat-denaturation and trypsin digestion was used to dissect bone biochemically into mineral-protected and mineral-unprotected pools of collagenous matrix. It was found that varying the particle size of the bone powder had a profound effect on the results. Using mature bovine cortical bone, the observed pool of "unmineralized" (mineral-unprotected) collagen could be varied from 2% to more than 60% of the total bone collagen simply by decreasing the particle size of the bone sample from greater than 1 mm to less than 38 micron. No major differences were seen in the contents of hydroxypyridinium cross-links between the collagens of the trypsin-soluble and trypsin-insoluble pools from the fine powders, contrary to earlier reports. A trend to a higher content of these cross-links was evident, however, in the very small collagen pool extracted from the coarsest bone particles. Similar extraction differences were noted using bacterial collagenase to probe for mineral-protected vs. mineral-unprotected domains of bone collagen. In summary, the biochemical dissection results appear largely to be an artifact of the powdering technique, the shear energies of which presumably destroy the intimate physical relationship between the mineral crystallites and the collagen fibrils at the fractured surfaces of the bone particles. As the fractured surface area increases with decreasing particle size so the fraction of protease degradable collagen increases. Since powdering is routinely adopted for many structural studies on both the mineral and organic phases of bone, the findings on finely powdered bone should be interpreted cautiously.

Quantitative analysis of the pyridinium crosslinks of collagen in urine using ion-paired reversed-phase high-performance liquid chromatography

A method has been developed for the simultaneous and rapid analysis of the 3-hydroxypyridinium crosslinks of mature collagen, pyridinoline and deoxypyridinoline, in samples of urine and tissue. After hydrolysis in 6 M HCl, samples were prefractionated by partition chromatography using columns packed with cellulose CF1. The appropriate fractions were freeze-dried and then subjected to high-performance liquid chromatography using a gradient system with 20 mM NH4Cl, pH 3.5, and acetonitrile, incorporating 1-octanesulfonic acid as an ion-pairing agent; the pyridinium crosslinks were detected fluorometrically. The limit of detection of both crosslinks was 1 pmol. The mean recoveries of added standard to samples ranged between 94.2 and 97.2%, and the reproducibility of the complete procedure was between 12 and 16%. An application of the method in the study of degenerative disorders of bone and connective tissue is illustrated.

Response of immature chicken meniscus to strenuous exercise: biochemical studies of proteoglycan and collagen

Male white Leghorn chickens were exercised on a treadmill at 70-80% of their maximal oxygen consumption starting at 4 weeks and continuing up to 20 weeks of age. The effect of the strenuous exercise regime on the extracellular matrix of menisci was followed through studies of proteoglycans and collagen. Avian menisci contain type I collagen, chondroitin sulfate proteoglycans, which increase with age in amount and degree of aggregation, and dermatan sulfate proteoglycans, which decrease with age. Five weeks of exercise cause a premature decrease of dermatan sulfate proteoglycans, while the chondroitin sulfate-containing molecules become significantly more aggregated than those of the tissue of age-matched controls. Strenuous exercise also causes a significant decrease in the number of pyridinoline crosslinks per mole of collagen in the menisci of young runners. The exercise-induced changes of proteoglycan and collagen occur only during the period of active growth, and all parameters return to normal when the animals reach skeletal maturity. The early proteoglycan aggregation and dermatan sulfate decrease induced by exercise are probably an adaptation to the increased loading. Although the mechanism by which strenuous exercise reduces or delays the formation of collagen pyridinoline crosslinks in menisci of skeletally immature animals is unknown, their decrease could negatively affect the mechanical properties of the tissue during the period of active growth.

Collagen cross-linking: distribution of hydroxypyridinium cross-links among invertebrate phyla and tissues

1. Using a specific HPLC assay, a wide variety of marine invertebrate connective tissues was screened for the 3-hydroxypyridinium amino acids that are prominent intermolecular cross-linking residues in the collagens of many vertebrate connective tissues. 2. One or both of the two structural forms that exist, hydroxylysyl pyridinoline (HP) and lysyl pyridinoline (LP), was found in organisms from the following phyla: coelenterata, Annelida, Echinodermata, Mollusca and Arthropoda. 3. Neither amino acid was found in tissues from representative species of Porifera and Chordata. 4. Of special note was an unusually high ratio of LP to HP in Limulus polyphemus gill cartilage.

Type VI collagen of the intervertebral disc. Biochemical and electron-microscopic characterization of the native protein

The collagen framework of the intervertebral disc contains two major fibril-forming collagens, types I and II. Smaller amounts of other types of collagen are also present. On examination of the nature and distribution of these minor collagens within bovine disc tissue, type VI collagen was found to be unusually abundant. It accounted for about 20% of the total collagen in calf nucleus pulposus, and about 5% in the annulus fibrosus. It was discovered by serially digesting disc tissue with chondroitin ABC lyase and Streptomyces hyaluronidase that native covalent polymers of type VI collagen could be extracted. Electron micrographs of this material prepared by rotary shadowing revealed the characteristic dimensions of tetramers and double tetramers of type VI molecules, with their central rods and terminal globular domains. Molecular-sieve column chromatography on agarose under non-reducing non-denaturing conditions gave a series of protein peaks with molecular sizes equivalent to the tetramer, double tetramer and higher multimers. On SDS/polyacrylamide-gel electrophoresis after disulphide cleavage, these fractions of type VI collagen all showed a main band at Mr 140,000 and four lesser bands between Mr 180,000 and 240,000. On electrophoresis without disulphide cleavage in agarose/2.4% polyacrylamide only dimeric (six chains) and tetrameric (12 chains) forms of type VI molecules were present. The ability to extract all the type VI collagen of the tissue in 4 M-guanidinium chloride, and absence of aldehyde-mediated cross-linking residues on direct analysis, showed that, in contrast with most matrix collagens, type VI collagen does not function as a covalently cross-linked structural polymer.

Pyridinium crosslinks of bone collagen and their location in peptides isolated from rat femur

The relative proportions of pyridinoline and deoxypyridinoline in bone showed large species variations, although the total number of pyridinium crosslinks in rat, rabbit and bovine bone collagen was only 25-30% of that found in articular cartilage. Three pyridinium-containing peptides were isolated from cyanogen bromide digests of rat femoral bone and were characterized by their Mr values and amino-acid compositions. The results showed that pyridinoline and its deoxy analogue were equally distributed at two locations stabilizing the 4D stagger through interactions involving both the N- and C-terminal telopeptide regions. Less than stoichiometric amounts of pyridinium crosslinks were present in the peptides, suggesting that the isolated peptides contained additional (unidentified) maturation products of the bifunctional, reducible crosslinks.

Chemistry of the collagen cross-links. Origin and partial characterization of a putative mature cross-link of collagen

The conversion of the reducible divalent cross-links in collagen to non-reducible multivalent cross-links in mature collagen has resulted in the identification of several new amino acids as the putative mature cross-link. None of these compounds has completely satisfied the necessary criteria. We have now isolated an amino acid of high Mr, derived from lysine, that is only present in high-Mr peptides derived from mature collagen. Its increase with age of the tissue correlates with the decrease in the reducible cross-links, and it is present both in mature skin and bone, which are initially cross-linked through the aldimine and oxo-imine divalent cross-link respectively. We propose that this amino acid, as yet incompletely characterized and designated compound M, is a major cross-link of mature collagen.

Characterisation of a type-I collagen trimeric cross-linked peptide from calf aorta and its cross-linked structure. Detection of pyridinoline by time-of-flight secondary ion-mass spectroscopy and evidence for a new cross-link

A collagenous trimeric cross-linked peptide has been isolated from the insoluble matrix of calf aorta, using trypsin solubilisation, and purified by gel filtration, cation-exchange chromatography and reversed-phase HPLC. Molecular mass and amino acid composition indicated that the C-terminal, non-helical region of type I collagen in its dimer form, designated as [ColC(I)]2, is cross-linked to a tryptic peptide TN(I) from the N-terminal helical cross-link region of an adjacent type I molecule, forming the cross-linked peptide [ColC(I)]2 X TN(I). Amino acid sequence analysis of the peptide yielded a series of sequences corresponding to the cross-linking domains ColC(I) and TN(I) and furnished the first direct chemical evidence for the 4D staggered arrangement of type I molecules within native fibers. The trifunctional cross-linking amino acid pyridinoline was shown to occur in the peptide, confirming the peptides three-chain structure. Pyridinoline was isolated from the cross-linked peptide by preparative amino acid analysis and reversed-phase HPLC and identified by its ultraviolet absorption spectra, its fluorescence excitation and emission spectra and, for the first time, its time-of-flight secondary ion-mass spectrum. The high sensitivity of the latter method, exceeding that of fast-atom-bombardment mass spectroscopy by three orders of magnitude, allowed detection of pyridinoline in the picomole range. The occurrence of pyridinoline in non-stoichiometric amounts, the presence of hydroxylysine in hydrolysates of all cross-linked peptides and the finding that hydrolysates also contained an unidentified component indicated that there is at least one cross-link form that is different from pyridinoline and is hydrolysable.

Structural crosslinking of lung connective tissue collagen in the blotchy mouse

Male mice with the sex-linked mutation Blotchy (Blo) have a defect in copper metabolism which results in deficient activity of a number of copper-containing enzymes. Inbred Blo/y mice spontaneously develop lung abnormalities which resemble emphysema and often die of ruptured aortic aneurysm. Lung, tail tendon, and tibial bone collagens from inbred Blo/y and their normal (+/y) litter mates were reduced with standardized [3H]NaBH4, acid and alkaline hydrolyzed, and chromatographed in order to quantify the aldehydic crosslink precursors, and the labile reducible and nonreducible stable mature covalent intermolecular crosslinks. Reducible lung collagen crosslinks were markedly (60%) decreased in the Blo/y mice and few, if any, mature nonreducible crosslinks were present. Total aldehydes were also decreased (65%) when Blo/y was compared to +/y. In tail tendon and bone, collagen crosslinks were decreased by only 28% and 15%, respectively. Selectively severe lack of activity of the copper-dependent enzyme level oxidase in lung with only partial lack in tendon and bone could account for the results obtained. Alternatively, insufficient reducible crosslinks, coupled with increased collagen turnover in the lung could prevent formation of the more mature stable crosslinks required to provide a proper connective tissue framework for the Blo/y lung.

Biosynthesis of collagen crosslinks: in vivo labelling of neonatal skin, tendon, and bone in rats

Collagen crosslinks in neonatal rats were labelled in vivo by a single intraperitoneal injection of 200 microCi of [14C]lysine. Rats were killed at times ranging from 30 minutes to 10 weeks after injection. Whole skin, tendon, and bone were analyzed, after reduction and hydrolysis, for collagen crosslink content by HPLC. Crosslinks and amino acids were visualized by their incorporation of radioactivity from [14C]lysine and also fluorometrically by post-column derivatization with o-phthalaldehyde. The incorporation of 14C from labelled lysine into the principal difunctional reducible crosslinks, N6.6'-dehydro-5,5'-dihydroxylysinonorleucine and N6.6'-dehydro-5-hydroxylysinonorleucine, increased most rapidly between 4 and 12 hours after injection, results similar to those observed by others studying crosslink biosynthesis in vitro. Incorporation of 14C into the tetrafunctional crosslink histidinohydroxymerodesmosine proceeded more slowly than it did for the difunctional crosslinks. Values for the amount of radioactivity incorporated into the various crosslinks reached an apparent constant value between 3 and 5 days after injection for all three tissues studied. These values remained approximately constant for the duration of the experiment except for HHMD in tendon, which showed an increase in incorporated radioactivity at 8 and 10 weeks after injection. Direct chemical quantification of these same crosslinks by determination of the fluorescence of their o-phthalaldehyde adducts was also performed. We conclude that in vivo labelling of collagen crosslinks can be studied, at least in rapidly growing neonates, after a single injection of radioactive lysine. The results of such studies support previous suggestions by others about the rate of formation of difunctional crosslinks based upon studies using in vitro systems. Our results further suggest that formation of the tetrafunctional reducible crosslink histidinohydroxymerodesmosine proceeds relatively rapidly in vivo. Finally, we conclude that such labelled crosslinks are apparently quite stable after biosynthesis, suggesting the possibility of studies of the metabolic fate of collagen crosslinks over appreciable fractions of the lifetime of a rat.

Effects of reperfusion after coronary artery occlusion on post-infarction scar tissue

Early reperfusion after a coronary occlusion may reduce myocardial infarct size, but late reperfusion into necrotic myocardium may alter post-infarction healing. In rabbits, we compared 1- or 3-week-old scars resulting from permanent coronary occlusion to those resulting from a 1- or 3-hour occlusion followed by reperfusion. Reperfusion at 1 hour post-occlusion did not affect scar mechanical properties assessed at 1 week post-infarction, but at 3 weeks post-infarction, these scars had a tensile strength significantly lower than those not reperfused (78 +/- 11 vs. 158 +/- 15 g/mm2, P less than 0.001). They also were composed of a mixture of fibrous tissue (58 +/- 8%) and myocytes (43 +/- 8%) with a hydroxyproline content of 23 +/- 2.5 mg/g dry weight. The nonreperfused scars had a higher proportion of fibrous tissue (73 +/- 3%) by histological evaluation and a 35% higher hydroxyproline content (31 +/- 2 mg/g dry weight, P less than 0.001) than the scars reperfused after 1 hour. In contrast, 3-week-old scars resulting from "late" reperfusion at 3 hours post-occlusion were similar to nonreperfused scars in fibrous tissue composition and hydroxyproline content. Nonetheless, the tensile strength of these scars reperfused 3 hours post-occlusion was significantly less than that of the nonreperfused scars (72 +/- 5 vs. 158 +/- 15 g/mm2, P less than 0.001). The lower tensile strength was associated with a lower collagen cross-link density in this reperfused group of scars. At physiological stress levels (approximately 3 g/mm2), all groups of reperfused and nonreperfused scars had similar mechanical properties in terms of natural strain, stiffness, creep, and stress relaxation. Thus, although the reperfused scars ruptured more easily at high stresses, when assessed at physiological stresses their mechanical properties were not significantly different from those of nonreperfused scars.

Content of the collagen and elastin cross-links pyridinoline and the desmosines in the human uterus in various reproductive states

During pregnancy the collagen content of the human uterus increases sevenfold and the elastin content increases fourfold to fivefold. The stable pyridinoline cross-link is found in uterine collagen at a level of 0.11 mol per mole of collagen. The same ratio, or a higher one, is found at the end of pregnancy, indicating that pyridinoline synthesis keeps pace with the rapid synthesis of collagen. This cross-link would participate in the maintenance of high mechanical strength of the uterus needed during parturition. Uterine elastin contains 2.4 residues of desmosine plus isodesmosine in 1000 residues of amino acids. This value falls to 0.95 at term, indicating that synthesis of desmosines does not keep pace with the synthesis of elastin. Therefore, desmosine measurements do not provide an accurate index of elastin changes in pregnancy. Collagen and elastin contents in nongravid uteri increase with successive pregnancies; the cross-links remain constant during this change.

An enzyme-linked immunosorbent assay to quantitate the elastin crosslink desmosine in tissue and urine samples

An enzyme-linked immunosorbent assay method has been developed for the determination of desmosine. The method is based on an inhibition immunoassay (under nonequilibrium conditions) and uses rabbit antisera directed against a desmosine-bovine serum albumin conjugate and microtiter plates coated with desmosine-gelatin conjugate. The assay quantitates desmosine in the range 2.5-50 pmol in tissue and urine samples. Important applications of this rapid and sensitive assay are in studying elastin metabolism and in screening for monoclonal antibodies against desmosine. Methods are described for obtaining a constant level of substitution of desmosine per molecule of bovine serum albumin and for preparing a desmosine-gelatin coating antigen. Five different antibody preparations directed against desmosine exhibit 15-20% cross-reactivity toward pyridinoline (3-hydroxypyridinium), a nonreducible collagen crosslinking compound also present in urine and many tissue samples.

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.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

Cartilage type IX collagen is cross-linked by hydroxypyridinium residues

Type IX collagen, a recently discovered, unusual protein of cartilage, has a segmented triple-helical structure containing interchain disulfides. Its polymeric form and function are unknown. When prepared by pepsin from bovine articular cartilage, type IX collagen was found to contain a high concentration of hydroxypyridinium cross-links, similar to that in type II collagen. Fluorescence spectroscopy located the hydroxylysyl pyridinoline and lysyl pyridinoline cross-linking residues exclusively in the high-molecular-weight collagen fraction, from which they were recovered predominantly in a single CNBr-derived peptide. The results point to a structural role for type IX collagen in cartilage matrix, possibly as an adhesion material to type II collagen fibrils.