All three chains of 1 alpha 2 alpha 3 alpha collagen from hyaline cartilage resist human collagenase

The fibronectin-like domain is required for the type V and XI collagenolytic activity of gelatinase B

Gelatinase B (matrix metalloproteinase-9) is able to degrade several extracellular matrix proteins, including gelatin, elastin, and collagen types IV, V, XI, and XIV. This enzyme contains a "fibronectin-like" domain which is composed of three tandem copies of a fibronectin type 2 homology unit inserted into its catalytic domain. We have studied the involvement of this domain in the substrate specificity of gelatinase B by expressing a mutant of the enzyme, in Escherichia coli, in which this domain has been deleted. This mutant enzyme retained its ability to cleave the peptide substrate Mca-PLGL(Dpa)AR-NH2, possessing K(m) and kcat values similar to those of the wild-type enzyme. In addition, the NH2-terminal, 14-kDa, inhibitory domain of recombinant tissue inhibitor of metalloproteinase-2 was able to inhibit the mutant and the wild-type enzymes with the same potency. The mutant's gelatinolytic activity was also retained but reduced in comparison to that of the wild-type enzyme. However, contrary to the wild-type enzyme, the mutant was not able to digest or bind fibrillar collagen types V and XI. These data indicate that the fibronectin-like domain of gelatinase B is an important determinant of the enzyme's fibrillar collagen substrate specificity. It allows the enzyme to bind to and cleave collagen types V and XI, events which are thought to be involved in several normal physiological and pathological processes such as metastasis and arthritis.

Matrix protein mRNA levels in canine meniscus cells in vitro

The resident cells of the meniscus synthesize a fibrocartilaginous extracellular matrix in vivo composed predominantly of type I collagen fibers. To increase our understanding of matrix biosynthesis by meniscus cells in vitro, we examined matrix protein mRNA levels in cultured meniscus cells isolated from skeletally mature dogs. The mRNA levels of five matrix protein genes (COL1A1, COL2A1, aggrecan, COL6A1, and fibronectin) were measured in meniscus cells by Northern blotting and compared with those of patellar tendon fibroblasts and femoral articular cartilage chondrocytes. In freshly isolated cells (Day 0 cells), COL1A1, COL2A1, and aggrecan mRNA levels were low or undetectable in both meniscus cells and tendon fibroblasts. In intact meniscus tissue, COL1A1 mRNA levels were also low or undetectable. COL2A1 and aggrecan mRNA transcripts were readily observed, however, in Day 0 articular chondrocytes. The levels of expression of COL6A1 and fibronectin mRNA transcripts in Day 0 meniscus cells were intermediate between higher articular chondrocyte levels and lower tendon fibroblast levels. After 1 week in monolayer culture (Day 7 cells), meniscus cells expressed readily detectable levels of COL1A1 mRNA transcripts, similar to that observed for cultured tendon fibroblasts. COL1A1 mRNA transcripts were either not detected or detected at very low levels in monolayer cultures of articular chondrocytes. COL2A1 and aggrecan mRNA transcripts were readily detected in cultured articular chondrocytes but not in meniscus cells or in tendon fibroblasts. All three types of cells continued to express COL6A1 and fibronectin mRNA transcripts after 1 week in culture. These results demonstrate that the patterns of expression of COL1A1 and COL2A1 mRNA transcripts by meniscus cells are similar to those of tendon fibroblasts and dissimilar to those of articular chondrocytes both in freshly isolated cells and in monolayer cultured cells. This mRNA expression pattern supports the idea that monolayer culture of meniscus cells results in the expression of a predominantly fibroblastic phenotype.

Characterisation of equine matrix metalloproteinase 2 and 9; and identification of the cellular sources of these enzymes in joints

The cellular production by resident articular cells and infiltrating inflammatory cells of the gelatinase matrix metalloproteinases (MMP) was investigated by tissue culture methods and analysis of cell supernatants by gelatin zymography. Peripheral blood neutrophils in short term culture produced MMP-9, as did peripheral blood monocytes in culture. Isolated articular chondrocytes in monolayer culture produced both MMP-2 and MMP-9, although articular cartilage maintained as explant culture produced MMP-2 alone. Synovial fibroblasts grown in monolayer culture produced MMP-2 alone, although synovial membrane in explant culture produced both MMP-2 and the active form of MMP-2. Lysis of blood polymorph neutrophils produced large quantities of MMP-9, but lysis of blood monocytes, synovial fibroblasts and articular chondrocytes produced little enzyme indicating that, unlike the other cell types, polymorph neutrophils store MMPs intracellularly. Equine MMP-2 was purified from synovial fibroblast cell culture supernatant, and equine MMP-9 from polymorph neutrophil cell culture supernatant, by gelatin-sepharose affinity chromatography. The 2 enzymes were identified from their molecular weights and by their respective N-terminal amino acid sequences which showed homology with the enzymes from other species. The demonstration that invasive cells and resident articular cells can produce enzymes which are capable of digestion of certain component molecules of the articular cartilage matrix, shows that therapeutic targeting of these enzymes could be a valid proposition in the prevention of cartilage destruction in osteoarthritis.

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).

Expression of type II collagen at the middle stages of chick embryonic and human fetal skin development

Using in situ hybridization techniques and RNase protection assays, type II collagen mRNA was transiently detected in the epidermis of chick embryonic skins during days 9-15 after fertilization, with a maximum expression at day 11. Immunohistochemical studies demonstrated that deposition of type II collagen was also transiently localized at the subepidermal region during days 10-15. Type II collagen gene and gene product concomitantly started to decline preferentially at the region where feather buds were being formed on day 12, and thereafter diminished at the region between feather buds. Using immunohistochemical methods, type II collagen was also detected in human fetal scalp skin at 17-23 fetal weeks at the subepidermal region, excluding the region beneath the hair follicles. These results indicate that the lack of type II collagen expression is related to the development of feather and hair at a certain stage of chick embryonic and human fetal skin development.

Type XI collagen-degrading activity in human osteoarthritic cartilage

Homogenates of 6 samples of human osteoarthritic cartilage were shown to degrade exogenous type XI collagen. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the cleavage products generated by each homogenate were similar, and they were identical to those obtained by cleavage of the substrate with purified gelatinase. Enzyme activity, which was inhibited by EDTA, was greater in extracts of fibrillated osteoarthritic cartilage than in extracts of grossly normal cartilage from the same joint or in extracts of cartilage from joints with osteonecrosis. Activation with APMA enhanced digestion, but breakdown was apparent in extracts of fibrillated osteoarthritic cartilage even without APMA. Enzymatic degradation of type XI collagen could play a significant role in the turnover of articular cartilage in health and disease states.

Degradation of cartilage collagens type II, IX, X and XI by enzymes derived from human articular chondrocytes

Conditioned culture medium derived from Interleukin-I alpha-activated human articular chondrocytes contained both collagen- and proteoglycan-degrading activities. Preparations of soluble type I collagen and the cartilage collagens type II, IX, X and XI were all degraded when incubated with the conditioned culture medium at 35 degrees C. Fractionation of the enzymic activities using column chromatography with Ultragel AcA 34 and Heparin-Sepharose allowed the separation and identification of neutral proteinase, collagenolytic and proteoglycan-degrading activities. Eluant fractions which contained type I collagenase activity effectively degraded collagen type II, but these fractions did not correspond precisely with those which degraded collagen types IX, X and XI. These observations indicate that chondrocytes have the potential to produce a conventional interstitial type II collagenase together with other enzymes having some specificity for the minor collagens. Thus IL-1-activated chondrocytes produce a range of collagenolytic and proteoglycan-degrading enzymes which can process most of the structural components of the cartilage matrix.

Production of collagens, collagenase and collagenase inhibitor during the dedifferentiation of articular chondrocytes by serial subcultures

Rabbit articular chondrocytes were cultured in monolayer and the progressive loss of their differentiated phenotype was monitored from passage to passage. The cell densities achieved in confluent cultures decreased abruptly between the primoculture and the second or third subculture, and more slowly thereafter, reflecting parallel morphological changes. The synthesis of collagen (but not that of other proteins) decreased sharply, and a smaller proportion of collagen was incorporated into the matrix. Cells in primoculture synthesized mainly the cartilage-specific collagens, types II and XI, which were mostly deposited in the matrix, but no type I nor III collagen. With increasing passages, the synthesis of type II collagen decreased progressively while that of types I and III collagens increased, the latter being almost completely released in the culture medium. Simultaneously, the production of type XI collagen was apparently switched to that of type V. Fully differentiated confluent chondrocytes in primoculture produced the collagenase inhibitor TIMP (tissue inhibitor of metalloproteinases) but no detectable procollagenase; their production of procollagenase was, however, induced by interleukin 1. The production of TIMP increased from passage to passage. A spontaneous production of procollagenase was only occasionally observed in confluent cultures of dedifferentiated chondrocytes. However, interleukin 1 induced an always higher production of procollagenase from dedifferentiated chondrocytes than from cells in primoculture.

Expression of type II and type XI collagens in canine mammary mixed tumors and demonstration of collagen production by tumor cells in collagen gel culture

The development of cartilaginous collagen types, II and XI, in canine mammary mixed tumors was studied biochemically and immunohistochemically. In mixed tumor, an alcian blue-positive myxomatous region appeared in the stroma, where round-shaped proliferating myoepithelial cells were scattered. Type II collagen was distributed in metaplastic cartilage matrix, while type XI was located only in the pericellular region, where proliferating cells were positively stained with anti-actin and anti-keratin antibodies. The accumulation of collagen types II and XI in the tumor mass was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblotting of the extract of the lesion using type-specific antibodies to collagen types II and XI. Tumor cells isolated from metaplastic tumor mass expressed both collagen types II and XI and myoepithelial types of cytoskeleton in gel culture, in which an alcian blue-positive substance became detectable in the pericellular region on day 3 and type II and type XI collagens on day 5. This may be a useful model for studying chondrocyte-type gene expression during tumorigenesis.

Problems in the immunolocalization of type IX collagen in fetal calf cartilage using a monoclonal antibody

Monoclonal antibodies were prepared against the pepsin-resistant fragments (X1-X3) of bovine type IX collagen. One of the five hybridomas that gave a positive reaction in an enzyme-linked immunosorbent assay was selected (H1a) for structural analysis and immunolocalization of type IX collagen. The location of the epitope for H1a was deducted from immunoblots and electron microscopic observations after rotary shadowing. The H1a antibody binds to one end of the longest X2, X3, X4 molecules, and preferentially 40-55nm from one end of X1 molecules thus, on or near the noncollagenous domain, NC2. Different immunolocalizations of type IX collagen in the superficial, middle and deep zones of fetal calf epiphyseal cartilage were observed depending on the thickness of the section and on hyaluronidase digestion conditions. In the middle and deep zones, staining with H1a throughout the matrix was obtained only with thin sections (5 microns) and digestion for 1 h at 37 degrees C. With thick sections (15 microns) or with digestion for 1 h at 24 degrees C, staining was restricted to the pericellular regions. Staining throughout the matrix was obtained in the superficial zone under all experimental conditions. Without hyaluronidase treatment, no immunofluorescent staining was seen with either H1a or polyclonal antibody to type II collagen, indicating that type IX collagen is present throughout the matrix in the different zones of fetal calf cartilage. This result is in good accordance with the recent demonstration of common cross-links between type II and type IX collagen in chicken and bovine cartilage. However, the preferential unmasking of type IX collagen antigenic sites in the pericellular regions of middle and deep zones of fetal calf cartilage does not preclude the presence in that region of a special pericellular organization of the collagenous network.

Susceptibility of cartilage collagens type II, IX, X, and XI to human synovial collagenase and neutrophil elastase

The action of purified rheumatoid synovial collagenase and human neutrophil elastase on the cartilage collagen types II, IX, X and XI was examined. At 25 degrees C, collagenase attacked type II and type X (45-kDa pepsin-solubilized) collagens to produce specific products reflecting one and at least two cleavages respectively. At 35 degrees C, collagenase completely degraded the type II collagen molecule to small peptides whereas a large fragment of the type X molecule was resistant to further degradation. In contrast, collagen type IX (native, intact and pepsin-solubilized type M) and collagen type XI were resistant to collagenase attack at both 25 degrees C and 35 degrees C even in the presence of excess enzyme. Mixtures of type II collagen with equimolar amounts of either type IX or XI did not affect the rate at which the former was degraded by collagenase at 25 degrees C. Purified neutrophil elastase, shown to be functionally active against soluble type III collagen, had no effect on collagen type II at 25 degrees C or 35 degrees C. At 25 degrees C collagen types IX (pepsin-solubilized type M) and XI were also resistant to elastase, but at 35 degrees C both were susceptible to degradation with type IX being reduced to very small peptides. Collagen type X (45-kDa pepsin-solubilized) was susceptible to elastase attack at 25 degrees C and 35 degrees C as judged by the production of specific products that corresponded closely with those produced by collagenase. Although synovial collagenase failed to degrade collagen types IX and XI, all the cartilage collagen species examined were degraded at 35 degrees C by conditioned culture medium from IL1-activated human articular chondrocytes. Thus chondrocytes have the potential to catabolise each cartilage collagen species, but the specificity and number of the chondrocyte-derived collagenase(s) has yet to be resolved.

The carboxypropeptide trimer of type II collagen is a prominent component of immature cartilages and intervertebral-disc tissue

Immature bovine cartilages and intervertebral-disc tissue all revealed a prominent protein, not present in the adult tissues, in non-denaturing extracts made with chondroitin ABC lyase (EC 4.2.2.4), Streptomyces hyaluronidase (EC 4.2.2.1) or 1 M NaCl. The protein ran on SDS-polyacrylamide electrophoresis, before disulphide reduction, as a close doublet of bands of apparent molecular weight 110,000 and 105,000. After reduction, they dissociated respectively into two protein bands at 37,000 and 35,000, indicating that the initial molecules were disulphide-bonded trimers. Amino-terminal sequence analysis established the identity of both proteins (Mr 110,000 and Mr 105,000) as forms of the carboxypropeptide of type II collagen. The larger molecule appeared to be the trimer of intact alpha 1(II) carboxypropeptides and the smaller, a version composed of chains that were ten residues shorter at their amino-terminal ends. The material appears to be identical to chondrocalcin, a protein previously found to be enriched in fetal growth plate and named on the basis that it may play a role in cartilage calcification. The present findings, however, indicate that the protein is equally abundant in all type II collagen-synthesizing young cartilages, including nucleus pulposus of the intervertebral disc and other cartilages that never calcify.

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.

Type VI collagen represents a major fraction of connective tissue collagens

A new method for the isolation of type VI collagen from peptic tissue digests is presented which gives tenfold higher yields than methods previously reported. From the amounts of purified protein obtained from human placenta, bovine uterus, chicken gizzard and entire mouse bodies we conclude that type VI collagen represents a major fraction of connective tissue collagens.

Electron microscopic study of chondroid tissue in the cat mandible

This paper deals with electron microscopic appearance of chondroid tissue. Samples from eight cat mandibles were studied without decalcification. The ultrastructural characteristics of the chondroid tissue cells are common with young osteocytes. The interterritorial matrix of chondroid tissue is mineralized, being constituted of large collagen fibrils and calcospherites. The compositions of these parts of the chondroid tissue matrix and of bony matrix are similar but they are two different tissues. The pericellular matrix of the chondroid tissue consists of finely branched filaments, thin collagen fibrils, and an abundant ground substance. It resembles a cartilage matrix and contains type II collagen which is not present in bony matrix.

Changes in the types of collagen synthesized during chondrogenesis of the mouse otic capsule

We have investigated the temporal relationship between the morphological differentiation of the mouse otic capsule and the pattern of collagen synthesis by mouse otocyst-mesenchyme complexes labeled in vitro. In 10.5- to 12-day embryos the mesenchyme surrounding the otocyst was loosely organized except for a few lateroventral condensations; explants from these embryos synthesized only small amounts of collagen. Collagen synthesis by whole explants increased by more than 50% between 12 and 13 days concomitant with metachromatic staining of the lateral periotic mesenchyme. Cartilage specific type II collagen was the predominant collagen synthesized by these explants as confirmed by SDS-PAGE, densitometry, CNBr cleavage, and V8 protease digestion. This biochemical expression of the cartilage phenotype preceded morphologic recognition of otic capsular cartilage by almost 2 days. Type II collagen synthesis continued to increase and predominate through Day 16 of gestation by which time the otic labyrinth was surrounded by mature cartilage. The minor cartilage collagen chains, 1 alpha, 2 alpha, and 3 alpha, first appeared on different days of gestation. The 1 alpha, and 3 alpha chains were synthesized by explants from 11-day embryos while the 2 alpha chain appeared during Day 13, just before overt differentiation of mature cartilage. These results suggested that the 1 alpha, 2 alpha, and 3 alpha chains may not form heterotrimers containing all three chains and that synthesis of the 2 alpha chain may be associated with stabilization of the cartilaginous matrix. Comparison of these data with the patterns of collagen production by mutant, diseased, or experimentally manipulated inner ear tissues may provide insights into the molecular basis of chondrogenic tissue interactions.

Distribution of type I, II, III and V in the pepsin solubilized collagens in bovine menisci

The inner one-third (IM) of both lateral and medial menisci resembles hyaline cartilage, both in gross appearance and histological examination, while the outer two-thirds (OM) is fibrocartilaginous in appearance. Collagen was extracted with pepsin, purified with anion and cation exchange column chromatographies and examined by differential salt precipitation, cyanogen bromide-peptide analysis and SDS gel electrophoresis. IM constitutes approximately 10% of the wet weight of whole meniscus, is made up of 70% collagen of which 34% is pepsin soluble. IM is composed of 60% type II and 40% type I collagen. OM is made up of 80% collagen of which 17% is pepsin soluble. The predominant collagen of OM is type I with a trace amount of types III and V (less than 1%).

Increased collagenase activity in human rheumatoid meniscus

Collagenase activity of the knee joint menisci of patients suffering from rheumatoid arthritis was approximately 3-fold higher than that found in menisci of control patients. The mean collagenase activity in the macroscopically more diseased parts of the rheumatoid menisci was significantly higher than that in the less damaged areas. The specific degradation products resulting from the cleavage of human meniscoid type II collagen by rheumatoid meniscoid collagenase were demonstrated by SDS-polyacrylamide gel electrophoresis. Addition of N-ethylmaleimide, which activates latent mammalian collagenases, did not further increase collagenase activity in rheumatoid menisci. Thus in rheumatoid meniscus, collagenase may be synthesized and then activated, probably by proteolytic enzymes involved in the inflammatory reaction.

Isolation and partial characterization of precursors to minor cartilage collagens

Suspension cultures of cartilage cells were prepared from 17-day chick embryo sterna and radiolabeled with [14C]-proline under conditions which sought to minimize proteolytic conversion of procollagen to collagen. Collagenous proteins were isolated from the culture medium and cell fraction, were purified in their native state by (NH4)2SO4 precipitation and DEAE-cellulose chromatography, and were characterized by protease susceptibility, SDS-gel-filtration and SDS-polyacrylamide gel electrophoresis. Qualitatively, the precursor components present in the medium were similar to those in the cell extract; quantitatively, it appeared that the minor cartilage collagen precursor components derived from 1 alpha, 2 alpha, 3 alpha and type IX collagens were more prevalent in the cell extract. SDS-PAGE of unreduced samples showed that precursors to both of these collagens migrated as distinct high-molecular-weight aggregates. After chymotrypsin digestion, unreduced type IX collagen migrated as two disulfide-bonded aggregates--a large one (Mr approximately 210K) and a small one (Mr approximately 43K); whereas 1 alpha, 2 alpha, 3 alpha chains migrated identically whether reduced or unreduced. Reduction of undigested type IX aggregate yielded two components of Mr approximately 97K and 78K; whereas reduction of the chymotrypsin resistant 210K and 43 K aggregates gave a single component of Mr approximately 61K and a component which migrated at the dye front, respectively. The molecular origin of these components was confirmed by differential NaCl precipitation. It was concluded that this culture system synthesized precursors to 1 alpha, 2 alpha, 3 alpha and type IX collagens in addition to type II; type X collagen was not detected even though the 17-day sternum contained a population of cells morphologically similar to hypertrophic chondrocytes. The precursor chains to 1 alpha, 2 alpha, 3 alpha collagen had an apparent Mr greater than pro-alpha (II) and could be isolated as a disulfide-bonded aggregate(s); the precursor chains to type IX collagen had an apparent Mr less than pro alpha (II) and could also be isolated as a disulfide-bonded aggregate. All of the cartilage collagen precursors had protease-susceptible regions, but those in type IX appeared to be more sensitive to pepsin than to chymotrypsin.

Identification and characterization of the human type II collagen gene (COL2A1)

The gene contained in the human cosmid clone CosHcol1, previously designated an alpha 1(I) collagen-like gene, has now been identified. CosHcol1 hybridizes strongly to a single 5.9-kilobase mRNA species present only in tissue in which type II collagen is expressed. DNA sequence analysis shows that this clone is highly homologous to the chicken alpha 1(II) collagen gene. These data together suggest that CosHcol1 contains the human alpha 1(II) collagen gene COL2A1. The clone appears to contain the whole gene (30 kilobases in length) and will be extremely useful in the study of cartilage development and for identifying those inherited chondrodystrophies in which defects occur in this gene.

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.