TRAMP (tyrosine rich acidic matrix protein), a protein that co-purifies with lysyl oxidase from porcine skin. Identification of TRAMP as the dermatan sulphate proteoglycan-associated 22K extracellular matrix protein

Tyrosine-sulfated dermatopontin shares multiple binding sites and recognition determinants on triple-helical collagens with proteins implicated in cell adhesion and collagen folding, fibrillogenesis, cross-linking, and degradation

Dermatopontin (DPT), a small extracellular matrix protein that stimulates collagen fibrillogenesis, contains sulfotyrosine residues but neither its level of sulfation nor its binding sites on fibrillar collagens are known. Here, we discovered that DPT is present in a relatively high mass concentration (~ 0.02%) in porcine corneal stroma, from which we purified five DPT charge variants (A-E) containing up to six sulfations. The major variant (C), containing four sulfotyrosine residues, was used to locate binding sites for DPT on triple-helical collagens II and III using the Collagen Toolkits. DPT-binding loci included the triple helix crosslinking sites and collagenase cleavage site. We find that strong DPT-binding sites on triple-helical collagen comprise an arginine-rich, positively-charged sequence that also contains hydrophobic residues. This collagen-binding signature of DPT is similar to that of the chaperone HSP47. Thus, we propose that DPT assumes the role of HSP47 as a collagen chaperone during and after the secretion. Peptide II-44, harbouring the conserved collagenase cleavage site, shows the strongest DPT-binding of the Collagen Toolkit II peptides. Substituting any of the three arginine residues (R) with alanine in the sequence GLAGQRGIVGLOGQRGER of II-44 resulted in almost complete loss of DPT binding. Since osteogenesis imperfecta, spondyloepiphyseal dysplasia, and spondyloepimetaphyseal dysplasia congenita are associated with missense mutations that substitute the corresponding arginine residues in collagens alpha-1(I) and alpha-1(II), we suggest that disrupted DPT binding to fibrillar collagens may contribute to these connective tissue disorders. In conclusion, the present work provides a cornerstone for further elucidation of the role of DPT.

Insights into the structure and dynamics of lysyl oxidase propeptide, a flexible protein with numerous partners

Lysyl oxidase (LOX) catalyzes the oxidative deamination of lysine and hydroxylysine residues in collagens and elastin, which is the first step of the cross-linking of these extracellular matrix proteins. It is secreted as a proenzyme activated by bone morphogenetic protein-1, which releases the LOX catalytic domain and its bioactive N-terminal propeptide. We characterized the recombinant human propeptide by circular dichroism, dynamic light scattering, and small-angle X-ray scattering (SAXS), and showed that it is elongated, monomeric, disordered and flexible (D_max: 11.7 nm, R_g: 3.7 nm). We generated 3D models of the propeptide by coarse-grained molecular dynamics simulations restrained by SAXS data, which were used for docking experiments. Furthermore, we have identified 17 new binding partners of the propeptide by label-free assays. They include four glycosaminoglycans (hyaluronan, chondroitin, dermatan and heparan sulfate), collagen I, cross-linking and proteolytic enzymes (lysyl oxidase-like 2, transglutaminase-2, matrix metalloproteinase-2), a proteoglycan (fibromodulin), one growth factor (Epidermal Growth Factor, EGF), and one membrane protein (tumor endothelial marker-8). This suggests new roles for the propeptide in EGF signaling pathway.

Comparing hydrazine-derived reactive groups as inhibitors of quinone-dependent amine oxidases

Lysyl oxidase has emerged as an important enzyme in cancer metastasis. Its activity has been reported to become upregulated in several types of cancer, and blocking its activity has been shown to limit the metastatic potential of various cancers. The small-molecules phenylhydrazine and β-aminopropionitrile are known to inhibit lysyl oxidase; however, issues of stability, toxicity, and poorly defined mechanisms limit their potential use in medical applications. The experiments presented herein evaluate three other families of hydrazine-derived compounds – hydrazides, alkyl hydrazines, and semicarbazides – as irreversible inhibitors of lysyl oxidase including determining the kinetic parameters and comparing the inhibition selectivities for lysyl oxidase against the topaquinone-containing diamine oxidase from lentil seedlings. The results suggest that the hydrazide group may be a useful core functionality that can be developed into potent and selective inhibitors of lysyl oxidase and eventually find application in cancer metastasis research.

Proteomic analysis of equine amniotic membrane: characterization of proteins

INTRODUCTION

Human amniotic membrane (AM) has been used as a biomaterial for surgical wound skin and ocular surface reconstruction for several years. Currently, equine AM has been used for corneal reconstruction in several animal species, and appears to have the same properties as human AM. Despite the observed positive healing abilities of this tissue in horses with ulcerative keratitis the proteins of equine AM have not been described.

OBJECTIVE

To identify proteins known to be associated with corneal healing from frozen equine AM.

PROCEDURES

Placentas were acquired from healthy live foal births from a local Thoroughbred breeding farm. The amnion was removed from the chorion by blunt dissection, washed with phosphate-buffered saline (PBS), and treated with 0.05% trypsin and 0.02% ethylene diaminetetraacetic acid in PBS. Amnion was attached to nitrocellulose paper (epithelial side up), and cut into 4 × 4 cm pieces. The sheets were frozen at -80 °C. The protein samples were solubilized, and analyzed by 2D gel electrophoresis and shotgun proteomics.

RESULTS

A reference identification map of the equine AM proteins was produced and 149 different proteins were identified. From gel-based proteomics, 49 spots were excised and 43 proteins identified by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). Shotgun proteomics identified 116 proteins with an overlap of 10 proteins in both analyses.

CONCLUSIONS

We have described a reference map for equine AM proteins that may provide a background to explain the positive results found in horses with ulcerative keratopathies using this biomaterial.

Spatiotemporal expression of the dermatopontin gene in zebrafish Danio rerio

There has been significant interest in the expression and function of dermatopontin (DPT) in mammals owing to recent evidence pointing to its critical role in collagen fibrillogenesis. Despite this interest, limited information is available about the site/s of DPT mRNA expression or changes in expression in vivo. We used reverse-transcription polymerase chain reaction and in situ hybridization to evaluate the spatial and temporal pattern of DPT mRNA expression in zebrafish, Danio rerio, a widely used vertebrate model. We observed that DPT transcripts were expressed in zebrafish embryos at all developmental stages in a range of tissues, including the brain and optic neuron cells. Based on our results, we hypothesize that DPT may also play a role in neural functions in vivo.

Dermatopontin interacts with fibronectin, promotes fibronectin fibril formation, and enhances cell adhesion

We report that dermatopontin (DP), an abundant dermal extracellular matrix protein, is found in the fibrin clot and in the wound fluid, which comprise the provisional matrix at the initial stage of wound healing. DP was also found in the serum but at a lower concentration than that in wound fluid. DP co-localized with both fibrin and fibronectin on fibrin fibers and interacted with both proteins. Both normal fibroblast and HT1080 cell adhesion to the fibrin-fibronectin matrix were dose-dependently enhanced by DP, and the adhesion was mediated by α5β1 integrin. The cytoskeleton was more organized in the cells that adhered to the fibrin-fibronectin-DP complex. When incubated with DP, fibronectin formed an insoluble complex of fibronectin fibrils as visualized by electron microscopy. The interacting sites of fibronectin with DP were the first, thirteenth, and fourteenth type III repeats (III(1), III(13), and III(14)), with III(13) and III(14) assumed to be the major sites. The interaction between III(2-3) and III(12-14) was inhibited by DP, whereas the interaction between I(1-5) and III(12-14) was specifically and strongly enhanced by DP. Because the interaction between III(2-3) and III(12-14) is involved in forming a globular conformation of fibronectin, and that between I(1-5) and III(12-14) is required for forming fibronectin fibrils, DP promotes fibronectin fibril formation probably by changing the fibronectin conformation. These results suggest that DP has an accelerating role in fibroblast cell adhesion to the provisional matrix in the initial stage of wound healing.

Dermatopontin promotes epidermal keratinocyte adhesion via alpha3beta1 integrin and a proteoglycan receptor

Dermatopontin, an extracellular matrix component initially purified from bovine dermis, promoted cell adhesion of the human epidermal keratinocyte cell line (HaCaT cells). HaCaT cells spread on dermatopontin and formed actin fibers. Adhesion of HaCaT cells to dermatopontin was inhibited by both EDTA and heparin and was mediated in part by alpha3beta1 integrin. A synthetic peptide (DP-4, PHGQVVVAVRS; bovine dermatopontin residues 33-43) specifically inhibited adhesion of cells to dermatopontin, and when the DP-4 peptide was coated on the well, it promoted cell adhesion in a dose-dependent manner. An active core sequence of the DP-4 peptide was localized to an eight-amino acid sequence (GQVVVAVR). These results indicate that dermatopontin is a novel epidermal cell adhesion molecule and suggest that the DP-4 sequence is critical for the cell adhesive activity of dermatopontin. Adhesion of cells to DP-4 was strongly inhibited by heparin. When HaCaT cells were treated with heparitinase I, the cells failed to adhere to DP-4 but chondroitinase ABC treatment did not influence the adhesion activity. DP-4 specifically interacted with biotinylated heparin, and this interaction was inhibited by unlabeled heparin. DP-4 peptide significantly promoted the adhesion of cells overexpressing syndecans, and syndecan bound to a DP-4 peptide affinity column. These results suggest that HaCaT cells adhere to dermatopontin through alpha3beta1 integrin and a heparan sulfate proteoglycan-type receptor, which is likely a syndecan. We conclude that dermatopontin plays a role as a multifunctional adhesion molecule for epidermal cells.

The role of dermatopontin in the stromal organization of the cornea

PURPOSE

Dermatopontin (DPT) is an abundant component of the stromal extracellular matrix; however, its function in the cornea is poorly understood. This study was conducted to determine whether DPT has a direct role in corneal matrix organization by investigating the ultrastructure of Dpt-null (Dpt(-/-)) mouse corneas.

METHODS

Conventional light microscopy was used to compare the corneal thickness of Dpt(-/-) mice with that of the wild type. Collagen fibril distribution was studied using transmission electron microscopy and the datasets analyzed using image analysis software to determine fibrillar volume, fibril diameter, and spacing.

RESULTS

Light microscopy demonstrated that Dpt(-/-) corneas in 2-month-old mice showed a 24% reduction in average stromal thickness compared with wild type (P < 0.001). The epithelium and Descemet's membrane appeared normal. Examination of Dpt(-/-) stroma by transmission electron microscopy indicated significant disruption of fibril spacing within the posterior lamellae, whereas the mid and anterior regions appeared largely unaffected compared with wild type. The collagen fibrils in Dpt(-/-) stroma showed a lower fibril volume fraction and a pronounced change in posterior fibrillar organization. There was no apparent difference in fibril diameter between Dpt(-/-) and wild-type mice.

CONCLUSIONS

Collectively, these data suggest that DPT plays a key role in collagen fibril organization. The defects in collagen organization in Dpt(-/-) cornea appear to be most severe in the posterior stroma. It is possible that DPT interacts with corneal proteoglycans and that this interaction is involved in the maintenance of stromal architecture.

Dermatopontin, a novel player in the biology of the extracellular matrix

Dermatopontin is a widely distributed small molecular weight protein in the extracellular matrix (ECM) and today its homologues are known in five mammals and several invertebrates. The structures of these homologues are relatively well conserved among the species. In the skin, dermatopontin is located mainly on the surface of the collagen fibers. It is found in the conditioned medium and also in the cytoplasm of cultured fibroblasts. Early studies focused on ECM assembly (collagen fibrillogenesis) and interactions (with the proteoglycan decorin). Subsequently, a targeted disruption of dermatopontin resulted in a phenotype similar to Ehlers-Danlos syndrome. In addition, a cell adhesion activity of this protein for dermal fibroblasts and several other cells was found, and this activity might suggest this protein's involvement in wound healing. The expression of dermatopontin around an infarct zone of experimental myocardial infarction may support this possibility. In invertebrates, dermatopontin homologues act mainly as adhesion/agglutination molecules. In addition, we found that transforming growth factor-beta1 interacts with dermatopontin and the function of this cytokine is modified by dermatopontin. Recently, the involvement of this protein in cell proliferation has been indicated. In this review we describe the reported functions of this protein and speculate on the multiple roles of this largely uncharacterized matrix molecule.

Diverse biological functions of extracellular collagen processing enzymes

Collagens are abundant proteins in higher organisms, and are formed by a complex biosynthetic pathway involving intracellular and extracellular post-translational modifications. Starting from simple soluble precursors, this interesting pathway produces insoluble functional fibrillar and non-fibrillar elements of the extracellular matrix. The present review highlights recent progress and new insights into biological regulation of extracellular procollagen processing, and some novel functions of byproducts of these extracellular enzymatic transformations. These findings underscore the notion that released propeptides and other proteolytic products of extracellular matrix proteins have important biological functions, and that structural proteins are multifunctional. An emerging concept is that a dynamic interplay exists between extracellular products and byproducts with cells that helps to maintain normal cellular phenotypes and tissue integrity.

Reduced dermatopontin expression is a molecular link between uterine leiomyomas and keloids

Uterine leiomyomas are prevalent estrogen-responsive clonal tumors, but the specific genetic alterations that contribute to their development have not been elucidated. To identify genes involved in the formation of leiomyomas, we used global expression profiling to compare clonal tumors with normal myometrium. Contrary to expectation, genes involved in estrogen action were not differentially expressed between leiomyoma and normal myometrium. Genes encoding extracellular-matrix proteins were prominently featured, suggesting their involvement in formation of a myofibroblast phenotype. Analysis of the extracellular matrix in the leiomyomas revealed a disordered collagen fibril orientation. Expression of the collagen-binding protein dermatopontin was found to be consistently decreased in leiomyoma by both reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time RT-PCR (mean underexpression = 9.41-fold) regardless of leiomyoma size, leiomyoma location, patient race, and patient age. This expression pattern was observed in 11 subjects and a total of 23 leiomyoma:myometrium pairs. Decreased expression of dermatopontin was also associated with keloid formation, a fibrotic disease that shares epidemiologic similarities with leiomyoma. Immunohistochemical studies of leiomyomas and keloids demonstrated reduced levels of dermatopontin in both tissues. In addition, ultrastructural analysis revealed that the orientation of the collagen fibrils in the keloid tissues strongly resembled that in the leiomyomas. Reduction in dermatopontin was associated with an increase in transforming growth factor-beta3 (TGFB3) mRNA levels in leiomyomas, whereas other genes involved in dermatopontin signaling were not differentially expressed. These findings suggest that leiomyoma development involves a myofibroblast cell phenotype characterized by dysregulation of genes encoding extracellular-matrix proteins. In particular, decreased expression of dermatopontin represents a molecular link between the leiomyoma and keloid phenotypes.

Autocrine growth factor regulation of lysyl oxidase expression in transformed fibroblasts

Lysyl oxidase catalyzes oxidative deamination of peptidyl-lysine and hydroxylysine residues in collagens and lysine residues in elastin to form peptidyl aldehydes that are required for the formation of covalent cross-links in normal extracellular matrix biosynthesis. Lysyl oxidase in addition has tumor suppressor activity, and phenotypic reversion of transformed cell lines is accompanied by increased lysyl oxidase expression. The mechanism of low expression of lysyl oxidase in tumor cells is unknown. The present study investigates the hypothesis that autocrine growth factor pathways maintain low lysyl oxidase expression levels in c-H-ras-transformed fibroblasts (RS485 cell line). Autocrine pathways were blocked with suramin, a general inhibitor of growth factor receptor binding, and resulted in more than a 10-fold increase in lysyl oxidase expression and proenzyme production. This regulation was found to be reversible and occurred at the transcriptional level determined using lysyl oxidase promoter/reporter gene assays. Function blocking anti-fibroblast growth factor-2 (FGF-2) antibody enhanced lysyl oxidase expression in the absence of suramin. Finally, the addition of FGF-2 to suramin-treated cells completely reversed suramin stimulation of lysyl oxidase mRNA levels. Data support that an FGF-2 autocrine pathway inhibits lysyl oxidase transcription in the tumorigenic-transformed RS485 cell line. This finding may be of therapeutic significance and, in addition, provides a new experimental approach to investigate the mechanism of the tumor suppressor activity of lysyl oxidase.

Lysyl oxidase-like protein from bovine aorta. Isolation and maturation to an active form by bone morphogenetic protein-1

Recently several cDNAs have been described encoding lysyl oxidase-like proteins. Their deduced amino acid sequences are characterized by a strong similarity in the C-terminal region, corresponding to the lysyl oxidase family catalytic domain, and by marked differences in the N-terminal regions. Different biological functions have been described for lysyl oxidases in addition to their traditionally assumed cross-linking role. To answer the question of whether these different functions are carried out by different lysyl oxidases, purified and active forms of these enzymes are required. At present only the classical form of lysyl oxidase has been purified and characterized. The purpose of this study was to isolate and characterize the lysyl oxidase-like protein. In view of the strong sequence homology with the C-terminal domain of other lysyl oxidases, we chose to purify the protein from bovine aorta using antibodies specific to the N-terminal domain of the proenzyme. We have isolated a 56-kDa protein identified by amino acid sequencing as the bovine lysyl oxidase-like precursor, which is cleaved at the Arg-Arg-Arg sequence at positions 89-91 by a furin-like activity, as revealed after deblocking of the N-terminal residue. The immunopurified protein was largely inactive, but further processing in vitro by bone morphogenetic protein-1 led to an enzyme that was active on elastin and collagen substrates.

The catalytic function of bovine lysyl oxidase in the absence of copper

Bovine lysyl oxidase (BLO) contains two different cofactors, copper (Kagan, H. M. (1986) in Biology of Extracellular Matrix (Mecham, R. P., ed) Vol. 1, pp. 321-398, Academic Press, Orlando, FL) and lysine tyrosyl quinone (LTQ) (Wang, S. X., Mure, M., Medzihradszky, K. F., Burlingame, A. L., Brown, D. E., Dooley, D. M., Smith, A. J., Kagan, H. M., and Klinman, J. P. (1996) Science 273, 1078-1084). By a combination of UV-visible spectroscopy, metal content analysis, and activity measurements, we find that copper-depleted BLO reacts in an irreversible manner with phenylhydrazine, an amine substrate analog, and catalyzes multiple turnovers of the substrate benzylamine. After removal of the majority of enzyme-bound copper, apoBLO exhibits a decrease in the LTQ content, as evidenced by the drop of the 510-520-nm absorbance, suggesting that the copper may play a structural role in stabilizing the LTQ. The remaining intact LTQ in the apoBLO reacted with phenylhydrazine, both in the presence and absence of the chelator, 10 mm 2,2'-dipyridyl. When benzylamine was used as the substrate, the apoBLO turned over at a rate of 50-60% of the native BLO (after correction for the residual copper and the change of LTQ content). Copper contamination from the assay buffer was ruled out by comparison of enzyme activity using different apoBLO concentrations. These studies demonstrate that the mature form of lysyl oxidase retains many of its functions in the absence of copper.

Multiple bone morphogenetic protein 1-related mammalian metalloproteinases process pro-lysyl oxidase at the correct physiological site and control lysyl oxidase activation in mouse embryo fibroblast cultures

Lysyl oxidase catalyzes the final enzymatic step required for collagen and elastin cross-linking in extracellular matrix biosynthesis. Pro-lysyl oxidase is processed by procollagen C-proteinase activity, which also removes the C-propeptides of procollagens I-III. The Bmp1 gene encodes two procollagen C-proteinases: bone morphogenetic protein 1 (BMP-1) and mammalian Tolloid (mTLD). Mammalian Tolloid-like (mTLL)-1 and -2 are two genetically distinct BMP-1-related proteinases, and mTLL-1 has been shown to have procollagen C-proteinase activity. The present study is the first to directly compare pro-lysyl oxidase processing by these four related proteinases. In vitro assays with purified recombinant enzymes show that all four proteinases productively cleave pro-lysyl oxidase at the correct physiological site but that BMP-1 is 3-, 15-, and 20-fold more efficient than mTLL-1, mTLL-2, and mTLD, respectively. To more directly assess the roles of BMP-1 and mTLL-1 in lysyl oxidase activation by connective tissue cells, fibroblasts cultured from Bmp1-null, Tll1-null, and Bmp1/Tll1 double null mouse embryos, thus lacking BMP-1/mTLD, mTLL-1, or all three enzymes, respectively, were assayed for lysyl oxidase enzyme activity and for accumulation of pro-lysyl oxidase and mature approximately 30-kDa lysyl oxidase. Wild type cells or cells singly null for Bmp1 or Tll1 all produced both pro-lysyl oxidase and processed lysyl oxidase at similar levels, indicating apparently normal levels of processing, consistent with enzyme activity data. In contrast, double null Bmp1/Tll1 cells produced predominantly unprocessed 50-kDa pro-lysyl oxidase and had lysyl oxidase enzyme activity diminished by 70% compared with wild type, Bmp1-null, and Tll1-null cells. Thus, the combination of BMP-1/mTLD and mTLL-1 is shown to be responsible for the majority of processing leading to activation of lysyl oxidase by murine embryonic fibroblasts, whereas in vitro studies identify pro-lysyl oxidase as the first known substrate for mTLL-2.

Relationships between bone protein and mineral in developing porcine long bone and calvaria

Several proteins in the bone matrix have been implicated in the regulation of mineral crystal formation and growth. To investigate the relationships between these proteins and the mineral phase at various stages of mineral maturation, fetal porcine calvariae and long bones were fragmented and the particles (20 microm) separated by density gradient sedimentation into fractions of increasing density (1.8 to >2.2 g/cm3). Samples from each fraction were analyzed by X-ray diffraction to obtain the average crystal size/strain and chemical composition. Other samples were sequentially extracted, first with 4.0 mol/L guanidium hydrochloride (GuHCl) (G1), then with 0.5 mol/L ethylene-diamine tetraacetic acid (EDTA) (E), and again with 4.0 mol/L Gu-HCI (G2), for analysis of proteins in different tissue compartments. Based on the mineral density distribution and crystal size, fetal porcine bone protein content was determined for tissue residue and each extract and the protein composition analyzed by sodium dodecyl-polyacrylamide gel electrophoresis (SDS-PAGE). Although the insoluble organic matrix decreased with mineral density the collagen and protein content remained fairly constant, representing approximately 10% of the tissue weight, except in the highest density fraction. Whereas the total extractable protein, representing predominantly noncollagenous proteins, did not show density-related differences, differences were observed for individual proteins on SDS-PAGE. Consistent with their presence in osteoid, the content of bone sialoprotein (BSP), tyrosine-rich acidic matrix protein (TRAMP), and a series of small proteins with cell attachment properties in the G1 extract decreased with mineral density, whereas TRAMP and BSP were increased in G2 extracts. Mineral-associated proteins, including alpha2HS-glycoprotein, BSP, osteopontin (OPN), and osteocalcin, increased with mineral density, whereas secreted protein acidic and rich in cysteine (SPARC)/osteonectin, and some minor proteins, appeared to decrease. Differences of individual proteins within and between the calvarial and long bones could be related to the role of these proteins in the formation and maturation of hydroxyapatite crystals. Collectively, these studies demonstrate mineral density-associated changes in protein composition that reflect a rapid maturation of mineral crystals in embryonic porcine bones.

Dermatopontin expression is decreased in hypertrophic scar and systemic sclerosis skin fibroblasts and is regulated by transforming growth factor-beta1, interleukin-4, and matrix collagen

Dermatopontin is a recently discovered extracellular matrix protein with proteoglycan and cell-binding properties and is assumed to play important roles in cell-matrix interactions and matrix assembly. In this study we examined the expression of dermatopontin mRNA and protein in skin fibroblast cultures from patients with hypertrophic scar and patients with systemic sclerosis. Dermatopontin mRNA and protein levels were reduced in fibroblast cultures from hypertrophic scar lesional skin compared with fibroblasts from normal skin of the same hypertrophic scar patient. Fibroblast cultures from systemic sclerosis patient involved skin also showed significantly reduced expression of dermatopontin compared with normal skin fibroblasts from healthy individuals. We also investigated the effects of cytokines and matrix collagen on dermatopontin expression in normal cultured fibroblasts. Transforming growth factor-beta1 increased dermatopontin mRNA and protein levels, while interleukin-4 reduced dermatopontin expression. Substrate coated with type I collagen reduced dermatopontin mRNA levels, the reduction being more prominent in three-dimensional collagen matrices. Our results suggest that the decreased expression of dermatopontin is associated with the pathogenesis of fibrosis in hypertrophic scar and systemic sclerosis, and that the effect of the cytokines and matrix collagen on dermatopontin may have important implications for skin fibrosis.

Identification of lysyl oxidase and TRAMP as the major proteins in dissociative extracts of the demineralized collagen matrix of porcine dentine

Carbonated apatite (dahllite) is formed within and between collagen fibrils in the mineralization of connective tissues. However, the mechanism of crystal nucleation at these sites has not been resolved. To identify non-collagenous proteins that may be involved in the nucleation process we have utilized a dissociative extraction procedure to isolate proteins associated non-covalently with the de-mineralized collagen matrix of dentine isolated from tooth roots of adult porcine incisors. Following extraction of dentine fragments with 4M GuHCl (G1-extract) and 0.5M EDTA (E-extract), de-mineralized collagen matrix-associated proteins were isolated with a second series of extractions with 4M GuHCl (G2-extract). Analysis of the G2-extracts on SDS-PAGE revealed two major 32 kDa and 24 kDa protein bands, comprising > 80% of the extracted non-collagenous proteins. The 32 kDa protein was purified by FPLC on hydroxyapatite and Mono Q resins, followed by HPLC reverse-phase chromatography. Small amounts of 26 kDa and 6 kDa proteins, which appear to represent proteolytically processed, disulphide-linked fragments of the 32 kDa protein, co-eluted with the major protein. The 32 kDa protein was identified as lysyl oxidase from amino acid sequence analysis of a 13 kDa CNBr peptide obtained from protein purified by preparative electrophoresis on SDS-PAGE. Fractionation of the 24 kDa protein on FPLC Mono Q resin generated < 5 closely eluting protein peaks. The proteins from these peaks were similar in size, staining properties, amino acid composition and CNBr digestion patterns. Each protein was immunoreactive with antibodies raised against a tyrosine-rich acidic matrix protein (TRAMP), reported previously to co-purify with lysyl oxidase. These studies, therefore, show that lysyl oxidase, which is important in collagen cross-link formation, and proteins with properties of TRAMP, a protein that can modulate collagen fibrillogenesis, are the major proteins in dissociative extracts of de-mineralized porcine dentine.

Irreversible inhibition of lysyl oxidase by homocysteine thiolactone and its selenium and oxygen analogues. Implications for homocystinuria

Homocysteine thiolactone, selenohomocysteine lactone, and homoserine lactone were found to be competitive, irreversible inhibitors of lysyl oxidase, with KI values of 21 +/- 3 microM, 8.3 +/- 2.2 microM, and 420 +/- 56 microM, respectively. The first order rate constants for inactivation (k2) of the enzyme varied over a much smaller range, ranging from 0.12 to 0.18 to 0.28 min-1 for the Se-, thio-, and O-lactones, respectively. Mutually exclusive labeling of the enzyme by [1-14C]beta-aminopropionitrile, [U-14C]phenylhydrazine, or [35S]homocysteine thiolactone was observed. These labeling results, together with the closely similar perturbations of the near UV-visible spectra of lysyl oxidase and of a model of its lysine tyrosylquinone cofactor by the thiolactone, indicate that the lactones likely derivatize and reduce the active site carbonyl cofactor. Substitution with deuterium at the alpha-carbon of the thiolactone caused a deuterium kinetic isotope effect on k2 of 3.2 +/- 0.2, consistent with the involvement of rate-limiting alpha-proton abstraction during lactone-induced inactivation of the enzyme. The activities of plasma amine oxidase and diamine oxidase were only minimally reduced at concentrations of the sulfur or selenium lactones that fully inhibited lysyl oxidase. Thus, these lactones constitute a new category of mechanism-based inactivators selective for lysyl oxidase. Further, these results may relate to the development of connective tissue defects seen in homocystinuria.

Incorporation of copper into lysyl oxidase

Lysyl oxidase is a copper-dependent enzyme involved in extracellular processing of collagens and elastin. Although it is known that copper is essential for the functional activity of the enzyme, there is little information on the incorporation of copper. In the present study we examined the insertion of copper into lysyl oxidase using 67Cu in cell-free transcription/translation assays and in normal skin fibroblast culture systems. When a full-length lysyl oxidase cDNA was used as a template for transcription/translation reactions in vitro, unprocessed prolysyl oxidase appeared to bind copper. To examine further the post-translational incorporation of copper into lysyl oxidase, confluent skin fibroblasts were incubated with inhibitors of protein synthesis (cycloheximide, 10 microg/ml), glycosylation (tunicamycin, 10 microg/ml), protein secretion (brefeldin A, 10 microg/ml) and prolysyl oxidase processing (procollagen C-peptidase inhibitor, 2.5 microg/ml) together with 300 microCi of carrier-free 67Cu. It was observed that protein synthesis was a prerequisite for copper incorporation, but inhibition of glycosylation by tunicamycin did not affect the secretion of 67Cu as lysyl oxidase. Brefeldin A inhibited the secretion of 67Ci-labelled lysyl oxidase by 46%, but the intracellular incorporation of copper into lysyl oxidase was not affected. In addition, the inhibition of the extracellular proteolytic processing of prolysyl oxidase to lysyl oxidase had minimal effects on the secretion of protein-bound 67Cu. Our results indicate that, similar to caeruloplasmin processing [Sato and Gitlin (1991) J. Biol. Chem. 266, 5128-5134], copper is inserted into prolysyl oxidase independently of glycosylation.

Expression of active, human lysyl oxidase in Escherichia coli

Lysyl oxidase (LO) is a copper amine oxidase of the extracellular matrix which initiates covalent cross-linking in collagens and elastin. Human LO was expressed in Escherichia coli. At 37 degrees C, large amounts of protein were obtained, but in the form of insoluble aggregates. Lowering the growth temperature, and reducing the amount of inducer, resulted in the production of soluble LO, which was active on a degrees [3H]lysine-labeled elastin substrate. LO was also targeted to the periplasm as a fusion protein with the pelb signal peptide. The periplasmic enzyme was soluble, active and inhibited by beta-aminopropionitrile. Production of the carbonyl co-factor is therefore not a limitation in the expression of active LO in bacteria.

A crosslinked cofactor in lysyl oxidase: redox function for amino acid side chains

A previously unknown redox cofactor has been identified in the active site of lysyl oxidase from the bovine aorta. Edman sequencing, mass spectrometry, ultraviolet-visible spectra, and resonance Raman studies showed that this cofactor is a quinone. Its structure is derived from the crosslinking of the epsilon-amino group of a peptidyl lysine with the modified side chain of a tyrosyl residue, and it has been designated lysine tyrosylquinone. This quinone appears to be the only example of a mammalian cofactor formed from the crosslinking of two amino acid side chains. This discovery expands the range of known quino-cofactor structures and has implications for the mechanism of their biogenesis.

The proteolytic processing site of the precursor of lysyl oxidase

The precise cleavage site of the N-terminal propeptide region of the precursor of lysyl oxidase has not yet been established, due to N-terminal blocking of the mature protein. Using a combination of peptide fragmentation, amino acid sequencing, time-of-flight m.s. and partial chemical unblocking procedures, it is shown that the mature form of lysyl oxidase begins at residue Asp-169 of the precursor protein (numbered according to the human sequence). The cleavage site is 28 residues to the C-terminal side of the site previously suggested on the basis of apparant molecular mass by SDS/PAGE, with the consequence that the two putative, N-linked glycosylation sites and the position of the Arg/Gln sequence polymorphism are now all in the precursor region.

Tyrosine-rich acidic matrix protein (TRAMP) is a tyrosine-sulphated and widely distributed protein of the extracellular matrix

Tyrosine-rich acidic matrix protein (TRAMP; 22 kDa extracellular matrix protein; dermatopontin) is a protein that co-purifies with lysyl oxidase and with dermatan sulphate proteoglycans, with possible functions in cell-matrix interactions and matrix assembly. Using a rabbit polyclonal antiserum raised against porcine TRAMP, which cross-reacts with both the human and murine forms of the protein, we show by immunoblotting that TRAMP has a widespread tissue distribution, including skin, skeletal muscle, heart, lung, kidney, cartilage and bone. In cultures of human skin fibroblasts, TRAMP incorporates both [35S]sulphate and [3H]tyrosine and is secreted into the medium, as shown by immunoprecipitation. Amino acid analysis of immunoprecipitated TRAMP demonstrates that many of the tyrosine residues in TRAMP are sulphated.