Identification of PCPE-2 as the endogenous specific inhibitor of human BMP-1/tolloid-like proteinases

BMP-1/tolloid-like proteinases (BTPs) are major players in tissue morphogenesis, growth and repair. They act by promoting the deposition of structural extracellular matrix proteins and by controlling the activity of matricellular proteins and TGF-β superfamily growth factors. They have also been implicated in several pathological conditions such as fibrosis, cancer, metabolic disorders and bone diseases. Despite this broad range of pathophysiological functions, the putative existence of a specific endogenous inhibitor capable of controlling their activities could never be confirmed. Here, we show that procollagen C-proteinase enhancer-2 (PCPE-2), a protein previously reported to bind fibrillar collagens and to promote their BTP-dependent maturation, is primarily a potent and specific inhibitor of BTPs which can counteract their proteolytic activities through direct binding. PCPE-2 therefore differs from the cognate PCPE-1 protein and extends the possibilities to fine-tune BTP activities, both in physiological conditions and in therapeutic settings.

Dynamics of the secreted frizzled related protein Sizzled and potential implications for binding to bone morphogenetic protein-1 (BMP-1)

Sizzled (Szl) is both a secreted frizzled related protein (sFRP) and a naturally occurring inhibitor of the zinc metalloproteinase bone morphogenetic protein-1 (BMP-1), a key regulator of extracellular matrix assembly and growth factor activation. Here we present a new crystal structure for Szl which differs from that previously reported by a large scale (90°) hinge rotation between its cysteine-rich and netrin-like domains. We also present results of a molecular docking analysis showing interactions likely to be involved in the inhibition of BMP-1 activity by Szl. When compared with known structures of BMP-1 in complex with small molecule inhibitors, this reveals features that may be helpful in the design of new inhibitors to prevent the excessive accumulation of extracellular matrix that is the hallmark of fibrotic diseases.

Comparison of extracellular matrix enrichment protocols for the improved characterization of the skin matrisome by mass spectrometry

A striking feature of skin organization is that the extracellular matrix (ECM) occupies a larger volume than the cells. Skin ECM also directly contributes to aging and most cutaneous diseases. In recent years, specific ECM enrichment protocols combined with in silico approaches allowed the proteomic description of the matrisome of various organs and tumor samples. Nevertheless, the skin matrisome remains under-studied and protocols allowing the efficient recovery of the diverse ECM found in skin are still to be described. Here, we compared four protocols allowing the enrichment of ECM proteins from adult mouse back skin and found that all protocols led to a significant enrichment (up to 65%) of matrisome proteins when compared to total skin lysates. The protocols based on decellularization and solubility profiling gave the best results in terms of numbers of proteins identified and confirmed that skin matrisome proteins exhibit very diverse solubility and abundance profiles. We also report the first description of the skin matrisome of healthy adult mice that includes 236 proteins comprising 95 core matrisome proteins and 141 associated matrisome proteins. These results provide a reliable basis for future characterizations of skin ECM proteins and their dysregulations in disease-specific contexts. SIGNIFICANCE: Extracellular matrix proteins are key players in skin physiopathology and have been involved in several diseases such as genetic disorders, wound healing defects, scleroderma and skin carcinoma. However, skin ECM proteins are numerous, diverse and challenging to analyze by mass spectrometry due to the multiplicity of their post-translational modifications and to the heterogeneity of their solubility profiles. Here, we performed the thorough evaluation of four ECM enrichment protocols compatible with the proteomic analysis of mouse back skin and provide the first description of the adult mouse skin matrisome in homeostasis conditions. Our work will greatly facilitate the future characterization of skin ECM alterations in preclinical mouse models and will inspire new optimizations to analyze the skin matrisome of other species and of human clinical samples.

Procollagen C-proteinase enhancer-1 (PCPE-1), a potential biomarker and therapeutic target for fibrosis

The correct balance between collagen synthesis and degradation is essential for almost every aspect of life, from development to healthy aging, reproduction and wound healing. When this balance is compromised by external or internal stress signals, it very often leads to disease as is the case in fibrotic conditions. Fibrosis occurs in the context of defective tissue repair and is characterized by the excessive, aberrant and debilitating deposition of fibril-forming collagens. Therefore, the numerous proteins involved in the biosynthesis of fibrillar collagens represent a potential and still underexploited source of therapeutic targets to prevent fibrosis. One such target is procollagen C-proteinase enhancer-1 (PCPE-1) which has the unique ability to accelerate procollagen maturation by BMP-1/tolloid-like proteinases (BTPs) and contributes to trigger collagen fibrillogenesis, without interfering with other BTP functions or the activities of other extracellular metalloproteinases. This role is achieved through a fine-tuned mechanism of action that is close to being elucidated and offers promising perspectives for drug design. Finally, the in vivo data accumulated in recent years also confirm that PCPE-1 overexpression is a general feature and early marker of fibrosis. In this review, we describe the results which presently support the driving role of PCPE-1 in fibrosis and discuss the questions that remain to be solved to validate its use as a biomarker or therapeutic target.

Latent TGF-β Activation Is a Hallmark of the Tenascin Family

Transforming growth factor-β (TGF-β) isoforms are secreted as inactive complexes formed through non-covalent interactions between bioactive TGF-β entities and their N-terminal pro-domains called latency-associated peptides (LAP). Extracellular activation of latent TGF-β within this complex is a crucial step in the regulation of TGF-β activity for tissue homeostasis and immune cell function. We previously showed that the matrix glycoprotein Tenascin-X (TN-X) interacted with the small latent TGF-β complex and triggered the activation of the latent cytokine into a bioactive TGF-β. This activation most likely occurs through a conformational change within the latent TGF-β complex and requires the C-terminal fibrinogen-like (FBG) domain of the glycoprotein. As the FBG-like domain is highly conserved among the Tenascin family members, we hypothesized that Tenascin-C (TN-C), Tenascin-R (TN-R) and Tenascin-W (TN-W) might share with TN-X the ability to regulate TGF-β bioavailability through their C-terminal domain. Here, we demonstrate that purified recombinant full-length Tenascins associate with the small latent TGF-β complex through their FBG-like domains. This association promotes activation of the latent cytokine and subsequent TGF-β cell responses in mammary epithelial cells, such as cytostasis and epithelial-to-mesenchymal transition (EMT). Considering the pleiotropic role of TGF-β in numerous physiological and pathological contexts, our data indicate a novel common function for the Tenascin family in the regulation of tissue homeostasis under healthy and pathological conditions.

A 9-kDa matricellular SPARC fragment released by cathepsin D exhibits pro-tumor activity in the triple-negative breast cancer microenvironment

Rationale: Alternative therapeutic strategies based on tumor-specific molecular targets are urgently needed for triple-negative breast cancer (TNBC). The protease cathepsin D (cath-D) is a marker of poor prognosis in TNBC and a tumor-specific extracellular target for antibody-based therapy. The identification of cath-D substrates is crucial for the mechanistic understanding of its role in the TNBC microenvironment and future therapeutic developments.

Methods: The cath-D substrate repertoire was investigated by N-Terminal Amine Isotopic Labeling of Substrates (TAILS)-based degradome analysis in a co-culture assay of TNBC cells and breast fibroblasts. Substrates were validated by amino-terminal oriented mass spectrometry of substrates (ATOMS). Cath-D and SPARC expression in TNBC was examined using an online transcriptomic survival analysis, tissue micro-arrays, TNBC cell lines, patient-derived xenografts (PDX), human TNBC samples, and mammary tumors from MMTV-PyMT Ctsd^-/-knock-out mice. The biological role of SPARC and its fragments in TNBC were studied using immunohistochemistry and immunofluorescence analysis, gene expression knockdown, co-culture assays, western blot analysis, RT-quantitative PCR, adhesion assays, Transwell motility, trans-endothelial migration and invasion assays.

Results: TAILS analysis showed that the matricellular protein SPARC is a substrate of extracellular cath-D. In vitro, cath-D induced limited proteolysis of SPARC C-terminal extracellular Ca²⁺ binding domain at acidic pH, leading to the production of SPARC fragments (34-, 27-, 16-, 9-, and 6-kDa). Similarly, cath-D secreted by TNBC cells cleaved fibroblast- and cancer cell-derived SPARC at the tumor pericellular acidic pH. SPARC cleavage also occurred in TNBC tumors. Among these fragments, only the 9-kDa SPARC fragment inhibited TNBC cell adhesion and spreading on fibronectin, and stimulated their migration, endothelial transmigration, and invasion.

Conclusions: Our study establishes a novel crosstalk between proteases and matricellular proteins in the tumor microenvironment through limited SPARC proteolysis, revealing a novel targetable 9-kDa bioactive SPARC fragment for new TNBC treatments. Our study will pave the way for the development of strategies for targeting bioactive fragments from matricellular proteins in TNBC.

BMP-1 disrupts cell adhesion and enhances TGF-β activation through cleavage of the matricellular protein thrombospondin-1

Bone morphogenetic protein 1 (BMP-1) is an important metalloproteinase that synchronizes growth factor activation with extracellular matrix assembly during morphogenesis and tissue repair. The mechanisms by which BMP-1 exerts these effects are highly context dependent. Because BMP-1 overexpression induces marked phenotypic changes in two human cell lines (HT1080 and 293-EBNA cells), we investigated how BMP-1 simultaneously affects cell-matrix interactions and growth factor activity in these cells. Increasing BMP-1 led to a loss of cell adhesion that depended on the matricellular glycoprotein thrombospondin-1 (TSP-1). BMP-1 cleaved TSP-1 between the VWFC/procollagen-like domain and the type 1 repeats that mediate several key TSP-1 functions. This cleavage induced the release of TSP-1 C-terminal domains from the extracellular matrix and abolished its previously described multisite cooperative interactions with heparan sulfate proteoglycans and CD36 on HT1080 cells. In addition, BMP-1-dependent proteolysis potentiated the TSP-1-mediated activation of latent transforming growth factor-β (TGF-β), leading to increased signaling through the canonical SMAD pathway. In primary human corneal stromal cells (keratocytes), endogenous BMP-1 cleaved TSP-1, and the addition of exogenous BMP-1 enhanced cleavage, but this had no substantial effect on cell adhesion. Instead, processed TSP-1 promoted the differentiation of keratocytes into myofibroblasts and stimulated production of the myofibroblast marker α-SMA, consistent with the presence of processed TSP-1 in human corneal scars. Our results indicate that BMP-1 can both trigger the disruption of cell adhesion and stimulate TGF-β signaling in TSP-1-rich microenvironments, which has important potential consequences for wound healing and tumor progression.

C-terminal proteolysis of the collagen VI α3 chain by BMP-1 and proprotein convertase(s) releases endotrophin in fragments of different sizes

The assembly of collagen VI microfibrils is a multistep process in which proteolytic processing within the C-terminal globular region of the collagen VI α3 chain plays a major role. However, the mechanisms involved remain elusive. Moreover, C5, the short and most C-terminal domain of the α3 chain, recently has been proposed to be released as an adipokine that enhances tumor progression, fibrosis, inflammation, and insulin resistance and has been named "endotrophin." Serum endotrophin could be a useful biomarker to monitor the progression of such disorders as chronic obstructive pulmonary disease, systemic sclerosis, and kidney diseases. Here, using biochemical and isotopic MS-based analyses, we found that the extracellular metalloproteinase bone morphogenetic protein 1 (BMP-1) is involved in endotrophin release and determined the exact BMP-1 cleavage site. Moreover, we provide evidence that several endotrophin-containing fragments are present in various tissues and body fluids. Among these, a large C2-C5 fragment, which contained endotrophin, was released by furin-like proprotein convertase cleavage. By using immunofluorescence microscopy and EM, we also demonstrate that these proteolytic maturations occur after secretion of collagen VI tetramers and during microfibril assembly. Differential localization of N- and C-terminal regions of the collagen VI α3 chain revealed that cleavage products are deposited in tissue and cell cultures. The detailed information on the processing of the collagen VI α3 chain reported here provides a basis for unraveling the function of endotrophin (C5) and larger endotrophin-containing fragments and for refining their use as biomarkers of disease progression.

A Scaffold- and Serum-Free Method to Mimic Human Stable Cartilage Validated by Secretome

A stabilized cartilage construct without signs of hypertrophy in chondrocytes is still a challenge. Suspensions of adipose stem/stromal cells (ASCs) and cartilage progenitor cells (CPCs) were seeded into micromolded nonadhesive hydrogel to produce spheroids (scaffold- and serum-free method) characterized by size, immunohistochemistry, fusion, and biomechanical properties. After cell dissociation, they were characterized for mesenchymal cell surface markers, cell viability, and quantitative real-time polymerase chain reaction. Both targeted and nontargeted (shotgun mass spectrometry) analyses were conducted on the culture supernatants. Induced ASC spheroids (ø = 350 μm) showed high cell viability and CD73 downregulation contrasting to CD90. The transforming growth factor (TGF)-β3/TGF-β1 ratio and SOX9 increased (p < 0.05), whereas interleukin (IL)-6, IL-8, RUNX2, and ALPL decreased. Induced ASC spheroids were able to completely fuse and showed a higher force required to compression at day 14 (p < 0.0001). Strong collagen type II in situ was associated with gradual decrease of collagen type X and a lower COLXA1 gene expression at day 14 compared with day 7 (p = 0.0352). The comparison of the secretome content of induced and non-induced ASCs and CPCs identified 138 proteins directly relevant to chondrogenesis of 704 proteins in total. Although collagen X was absent, thrombospondin-1 (TSP-1), described as antiangiogenic and antihypertrophic, and cartilage oligomeric matrix protein (COMP), a biomarker of chondrogenesis, were upregulated in induced ASC spheroids. Our scaffold- and serum-free method mimics stable cartilage acting as a tool for biomarker discovery and for regenerative medicine protocols. Impact Statement Promising adult stem cell sources for cartilage regeneration include adipose stem/stromal cells (ASCs) from subcutaneous adipose tissue. Our main objective was the development of a reproducible and easy-to-handle scaffold- and serum-free method to obtain stable cartilage from induced ASC spheroids. In addition to targeted protein profiling and biomechanical analysis, we provide the first characterization of the secretome composition for ASC spheroids, providing a useful tool to monitor in vitro chondrogenesis and a noninvasive quality control of tissue-engineered constructs. Furthermore, our secretome analysis revealed a potential novel biomarker-thrombospondin-1 (TSP-1), known by its antiangiogenic properties and recently described as an antihypertrophic protein.

COL1A1 C-propeptide mutations cause ER mislocalization of procollagen and impair C-terminal procollagen processing

Mutations in the type I procollagen C-propeptide occur in ~6.5% of Osteogenesis Imperfecta (OI) patients. They are of special interest because this region of procollagen is involved in α chain selection and folding, but is processed prior to fibril assembly and is absent in mature collagen fibrils in tissue. We investigated the consequences of seven COL1A1 C-propeptide mutations for collagen biochemistry in comparison to three probands with classical glycine substitutions in the collagen helix near the C-propeptide and a normal control. Procollagens with C-propeptide defects showed the expected delayed chain incorporation, slow folding and overmodification. Immunofluorescence microscopy indicated that procollagen with C-propeptide defects was mislocalized to the ER lumen, in contrast to the ER membrane localization of normal procollagen and procollagen with helical substitutions. Notably, pericellular processing of procollagen with C-propeptide mutations was defective, with accumulation of pC-collagen and/or reduced production of mature collagen. In vitro cleavage assays with BMP-1 ± PCPE-1 confirmed impaired C-propeptide processing of procollagens containing mutant proα1(I) chains. Overmodified collagens were incorporated into the matrix in culture. Dermal fibrils showed alterations in average diameter and diameter variability and bone fibrils were disorganized. Altered ER-localization and reduced pericellular processing of defective C-propeptides are expected to contribute to abnormal osteoblast differentiation and matrix function, respectively.

Inhibitors of BMP-1/tolloid-like proteinases: efficacy, selectivity and cellular toxicity

BMP‐1/tolloid‐like proteinases belong to the astacin family of human metalloproteinases, together with meprins and ovastacin. They represent promising targets to treat or prevent a wide range of diseases such as fibrotic disorders or cancer. However, the study of their pathophysiological roles is still impaired by the lack of well‐characterized inhibitors and the questions that remain regarding their selectivity and in vivo efficiency. As a first step towards the identification of suitable tools to be used in functional studies, we have undertaken a systematic comparison of seven molecules known to affect the proteolytic activity of human astacins including three hydroxamates (FG‐2575, UK383,367, S33A), the protein sizzled, a new phosphinic inhibitor (RXP‐1001) and broad‐spectrum protease inhibitors (GM6001, actinonin). Their efficacy in vitro, their cellular toxicity and efficacy in cell cultures were thoroughly characterized. We found that these molecules display very different potency and selectivity profiles, with hydroxamate FG‐2575 and the protein sizzled being very powerful and selective inhibitors of BMP‐1, whereas phosphinic peptide RXP‐1001 behaves as a broad‐spectrum inhibitor of astacins. Their use should therefore be carefully considered in agreement with the aim of the study to avoid result misinterpretation.

Structural Basis for the Acceleration of Procollagen Processing by Procollagen C-Proteinase Enhancer-1

Procollagen C-proteinase enhancer-1 (PCPE-1) is a secreted protein that specifically accelerates proteolytic release of the C-propeptides from fibrillar procollagens, a crucial step in fibril assembly. As such, it is a potential therapeutic target to improve tissue repair and prevent fibrosis, a major cause of mortality worldwide. Here we present the crystal structure of the active CUB1CUB2 fragment of PCPE-1 bound to the C-propeptide trimer of procollagen III (CPIII). This shows that the two CUB domains bind to two different chains of CPIII and that the N-terminal region of one CPIII chain, close to the proteolytic cleavage site, lies in the cleft between CUB1 and CUB2. This suggests that enhancing activity involves unraveling of this chain from the rest of the trimer, thus facilitating the action of the proteinase involved. Support for this hypothesis comes from site-directed mutagenesis, enzyme assays, binding studies, and molecular modeling.

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The crystal structure of PCPE-1 bound to the C-propeptides has been determined

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The N terminus of one propeptide chain binds to the CUB1CUB2 fragment of PCPE-1

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PCPE-1 seems to unravel the propeptide trimer to enable proteolytic release

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Molecular modeling with the proteinase and its substrate supports this hypothesis

Interaction of Complement Defence Collagens C1q and Mannose-Binding Lectin with BMP-1/Tolloid-like Proteinases

The defence collagens C1q and mannose-binding lectin (MBL) are immune recognition proteins that associate with the serine proteinases C1r/C1s and MBL-associated serine proteases (MASPs) to trigger activation of complement, a major innate immune system. Bone morphogenetic protein-1 (BMP-1)/tolloid-like proteinases (BTPs) are metalloproteinases with major roles in extracellular matrix assembly and growth factor signalling. Despite their different functions, C1r/C1s/MASPs and BTPs share structural similarities, including a specific CUB-EGF-CUB domain arrangement found only in these enzymes that mediates interactions with collagen-like proteins, suggesting a possible functional relationship. Here we investigated the potential interactions between the defence collagens C1q and MBL and the BTPs BMP-1 and mammalian tolloid-like-1 (mTLL-1). C1q and MBL bound to immobilized BMP-1 and mTLL-1 with nanomolar affinities. These interactions involved the collagen-like regions of the defence collagens and were inhibited by pre-incubation of C1q or MBL with their cognate complement proteinases. Soluble BMP-1 and mTLL-1 did not inhibit complement activation and the defence collagens were neither substrates nor inhibitors of BMP-1. Finally, C1q co-localized with BMP-1 in skin biopsies following melanoma excision and from patients with recessive dystrophic epidermolysis bullosa. The observed interactions provide support for a functional link between complement and BTPs during inflammation and tissue repair.

Structural basis of homo- and heterotrimerization of collagen I

Fibrillar collagen molecules are synthesized as precursors, procollagens, with large propeptide extensions. While a homotrimeric form (three α1 chains) has been reported in embryonic tissues as well as in diseases (cancer, fibrosis, genetic disorders), collagen type I usually occurs as a heterotrimer (two α1 chains and one α2 chain). Inside the cell, the role of the C-terminal propeptides is to gather together the correct combination of three α chains during molecular assembly, but how this occurs for different forms of the same collagen type is so far unknown. Here, by structural and mutagenic analysis, we identify key amino acid residues in the α1 and α2 C-propeptides that determine homo- and heterotrimerization. A naturally occurring mutation in one of these alters the homo/heterotrimer balance. These results show how the C-propeptide of the α2 chain has specifically evolved to permit the appearance of heterotrimeric collagen I, the major extracellular building block among the metazoa.

Deficiency of the DSPP-cleaving enzymes meprin α and meprin β does not result in dentin malformation in mice

Formation of dentin requires the maturation of procollagen I and the proteolytic processing of the dentin sialophosphoprotein (DSPP). These cleavage events can be facilitated by the metalloproteinases meprin α and meprin β as well as by bone morphogenetic protein 1 (BMP-1). All three enzymes have been shown to play important roles during collagen I maturation in vivo and their potential in cleaving DSPP was demonstrated in vitro. Hence, it has been discussed whether meprin α, meprin β, BMP-1 or all three are crucial factors in the onset and progression of dentin-related diseases and this issue is addressed here. In this study, we compare the incisors and molars of meprin α (Mep1a ^-/-)- and meprin β (Mep1b ^-/-)-deficient mice with wild-type (WT) controls on the macroscopic and microscopic level. The dentin was evaluated towards the bone mineral density, dentin volume, calcification and collagen matrix integrity. Using immunohistochemistry, we could identify meprin β, BMP-1 and DSPP/DSP in the pre-dentin of WT mice. Nevertheless, no significant dentin malformation was observed in Mep1b ^-/- or Mep1a ^-/- deficient mice.

Determination of the substrate repertoire of ADAMTS2, 3, and 14 significantly broadens their functions and identifies extracellular matrix organization and TGF-β signaling as primary targets

A disintegrin and metalloproteinase with thrombospondin type I motif (ADAMTS)2, 3, and 14 are collectively named procollagen N-proteinases (pNPs) because of their specific ability to cleave the aminopropeptide of fibrillar procollagens. Several reports also indicate that they could be involved in other biological processes, such as blood coagulation, development, and male fertility, but the potential substrates associated with these activities remain unknown. Using the recently described N-terminal amine isotopic labeling of substrate approach, we analyzed the secretomes of human fibroblasts and identified 8, 17, and 22 candidate substrates for ADAMTS2, 3, and 14, respectively. Among these newly identified substrates, many are components of the extracellular matrix and/or proteins related to cell signaling such as latent TGF-β binding protein 1, TGF-β RIII, and dickkopf-related protein 3. Candidate substrates for the 3 ADAMTS have been biochemically validated in different contexts, and the implication of ADAMTS2 in the control of TGF-β activity has been further demonstrated in human fibroblasts. Finally, the cleavage site specificity was assessed showing a clear and unique preference for nonpolar or slightly hydrophobic amino acids. This work shows that the activities of the pNPs extend far beyond the classically reported processing of the aminopropeptide of fibrillar collagens and that they should now be considered as multilevel regulators of matrix deposition and remodeling.-Bekhouche, M., Leduc, C., Dupont, L., Janssen, L., Delolme, F., Vadon-Le Goff, S., Smargiasso, N., Baiwir, D., Mazzucchelli, G., Zanella-Cleon, I., Dubail, J., De Pauw, E., Nusgens, B., Hulmes, D. J. S., Moali, C., Colige, A. Determination of the substrate repertoire of ADAMTS2, 3, and 14 significantly broadens their functions and identifies extracellular matrix organization and TGF-β signaling as primary targets.

Development of ex vivo organ culture models to mimic human corneal scarring

PURPOSE

To develop ex vivo organ culture models of human corneal scarring suitable for pharmacological testing and the study of the molecular mechanisms leading to corneal haze after laser surgery or wounding.

METHODS

Corneas from human donors were cultured ex vivo for 30 days, either at the air-liquid interface (AL) or immersed (IM) in the culture medium. Histological features and immunofluorescence for fibronectin, tenascin C, thrombospondin-1, and α-smooth muscle actin were graded from 0 to 3 for control corneas and for corneas wounded with an excimer laser. The effects of adding 10 ng/ml transforming growth factor-β1 (TGF-β1) to the culture medium and of prior complete removal of the epithelium and limbus, thus preventing reepithelialization, were also analyzed on wounded corneas. Collagen III expression was detected with real-time PCR.

RESULTS

Wounding alone was sufficient to induce keratocyte activation and stromal disorganization, but it was only in the presence of added TGF-β1 that intense staining for fibronectin and tenascin C was found in the AL and IM models (as well as thrombospondin-1 in the AL model) and that α-smooth muscle actin became detectable. The scar-like appearance of the corneas was exacerbated when TGF-β1 was added and reepithelialization was prevented, resulting in the majority of corneas becoming opaque and marked upregulation of collagen III.

CONCLUSIONS

THE MAIN FEATURES OF CORNEAL SCARRING WERE REPRODUCED IN THESE TWO COMPLEMENTARY MODELS: the AL model preserved differentiation of the epithelium and permits the topical application of active molecules, while the IM model ensures better perfusion by soluble compounds.

Structural basis of fibrillar collagen trimerization and related genetic disorders

The C propeptides of fibrillar procollagens have crucial roles in tissue growth and repair by controlling both the intracellular assembly of procollagen molecules and the extracellular assembly of collagen fibrils. Mutations in C propeptides are associated with several, often lethal, genetic disorders affecting bone, cartilage, blood vessels and skin. Here we report the crystal structure of a C-propeptide domain from human procollagen III. It reveals an exquisite structural mechanism of chain recognition during intracellular trimerization of the procollagen molecule. It also gives insights into why some types of collagen consist of three identical polypeptide chains, whereas others do not. Finally, the data show striking correlations between the sites of numerous disease-related mutations in different C-propeptide domains and the degree of phenotype severity. The results have broad implications for understanding genetic disorders of connective tissues and designing new therapeutic strategies.

Production and crystallization of the C-propeptide trimer from human procollagen III

The C-propeptide domains of the fibrillar procollagens, which are present throughout the Metazoa in the form of ∼90 kDa trimers, play crucial roles in both intracellular molecular assembly and extracellular formation of collagen fibrils. The first crystallization of a C-propeptide domain, that from human procollagen III, is described. Following transient expression in mammalian 293T cells of both the native protein and a selenomethionine derivative, two crystal forms of the homotrimer were obtained: an orthorhombic form (P2(1)2(1)2(1)) that diffracted to 1.7 Å resolution and a trigonal form (P321) that diffracted to 3.5 Å resolution. Characterization by MALDI-TOF mass spectrometry allowed the efficiency of selenomethionine incorporation to be determined.

Sizzled is unique among secreted frizzled-related proteins for its ability to specifically inhibit bone morphogenetic protein-1 (BMP-1)/tolloid-like proteinases

BMP-1/tolloid-like proteinases (BTPs) are major enzymes involved in extracellular matrix assembly and activation of bioactive molecules, both growth factors and anti-angiogenic molecules. Although the control of BTP activity by several enhancing molecules is well established, the possibility that regulation also occurs through endogenous inhibitors is still debated. Secreted frizzled-related proteins (sFRPs) have been studied as possible candidates, with highly contradictory results, after the demonstration that sizzled, a sFRP found in Xenopus and zebrafish, was a potent inhibitor of Xenopus and zebrafish tolloid-like proteases. In this study, we demonstrate that mammalian sFRP-1, -2, and -4 do not modify human BMP-1 activity on several of its known substrates including procollagen I, procollagen III, pN-collagen V, and prolysyl oxidase. In contrast, Xenopus sizzled appears as a tight binding inhibitor of human BMP-1, with a K(i) of 1.5 ± 0.5 nM, and is shown to strongly inhibit other human tolloid isoforms mTLD and mTLL-1. Because sizzled is the most potent inhibitor of human tolloid-like proteinases known to date, we have studied its mechanism of action in detail and shown that the frizzled domain of sizzled is both necessary and sufficient for inhibitory activity and that it acts directly on the catalytic domain of BMP-1. Residues in sizzled required for inhibition include Asp-92, which is shared by sFRP-1 and -2, and also Phe-94, Ser-43, and Glu-44, which are specific to sizzled, thereby providing a rational basis for the absence of inhibitory activity of human sFRPs.

Procollagen C-proteinase enhancer stimulates procollagen processing by binding to the C-propeptide region only

Bone morphogenetic protein-1 (BMP-1) and the tolloid-like metalloproteinases control several aspects of embryonic development and tissue repair. Unlike other proteinases whose activities are regulated mainly by endogenous inhibitors, regulation of BMP-1/tolloid-like proteinases relies mostly on proteins that stimulate activity. Among these, procollagen C-proteinase enhancers (PCPEs) markedly increase BMP-1/tolloid-like proteinase activity on fibrillar procollagens, in a substrate-specific manner. Here, we performed a detailed quantitative study of the binding of PCPE-1 and of its minimal active fragment (CUB1-CUB2) to three regions of the procollagen III molecule: the triple helix, the C-telopeptide, and the C-propeptide. Contrary to results described elsewhere, we found the PCPE-1-binding sites to be located exclusively in the C-propeptide region. In addition, binding and enhancing activities were found to be independent of the glycosylation state of the C-propeptide. These data exclude previously proposed mechanisms for the action of PCPEs and also suggest new mechanisms to explain how these proteins can stimulate BMP-1/tolloid-like proteinases by up to 20-fold.

Processing of procollagen III by meprins: new players in extracellular matrix assembly?

Meprins α and β, a subgroup of zinc metalloproteinases belonging to the astacin family, are known to cleave components of the extracellular matrix, either during physiological remodeling or in pathological situations. In this study we present a new role for meprins in matrix assembly, namely the proteolytic processing of procollagens. Both meprins α and β release the N- and C-propeptides from procollagen III, with such processing events being critical steps in collagen fibril formation. In addition, both meprins cleave procollagen III at exactly the same site as the procollagen C-proteinases, including bone morphogenetic protein-1 (BMP-1) and other members of the tolloid proteinase family. Indeed, cleavage of procollagen III by meprins is more efficient than by BMP-1. In addition, unlike BMP-1, whose activity is stimulated by procollagen C-proteinase enhancer proteins (PCPEs), the activity of meprins on procollagen III is diminished by PCPE-1. Finally, following our earlier observations of meprin expression by human epidermal keratinocytes, meprin α is also shown to be expressed by human dermal fibroblasts. In the dermis of fibrotic skin (keloids), expression of meprin α increases and meprin β begins to be detected. Our study suggests that meprins could be important players in several remodeling processes involving collagen fiber deposition.

Role of the netrin-like domain of procollagen C-proteinase enhancer-1 in the control of metalloproteinase activity

The netrin-like (NTR) domain is a feature of several extracellular proteins, most notably the N-terminal domain of tissue inhibitors of metalloproteinases (TIMPs), where it functions as a strong inhibitor of matrix metalloproteinases and some other members of the metzincin superfamily. The presence of a C-terminal NTR domain in procollagen C-proteinase enhancers (PCPEs), proteins that stimulate the activity of astacin-like tolloid proteinases, raises the possibility that this might also have inhibitory activity. Here we show that both long and short forms of the PCPE-1 NTR domain, the latter beginning at the N-terminal cysteine known to be critical for TIMP activity, show no inhibition, at micromolar concentrations, of several members of the metzincin superfamily, including matrix metalloproteinase-2, bone morphogenetic protein-1 (a tolloid proteinase), and different ADAMTS (a disintegrin and a metalloproteinase with thrombospondin motifs) proteinases from the adamalysin family. In contrast, we report that the NTR domain within PCPE-1 leads to superstimulation of bone morphogenetic protein-1 activity in the presence of heparin and heparan sulfate. These observations point to a new mechanism whereby binding to cell surface-associated or extracellular heparin-like sulfated glycosaminoglycans might provide a means to accelerate procollagen processing in specific cellular and extracellular microenvironments.

Strong cooperativity and loose geometry between CUB domains are the basis for procollagen c-proteinase enhancer activity

Procollagen C-proteinase enhancers (PCPE-1 and -2) specifically activate bone morphogenetic protein-1 (BMP-1) and other members of the tolloid proteinase family during C-terminal processing of fibrillar collagen precursors. PCPEs consist of two CUB domains (CUB1 and CUB2) and one NTR domain separated by one short and one long linker. It was previously shown that PCPEs can strongly interact with procollagen molecules, but the exact mechanism by which they enhance BMP-1 activity remains largely unknown. Here, we used a series of deletion mutants of PCPE-1 and two chimeric constructs with repetitions of the same CUB domain to study the role of each domain and linker. Out of all the forms tested, only those containing both CUB1 and CUB2 were capable of enhancing BMP-1 activity and binding to a mini-procollagen substrate with nanomolar affinity. Both these properties were lost by individual CUB domains, which had dissociation constants at least three orders of magnitude higher. In addition, none of the constructs tested could inhibit PCPE activity, although CUB2CUB2NTR was found to modulate BMP-1 activity through direct complex formation with the enzyme, resulting in a decreased rate of substrate processing. Finally, increasing the length of the short linker between CUB1 and CUB2 was without detrimental effect on both activity and substrate binding. These data support the conclusion that CUB1 and CUB2 bind to the procollagen substrate in a cooperative manner, involving the short linker that provides a flexible tether linking the two binding regions.

Insights into how CUB domains can exert specific functions while sharing a common fold: conserved and specific features of the CUB1 domain contribute to the molecular basis of procollagen C-proteinase enhancer-1 activity

Procollagen C-proteinase enhancers (PCPE-1 and -2) are extracellular glycoproteins that can stimulate the C-terminal processing of fibrillar procollagens by tolloid proteinases such as bone morphogenetic protein-1. They consist of two CUB domains (CUB1 and -2) that alone account for PCPE-enhancing activity and one C-terminal NTR domain. CUB domains are found in several extracellular and plasma membrane-associated proteins, many of which are proteases. We have modeled the structure of the CUB1 domain of PCPE-1 based on known three-dimensional structures of CUB-containing proteins. Sequence alignment shows conserved amino acids, notably two acidic residues (Asp-68 and Asp-109) involved in a putative surface-located calcium binding site, as well as a conserved tyrosine residue (Tyr-67). In addition, three residues (Glu-26, Thr-89, and Phe-90) are found only in PCPE CUB1 domains, in putative surface-exposed loops. Among the conserved residues, it was found that mutations of Asp-68 and Asp-109 to alanine almost completely abolished PCPE-1 stimulating activity, whereas mutation of Tyr-67 led to a smaller reduction of activity. Among residues specific to PCPEs, mutation of Glu-26 and Thr-89 had little effect, whereas mutation of Phe-90 dramatically decreased the activity. Changes in activity were paralleled by changes in binding of different PCPE-1 mutants to a mini-procollagen III substrate, as shown by surface plasmon resonance. We conclude that PCPE-stimulating activity requires a calcium binding motif in the CUB1 domain that is highly conserved among CUB-containing proteins but also that PCPEs contain specific sites that could become targets for the development of novel anti-fibrotic therapies.

Substrate-specific Modulation of a Multisubstrate Proteinase

Members of the bone morphogenetic protein-1/tolloid (BMP-1/Tld) family of metalloproteinases, also known as procollagen C-proteinases (PCPs), control multiple biological events (including matrix assembly, cross-linking, cell adhesion/migration and pattern formation) through enzymatic processing of several extracellular substrates. PCP activities on fibrillar procollagens can be stimulated by another family of extracellular proteins, PCP enhancers (PCPE-1, PCPE-2), which lack intrinsic enzymatic activity. While PCPs have multiple substrates, the extent to which PCPEs is involved in the processing of proteins other than fibrillar procollagens is unknown. In the experiments reported here, PCPE-1 was found to have no effect on the in vitro BMP-1 processing of procollagen VII, the procollagen V N-propeptide, the laminin 5 gamma2 chain, osteoglycin, prolysyl oxidase, or chordin. In contrast, PCPE-1 enhanced C-terminal processing of human fibrillar procollagen III but only when this substrate was in its native, disulfide-bonded conformation. Surprisingly, processing of procollagen III continued to be enhanced when essentially all the triple-helical region was removed. These and previous results (Ricard-Blum, S., Bernocco, S., Font, B., Moali, C., Eichenberger, D., Farjanel, J., Burchardt, E. R., van der Rest, M., Kessler, E., and Hulmes, D. J. S. (2002) J. Biol. Chem. 277, 33864-33869; Bernocco, S., Steiglitz, B. M., Svergun, D. I., Petoukhov, M. V., Ruggiero, F., Ricard-Blum, S., Ebel, C., Geourjon, C., Deleage, G., Font, B., Eichenberger, D., Greenspan, D. S., and Hulmes, D. J. S. (2003) J. Biol. Chem. 278, 7199-7205) indicate that the mechanism of PCPE-1 action involves recognition sites in both the C-propeptide domain and in the C-telopeptide region of the procollagen molecule. PCPEs therefore define a new class of extracellular adaptor proteins that stimulate proteinase activity in a substrate-specific manner, thereby providing a new target for the selective regulation of PCP activity on fibrillar procollagen substrates.

Analysis of the importance of the metallo-beta-lactamase active site loop in substrate binding and catalysis

The role of the mobile loop comprising residues 60-66 in metallo-beta-lactamases has been studied by site-directed mutagenesis, determination of kinetic parameters for six substrates and two inhibitors, pre-steady-state characterization of the interaction with chromogenic nitrocefin, and molecular modeling. The W64A mutation was performed in IMP-1 and BcII (after replacement of the BcII 60-66 peptide by that of IMP-1) and always resulted in increased K(i) and K(m) and decreased k(cat)/K(m) values, an effect reinforced by complete deletion of the loop. k(cat) values were, by contrast, much more diversely affected, indicating that the loop does not systematically favor the best relative positioning of substrate and enzyme catalytic groups. The hydrophobic nature of the ligand is also crucial to strong interactions with the loop, since imipenem was almost insensitive to loop modifications.

Interaction properties of the procollagen C-proteinase enhancer protein shed light on the mechanism of stimulation of BMP-1

Procollagen C-proteinase enhancer (PCPE) is an extracellular matrix glycoprotein that binds to the C-propeptide of procollagen I and can enhance the activities of procollagen C-proteinases up to 20-fold. To determine the molecular mechanism of PCPE activity, the interactions of the recombinant protein with the procollagen molecule as well as with its isolated C-propeptide domain were studied using surface plasmon resonance (BIAcore) technology. Binding required the presence of divalent metal cations such as calcium and manganese. By ligand blotting, calcium was found to bind to the C-propeptide domains of procollagens I and III but not to PCPE. By chemical cross-linking, the stoichiometry of the PCPE/C-propeptide interaction was found to be 1:1 in accordance with enzyme kinetic data. The use of a monoclonal antibody directed against the N-terminal region of the C-propeptide suggested that this region is probably not involved in binding to PCPE. Association and dissociation kinetics of the C-propeptide domains of procollagens I and III on immobilized PCPE were rapid. Extrapolation to saturation equilibrium yielded apparent equilibrium dissociation constants in the range 150-400 nM. In contrast, the association/dissociation kinetics of intact procollagen molecules on immobilized PCPE were relatively slow, corresponding to a dissociation constant of 1 nM. Finally, pN-collagen (i.e. procollagen devoid of the C-terminal propeptide domain) was also found to bind to immobilized PCPE, suggesting that PCPE binds to sites on either side of the procollagen cleavage site, thereby facilitating the action of procollagen C-proteinases.

Thiomandelic acid, a broad spectrum inhibitor of zinc beta-lactamases: kinetic and spectroscopic studies

Resistance to beta-lactam antibiotics mediated by metallo-beta-lactamases is an increasingly worrying clinical problem. Candidate inhibitors include mercaptocarboxylic acids, and we report studies of a simple such compound, thiomandelic acid. A series of 35 analogues were synthesized and examined as metallo-beta-lactamase inhibitors. The K(i) values (Bacillus cereus enzyme) are 0.09 microm for R-thiomandelic acid and 1.28 microm for the S-isomer. Structure-activity relationships show that the thiol is essential for activity and the carboxylate increases potency; the affinity is greatest when these groups are close together. Thioesters of thiomandelic acid are substrates for the enzyme, liberating thiomandelic acid, suggesting a starting point for the design of "pro-drugs." Importantly, thiomandelic acid is a broad spectrum inhibitor of metallo-beta-lactamases, with a submicromolar K(i) value for all nine enzymes tested, except the Aeromonas hydrophila enzyme; such a wide spectrum of activity is unprecedented. The binding of thiomandelic acid to the B. cereus enzyme was studied by NMR; the results are consistent with the idea that the inhibitor thiol binds to both zinc ions, while its carboxylate binds to Arg(91). Amide chemical shift perturbations for residues 30-40 (the beta(3)-beta(4) loop) suggest that this small inhibitor induces a movement of this loop of the kind seen for other larger inhibitors.

CENTA as a chromogenic substrate for studying beta-lactamases

CENTA, a chromogenic cephalosporin, is readily hydrolyzed by beta-lactamases of all classes except for the Aeromonas hydrophila metalloenzyme. Although it cannot practically be used for the detection of beta-lactamase-producing strains on agar plates, it should be quite useful for kinetic studies and the detection of the enzymes in crude extracts and chromatographic fractions.

Oxidations of N(omega)-hydroxyarginine analogues and various N-hydroxyguanidines by NO synthase II: key role of tetrahydrobiopterin in the reaction mechanism and substrate selectivity

Oxidations of L-arginine 2, homo-L-arginine 1, their N(omega)-hydroxy derivatives 4 and 3 (NOHA and homo-NOHA, respectively), and four N-hydroxyguanidines, N(omega)-hydroxynor-L-arginine 5 (nor-NOHA), N(omega)-hydroxydinor-L-arginine 6 (dinor-NOHA), N-(4-chlorophenyl)-N'-hydroxyguanidine (8), and N-hydroxyguanidine (7) itself, by either NOS II or (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4)-free NOS II, have been studied in a comparative manner. Recombinant BH4-free NOS II catalyzes the oxidation of all N-hydroxyguanidines by NADPH and O2, with formation of NO2(-) and NO3(-) at rates between 20 and 80 nmol min(-1) (mg of protein)(-1). In the case of compound 8, formation of the corresponding urea and cyanamide was also detected besides that of NO2(-) and NO3(-). These BH4-free NOS II-dependent reactions are inhibited by modulators of electron transfer in NOS such as thiocitrulline (TC) or imidazole (ImH), but not by Arg, and are completely suppressed by superoxide dismutase (SOD). They exhibit characteristics very similar to those previously reported for microsomal cytochrome P450-catalyzed oxidation of N-hydroxyguanidines. Both P450 and BH4-free NOS II reactions appear to be mainly performed by O2(.-) derived from the oxidase function of those heme proteins. In the presence of increasing concentrations of BH4, these nonselective oxidations progressively disappear while a much more selective monooxygenation takes place only with the N-hydroxyguanidines that are recognized well by NOS II, NOHA, homo-NOHA, and 8. These monooxygenations are much more chemoselective (8 being selectively transformed into the corresponding urea and NO) and are inhibited by Arg but not by SOD, as expected for reactions performed by the NOS Fe(II)-O2 species. Altogether, these results provide a further clear illustration of the key role of BH4 in regulating the monooxygenase/oxidase ratio in NOS. They also suggest a possible implication of NOSs in the oxidative metabolism of certain classes of xenobiotics such as N-hydroxyguanidines, not only via their monooxygenase function but also via their oxidase function.

Recognition of alpha-amino acids bearing various C=NOH functions by nitric oxide synthase and arginase involves very different structural determinants

Several alpha-amino acids bearing a C=NOH function separated from the Calpha carbon by two to five atoms have been synthesized and tested as substrates or inhibitors of recombinant nitric oxide synthases (NOS) I and II and as inhibitors of rat liver arginase (RLA). These include four N-hydroxyguanidines, N(omega)-hydroxy-L-arginine (NOHA) and its analogues homo-NOHA, nor-NOHA, and dinor-NOHA, two amidoximes bearing the -NH-C(CH(3))=NOH group, and two amidoximes bearing the -CH(2)-C(NH(2))=NOH group. Their behavior toward NOS and RLA was compared to that of the corresponding compounds bearing a C=NH function instead of the C=NOH function. The results obtained clearly show that efficient recognition of these alpha-amino acids by NOS and RLA involves very different structural determinants. NOS favors molecules bearing a -NH-C(R)=NH motif separated from Calpha by three or four CH(2) groups, such as arginine itself, with the necessary presence of delta-NH and omega-NH groups and a more variable R substituent. The corresponding molecules with a C=NOH function exhibit a much lower affinity for NOS. On the contrary, RLA best recognizes molecules bearing a C=NOH function separated from Calpha by three or four atoms, the highest affinity being observed in the case of three atoms. The presence of two omega-nitrogen atoms is important for efficient recognition, as in the two best RLA inhibitors, N(omega)-hydroxynorarginine and N(omega)-hydroxynorindospicine, which exhibit IC(50) values at the micromolar level. However, contrary to what was observed in the case of NOS, the presence of a delta-NH group is not important. These different structural requirements of NOS and RLA may be directly linked to the position of crucial residues that have been identified from crystallographic data in the active sites of both enzymes. Thus, binding of arginine analogues to NOS particularly relies on strong interactions of their delta-NH and omega-NH(2) groups with glutamate 371 (of NOS II), whereas binding of C=NOH molecules to RLA is mainly based on interactions of their terminal OH group with the binuclear Mn(II).Mn(II) cluster of the enzyme and on possible additional bonds between their omega-NH(2) group with histidine 141, glutamate 277, and one Mn(II) ion. The different modes of interaction displayed by both enzymes depend on their different catalytic functions and give interesting opportunities to design useful molecules to selectively regulate NOS and arginase.

Effects of the new arginase inhibitor N(omega)-hydroxy-nor-L-arginine on NO synthase activity in murine macrophages

In stimulated murine macrophage, arginase and nitric oxide synthase (NOS) compete for their common substrate, l-arginine. The objectives of this study were (i) to test the new alpha-amino acid N(omega)-hydroxy-nor-l-arginine (nor-NOHA) as a new selective arginase inhibitor and (ii) to elucidate the effects of arginase inhibition on l-arginine utilization by an inducible NOS. Nor-NOHA is about 40-fold more potent than N(omega)-hydroxy-l-arginine (NOHA), an intermediate in the l-arginine/NO pathway, to inhibit the hydrolysis of l-arginine to l-ornithine catalyzed by unstimulated murine macrophages (IC(50) values 12 +/- 5 and 400 +/- 50 microM, respectively). Stimulation of murine macrophages with interferon-gamma and lipopolysaccharide (IFN-gamma + LPS) results in clear expression of an inducible NOS (iNOS) and to an increase in arginase activity. Nor-NOHA is also a potent inhibitor of arginase in IFN-gamma + LPS-stimulated macrophage (IC(50) value 10 +/- 3 microM). In contrast to NOHA, nor-NOHA is neither a substrate nor an inhibitor for iNOS and it appears as a useful tool to study the interplays between arginase and NOS. Inhibition of arginase by nor-NOHA increases nitrite and l-citrulline accumulation for incubation times higher than 12 h, under our conditions. Our results allow the determination of the kinetic parameters of the two competitive pathways and the proposal of a simple model which readily explains the differences observed between experiments. This model readily accounts for the observed effects and should be useful to predict the consequences of arginase inhibition in the presence of an active NOS on l-arginine availability.

Nitric oxide biosynthesis, nitric oxide synthase inhibitors and arginase competition for L-arginine utilization

Nitric oxide (NO) is a recently discovered mediator produced by mammalian cells. It plays a key role in neurotransmission, control of blood pressure, and cellular defense mechanisms. Nitric oxide synthases (NOSs) catalyze the oxidation of L-arginine to NO and L-citrulline. NOSs are unique enzymes in that they possess on the same polypeptidic chain a reductase domain and an oxygenase domain closely related to cytochrome P450s. NO and superoxide formation as well as NOS stability are finely regulated by Ca2+/calmodulin interactions, by the cofactor tetrahydrobiopterin, and by substrate availability. Strong interactions between the L-arginine-metabolizing enzymes are clearly demonstrated by competition between NOSs and arginases for L-arginine utilization, and by potent inhibition of arginase activity by N(omega)-hydroxy-L-arginine, an intermediate in the L-arginine to NO pathway.

Detection of a nitric oxide synthase possibly involved in the regulation of the Rhodococcus sp R312 nitrile hydratase

Crude homogenates from Rhodococcus sp 312 catalyze the conversion of L-arginine into L-citrulline and NO2-, the usual oxidation product of NO under aerobic conditions. They also catalyze the conversion of N omega-hydroxy-L-arginine (NOHA) into L-citrulline and NO2- with similar rates (10-15 and 100-150 nmol of product.min-1.(mg of protein)-1 respectively for the crude homogenate and for a fraction obtained from ammonium sulfate precipitation). L-citrulline formation is strongly inhibited by classical inhibitors of mammalian nitric oxide synthases (NOSs) such as N omega-methyl-L-arginine (NMA) and thio-L-citrulline (TC). Finally, the lack of inhibitory effects of EGTA, a classical inhibitor of constitutive mammalian NOSs, and the specific immunodetection of a 100 kD protein from Rhodococcus cytosol by an antibody raised against human inducible NOS, is in favor of the presence of a NOS similar to inducible mammalian NOSs in Rhodococcus sp 312. This NOS should be responsible for the NO-dependent inactivation of Rhodococcus Nitrile Hydratase (NHase) in the absence of light; it could regulate the activity of the latter enzyme.

Substrate specificity of NO synthases: detailed comparison of L-arginine, homo-L-arginine, their N omega-hydroxy derivatives, and N omega-hydroxynor-L-arginine

A detailed comparison of the oxidation of five compounds closely related to L-arginine (Arg) by purified recombinant neuronal and macrophage NO synthases (NOS I and NOS II) was performed. Homo-L-arginine (homo-Arg) is oxidized by both NOSs in the presence of NADPH with major formation of NO and homo-L-citrulline, with a molar ratio of close to 1, and minor formation of N omega-hydroxyhomo-L-arginine (homo-NOHA). Oxidation of homo-NOHA by the two NOSs also leads to NO and homocitrulline in a 1:1 molar ratio. On the contrary, N omega-hydroxynor-L-arginine (nor-NOHA) is a very poor substrate of NOS I and II, which fails to produce significant amounts of nitrite. The catalytic efficiency of both NOSs markedly decreases in the order Arg > NOHA > homo-Arg > homo-NOHA, as shown by the 20- and 10-fold decrease of kcat/Km observed for NOS I and NOS II, respectively, when comparing Arg to homo-NOHA. The greater loss of catalytic efficiency for homo-Arg, when compared to that for Arg, appears to occur at the first step (N-hydroxylation) of the reaction. In that regard, it is noteworthy that the Vm values for NOHA and homo-NOHA oxidation are very similar (about 1 and 2 micromol of NO min-1 mg of protein-1 for NOS I and II, respectively). In fact, lengthening of the Arg chain by one CH2 leads not only to markedly decreased kcat/Km but also to clear disturbances in NOS functioning. This is shown by a greater accumulation of the N omega-hydroxyguanidine intermediate (homo-NOHA:homocitrulline ratio between 0.2 and 0.4) and an increased consumption of NADPH for NO formation (between 2.0 and 2.6 mol of NADPH consumed for the formation of 1 mol of NO in the case of homo-Arg, instead of 1.5 mol in the case of Arg). Most of the above results could be interpreted by comparing the possible positionings of the various substrates relative to the two NOS active oxygen species which are believed to be responsible for the two steps of the reaction.

Highly efficient control of iron-containing nitrile hydratases by stoichiometric amounts of nitric oxide and light

The reaction of two iron-containing nitrile hydratases (NHase) with NO has been studied: NHase from Rhodococcus sp. R312, which is probably similar to the photosensitive N771 NHase, and the new NHase from Comamonas testosteroni NI1 whose aminoacid sequence is quite different from those of BR312 and N771 NHases. Both enzymes are equally inactivated after addition of stoichiometric amounts of NO added as an anaerobic solution or produced in situ under physiological conditions by a rat brain NO-synthase. Both enzymes are reactivated by photoirradiation, and two cycles of NO inactivation/photoactivation can be performed without significant loss of activity. Both iron-containing NHases have a high affinity for NO, similar to that of methemoglobin.