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

Lysyl Oxidases as Targets for Cancer Therapy and Diagnostic Imaging

The understanding of the contribution of the tumour microenvironment to cancer progression and metastasis, in particular the interplay between tumour cells, fibroblasts and the extracellular matrix has grown tremendously over the last years. Lysyl oxidases are increasingly recognised as key players in this context, in addition to their function as drivers of fibrotic diseases. These insights have considerably stimulated drug discovery efforts towards lysyl oxidases as targets over the last decade. This review article summarises the biochemical and structural properties of theses enzymes. Their involvement in tumour progression and metastasis is highlighted from a biochemical point of view, taking into consideration both the extracellular and intracellular action of lysyl oxidases. More recently reported inhibitor compounds are discussed with an emphasis on their discovery, structure-activity relationships and the results of their biological characterisation. Molecular probes developed for imaging of lysyl oxidase activity are reviewed from the perspective of their detection principles, performance and biomedical applications.

Semicarbazide-Sensitive Amine Oxidase (SSAO) and Lysyl Oxidase (LOX) Association in Rat Aortic Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) are the main stromal cells in the medial layer of the vascular wall. These cells produce the extracellular matrix (ECM) and are involved in many pathological changes in the vascular wall. Semicarbazide-sensitive amine oxidase (SSAO) and lysyl oxidase (LOX) are vascular enzymes associated with the development of atherosclerosis. In the vascular smooth muscle cells, increased SSAO activity elevates reactive oxygen species (ROS) and induces VSMCs death; increased LOX induces chemotaxis through hydrogen peroxide dependent mechanisms; and decreased LOX contributes to endothelial dysfunction. This study investigates the relationship between SSAO and LOX in VSMCs by studying their activity, protein, and mRNA levels during VSMCs passaging and after silencing the LOX gene, while using their respective substrates and inhibitors. At the basal level, LOX activity decreased with passage and its protein expression was maintained between passages. βAPN abolished LOX activity (** p < 0.01 for 8 vs. 3 and * p < 0.05 for 5 vs. 8) and had no effect on LOX protein and mRNA levels. MDL72527 reduced LOX activity at passage 3 and 5 (## p < 0.01) and had no effect on LOX protein, and mRNA expression. At the basal level, SSAO activity also decreased with passage, and its protein expression was maintained between passages. MDL72527 abolished SSAO activity (**** p < 0.0001 for 8 vs. 3 and * p < 0.05 for 5 vs. 8), VAP-1 expression at passage 5 (** p < 0.01) and 8 (**** p < 0.0001), and Aoc3 mRNA levels at passage 8 (* p < 0.05). βAPN inhibited SSAO activity (**** p < 0.0001 for 5 vs. 3 and 8 vs. 3 and * p < 0.05 for 5 vs. 8), VAP-1 expression at passage 3 (* p < 0.05), and Aoc3 mRNA levels at passage 3 (* p < 0.05). Knockdown of the LOX gene (**** p < 0.0001 for Si6 vs. Sictrl and *** p < 0.001 for Si8 vs. Sictrl) and LOX protein (** p < 0.01 for Si6 and Si8 vs. Sictrl) in VSMCs at passage 3 resulted in a reduction in Aoc3 mRNA (#### p < 0.0001 for Si6 vs. Sictrl and ### p < 0.001 for Si8 vs. Sictrl) and VAP-1 protein (# p < 0.05 for Si8 vs. Sictrl). These novel findings demonstrate a passage dependent decrease in LOX activity and increase in SSAO activity in rat aortic VSMCs and show an association between both enzymes in early passage rat aortic VSMCs, where LOX was identified as a regulator of SSAO activity, protein, and mRNA expression.

Reaction mechanism of lysyl oxidase-like 2 (LOXL2) studied by computational methods

Lysyl oxidase-like 2 (LOXL2) is a copper-dependent amine oxidase that catalyzes the oxidative deamination of the ε-amino group of lysines/hydroxylysines on substrate proteins (collagen and elastin) to form aldehyde groups. The generated aldehyde groups are of significance in crosslinking with the adjacent aldehyde or ε-amino group on proteins in extracellular matrix. In this paper, we have studied the reaction mechanism of LOXL2 by means of quantum mechanics (QM) and combined QM and molecular mechanics (QM/MM) methods. This study is divided into two parts, i.e. the biosynthesis of lysine tyrosylquinone (LTQ) cofactor and oxidative deamination of ε-amino group of lysine by LTQ. For the former part, the reaction is driven by a large exothermicity of about 284 kJ/mol. Dopaquinone radical (DPQr) is suggested to be an intermediate state in this reaction. In addition, His652 residue is predicted to serve as proton acceptor. The rate-determining step for the biosynthesis of LTQ is found to be hydrogen-atom abstraction from the benzene ring on substrate by Cu²⁺-hydroxide, which is a proton-coupled electron transfer (PCET) process with an energy barrier of 84 kJ/mol. For the latter part, the reaction is exothermic by about 145 kJ/mol, and the copper ion is proposed to play a role of redox catalyst in the last step to generate the product of aldehyde. However, the copper ion might not be indispensable for the latter part, which is consistent with the previous study.

Moving Through Barriers in Science and Life

This first serious attempt at an autobiographical accounting has forced me to sit still long enough to compile my thoughts about a long personal and scientific journey. I especially hope that my trajectory will be of interest and perhaps beneficial to much younger women who are just getting started in their careers. To paraphrase from Virginia Woolf's writings in A Room of One's Own at the beginning of the 20th century, "for most of history Anonymous was a Woman." However, Ms. Woolf is also quoted as saying "nothing has really happened until it has been described," a harbinger of the enormous historical changes that were about to be enacted and recorded by women in the sciences and other disciplines. The progress in my chosen field of study-the chemical basis of enzyme action-has also been remarkable, from the first description of an enzyme's 3D structure to a growing and deep understanding of the origins of enzyme catalysis.

Targeting the lysyl oxidases in tumour desmoplasia

The extracellular matrix (ECM) is a fundamental component of tissue microenvironments and its dysregulation has been implicated in a number of diseases, in particular cancer. Tumour desmoplasia (fibrosis) accompanies the progression of many solid cancers, and is also often induced as a result of many frontline chemotherapies. This has recently led to an increased interest in targeting the underlying processes. The major structural components of the ECM contributing to desmoplasia are the fibrillar collagens, whose key assembly mechanism is the enzymatic stabilisation of procollagen monomers by the lysyl oxidases. The lysyl oxidase family of copper-dependent amine oxidase enzymes are required for covalent cross-linking of collagen (as well as elastin) molecules into the mature ECM. This key step in the assembly of collagens is of particular interest in the cancer field since it is essential to the tumour desmoplastic response. LOX family members are dysregulated in many cancers and consequently the development of small molecule inhibitors targeting their enzymatic activity has been initiated by many groups. Development of specific small molecule inhibitors however has been hindered by the lack of crystal structures of the active sites, and therefore alternate indirect approaches to target LOX have also been explored. In this review, we introduce the importance of, and assembly steps of the ECM in the tumour desmoplastic response focussing on the role of the lysyl oxidases. We also discuss recent progress in targeting this family of enzymes as a potential therapeutic approach.

MftD Catalyzes the Formation of a Biologically Active Redox Center in the Biosynthesis of the Ribosomally Synthesized and Post-translationally Modified Redox Cofactor Mycofactocin

Mycofactocin (MFT) is a putative ribosomally synthesized and post-translationally modified (RiPP) redox cofactor. The biosynthesis of MFT is encoded by the gene cluster mftABCDEF. While processing of the precursor peptide by MftB, MftC, and MftE has been shown to result in the formation of the small molecule 3-amino-5-[(p-hydroxyphenyl)methyl]-4,4-dimethyl-2-pyrrolidinone (AHDP), no activity has been shown for the putative dehydrogenase MftD and the putative glycosyltransferase MftF. In addition, evidence demonstrating that MFT is a redox cofactor has only been limited to the requirement of mft genes for ethanol assimilation in Mycobacterium smegmatis mc²155. Here, we demonstrate that MftD catalyzes the oxidative deamination of AHDP, forming an α-keto moiety on the resulting molecule, which we call pre-mycofactocin (PMFT). We characterize PMFT by 1D and 2D NMR spectroscopy techniques and by high-resolution mass spectrometry data to solve its structure. We further characterized PMFT by cyclic voltammetry and found its midpoint potential to be ∼255 mV. Lastly, we demonstrate that PMFT is a biologically active redox cofactor that oxidizes NADH bound by M. smegmatis carveol dehydrogenase (MsCDH) and can be used by MsCDH in the oxidation of carveol. These data demonstrate for the first time that PMFT functions as a biologically active redox mediator and provides the most direct evidence to date that MFT is a RiPP-derived redox cofactor.

Recombinant human diamine oxidase activity is not inhibited by ethanol, acetaldehyde, disulfiram, diethyldithiocarbamate or cyanamide

Human diamine oxidase (hDAO, EC 1.4.3.22) is the key enzyme in the degradation of extracellular histamine. Consumption of alcohol is a known trigger of mast cell degranulation in patients with mast cell activation syndrome. Ethanol may also interfere with enzymatic histamine degradation, but reports on the effects on DAO activity are controversial. There are also conflicting reports whether disulfiram, an FDA-approved agent in the treatment of alcohol dependence, inhibits DAO. We therefore investigated the inhibitory potential of ethanol and disulfiram and their metabolites on recombinant human DAO (rhDAO) in three different assay systems. Relevant concentrations of ethanol, acetaldehyde, and acetate did not inhibit rhDAO activity in an in vitro assay system using horseradish peroxidase (HRP) -mediated luminol oxidation. The aldehyde dehydrogenase (ALDH; EC 1.2.1.3) inhibitors cyanamide and its dimer dicyanamide also had no effect on DAO activity. In one assay system, the irreversible ALDH inhibitor disulfiram and its main metabolite diethyldithiocarbamate seemed to inhibit DAO activity. However, the decreased product formation was not due to a direct block of DAO activity but resulted from inhibition of peroxidase employed in the coupled system. Our in vitro data do not support a direct blocking effect of ethanol, disulfiram, and their metabolites on DAO activity in vivo.

Lysyl Oxidase Isoforms and Potential Therapeutic Opportunities for Fibrosis and Cancer

INTRODUCTION

The lysyl oxidase family of enzymes is classically known as being required for connective tissue maturation by oxidizing lysine residues in elastin and lysine and hydroxylysine residues in collagen precursors. The resulting aldehydes then participate in cross-link formation, which is required for normal connective tissue integrity. These enzymes have biological functions that extend beyond this fundamental biosynthetic role, with contributions to angiogenesis, cell proliferation, and cell differentiation. Dysregulation of lysyl oxidases occurs in multiple pathologies including fibrosis, primary and metastatic cancers, and complications of diabetes in a variety of tissues.

AREAS COVERED

This review summarizes the major findings of novel roles for lysyl oxidases in pathologies, and highlights some of the potential therapeutic approaches that are in development and which stem from these new findings.

EXPERT OPINION

Fundamental questions remain regarding the mechanisms of novel biological functions of this family of proteins, and regarding functions that are independent of their catalytic enzyme activity. However, progress is underway in the development of isoform-specific pharmacologic inhibitors, potential therapeutic antibodies and gaining an increased understanding of both tumor suppressor and metastasis promotion activities. Ultimately, this is likely to lead to novel therapeutic agents.

Areca nut and its role in oral submucous fibrosis

Areca nut, commonly called as betel nut or supari, is a fruit of areca catechu palm tree, which is native of South Asia and Pacific Islands. The seed or endosperm is consumed fresh, boiled or after sun drying or curing. Chewing areca nut is thought to have central nervous system stimulating effect and along with this it is known to have salivary stimulating and digestive properties. According to the traditional Ayurvedic medicine, chewing areca nut and betel leaf is a good remedy against halitosis. It is also used for its deworming property. Along with these beneficial effects of areca nut one of its most harmful effects on the human body in general and oral cavity in particular is the development of potentially malignant disorder called Oral Submucous Fibrosis. The present paper discusses in detail the effects of the components of areca nut on pathogenesis of Oral Submucous Fibrosis. Key words:Areca nut, oral submucous fibrosis, potentially malignant disorder, supari.

Human lysyl oxidase-like 2

Lysyl oxidase like-2 (LOXL2) belongs to the lysyl oxidase (LOX) family, which comprises Cu(2+)- and lysine tyrosylquinone (LTQ)-dependent amine oxidases. LOXL2 is proposed to function similarly to LOX in the extracellular matrix (ECM) by promoting crosslinking of collagen and elastin. LOXL2 has also been proposed to regulate extracellular and intracellular cell signaling pathways. Dysregulation of LOXL2 has been linked to many diseases, including cancer, pro-oncogenic angiogenesis, fibrosis and heart diseases. In this review, we will give an overview of the current understandings and hypotheses regarding the molecular functions of LOXL2.

Peptides derived from the copper-binding region of lysyl oxidase exhibit antiangiogeneic properties by inhibiting enzyme activity: an in vitro study

Despite the rigorous research on abnormal angiogenesis, there is a persistent need for the development of new and efficient therapies against angiogenesis-related diseases. The role of Lysyl oxidase (LOX) in angiogenesis and cancer has been established in prior studies. Copper is known to induce the synthesis of LOX, and hence regulates its activity. Hypoxia-induced metastasis is dependent on LOX expression and activity. It has been believed that the inhibition of LOX would be a therapeutic strategy to inhibit angiogenesis. To explore this, we designed peptides (M peptides) from the copper-binding region of LOX and hypothesized them to modulate LOX. The peptides were characterized, and their copper-binding ability was confirmed by mass spectrometry. The M peptides were found to reduce the levels of intracellular copper when the cells were co-treated with copper. The peptides showed promising effect on aortic LOX, recombinant human LOX and LOX produced by human umbilical vein endothelial cells (HUVECs). The study also explores the effect of these peptides on copper and hypoxia-stimulated angiogenic response in HUVECs. It was found that the M peptides inhibited copper/hypoxia-induced LOX activity and inhibited stimulated HUVEC tube formation and migration. This clearly indicated the potential of M peptides in inhibiting angiogenesis, highlighting their role in the formulation of drugs for the same.

MCF-7 cells expressing nuclear associated lysyl oxidase-like 2 (LOXL2) exhibit an epithelial-to-mesenchymal transition (EMT) phenotype and are highly invasive in vitro

LOXL2 is a copper- and lysine tyrosylquinone-dependent amine oxidase that has been proposed to function both extracellularly and intracellularly to activate oncogenic signaling pathways leading to EMT and invasion of breast cancer cells. In this study, we selected MCF-7 cells that stably express forms of recombinant LOXL2 differing in their subcellular localizations and catalytic competencies. This enabled us to dissect the molecular functions of intracellular and extracellular LOXL2s and examine their contributions to breast cancer metastasis/invasion. We discovered that secreted LOXL2 (~100-kDa) is N-glycosylated at Asn-455 and Asn-644, whereas intracellular LOXL2 (~75-kDa) is nonglycosylated and N-terminally processed, and is primarily associated with the nucleus. Both forms of LOXL2 can oxidize lysine in solution. However, we found that expression of intracellular LOXL2 is more strongly associated with EMT and invasiveness than secreted LOXL2 in vitro. The results indicate that nuclear associated LOXL2 contributes to the stabilization of Snail1 transcription factor at the protein level to induce EMT and promote invasion in vitro, through repression of E-cadherin, occludin, and estrogen receptor-α, and up-regulation of vimentin, fibronectin, and MT1-MMP.

Purification of high yields of catalytically active lysyl oxidase directly from Escherichia coli cell culture

Lysyl oxidase is a highly insoluble enzyme requiring high concentrations of urea to solubilize. A method to obtain lysyl oxidase in high yields directly from an Escherichia coli culture without the need for refolding of inclusion bodies has been developed using nutrient rich media. pET21b was used to overexpress the lysyl oxidase enzyme and to introduce a C-terminal 6X histidine tag for purification. Lysyl oxidase yields of 10 mg of active and properly folded enzyme per liter of media have been obtained. Purification was achieved via affinity chromatography using a Ni-NTA column. Copper content was found to be 19%. LTQ cofactor formation in LOX is a self-processing event in the presence of copper. LTQ content was determined to be 24% based on reaction with phenylhydrazine to form a phenylhydrazone adduct. Quantification of this adduct was attained using the previously reported extinction coefficient of 15.4 mM(-1)cm(-1). LTQ presence was also verified by redox cycling. Specific enzymatic activity was measured to be 0.31 U/mg, one of the highest activities reported.

LOXL1 deficiency negatively impacts the biomechanical properties of the mouse vagina and supportive tissues

Mice deficient in lysyl oxidase-like1 protein (LOXL1(-/-)) develop pelvic organ prolapse (POP). We sought to determine the impact of LOXL1(-/-) on the biomechanical properties of the vagina and its supportive tissues tested as a complex. Tissues of nulliparous LOXL1(-/-) and age-matched wild type (WT) mice were tested to failure to obtain load-distension curves. Data were compared utilizing one-way analysis of variance and appropriate post hoc tests. The groups demonstrated different biomechanical behavior, with LOXL1(-/-) animals displaying a 31% decrease in ultimate load at failure (p=0.001). Experimental disruption of specific levels of support in WT mice failed to generate load-distension curves similar to the LOXL1(-/-) mice indicating a global instead of a site-specific tissue defect. The decrease in the ultimate load at failure in the LOXL1(-/-) mice suggests mechanically weaker tissues. LOXL1 mutation results in a global defect in connective tissues and correlates with altered biomechanical behavior of the vagina and supportive tissues.

Trapping of a dopaquinone intermediate in the TPQ cofactor biogenesis in a copper-containing amine oxidase from Arthrobacter globiformis

The biogenesis of the topaquinone (TPQ) cofactor of copper amine oxidase (CAO) is self-catalyzed and requires copper and molecular oxygen. A dopaquinone intermediate has been proposed to undergo 1,4-addition of a copper-associated water molecule to form the reduced form of TPQ (TPQ(red)), followed by facile oxidation by O(2) to yield the mature TPQ (TPQ(ox)). In this study, we have incorporated a lysine residue in the active site of Arthrobacter globiformis CAO (AGAO) by site-directed mutagenesis to produce D298K-AGAO. The X-ray crystal structure of D298K-AGAO at 1.7-A resolution revealed that a covalent linkage formed between the epsilon-amino side chain of Lys298 and the C2 position of a dopaquinone derived from Tyr382, a precursor to TPQ(ox). We assigned the species as an iminoquinone tautomer (LTI) of lysine tyrosylquinone (LTQ), the organic cofactor of lysyl oxidase (LOX). The time course of the formation of LTI at pH 6.8 was followed by UV/vis and resonance Raman spectroscopies. In the early phase of the reaction, an LTQ-like intermediate was observed. This intermediate then slowly converted to LTI in an isosbestic manner. Not only is the presence of a dopaquinone intermediate in the TPQ biogenesis confirmed, but it also provides strong support for the proposed intermediacy of a dopaquinone in the biogenesis of LTQ in LOX. Further, this study indicates that the dopaquinone intermediate in AGAO is mobile and can swing from the copper site into the active-site wedge to react with Lys298.

Chemical genetics suggests a critical role for lysyl oxidase in zebrafish notochord morphogenesis

As a result of a chemical genetic screen for modulators of metalloprotease activity, we report that 2-mercaptopyridine-N-oxide induces a conspicuous undulating notochord defect in zebrafish embryos, a phenocopy of the leviathan mutant. The location of the chemically-induced wavy notochord correlated with the timing of application, thus defining a narrow chemical sensitivity window during segmentation stages. Microscopic observations revealed that notochord undulations appeared during the phase of notochord cell vacuolation and notochord elongation. Notochord cells become swollen as well as disorganized, while electron microscopy revealed disrupted organization of collagen fibrils in the surrounding sheath. We demonstrate by assay in zebrafish extracts that 2-mercaptopyridine-N-oxide inhibits lysyl oxidase. Thus, we provide insight into notochord morphogenesis and reveal novel compounds for lysyl oxidase inhibition. Taken together, these data underline the utility of small molecules for elucidating the dynamic mechanisms of early morphogenesis and provide a potential explanation for the recently established role of copper in zebrafish notochord formation.

A novel type of lysine oxidase: L-lysine-epsilon-oxidase

The melanogenic marine bacterium M. mediterranea synthesizes marinocine, a protein with antibacterial activity. We cloned the gene coding for this protein and named it lodA [P. Lucas-Elío, P. Hernández, A. Sanchez-Amat, F. Solano, Purification and partial characterization of marinocine, a new broad-spectrum antibacterial protein produced by Marinomonas mediterranea. Biochim. Biophys. Acta 1721 (2005) 193-203; P. Lucas-Elío, D. Gómez, F. Solano, A. Sanchez-Amat, The antimicrobial activity of marinocine, synthesized by M. mediterranea, is due to the hydrogen peroxide generated by its lysine oxidase activity. J. Bacteriol. 188 (2006) 2493-2501]. Now, we show that this protein is a new type of lysine oxidase which catalyzes the oxidative deamination of free L-lysine into 6-semialdehyde 2-aminoadipic acid, ammonia and hydrogen peroxide. This new enzyme is compared to other enzymes related to lysine transformation. Two different groups have been used for comparison. Enzymes in the first group lead to 2-aminoadipic acid as a final product. The second one would be enzymes catalyzing the oxidative deamination of lysine releasing H2O2, namely lysine-alpha-oxidase (LalphaO) and lysyl oxidase (Lox). Kinetic properties, substrate specificity and inhibition pattern show clear differences with all above mentioned lysine-related enzymes. Thus, we propose to rename this enzyme lysine-epsilon-oxidase (lod for the gene) instead of marinocine. Lod shows high stereospecificity for free L-lysine, it is inhibited by substrate analogues, such as cadaverine and 6-aminocaproic acid, and also by beta-aminopropionitrile, suggesting the existence of a tyrosine-derived quinone cofactor at its active site.

Molecular pathogenesis of oral submucous fibrosis - a collagen metabolic disorder

Oral submucous fibrosis (OSF) is a chronic debilitating disease and a premalignant condition of the oral cavity. It is characterized by a generalized submucosal fibrosis. The pathogenesis of the disease is not well established. Epidemiological evidences strongly indicate the association of the betel quid (BQ) habit and OSF. Various findings indicate the disease to be a consequence of disturbances in the homeostatic equilibrium between synthesis and degradation of extracellular matrix (ECM), wherein collagen forms a major component, thus can be considered as a collagen-metabolic disorder. Transforming growth factor-beta (TGF-beta) is a potent stimulator of production and deposition of the ECM. The objectives of this review are to highlight the molecular events involved in the overproduction of insoluble collagen and decreased degradation of collagen occurring via exposure to BQ and stimulation of the TGF-beta pathway, and elucidate the cell signaling that is involved in the etiopathogenesis of the disease process.

Mechanism of post-translational quinone formation in copper amine oxidases and its relationship to the catalytic turnover

Copper amine oxidases (CAOs) post-translationally construct a redox-active quinone from an amino acid side chain in their polypeptide chain. As such, these enzymes illustrate how nature is able to expand upon naturally-occurring side chains to create new, catalytically powerful functionalities. The active sites of the CAOs are highly unusual in their ability to catalyze two very different reactions: single-turnover, oxygen-dependent quinone formation, followed by catalytic oxidation (formally dehydrogenation) of amines. This review summarizes our current understanding of the pathway whereby the 2,4,5-trihydroxyphenylalanyl quinone (TPQ) cofactor is generated from the phenolic side chain of tyrosine. This reaction occurs spontaneously intermediates in the presence of O(2) and active site bound Cu(II), without the assistance of other proteins or cofactors. Ongoing work has focused on uncovering the details of the TPQ formation mechanism. A larger goal is to understand how a single active site is capable of supporting both quinone formation and subsequent catalytic turnover.

Metavanadate causes cellular accumulation of copper and decreased lysyl oxidase activity

Selected indices of copper metabolism in weanling rats and fibroblast cultures were progressively altered in response to increased levels of sodium metavanadate. In diets, vanadium was added in amounts ranging from 0 to 80 microg V/g of diet, that is, 0-1.6 micromol V/g of diet. In fibroblast cultures, vanadium ranged from 0 to 400 nmol V/ml. The inhibition of P-ATPase-7A activity by metavanadate, important to copper egress from cells, was a primary focus. In skin, and tendon, the copper concentration was increased in response to increased dietary levels of metavanadate, whereas lysyl oxidase activity, a secreted cuproprotein, was reduced. The reduction in lysyl oxidase activity was also accompanied by reduced redox cycling potential of isolated fractions of lysyl oxidase, presumably due to reduced lysyltyrosyl quinone (LTQ) formation at the active site of lysyl oxidase. In contrast, liver copper concentrations and plasma ceruloplasmin activity were not affected by metavanadate exposure. However, semicarbazide-sensitive benzylamine oxidase (SCBO) activity, which was taken as an indirect measure of vascular adhesive protein-1 (VAP-1), was increased. In cultured fibroblasts, cellular copper was also increased and lysyl oxidase decreased in response to metavanadate. Moreover, the steady-state levels of atp7a and lysyl oxidase mRNAs were not affected by addition of metavanadate to culture medium up to 200 nmol/ml. Taken together, these data suggest that pathways involving copper egress and lysyl oxidase activation are particularly sensitive to metavanadate exposure through processes that are predominately posttranslational.

Evaluation of dose-response curve analysis in delineating shared or different molecular sites of action for osteolathyrogens

Single-chemical and mixture concentration-response curves generated using a frog embryo model were examined for value in assessing whether chemicals exert toxic effects at the same or at different molecular sites of action. Toxicity tests were conducted on a series of osteolathyrogens, i.e. chemicals that inhibit cross-linking of developing connective tissue fibers. Induction of osteolathyrism, which manifests as lesions in the notochord of exposed tadpoles, has several possible molecular sites of action, including agent-cofactor reactivity during the enzyme-mediated cross-linking process. UV-VIS spectrophotometry of osteolathyrogen-cofactor reactivity (i.e. in vitro analysis) was coupled with the 96-h frog embryo mixture toxicity assay (i.e. in vivo toxicity) to compare molecular sites of action for several osteolathyrogens with the combined osteolathyritic effects of the agents. Single-chemical concentration-response curves were used to calculate theoretical curves for the dose-addition model of combined effect. Slope and EC(50) values for both theoretical and experimental mixture curves were then generated to statistically examine the hypothesis that agents with shared sites of action have dose-response curve (DRC) slopes that are similar when given alone and in combination, and slope and EC(50) values that, when administered together, are consistent with those calculated for dose-addition. For combinations of cofactor-binding agents (semicarbazide, thiosemicarbazide, aminoacetonitrile), slope values were generally similar with additivity quotients near 1.0 (1.0=dose-additive) and combined osteolathyritic effects that were consistent with dose-addition. None of these were true for combinations that included agents that did not show rapid cofactor binding (β-aminopropionitrile, methyleneaminoacetonitrile). The results suggest that DRC analysis could be a useful tool for delineating common or different molecular sites of toxic action and that the approaches used warrant further study for evaluating the mechanistic basis for combined effects of toxicants.

Tyrosine-derived quinone cofactors

Copper amine oxidases (CAOs) and lysyl oxidase (LOX) both contain Cu(2+) and quinone cofactors that are derived from a tyrosine residue in the active site. In CAOs, the cofactor is 2,4,5-trihydroxyphenylalanine quinone (TPQ), and in LOX it is lysine tyrosyl quinone (LTQ). The mechanism of oxidative deamination by CAOs is well understood, but there is a controversy surrounding the role of Cu(2+) in cofactor reoxidation. The chemistry of LTQ in LOX, by contrast, has not been as extensively studied. This Account discusses the strategies that CAOs have evolved to control the mobility of TPQ to optimize activity. In addition, some recent studies on CAOs whose active-site Cu(2+) has been replaced with Co(2+) or Ni(2+) are summarized. Finally, there is a discussion on the properties of a model compound of LTQ and their relevance to the chemistry of LOX.

Expression and Purification of Enzymatically Active Forms of the Human Lysyl Oxidase-like Protein 4

The lysyl oxidase-like protein 4 (LOXL4) is the latest member of the emerging family of lysyl oxidases, several of which were shown to function as copper-dependent amine oxidases catalyzing lysine-derived cross-links in extracellular matrix proteins. LOXL4 contains four scavenger receptor cysteine-rich domains in addition to the characteristic domains of the LOX family, including the copper-binding domain, the cytokine receptor-like domain, and the residues of the lysyl-tyrosyl quinone cofactor. In an effort to assess its amine oxidase activity, we expressed LOXL4 as recombinant forms attached with hexa-histidine residues at the carboxyl terminus by using an Escherichia coli expression system. The recombinant proteins were purified with nickel-chelating affinity chromatography and converted into enzymatically active forms by stepwise dialysis. The purified LOXL4 proteins showed beta-aminopropionitrile-inhibitable activity of 0.022-0.032 units/mg toward a nonpeptidyl substrate, benzylamine. These results indicate that LOXL4, with the four scavenger receptor cysteine rich domains, may also function as an active amine oxidase. Availability of the pure and active forms of LOXL4 will be significantly helpful in functional studies related to substrate specificity and crystal structure of this amine oxidase, which should provide significant insights into functional differences within the LOX family members.

Trace elements in human physiology and pathology. Copper

Copper is a trace element, important for the function of many cellular enzymes. Copper ions can adopt distinct redox states oxidized Cu(II) or reduced (I), allowing the metal to play a pivotal role in cell physiology as a catalytic cofactor in the redox chemistry of enzymes, mitochondrial respiration, iron absorption, free radical scavenging and elastin cross-linking. If present in excess, free copper ions can cause damage to cellular components and a delicate balance between the uptake and efflux of copper ions determines the amount of cellular copper. In biological systems, copper homeostasis has been characterized at the molecular level. It is coordinated by several proteins such as glutathione, metallothionein, Cu-transporting P-type ATPases, Menkes and Wilson proteins and by cytoplasmic transport proteins called copper chaperones to ensure that it is delivered to specific subcellular compartments and thereby to copper-requiring proteins.

Purification of enzymatically active human lysyl oxidase and lysyl oxidase-like protein from Escherichia coli inclusion bodies

Lysyl oxidase (LOX) is an extracellular copper dependent enzyme catalyzing lysine-derived cross-links in extracellular matrix proteins. Recent molecular cloning has revealed the existence of a LOX family consisting of LOX and four lysyl oxidase-like proteins (LOXLs; LOXL, LOXL2, LOXL3, and LOXL4). Each member of the LOX family contains a copper-binding domain, residues for lysyl-tyrosyl quinone, and a cytokine receptor-like domain. Very recently, novel functions, such as tumor suppression, cellular senescence, and chemotaxis, have been attributed to this family of amine oxidases, but functional differences among the family members have yet to be determined. For efficient expression and purification, we cloned the cDNAs corresponding to proteolytically processed forms of LOX (LOX-p) and LOXL (LOXL-p1 and LOXL-p2) into a bacterial expression vector pET21a with six continuous histidine codons attached to the 3' of the gene. The recombinant proteins were purified with nickel-chelating affinity chromatography and converted into enzymatically active forms by stepwise dialysis in the presence of N-lauroylsarcosinate and Cu2+. The purified LOX-p, LOXL-p1, and LOXL-p2 proteins showed specific amine oxidase activity of 0.097, 0.054, and 0.150 U/mg, respectively, which was inhibited by beta-aminopropionitrile (BAPN), a specific inhibitor of LOX. Availability of these pure and active forms of LOX and LOXLs will be significantly helpful in functional studies related to substrate specificity and crystal structures of this family of amine oxidases.

The multi-functional topa-quinone copper amine oxidases

The mature copper amine oxidases (CAOs) contain a tyrosine-derived 2,4,5-trihydroxyphenylalanyl quinone (topa quinone or TPQ) and a cupric ion in close proximity. Through a combination of structural, spectroscopic and kinetic analyses, a chemical mechanism for the self-processing of an active site tyrosine to TPQ has been proposed. Once formed, TPQ acts as a switch between the heterolytic transformation of amine substrates to aldehydes, via a pyridoxal phosphate-like Schiff base complex, and one electron chemistry involving reduction of molecular oxygen. The relationship between the biogenetic and catalytic processes is discussed.

Lysyl oxidase: properties, specificity, and biological roles inside and outside of the cell

Lysyl oxidase (LO) plays a critical role in the formation and repair of the extracellular matrix (ECM) by oxidizing lysine residues in elastin and collagen, thereby initiating the formation of covalent crosslinkages which stabilize these fibrous proteins. Its catalytic activity depends upon both its copper cofactor and a unique carbonyl cofactor and has been shown to extend to a variety of basic globular proteins, including histone H1. Although the three-dimensional structure of LO has yet to be determined, the present treatise offers hypotheses based upon its primary sequence, which may underlie the prominent electrostatic component of its unusual substrate specificity as well as the catalysis-suppressing function of the propeptide domain of prolysyl oxidase. Recent studies have demonstrated that LO appears to function within the cell in a manner, which strongly modifies cellular activity. Newly discovered LO-like proteins also likely play unique roles in biology.

Biochemical and toxicological evaluation of agent-cofactor reactivity as a mechanism of action for osteolathyrism

In vitro reactivity for each of four osteolathyrogens with a model compound for the lysyl oxidase (LO) cofactor was evaluated and coupled with mixture toxicity testing to evaluate agent-cofactor reactivity as a potential mechanism of action for osteolathyrism. Reactivity of the model cofactor (mLTQ: 4-butylamino-5-methyl-o-quinone), with each of two ureides, semicarbazide (SC) and thiosemicarbazide (TSC), and each of two aminonitriles, aminoacetonitrile (AAN) and beta-aminopropionitrile (betaAPN), was assessed using UV-vis spectrophotometry; both in the absence and presence of Cu(II)-bipyridine (bipy) complex. Two sets of mixture toxicity experiments were conducted using a frog embryo assay that assessed the incidence of osteolathyrism in the notochord of tadpoles after 96-h exposure. The resulting concentration-response curves for each set were evaluated (chi(2) goodness-of-fit test) against theoretical curves for two combined effects models: dose-addition and independence, to determine the combined effect of each osteolathyrogen combination. The agents SC, TSC and AAN each showed rapid, irreversible reactivity with mLTQ, both in the absence and presence of Cu(II)-bipy complex, as indicated by bleaching of the mLTQ peak (504 nm) and formation of an adduct at 350 nm. betaAPN showed no apparent reactivity in the absence of prolonged incubation with mLTQ, whether Cu(II)-bipy complex was present or not. After prolonged incubation (24-144 h) a new peak formed at 350 nm, suggesting that betaAPN reacts weakly with the cofactor, but in a manner different from the other agents examined. The toxicity tests indicated a dose-additive combined effect for the SC:TSC, AAN:SC and AAN:SC:TSC mixtures (0.1<P<0.9; with slope and EC(50) additivity quotient values between 0.93-1.14), suggesting these agents induce osteolathyrism via a common mechanism. The betaAPN:SC and betaAPN: SC:TSC mixtures showed a greater-than dose-additive combined effect (P<0.001; slope and EC(50) additivity quotient values from 0.74 to 0.89), suggesting a different or additional mechanism of osteolathyrism for betaAPN. Taken together the results indicate that SC, TSC and AAN induce osteolathyrism primarily, if not completely, by binding to the LO cofactor, thereby inhibiting proper connective tissue fiber cross-linking. While betaAPN also has the potential to react with the cofactor, the nature of the reactivity observed suggests that betaAPN-cofactor binding is, at best, a secondary mechanism for induction of osteolathyrism in vivo.