Catalytic properties and structural components of lysyl oxidase

Key aspects of the biosynthesis and catalytic specificity of lysyl oxidase (LO) have been explored. Oxidation of peptidyl lysine in synthetic oligopeptides is markedly sensitive to the presence of vicinal dicarboxylic ami/no acid residues. Optimal activity is obtained with the -Glu-Lys- sequence within a polyglycine 11-mer, whereas the -Lys-Glu- sequence is much less efficiently oxidized. The -Asp-Glu-Lys- sequence is a very poor substrate, although this sequence is oxidized in type I collagen fibrils. These results are considered in the light of a model requiring collagen to be assembled as fibrils prior to oxidation by LO. An in vitro system for the expression of catalytically active LO has been devised. Deletion or inclusion of the cDNA coding for the propeptide region in the expressed construct results in apparently identical, catalytically active enzyme products, indicating the lack of essentiality of this region for active enzyme production. These effects are considered with respect to the conservation of the amino acid sequence of LO produced by different species.

Lysyl oxidase: an oxidative enzyme and effector of cell function

Lysyl oxidase (LOX) oxidizes the side chain of peptidyl lysine converting specific lysine residues to residues of alpha-aminoadipic-delta-semialdehyde. This posttranslational chemical change permits the covalent crosslinking of the component chains of collagen and those of elastin, thus stabilizing the fibrous deposits of these proteins in the extracellular matrix. Four LOX-like (LOXL) proteins with varying degrees of similarity to LOX have been described, constituting a family of related proteins. LOX is synthesized as a preproprotein which emerges from the cell as proLOX and then is processed to the active enzyme by proteolysis. In addition to elastin and collagen, LOX can oxidize lysine within a variety of cationic proteins, suggesting that its functions extend beyond its role in the stabilization of the extracellular matrix. Indeed, recent findings reveal that LOX and LOXL proteins markedly influence cell behavior including chemotactic responses, proliferation, and shifts between the normal and malignant phenotypes.

Lysyl oxidase, the extracellular matrix-forming enzyme, in rat brain injury sites

Lysyl oxidase is an extracellular enzyme that catalyzes cross-linkages of extracellular matrix proteins. We hypothesized that this enzyme is secreted by cells attracted to central nervous system injury sites and is involved in extracellular matrix modulation and in scar formation. Specific antibodies for immunohistochemistry and enzyme activity measurements were used to detect the presence of lysyl oxidase after longitudinal knife cuts in adult rat forebrain. Immunoreactivity was observed within the core of injury sites from 1 and up to 30 days postoperative, with less staining at 2 and 5 days, and was not associated with glial fibrillary acidic protein-positive astrocytes. Enzyme activity increased transiently in injury site regions with a peak (200% of control) at 10 days postoperative. These results are the first to provide evidence for a time-dependent appearance of active extracellular lysyl oxidase in brain injury sites. They imply that enzyme molecules are synthesized and secreted by cells attracted to brain injury sites and participate in extracellular matrix modulation.

Salivary histatin 5 is a potent competitive inhibitor of the cysteine proteinase clostripain

Histatin 5 is a low molecular weight salivary protein which is known to exhibit inhibitory activity against several proteinases, including the cysteine proteinases gingipains. The purpose of this study was to characterize the effect of salivary histatin on the proteolytic activity of the cysteine proteinase clostripain derived from the pathogen Clostridium histolyticum. Using a synthetic nitroanilide substrate, we studied in detail the inhibition of clostripain by histatin 5 and compared the effect of this peptide to that of leupeptin, a known competitive inhibitor of clostripain. It was found that the concentration of histatin 5 required to inhibit 50% of clostripain activity was 23.6+/-1.6 nM. Kinetic analysis revealed that histatin 5 is a competitive inhibitor of clostripain with an inhibition constant (K(i)) of 10 nM. The K(i) for the inhibition of clostripain activity against nitroanilide substrate by leupeptin was found to be 60 nM, significantly higher than that of histatin 5. Thus, histatin 5 inhibits clostripain more effectively than leupeptin and other cysteine protease inhibitors studied here. No significant proteolysis of histatin 5 was observed when histatin 5 was incubated at physiologic concentrations with clostripain. The potent inhibition of clostripain by histatin 5 points towards the possibility that this protein may prevent establishment of clostridial infections and therefore may have significant potential for the treatment of diseases associated with this enzyme.

Intra- and extracellular enzymes of collagen biosynthesis as biological and chemical targets in the control of fibrosis

The several steps in the pathway for the biosynthesis of fibrillar collagen are reviewed to illustrate potential sites for the chemotherapeutic control of fibrosis. Particular emphasis is placed upon the properties and inhibition of lysyl oxidase, the enzyme which initiates the covalent crosslinking of extracellular collagen molecules converting these to insoluble fibers, and upon the properties and inhibition of prolyl hydroxylase, the intracellular enzyme which hyroxylates proline residues within collagen.

Fully processed lysyl oxidase catalyst translocates from the extracellular space into nuclei of aortic smooth-muscle cells

Lysyl oxidase (LO), a secreted protein, was recently identified within the nuclei of vascular smooth-muscle cells (SMC) and 3T3 fibroblasts. A possible pathway by which LO can enter cell nuclei was explored in the present study. SMC were incubated with purified 32-kDa bovine aorta LO that had been fluorescently labeled with rhodamine (TRITC-LO). TRITC-LO entered the cytosol and then rapidly concentrated within the nuclei of preconfluent cultures of these cells, whereas carbonic anhydrase, a protein of similar molecular weight and similarly labeled, did not enter the cells under these conditions. LO that had been reductively methylated at lysine residues with [(14)C]HCHO was also taken up into the cytosolic and nuclear compartments. Intracellular uptake and intracellular distribution were not altered by inhibiting LO activity with beta-aminopropionitrile. An excess of native LO but not of carbonic anhydrase competitively inhibited the uptake of the isotopically labeled enzyme. Thus, once secreted and proteolytically processed, mature LO can enter the cells and concentrate within nuclei in a manner that appears to be specific and independent of its catalytic activity.

Hydrogen peroxide-mediated, lysyl oxidase-dependent chemotaxis of vascular smooth muscle cells

Lysyl oxidase (LO), an enzyme secreted by vascular smooth muscle cells (VSMC), initiates the covalent crosslinking of polypeptide chains within collagen and elastin. The present study reveals that purified LO strongly induces directional migration of VSMC in an in vitro assay system. LO-dependent chemotaxis, but not chemokinesis, was abolished by beta-aminopropionitrile, an active site inhibitor of LO, or by catalase, as well as by prior heat denaturation. This indicates that the H(2)O(2) product of amine oxidation by LO is critical to the expression of its chemotactic activity. The results indicate that the chemotactic response requires direct access between LO and a substrate molecule (or molecules) tightly associated with the VSMC. The addition of LO to VSMC elevated the levels of intracellular H(2)O(2), enhanced stress fiber formation, and focal adhesion assembly, is consistent with the induction of the chemotactic response.

Proteins secreted by vascular smooth muscle cells as substrates of lysyl oxidase

The mixture of proteins secreted by neonatal rat aorta smooth muscle cells cultured in the presence of beta-aminopropionitrile was readily oxidized and polymerized upon incubation with purified or crude preparations of lysyl oxidase. Western blot analysis indicated that these substrates included 30-60kDa protein bands reactive with anti-elastin, presumed to be fragments derived from tropoelastin. Thus, truncated, elastin-like as well as other proteins accumulate in the media of these cultures which, in toto, can serve as a conveniently prepared, highly efficient substrate for the routine assay of lysyl oxidase activity.

Upregulation of lysyl oxidase in vascular smooth muscle cells by cAMP: role for adenosine receptor activation

Lysyl oxidase (LO) is a key participant in the accumulation of insoluble fibers of elastin and collagen by virtue of its role in the initiation of the covalent cross-linkages between and within individual molecules comprising these fibers. In view of the essential role played by LO in the accumulation of the fibrotic components of occlusive arterial lesions in atherosclerosis, identification of the signaling molecules which can affect the expression of the LO gene in vascular smooth muscle is of considerable interest. In the present report, we describe evidence for the role of the second messenger, cAMP, in the modulation of the levels of LO in vascular smooth muscle cells. Elevated intracellular cAMP induces the transcription of the LO gene, as revealed by Northern blot analysis and nuclear run on assays. Transient transfection experiments performed with the wild-type LO promoter and with this promoter mutated at a consensus CREB site, located within the region -100 to -93 base pairs relative to the start of transcription, indicate that cAMP-induced transcriptional activation is partially due to the presence of this CREB site within the promoter. Activation of stimulatory adenosine receptors in vascular smooth muscle cells with 5'-N-ethylcarboxamido adenosine (NECA) increases cAMP, LO mRNA, and enzyme activity. These findings point to the importance of cAMP signaling, potentially initiated by a variety of physiological agents, in the upregulation of LO expression in vascular smooth muscle cells.

Aminoalkylaziridines as substrates and inhibitors of lysyl oxidase: specific inactivation of the enzyme by N-(5-aminopentyl)aziridine

The reaction of lysyl oxidase was assessed with members of a series of aminoalkylaziridines in which the primary amino group and the aziridinyl nitrogen were separated by 3-7 methylene carbons. Among these, N-(5-aminopentyl)aziridine proved to be the poorest substrate by far and to inhibit the enzyme activity. Aminoalkylaziridines with chain lengths shorter or longer than five carbons did not inhibit the enzyme. The resulting inhibition was competitive with productive substrates and became irreversible with time, following pseudo first order kinetics with a KI of 0.22 mM. N-(5-aminopentyl)aziridine appears to act as a bifunctional affinity label covalently interacting with the active site of this enzyme.

Lysyl oxidase: properties, regulation and multiple functions in biology

Lysyl oxidase (LO) is a copper-dependent amine oxidase that plays a critical role in the biogenesis of connective tissue matrices by crosslinking the extracellular matrix proteins, collagen and elastin. Levels of LO increase in many fibrotic diseases, while expression of the enzyme is decreased in certain diseases involving impaired copper metabolism. While the three-dimensional structure of the enzyme is not yet available, many of its physical-chemical properties, its novel carbonyl cofactor, and its catalytic mechanism have been described. Lysyl oxidase is synthesized as a preproprotein, secreted as a 50 kDa, N-glycosylated proenzyme and then proteolytically cleaved to the 32 kDa, catalytically active, mature enzyme. Within the past decade, the gene encoding LO has been cloned, facilitating investigations of the regulation of expression of the enzyme in response to diverse stimuli and in numerous disease states. Transforming growth factor-beta, platelet-derived growth factor, angiotensin II, retinoic acid, fibroblast growth factor, altered serum conditions, and shear stress are among the effectors or conditions that regulate LO expression. New, LO-like genes have also been identified and cloned, suggesting the existence of a multigene family. It has also become increasingly evident that LO may have other important biological functions in addition to its role in the crosslinking of elastin and collagen in the extracellular matrix.

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

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

Localization and activity of lysyl oxidase within nuclei of fibrogenic cells

Lysyl oxidase (EC 1.4.3.13) oxidizes peptidyl lysine to peptidyl aldehyde residues within collagen and elastin, thus initiating formation of the covalent cross-linkages that insolubilize these extracellular proteins. Recent findings raise the possibility that this enzyme may also function intracellularly. The present study provides evidence by immunocytochemical confocal microscopy, Western blot analysis, enzyme assays, and chemical analyses for lysyl oxidase reaction products that this enzyme is present and active within rat vascular smooth muscle cell nuclei. Confocal microscopy indicates its presence within nuclei of 3T3 fibroblasts, as well.

Transcriptional and post-transcriptional control of lysyl oxidase expression in vascular smooth muscle cells: effects of TGF-beta 1 and serum deprivation

Transforming growth factor-beta 1 (TGF-beta 1) markedly reduced cell proliferation and elevated steady state lysyl oxidase (LO) mRNA 3-fold in neonatal rat aorta smooth muscle cells cultured in medium containing 10% fetal bovine serum. The increase in LO mRNA was prevented by the presence of cycloheximide, indicative of controlling events at the level of protein synthesis. The basal level of mRNA in cells proliferating in 10% fetal bovine serum in the absence of TGF-beta 1 was enhanced 7-fold upon decreasing growth by shifting to medium containing 0.5% serum. Changes in LO activity paralleled those in LO mRNA. Nuclear run-on assays revealed that the stimulation of expression in 0.5% serum involved increased gene transcription whereas that caused by TGF-beta 1 was mostly post-transcriptional in origin. LO mRNA was quite labile (t1/2 approximately 3 h) in 10% serum but was markedly stabilized (t1/2 > 12 h) by the presence of TGF-beta 1 in the 10% serum medium. LO mRNA was also considerably more stable under retarded growth conditions (0.5% serum) in the absence of TGF-beta 1. LO promoter activity in luciferase reporter constructs transfected into these cells was low and not significantly affected by the addition of TGF-beta 1 to the 10% serum medium but was markedly elevated by shifting from 10 to 0.5% serum in the absence of TGF-beta 1. Thus, LO expression is inversely correlated with cell proliferation, and is subject to control at transcriptional and post-transcriptional levels. TGF-beta 1 enhances LO expression in these cells by dramatically stabilizing LO mRNA.

Detection of elastin in the human fetal membranes: proposed molecular basis for elasticity

The human fetal membranes provide a sterile biomechanical container which adjust by growth to mid-pregnancy to the increase in fetal size, and by elasticity to the forceful movements of the fetus. The molecular basis for this elasticity is not known, yet reduced elasticity may lead to their premature rupture and preterm birth, a major problem in perinatal medicine. Classically, elastin confers the property of elastic recoil to elastic fibres which are assembled from a family of tropoelastin precursors. These are covalently cross-linked to form insoluble elastin by formation of desmosine and isodesmosine, catalysed by the enzyme lysyl oxidase. The amnion, chorion and decidua were shown by Northern analysis and RT-PCR to contain detectable levels of tropoelastin mRNA and the mRNA encoding lysyl oxidase. The proteins encoded by these mRNAs were also identified by Western blotting and immunolocalization. Further, insoluble elastin was extracted from the human fetal membranes and shown by comparison to elastin preparations from other elastic tissues to have a reasonable desmosine content. Finally, scanning electron microscopy confirmed the presence of multiple layers of an apparently very thin elastic system in this tissue. This biochemical and histopathologic study has demonstrated therefore that the human fetal membranes synthesize and deposit a novel elastic fibre. The presence of such an elastic system in these tissues provides, for the first time, a probable molecular basis for the elastic properties of this tissue.

Regulation of lysyl oxidase and cyclooxygenase expression in human lung fibroblasts: interactions among TGF-beta, IL-1 beta, and prostaglandin E

Prostaglandin E2, transforming growth factor-beta, and interleukin-1 beta variably regulate the expression of cyclooxygenase 1, cyclooxygenase 2, and lysyl oxidase in IMR90, human embryo lung fibroblasts. Prostaglandin E2 at 100 nM upregulates cyclooxygenase 1 mRNA by approximately three-fold while it downregulates lysyl oxidase mRNA levels. Notably, prostaglandin E2 suppresses the enhancing effect of TGF-beta on basal levels of lysyl oxidase mRNA. These changes in steady state mRNA levels reflect transcriptional level control, at least in part. Corresponding changes are seen in the protein levels of lysyl oxidase, cyclooxygenase 1 and cyclooxygenase 2 and catalytic activities of these enzymes, including net prostaglandin E2 synthesis. Cyclooxygenase 2 mRNA(t1/2, 30 min) is considerably less stable than that of cyclooxygenase 1 (t1/2, 4 h) while lysyl oxidase mRNA is unusually stable (t1/2 > 14 h). Taken together with the differing kinetics with which these genes respond to perturbation by these cytokines, the present results suggest a coordinated, autocrine-like mechanism of regulation of cyclooxygenase 1 and cyclooxygenase 2 and further point to the potential of their metabolic product, prostaglandin E2, to suppress the expression of lysyl oxidase in the inflammatory response to injury.

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

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

Metalloproteinase activity secreted by fibrogenic cells in the processing of prolysyl oxidase. Potential role of procollagen C-proteinase

Lysyl oxidase is secreted from fibrogenic cells as a 50-kDa proenzyme that is proteolytically processed to the mature enzyme in the extracellular space. To characterize the secreted proteinase activity, a truncated, recombinant form of lysyl oxidase was prepared as a proteinase substrate containing the sequence of the propeptide cleavage region. The processing proteinase activity secreted by cultured fibrogenic cells resists inhibitors of serine or aspartyl proteinases as well as tissue inhibitor of matrix metalloproteinases-2 (MMP-2) but is completely inhibited by metal ion chelators. Known metalloproteinases were tested for their activity toward this substrate. Carboxyl-terminal procollagen proteinase (C-proteinase), MMP-2, and conditioned fibrogenic cell culture medium cleave the lysyl oxidase substrate to the size of the mature enzyme. The NH2-terminal sequence generated by arterial smooth muscle conditioned medium and the C-proteinase but not by MMP-2, i.e. Asp-Asp-Pro-Tyr, was identical to that previously identified in mature lysyl oxidase isolated from connective tissue. The C-proteinase activity against the model substrate was inhibited by a synthetic oligopeptide mimic of the cleavage sequence (Ac-Met-Val-Gly-Asp-Asp-Pro-Tyr-Asn-amide), whereas this peptide also inhibited the generation of lysyl oxidase activity in the medium of fetal rat lung fibroblasts in culture. In toto, these results identify a secreted metalloproteinase activity participating in the activation of prolysyl oxidase, identify inhibitors of the processing activity, and implicate procollagen C-proteinase in this role.

Induction of human monocyte motility by lysyl oxidase

Lysyl oxidase highly purified from calf aorta was found to be a potent chemotactic agent for unstimulated human peripheral blood mononuclear cells, determined in in vitro assays in Boyden chambers. A typical chemotactic bell-shaped curve was observed, with a maximal migratory response of 237% of control occurring at 10(-10) M lysyl oxidase. The chemotactic response was prevented by prior heat inactivation of the enzyme, by treatment of the enzyme with beta-aminopropionitrile or ethylenediamine, which are active site-directed inhibitors of lysyl oxidase, and by a competing, lysine-containing peptide substrate of lysyl oxidase. The chemoattractant response to lysyl oxidases was characterized by both chemokinetic and chemotactic components. These results raise the possibility that extracellular lysyl oxidase may have important roles to play in biology in addition to its established function in the crosslinking of elastin and collagen.

Expression of lysyl oxidase from cDNA constructs in mammalian cells: the propeptide region is not essential to the folding and secretion of the functional enzyme

Rat aortic lysyl oxidase cDNA was expressed under a metallothionein promoter in Chinese hamster ovary cells using a dihydrofolate reductase selection marker. One methotrexate-resistant cell line, LOD-06, generated by transfecting with full-length cDNA, yielded lysyl oxidase proteins consistent with the 50 kDa proenzyme and a 29 kDa mature catalyst. A second cell line, LOD32-2, was generated by transfection with a truncated cDNA lacking sequences which code for the bulk of the propeptide region. Both cell lines secreted apparently identical, 29 kDa forms of mature lysyl oxidase each of which catalyzed the deamination of human recombinant tropoelastin and alkylamines, consistent with the known specificity of lysyl oxidase. The secreted enzyme forms were inhibited by chemical inhibitors of lysyl oxidase activity, including beta-aminopropionitrile, phenylhydrazine, ethylenediamine, alpha, alpha'-dipyridyl, and diethyldithiocarbamate. Sensitivity to these agents is consistent with the presence of copper and carbonyl cofactors in the expressed enzymes, characteristic of lysyl oxidase from connective tissues. These results indicate the lack of essentiality of the deleted proprotein sequence for the proper folding, generation of catalytic function, and secretion of lysyl oxidase.

Downregulation of lysyl oxidase in cadmium-resistant fibroblasts

Lysyl oxidase, a copper-dependent metalloenzyme, plays a central role in crosslinking of collagen and elastin in the extracellular matrix. Notably, lung lysyl oxidase activity is markedly stimulated in rats exposed to cadmium vapors. To further understand the mechanism of cadmium toxicity, the mRNA expression, synthesis, post-translational processing, and catalytic activity of lysyl oxidase were examined in cadmium-resistant (CdR) cells and the cadmium-sensitive Swiss mouse 3T3 cells from which they were derived. These CdR cells synthesized and accumulated markedly elevated levels of metallothionein, a known marker for cadmium resistance, whereas the expression of lysyl oxidase was reduced considerably. In comparison to the parental, cadmium-sensitive cells, the suppression of enzyme production in the CdR cells was seen at the mRNA level, at the levels of intracellular proprotein production and mature enzyme secreted into the medium, and in terms of total enzyme activity in the culture. The presence of cupric chloride in the culture medium during the incubation of the CdR cells for 16 h significantly enhanced lysyl oxidase activity accumulating in the medium, suggesting that lysyl oxidase deficiency in CdR cells may be related to abnormal copper metabolism.

Identification of lysyl oxidase and other platelet-derived growth factor-inducible genes in vascular smooth muscle cells by differential screening

BACKGROUND

Atherosclerosis and arterial injury are characterized by vascular smooth muscle cell (VSMC) migration and growth and an increase in synthesis of extracellular matrix. Platelet-derived growth factor (PDGF) has been implicated in these processes. This study was designed to identify additional PDGF-regulated genes in VSMC.

EXPERIMENTAL DESIGN

A cDNA library prepared from PDGF-stimulated rat aortic VSMC was screened by differential hybridization to identify clones representing PDGF-inducible genes. The time course of growth factor-induced changes in gene expression was examined by RNA blot hybridization. Assays of protein activity were also performed for selected gene products.

RESULTS

Four PDGF-regulated cDNA clones were identified by DNA sequencing. These encoded the extracellular matrix proteins lysyl oxidase (LO), thrombospondin, and osteopontin and the intracellular enzyme lactate dehydrogenase (LDH). Levels of mRNA corresponding to all four genes were low in quiescent VSMC and were markedly induced by PDGF, angiotensin II, and 10% calf serum. The regulation of LO and LDH mRNA by these agonists in VSMC has not been previously reported. LO enzymatic activity in the culture media was increased by approximately equals to 700% after exposure to PDGF. In contrast, LDH activity was not increased by PDGF treatment.

CONCLUSIONS

The induction of LO mRNA and its secretion by VSMC is an early event accompanying growth factor stimulation and may contribute to organization of the extracellular matrix.

Regulation of lysyl oxidase expression in lung fibroblasts by transforming growth factor-beta 1 and prostaglandin E2

The regulation of lysyl oxidase produced by cultured, lipid-enriched, neonatal rat lung fibroblasts was explored. The presence of 40 pM of transforming growth factor-beta 1 (TGF-beta 1) in overnight cultures increased levels of enzyme secreted into the medium by 1.6-fold while steady-state levels of lysyl oxidase mRNA increased similarly. In contrast, incubation of these cultures with 100 nM of prostaglandin E2 (PGE2) reduced enzyme activity levels by 40 to 50% although steady-state mRNA was not changed. Consistent with the effect of PGE2, the presence of indomethacin stimulated levels of secreted enzyme activity. When present in cultures simultaneously with TGF-beta 1, PGE2 prevented the stimulation beyond control levels seen with TGF-beta 1 alone. Densitometry of protein bands immunoprecipitated by antibody to lysyl oxidase indicated that the degree of conversion of the 50 kD proenzyme to the 29 kD enzyme was not significantly altered by TGF-beta 1 or PGE2. However, the net accumulation of all forms of lysyl oxidase protein was increased by TGF-beta 1 and decreased by PGE2. These results indicate that TGF-beta 1 and specific prostaglandin(s) exert opposing effects on the expression of lysyl oxidase in these lung fibroblasts.

Retinoic acid prevents downregulation of ras recision gene/lysyl oxidase early in adipocyte differentiation

Adipocyte differentiation of 3T3-L1 cells is a complex process which is inhibited by retinoic acid (RA). Since RA acts by nuclear receptors which directly regulate gene expression, we postulate that the primary targets of RA action in this system are genes which are regulated early in adipose conversion. In this study, we demonstrate the use of the differential display technique to search for early events in adipose commitment which are sensitive to RA. A mRNA was identified on the basis of its RA-dependent gene expression 24 h after initiation of a standard differentiation protocol. Molecular cloning of the cDNA revealed it to be identical to the ras recision gene (rrg), for lysyl oxidase. Indeed, two mRNAs identical to those recognized by lysyl oxidase probes were expressed in preadipocytes and tandemly repressed with 24 h of exposure to differentiation conditions. Lysyl oxidase activity was similarly reduced in the media of differentiated cells. RA completely blocked the differ-entiation-related reduction in rrg/lysyl oxidase gene expression, although RA had no independent stimulatory effect on rrg/lysyl oxidase expression in cells not exposed to differentiating conditions. Thus, differential display has been successfully used to identify rrg/lysyl oxidase as an early marker for adipose conversion that is responsive to RA.

Lysyl oxidase: mechanism, regulation and relationship to liver fibrosis

Lysyl oxidase oxidizes peptidyl lysine in collagen and elastin substrates to residues of alpha-aminoadipic-delta-semialdehyde. The peptidyl aldehydes can then undergo spontaneous condensations with unreacted epsilon-amino groups and with neighboring aldehyde functions, thus forming the covalent crosslinkages which convert elastin and collagen into insoluble fibers. The unique role of lysyl oxidase in the post-translational modification of these proteins qualifies this enzymatic reaction as a potentially pivotal site of biological and/or chemotherapeutic control of collagen fiber deposition. Recent advances in the study of the catalytic mechanism, in the development of active site inhibitors, and in the biosynthesis and regulation of this unusual catalyst are reviewed as are studies on the response of lysyl oxidase in fibrotic liver.

Modulation of lysyl oxidase activity toward peptidyl lysine by vicinal dicarboxylic amino acid residues. Implications for collagen cross-linking

The substrate specificity of lysyl oxidase has been explored with synthetic oligopeptides. kcat/Km increased with increasing peptide length in Ac-(Gly)n-Lys-(Gly)n-CONH2 (n = 1-5). Using 11-mers as the standard peptide length, Glu immediately N-terminal to Lys increased kcat/Km 8.8-fold over that for the -Lys-Glu- sequence and 4.9-fold over the glutamate-free control. Kinetic constants were significantly less perturbed when Glu was 2 or more residues distant from Lys. Replacement of Glu in -Glu-Lys- with Gln significantly increased Km and lowered kcat/Km. Asp rather than Glu N-terminal to Lys decreased Km similar to that of the -Glu-Lys- 11-mer, although the kcat decreased considerably, indicating that lysyl oxidase responds to the side chain length of vicinal Asp or Glu at this position. -Asp-Glu-Lys- within an 11-mer was not oxidized, although this sequence is oxidized within the N-terminal telopeptide of the alpha 1(I) chain in type I collagen fibrils. Thus, lysyl oxidase exhibits distinct preferences for sequences vicinal to lysine. These results are discussed with respect to a model requiring collagen fibril formation prior to oxidation of lysine in collagen by lysyl oxidase.

Alterations in cytoskeletal organization and homeostasis of cellular thiols in cadmium-resistant cells

To understand further the mechanisms of cadmium toxicity, cytoskeletal organization and homeostasis of cellular thiols were examined in cadmium-resistant cells isolated from Swiss mouse 3T3 cells by incubation in graded concentrations of CdCl2 (Cd2+) in the culture medium. Cd(2+)-resistant cells displayed profound alterations in their cytoskeletal organization characterized by the appearance of many elongated, tadpole-shaped cells with a high density of microtubules (MT) and microfilaments (MF), with the former being mainly distributed along the long axis of the cell. Exposure of Cd(2+)-resistant cells to 50 microM Cd2+ for 16 hr did not cause apparent cytoskeletal perturbations, whereas treatment of parental cells with 5 microM Cd2+ for the same duration produced a severe loss of MT and smeared patches of MF. Thus, the cytoskeleton of Cd(2+)-resistant cells is markedly more preserved and protected against Cd2+ damage than that of their parental counterparts. Cd(2+)-resistant cells contained a higher basal level of protein sulfhydryls (PSH) in both the cytoskeletal and cytosolic fractions than the parental cells. Exposure to 50 microM Cd2+ further increased cellular PSH contents, reaching 192 and 215% of the basal levels for the cytoskeletal and cytosolic fractions, respectively. Although 5 microM Cd2+ exposure also elevated the amounts of PSH in parental cells, the "absolute" values were still below the corresponding basal levels in Cd(2+)-resistant cells. Furthermore, Cd(2+)-resistant cells also exhibited enhanced basal levels of metallothionein and cellular glutathione (GSH), amounting to 19- and 2.1-fold of the parental basal levels, respectively. Thus, the Cd(2+)-resistant cells produced larger quantities of both protein and nonprotein thiol-containing elements than the parental cells. Interestingly, exposure of Cd(2+)-resistant cells to 50 microM Cd2+ also further increased metallothionein and cellular GSH to 178 and 138% of the basal levels, respectively. Based on the affinity of Cd2+ for sulfhydryls as a mechanism of Cd2+ toxicity, we propose that the coordinately increased levels of metallothionein, GSH, and PSH in Cd(2+)-resistant cells would provide a mechanistic basis for the homeostasis of cellular thiols which may collectively contribute to the cytoskeletal preservation by protecting the cytoskeleton from Cd2+ insult.

Kinetics and stereospecificity of the lysyl oxidase reaction

The structural specificity of amine oxidation by lysyl oxidase was investigated using kinetic and NMR spectroscopic analyses. Substrate efficiency increased with increasing molecular distance from the alpha-carbon of the aromatic moiety substituted on the aliphatic chains of a series of primary amines. The p-hydroxyl substituent of p-hydroxybenzylamine significantly increased kcat over that of benzylamine, whereas this was not the case when tyramine and phenethylamine were compared. Direct spectrophotometric measurement of p-hydroxybenzaldehyde formation yielded burst kinetics, the second, slower phase of which was eliminated under anaerobic conditions. Thus, enzyme reoxidation is the more rate-limiting of the two half-reactions catalyzed with this substrate by this ping-pong enzyme. 1H NMR spectroscopy of the alcohol reductively derived from the aldehyde product of the lysyl oxidase-catalyzed oxidation of deuterated tyramine indicated that the pro-S but not the pro-R alpha-deuteron was catalytically abstracted. Moreover, lysyl oxidase catalyzed solvent exchange of protons at the C-2 position. Such stereospecificity and proton exchange uniquely differentiates lysyl oxidase from all but an aortic semicarbazide-sensitive amine oxidase among the pro-S-specific copper-dependent amine oxidases analyzed thus far.

Reaction of lysyl oxidase with trans-2-phenylcyclopropylamine

trans-2-Phenylcyclopropylamine hydrochloride (tranylcypromine; TCP) was found to be both an inhibitor and a substrate of lysyl oxidase, the enzyme which oxidizes peptidyl lysine in elastin and collagen to initiate cross-linking in these proteins. The reaction of TCP with this enzyme was further characterized in view of the potential interference that chronic administration of this antidepressant compound may exert on the development and repair of connective tissues. In contrast to the irreversible and/or competitive inhibitors of lysyl oxidase previously described, TCP noncompetitively and reversibly inhibited the oxidation of both alkylamine and elastin substrates with Ki values of 386 and 375 microM, respectively. The noncompetitive mode of interaction affected the accessibility of the active site to productive amine substrates since the reductive trapping of n-hexylamine to lysyl oxidase was largely prevented by the presence of TCP. It was of additional interest that lysyl oxidase catalyzed a limited degree of conversion of TCP to cinnamaldehyde accompanied by the production of hydrogen peroxide. The lack of significant incorporation of protein-bound tritium accompanying reduction of the enzyme-TCP complex with [3H]NaBH4 argued against the formation of a Schiff base between the enzyme and the cinnamaldehyde product as the basis of the inhibitory effect. Spectral evidence was also obtained for an additional interaction between TCP and lysyl oxidase that was independent of the inhibitory effect of TCP. Cyclopropylamine, lacking the benzene moiety of TCP, inhibited lysyl oxidase irreversibly and competitively, and was not a substrate, pointing toward a defining role for the benzene moiety in the interaction of TCP with lysyl oxidase.

A developmentally regulated program restricting insolubilization of elastin and formation of laminae in the fetal lamb ductus arteriosus

BACKGROUND

The ductus arteriosus (DA) is a fetal vessel in which the elastic laminae fail to assemble normally in late gestation. This feature is associated with the development of intimal cushions, structures that partially occlude the DA lumen and assure that the vessel will close completely when it constricts postnatally.

EXPERIMENTAL DESIGN

We studied the fetal lamb DA at two different gestational time-points, 100 days before, and 138 days coincident with intimal cushion formation (term = 145 days) to establish the ultrastructural basis for the 'disassembly' of elastic laminae apparent on light microscopy and to determine further whether the mechanism was due to increased elastolytic activity, decreased synthesis of tropoelastin, or impaired insolubilization of tropoelastin.

RESULTS

Morphometric ultrastructural analyses of tissue from the 138-day gestation fetal lambs revealed that the volume density of elastin in the DA vessel wall was only 40% of that in the aorta (Ao) and 50% of that in the pulmonary artery (PA). Moreover, only 16% of the elastin present contributed to the formation of laminae when compared to 80% in the Ao and 50% in the PA. Despite the morphologic appearance of 'fragmented' elastin, there was no evidence of increased elastolytic activity in the DA at either gestational time-point as judged by solubilization of a [3H] elastin substrate. The reduced elastin apparent was morphologically accompanied by an increase in soluble (tropo) elastin in DA compared with Ao and PA, as measured by enzyme linked immunosorbent assay, in tissue from both 100- and 138-day gestation lambs. Lack of differences in tropoelastin mRNA levels when comparing the 3 vessels suggested that the enzyme linked immunosorbent assay measurements reflected increased DA tropoelastin accumulation owing to lack of insolubilization rather than an increase in synthesis. Reduced insolubilization of newly synthesized elastin was evident in the DA compared with the Ao at 100 days gestation and in the DA compared with both Ao and PA at 138 days gestation in association with reduced desmosine levels.

CONCLUSIONS

The mechanism of the decrease in tropoelastin insolubilization was unrelated to lysyl oxidase activity in the tissue and represents a unique developmental program.

Characterization and developmental expression of chick aortic lysyl oxidase

The complete primary structure of chick lysyl oxidase was determined by recombinant DNA techniques. The nucleotide sequence of contiguous chick lysyl oxidase cDNA clones contained an open reading frame of 1260 bases which encodes a predicted protein of 420 amino acid residues (48,150 Da). In comparison to the deduced primary structure of rat lysyl oxidase, the chick enzyme is larger in size and exhibits a strong conservation of sequence within the latter two thirds of the molecule (92% identity) and a high degree of divergence in the first 150 amino acid residues (60% identity allowing for several insertions in both sequences). The developmental steady-state levels of lysyl oxidase mRNA together with the mRNAs encoding two of the enzyme's substrates (tropoelastin and type I collagen) increased between 8 and 16 days of embryonic development. Although levels of lysyl oxidase mRNA increased during aortic embryogenesis, the specific activity of the enzyme remained fairly constant suggesting that lysyl oxidase activity increases in direct proportion to total protein synthesis and cell number. In situ hybridization showed that the spatial expressions of lysyl oxidase and tropoelastin transcripts differ suggesting that the enzyme and substrate genes are differentially regulated within the cells of the arterial wall.

Oxidation of peptidyl lysine by copper complexes of pyrroloquinoline quinone and other quinones. A model for oxidative pathochemistry

Various o- and p-quinones were assessed as oxidants of peptidyl lysine in elastin and collagen substrates in the presence and absence of divalent copper as paradigms of protein-lysine 6-oxidase (lysyl oxidase) which contains both quinone and copper cofactors. Pyrroloquinoline quinone was among the most active in the absence and the most active of the o- and p-quinones tested in the presence of copper. The optimal rate of elastin oxidation occurred at a 2:1 PQQ/Cu(II) ratio while Cu(II) itself oxidized elastin relatively slightly. Elastin oxidation by 2:1 PQQ/Cu(II) required aerobic conditions consistent with oxygen-dependent turnover of this catalytic pair. Dimethylsulfoxide and catalase individually or in combination inhibited elastin oxidation by PQQ/Cu(II) by approx. 50%, suggesting that oxygen free radical species participate in the reaction. Amino-acid analysis of elastin and collagen substrates oxidized by 2:1 PQQ/Cu and then reduced with borohydride revealed that alpha-aminoadipic-delta-semialdehyde and lesser amounts of covalent cross-linkages were generated by this oxidant. In contrast, lysine oxidase produced aldehydes and significantly greater quantities of cross-linkage products, consistent with the known specificity of the enzyme. These data, thus, indicate the potential for free quinones, such as PQQ, particularly when stimulated by appropriate metal ions, to act as adventitious oxidants of lysine side-chains in proteins.

The complete derived amino acid sequence of human lysyl oxidase and assignment of the gene to chromosome 5 (extensive sequence homology with the murine ras recision gene)

Lysyl oxidase catalyzes the oxidation of lysine residues to alpha-aminoadipic-delta-semialdehyde. This is the first step in the covalent cross-linking of collagen and tropoelastin and results in the formation of insoluble collagen and elastic fibers in the extracellular matrix. We have characterized the complete nucleotide sequence of human lysyl oxidase (EC 1.4.3.13) and compared the derived amino acid sequence (417-amino acids) to rat lysyl oxidase and the mouse ras recision gene (rrg). 88% of amino acids and 83% of nucleotides were conserved between human and rat lysyl oxidase. The mouse ras recision gene demonstrated 89% conservation of amino acids with human lysyl oxidase. The sequence conservation was not evenly distributed along the molecule. The carboxy terminus of the protein, which contains the putative copper binding sites and is likely to be the catalytically active domain, was more highly conserved than the amino terminus. The 89% amino acid sequence similarity between the murine ras recision gene and human lysyl oxidase suggests that they are the same gene product. Therefore, in addition to cross linking of extracellular matrix proteins, lysyl oxidase may have a direct role in tumor suppression. Northern blot analysis of poly A+RNA from cultured skin fibroblasts revealed at least three-distinct transcripts, sized 4.8 kb, 3.8 kb and 2.0 kb. In addition, using a panel of human mouse cell hybrids, the lysyl oxidase gene was assigned to human chromosome 5.

Post-translational glycosylation and proteolytic processing of a lysyl oxidase precursor

The synthesis and post-translational processing of a lysyl oxidase precursor protein predicted by the cDNA sequence of rat aorta lysyl oxidase were investigated. In vitro transcription of the cloned lysyl oxidase cDNA and cell-free translation of its mRNA transcript yielded a 47-kDa protein in the absence and a 50-kDa glycosylated protein in the presence of pancreatic membranes, each of which were immunoprecipitated with antibody against the 32-kDa bovine enzyme. Similarly, an N-glycosylated, 50-kDa protein band was synthesized by and immunoprecipitated from cultured neonatal rat aorta smooth muscle cells labeled with [35S]methionine. Pulse-chase studies of proteins newly synthesized by these cells demonstrated that the glycosylated 50-kDa precursor is secreted and that it is processed to the 32-kDa molecular form of lysyl oxidase principally in the medium. The presence of an extracellular processing enzyme activity which converts the 50-kDa precursor to the 32-kDa species was demonstrated by incubating conditioned medium of neonatal rat aorta smooth muscle cell cultures as a source of processing activity with conditioned medium containing 35S-labeled precursor synthesized but not processed by a tumorigenic cell line transfected with a lysyl oxidase expression vector. In contrast to the 50-kDa species, the 32-kDa protein does not appear to be N-glycosylated consistent with the loss of N-linked oligosaccharide units during the processing to the smaller species. The modes of biosynthesis and secretion of lysyl oxidase are discussed in terms of other nonproteolytic activities required for activation of prolysyl oxidase.

Properties and function of lysyl oxidase

Lysyl oxidase catalyzes the oxidation of peptidyl lysine to alpha-aminoadipic-delta-semialdehyde, the precursor to the covalent crosslinkages that stabilize fibers of elastin and collagen. This enzyme contains both copper and a carbonyl cofactor consistent with an o-quinone. The proposed mechanism of action is derived from available kinetic and chemical data and also can account for mechanism-based inhibition of the enzyme by specific monoamines and diamines. Recent evidence for biosynthetic precursors and for the regulation of lysyl oxidase in fibrotic and malignant diseases is discussed.

Specific detection of quinoproteins by redox-cycling staining

Quinones and related quinonoid substances catalyze redox cycling at an alkaline pH in the presence of excess glycine as reductant. With nitroblue tetrazolium and oxygen present there is concomitant reduction of the tetrazolium to formazan. This property of quinonoid compounds is used for the specific staining of quinoproteins, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electroblotted onto nitrocellulose. The dopa-containing vitelline proteins and the 6-hydroxydopa-containing bovine serum amine oxidase are stained with the nitroblue tetrazolium/glycinate reagent. Also, the mammalian quinoproteins, diamine oxidase and lysyl oxidase, purported to contain pyrroloquinoline quinone, tested positive in this procedure. No quinonoid components were detected in three putative pyrroloquinoline quinone-containing quinoproteins, dopamine beta-hydroxylase, lipoxygenase, and peptidylglycine-amidating monoxygenase. Redox-cycling staining therefore confirms the presence of covalently bound quinones in the copper-dependent amine oxidases, but not in two putative quinoprotein oxygenases. Clarification of the biological significance of quinolation should be facilitated by identification of quinoproteins using this approach.

Induction of lung lysyl oxidase activity and lysyl oxidase protein by exposure of rats to cadmium chloride: properties of the induced enzyme

Inspiration of CdCl2 results in a focally fibrotic response in rat lungs and markedly increases the activity of lung lysyl oxidase. Western blot analyses of urea-extractable rat lung proteins revealed that the levels of an immunoreactive, 32,000-Da protein were markedly increased in the cadmium-exposed rat lung tissue, consistent with the induction of lysyl oxidase protein. Anion exchange chromatography revealed low levels of multiple peaks of catalytically functional lysyl oxidase in control rat lung extracts, while the profile of cadmium-exposed rat lung extracts displayed markedly elevated levels of multiple peaks of enzyme activity indicating that the charge heterogeneity is expressed in the activated enzyme. The cadmium-induced enzyme was purified as a species of 32 kDa, without resolving individual ionic variants. The catalytic and physical properties of the isolated enzyme were very similar to those of previously well characterized basal enzyme of bovine aorta, including the presence of a pyrroloquinoline quinone-like carbonyl cofactor. The copper and cadmium content of the cadmium-induced enzyme indicated little if any replacement of tightly-bound copper by cadmium in the exposed lung.

Effect of dietary cadmium and/or copper on the bone lysyl oxidase in copper-deficient rats relative to the metabolism of copper in the bone

Effects of cadmium (Cd) on lysyl oxidase activity and copper (Cu) metabolism in bone were studied using Cu-deficient rats supplemented with Cu and/or Cd in a diet. When fed for 8 weeks on a diet containing 0.3 ppm or less Cu (-Cu diet), weanling rats revealed anemia, and markedly decreased plasma ceruloplasmin activity and serum Cu to less than 15% of normal level, showing features of Cu-deficiency. These rats were divided into four groups and refed for another 2 weeks on the following diets: Group I, -Cu diet; Group II, -Cu diet with 50 ppm Cd (+Cd diet); Group III, -Cu diet supplemented with 15 ppm Cu (+Cu diet); group IV, -Cu diet with both Cu and Cd (+Cu/+Cd diet). After 2 weeks, serum Cu levels of Groups I, II, III and IV were 1.8, 0.8, 78 and 74% of the normal control level (1.438 +/- 0.060 micrograms/ml), respectively. Concentrations of Cu in epi- and metaphyses of the control group, Groups I, II, III and IV were 1.45 +/- 0.20, 0.67 +/- 0.08, 0.76 +/- 0.12, 1.40 +/- 0.31 and 1.22 +/- 0.05 micrograms/g wet tissue, in that order. Concentrations of Cd in epi- and metaphysis increased in only Groups II and IV and were 0.15 +/- 0.03 and 0.18 +/- 0.01 micrograms/g wet tissue, respectively. Thus, having both Cd and Cu supplements in a diet did not inhibit each other's uptake into the tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning of rat aorta lysyl oxidase cDNA: complete codons and predicted amino acid sequence

Lysyl oxidase cDNA clones were identified by their reactivity with anti-bovine lysyl oxidase in a neonatal rat aorta cDNA lambda gt11 expression library. A 500-bp cDNA sequence encoding four of six peptides derived from proteolytic digests of bovine aorta lysyl oxidase was found from the overlapping cDNA sequences of two positive clones. The library was rescreened with a radiolabeled cDNA probe made from one of these clones, thus identifying an additional 13 positive clones. Sequencing of the largest two of these overlapping clones resulted in 2672 bp of cDNA sequence containing partial 5'- and 3'-untranslated sequences of 286 and 1159 nucleotides, respectively, and a complete open reading frame of 1227 bp encoding a polypeptide of 409 amino acids (46 kDa), consistent with the 48 +/- 3 kDa cell-free translation product of rat smooth muscle cell RNA that was immunoprecipitated by anti-bovine lysyl oxidase. The rat aorta cDNA-derived amino acid sequence contains the sequence of each of the six peptides isolated and sequenced from the 32-kDa bovine aorta enzyme, including the C-terminal peptide with sequence identity of 96%. Northern blots screened with lysyl oxidase cDNA probes identified hybridizing species of 5.8 and 4.5 kb in mRNA of rat aorta and lung, while dot blot analyses were negative for lysyl oxidase mRNA in preparations of rat brain, liver, kidney, and heart. A 258-bp segment of the 3'-untranslated region of lysyl oxidase cDNA is 93% identical with a highly conserved region of the 3'-untranslated sequence of rat elastin cDNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Vicinal diamines as pyrroloquinoline quinone-directed irreversible inhibitors of lysyl oxidase

The observation that aliphatic diamines become poor substrates as the carbon chain length decreases and that ethylenediamine, the shortest diamine, is an irreversible inhibitor of lysyl oxidase led to the investigation of the mechanism of inhibition by ethylenediamine. The cis but not the trans isomer of 1,2-diaminocyclohexane was also a potent irreversible inhibitor of lysyl oxidase, consistent with the interaction of both amino groups of vicinal diamines with an enzyme moiety. Both cis-1,2-diaminocyclohexane and ethylenediamine but not trans-1,2-diaminocyclohexane markedly perturbed the spectrum of free pyrroloquinoline quinone (PQQ), a covalently linked form of which is the carbonyl cofactor of lysyl oxidase. cis-1,2-Diaminocyclohexane also induced similar changes in the spectrum of lysyl oxidase. The perturbations of the spectra of PQQ or of lysyl oxidase by cis-1,2-diaminocyclohexane or ethylenediamine as well as the development of irreversible inhibition of the enzyme by cis-1,2-diaminocyclohexane or ethylenediamine were all markedly reduced under anaerobic conditions. Moreover, approximately 1 mol of H2O2 was released per mol of PQQ or lysyl oxidase upon aerobic incubation with cis-1,2-diaminocyclohexane, while approximately 2 mol of 3H+ were released from cis-[1,2-3H] 1,2-diaminocyclohexane per mol of PQQ or lysyl oxidase under corresponding conditions. A proposal for the mechanism of inhibition of lysyl oxidase by vicinal diamines is presented which involves limited oxidation of the diamine linked to PQQ at the active site so that the PQQ-diamine complex is finally stabilized by a conjugated 6-membered ring.

PQQ, the elusive coenzyme

The recently discovered redox coenzyme, PQQ (methoxatin), is widely distributed. Quantitation of protein-bound PQQ has been difficult, but unique redox cycling reactions, which reflect its striking biological properties, reveal trace amounts. PQQ is a potential target for drugs.

Comparative sensitivities of purified preparations of lysyl oxidase and other amine oxidases to active site-directed enzyme inhibitors

Recent evidence has revealed that lysyl oxidase, plasma amine oxidase and diamine oxidase each contain copper and pyrroloquinoline quinone at their active sites as cofactors essential to their catalytic functions. It thus seems likely that these enzymes will share similar mechanisms of action. Since mechanism-based inhibitors of lysyl oxidase have important chemotherapeutic potential for the control of fibrotic disease, the relative inhibitory potential of such agents toward catalytically similar amine oxidases was assessed in the present study using purified preparations of lysyl oxidase, diamine oxidase, plasma amine oxidase and the flavin-dependent mitochondrial monoamine oxidase A and B. The results indicate that there is sufficient difference between the sensitivities of lysyl oxidase and the other amine oxidases to beta-aminopropionitrile to warrant its consideration as an antifibrotic agent in vivo, while also revealing that aminoguanidine, clorgyline and deprenyl are sufficiently selective for diamine oxidase, monoamine oxidase A and monoamide oxidase B, respectively, to differentiate between lysyl oxidase and these enzymes at appropriate concentrations.

Evidence for a functional role for histidine in lysyl oxidase catalysis

The pH-dependent kinetics of lysyl oxidase catalysis was examined for evidence of an ionizable enzyme residue which might function as a general base catalyzing proton abstraction previously shown to be a component of the mechanism of substrate processing by this enzyme. Plots of log Vmax/Km for the oxidation of n-hexylamine versus pH yielded pKa values of 7.0 +/- 0.1 and 10.4 +/- 0.1. The higher pKa varied with different substrates, reflecting ionization of the substrate amino group. A van't Hoff plot of the temperature dependence of the lower pKa yielded a value of 6.1 kcal mol-1 for the enthalpy of ionization. This value as well as the pKa of 7.0 are consistent with those of histidine residues previously implicated as general base catalysts in enzymes. Incubation of lysyl oxidase with low concentrations of diethyl pyrocarbonate, a histidine-selective reagent, at 22 degrees C and pH 7.0 irreversibly inhibited enzyme activity by a pseudo first-order kinetic process. The inactivation of lysyl oxidase correlated with spectral and pH-dependent kinetic evidence for the chemical modification of 1 histidine residue/mol of enzyme, the pKa of which was 6.9 +/- 0.1, within experimental error of that seen in the plot of log Vmax/Km versus pH. Enzyme activity was restored by incubation of the modified enzyme with hydroxylamine, consistent with the ability of this nucleophile to displace the carbethoxy group from N-carbethoxyhistidine. The presence of the n-hexylamine substrate largely protected against enzyme inactivation by diethyl pyrocarbonate. These results thus indicate a functional role for histidine in lysyl oxidase catalysis consistent with that of a general base in proton abstraction.

Inhibition by heparin of the oxidation of lysine in collagen by lysyl oxidase

The generation of covalent cross-linkages in collagen is initiated by the deamination by lysyl oxidase of specific lysine residues in this connective tissue protein. Since lysyl oxidase activity is influenced by ionic ligands bound to its protein substrates, the effect of heparin, an anionic glycosaminoglycan known to bind to collagen, was explored by using collagen and elastin substrates and highly purified lysyl oxidase. Concentrations of heparin up to 1 mg mL-1 had little effect on the enzymatic rate of oxidation if it was added prior to the addition of enzyme to a preformed fibrillar collagen substrate or to an insoluble elastin substrate. However, collagen oxidation was inhibited by 85% if this glycosaminoglycan was present at 0.4 mg mL-1 during collagen fibril formation before addition of the enzyme. Similarly, the rate and extent of collagen fibrillogenesis in the absence of lysyl oxidase were each markedly inhibited in the presence of 0.4 mg mL-1 heparin. Heparin also inhibited the extent of tight binding of lysyl oxidase to preformed fibrils by about 40% under conditions where enzyme activity against preformed fibrils was hardly affected. These results suggest that heparin may modulate the oxidation and thus the insolubilization of extracellular collagen fibers, possibly under conditions where elastin fiber synthesis is not affected, and that the tight binding of lysyl oxidase to collagen is not completely related to the expression of enzyme activity toward this substrate. These results also have mechanistic implications for the retarding effect of heparin on postoperative wound healing.

Electronegativity of aromatic amines as a basis for the development of ground state inhibitors of lysyl oxidase

Benzylamine derivatives containing para substituents of differing electronegativity as well as isomers of aminomethylpyridine have been assessed for their substrate and inhibitor potentials toward lysyl oxidase. Substituted benzylamines with increasingly electronegative para substituents had the lowest KI values and thus were the most effective inhibitors of the oxidation of elastin by lysyl oxidase. The kcat values for these compounds as substrates of lysyl oxidase were also reduced with increasingly electronegative para substituents. Both the Dkcat and D(kcat/Km) kinetic isotope effects decreased with increasingly electronegative p-substituents in [alpha, alpha'-2H]benzylamines. In contrast, there was no Dkcat solvent isotope effect with [2H] H2O while the D(kcat/Km) solvent isotope effect tended to increase with increasingly electronegative p-substituents. These results are consistent with the stabilization of an enzyme-generated substrate carbanion and thus the retardation of substrate oxidation by electronegative substituents. Such ground state stabilization thus can result in compounds with increased potential for the inhibition of the oxidation of protein substrates of lysyl oxidase.