Competitive inhibition of swine kidney copper amine oxidase by drugs: amiloride, clonidine, and gabexate mesylate

Efficient Oxidation of 5-Hydroxymethylfurfural Using a Flavoprotein Oxidase from the Honeybee Apis mellifera

The chemical 5-hydroxymethylfurfural (HMF) can be derived from lignocellulose and is an interesting bio-based platform chemical as it has the potential to be transformed into numerous valuable building blocks such as the polymer-precursor 2,5-diformylfuran (DFF). To date, only a few oxidases acting on HMF are known and by sampling atypical species, we discovered a novel flavin-dependent oxidoreductase from the honeybee Apis mellifera (beeHMFO). The enzyme can perform the chemoselective oxidation of HMF to DFF but can also readily accept other aromatic alcohols as substrates. The function of the enzyme may well be the antimicrobial generation of hydrogen peroxide using HMF, which is very abundant in honey. The discovery of this insect-derived flavoprotein oxidase holds promising potential in the synthesis of renewable products and demonstrates that insects can be an interesting source of novel biocatalysts.

Catalytic and spectroscopic properties of the halotolerant soluble methane monooxygenase reductase from Methylomonas methanica MC09

The soluble methane monooxygenase receives electrons from NADH via its reductase MmoC for oxidation of methane, which is itself an attractive C1 building block for a future bioeconomy. Herein, we present biochemical and spectroscopic insights into the reductase from the marine methanotroph Methylomonas methanica MC09. The presence of a flavin adenine dinucleotide (FAD) and [2Fe2S] cluster as its prosthetic group were revealed by reconstitution experiments, iron determination and electron paramagnetic resonance spectroscopy. As a true halotolerant enzyme, MmoC still showed 50 % of its specific activity at 2 M NaCl. We show that MmoC produces only trace amounts of superoxide, but mainly hydrogen peroxide during uncoupled turnover reactions. The characterization of a highly active reductase is an important step for future biotechnological applications of a halotolerant sMMO.

Structure-Based Engineering of Phanerochaete chrysosporium Alcohol Oxidase for Enhanced Oxidative Power toward Glycerol

Glycerol is a major byproduct of biodiesel production, and enzymes that oxidize this compound have been long sought after. The recently described alcohol oxidase from the white-rot basidiomycete Phanerochaete chrysosporium (PcAOX) was reported to feature very mild activity on glycerol. Here, we describe the comprehensive structural and biochemical characterization of this enzyme. PcAOX was expressed in Escherichia coli in high yields and displayed high thermostability. Steady-state kinetics revealed that PcAOX is highly active toward methanol, ethanol, and 1-propanol (k_cat = 18, 19, and 11 s^–1, respectively), but showed very limited activity toward glycerol (k_obs = 0.2 s^–1 at 2 M substrate). The crystal structure of the homo-octameric PcAOX was determined at a resolution of 2.6 Å. The catalytic center is a remarkable solvent-inaccessible cavity located at the re side of the flavin cofactor. Its small size explains the observed preference for methanol and ethanol as best substrates. These findings led us to design several cavity-enlarging mutants with significantly improved activity toward glycerol. Among them, the F101S variant had a high k_cat value of 3 s^–1, retaining a high degree of thermostability. The crystal structure of F101S PcAOX was solved, confirming the site of mutation and the larger substrate-binding pocket. Our data demonstrate that PcAOX is a very promising enzyme for glycerol biotransformation.

Agmatine: multifunctional arginine metabolite and magic bullet in clinical neuroscience?

Agmatine, the decarboxylation product of arginine, was largely neglected as an important player in mammalian metabolism until the mid-1990s, when it was re-discovered as an endogenous ligand of imidazoline and α2-adrenergic receptors. Since then, a wide variety of agmatine-mediated effects have been observed, and consequently agmatine has moved from a wallflower existence into the limelight of clinical neuroscience research. Despite this quantum jump in scientific interest, the understanding of the anabolism and catabolism of this amine is still vague. The purification and biochemical characterization of natural mammalian arginine decarboxylase and agmatinase still are open issues. Nevertheless, the agmatinergic system is currently one of the most promising candidates in order to pharmacologically interfere with some major diseases of the central nervous system, which are summarized in the present review. Particularly with respect to major depression, agmatine, its derivatives, and metabolizing enzymes show great promise for the development of an improved treatment of this common disease.

Ability of Kocuria varians LTH 1540 To Degrade Putrescine: Identification and Characterization of a Novel Amine Oxidase

This work describes the identification and characterization of an amine oxidase from Kocuria varians LTH 1540 (syn. Micrococcus varians) primarily acting on putrescine. Data from MALDI-TOF MS/MS and the identification of Δ(1)-pyrroline as degradation product from putrescine indicate that the enzyme is a flavin-dependent putrescine oxidase (PuO). Properties of partially purified enzyme have been determined. The enzyme oxidizes diamines, putrescine and cadaverine, and, to a lesser extent, polyamines, such as spermidine, but not monoamines. The kinetic constants (Km and Vmax) for the two major substrates were 94 ± 10 μM and 2.3 ± 0.1 μmol/min·mg for putrescine and 75 ± 5 μM and 0.15 ± 0.02 μmol/min·mg for cadaverine. Optimal temperature and pH were 45 °C and 8.5, respectively. Enzyme was stable until 50 °C. K. varians PuO is sensitive to human flavin-dependent amine oxidase inhibitors and carboxyl-modifying compounds. The new enzyme has been isolated from a bacterial starter used in the manufacture of fermented meat. One of the problems of fermented foods or beverages is the presence of toxic biogenic amines produced by bacteria. The importance of this works lies in the description of a new enzyme able to degrade two of the most abundant biogenic amines (putrescine and cadaverine), the use of which could be envisaged to diminish biogenic amines content in foods in the future.

Hot or not? Discovery and characterization of a thermostable alditol oxidase from Acidothermus cellulolyticus 11B

We describe the discovery, isolation and characterization of a highly thermostable alditol oxidase from Acidothermus cellulolyticus 11B. This protein was identified by searching the genomes of known thermophiles for enzymes homologous to Streptomyces coelicolor A3(2) alditol oxidase (AldO). A gene (sharing 48% protein sequence identity to AldO) was identified, cloned and expressed in Escherichia coli. Following 6xHis tag purification, characterization revealed the protein to be a covalent flavoprotein of 47 kDa with a remarkably similar reactivity and substrate specificity to that of AldO. A steady-state kinetic analysis with a number of different polyol substrates revealed lower catalytic rates but slightly altered substrate specificity when compared to AldO. Thermostability measurements revealed that the novel AldO is a highly thermostable enzyme with an unfolding temperature of 84 °C and an activity half-life at 75 °C of 112 min, prompting the name HotAldO. Inspired by earlier studies, we attempted a straightforward, exploratory approach to improve the thermostability of AldO by replacing residues with high B-factors with corresponding residues from HotAldO. None of these mutations resulted in a more thermostable oxidase; a fact that was corroborated by in silico analysis.

Reaction of vascular adhesion protein-1 (VAP-1) with primary amines: mechanistic insights from isotope effects and quantitative structure-activity relationships

Human vascular adhesion protein-1 (VAP-1) is an endothelial copper-dependent amine oxidase involved in the recruitment and extravasation of leukocytes at sites of inflammation. VAP-1 is an important therapeutic target for several pathological conditions. We expressed soluble VAP-1 in HEK293 EBNA1 cells at levels suitable for detailed mechanistic studies with model substrates. Using the model substrate benzylamine, we analyzed the steady-state kinetic parameters of VAP-1 as a function of solution pH. We found two macroscopic pK(a) values that defined a bell-shaped plot of turnover number k(cat,app) as a function of pH, representing ionizable groups in the enzyme-substrate complex. The dependence of (k(cat)/K(m))(app) on pH revealed a single pK(a) value (∼9) that we assigned to ionization of the amine group in free benzylamine substrate. A kinetic isotope effect (KIE) of 6 to 7.6 on (k(cat)/K(m))(app) over the pH range of 6 to 10 was observed with d(2)-benzylamine. Over the same pH range, the KIE on k(cat) was found to be close to unity. The unusual KIE values on (k(cat)/K(m))(app) were rationalized using a mechanistic scheme that includes the possibility of multiple isotopically sensitive steps. We also report the analysis of quantitative structure-activity relationships (QSAR) using para-substituted protiated and deuterated phenylethylamines. With phenylethylamines we observed a large KIE on k(cat,app) (8.01 ± 0.28 with phenylethylamine), indicating that C-H bond breakage is limiting for 2,4,5-trihydroxyphenylalanine quinone reduction. Poor correlations were observed between steady-state rate constants and QSAR parameters. We show the importance of combining KIE, QSAR, and structural studies to gain insight into the complexity of the VAP-1 steady-state mechanism.

Cell wall peroxidases in the liverwort Dumortiera hirsuta are responsible for extracellular superoxide production, and can display tyrosinase activity

In our earlier work, we showed that the liverwort Dumortiera hirsuta produces an extracellular oxidative burst of superoxide radicals during rehydration following desiccation stress. The oxidative burst is a common early response of organisms to biotic and abiotic stresses, with suggested roles in signal transduction, formation of protective substances such as suberin, melanin and lignin and defense against pathogens. To discover which enzymes are responsible for the extracellular superoxide production, we isolated apoplastic fractions from D. hirsuta, surveyed for the presence of potential redox enzymes, and performed non-denaturing polyacrylamide gel electrophoresis activity stains. Various isoforms of peroxidase (EC 1.11.1.7) and tyrosinase (o-diphenolase) (EC 1.10.3.1) were present at significant levels in the apoplast. In-gel activity staining revealed that some peroxidases isoforms could produce superoxide, while tryosinases could readily metabolize 3,4-dihydroxy phenyl l-alanine (l-dopa) into melanins. Interestingly, some peroxidase isoforms could oxidize the native tyrosinase substrate l-dopa at significant levels, even in the absence of hydrogen peroxide, while others could do so only in the presence of hydrogen peroxide. In D. hirsuta, peroxidases may play an important role in melanin formation. Possible functions for these diverse oxidases in liverwort biology are discussed.

Export of functional Streptomyces coelicolor alditol oxidase to the periplasm or cell surface of Escherichia coli and its application in whole-cell biocatalysis

Streptomyces coelicolor A3(2) alditol oxidase (AldO) is a soluble monomeric flavoprotein in which the flavin cofactor is covalently linked to the polypeptide chain. AldO displays high reactivity towards different polyols such as xylitol and sorbitol. These characteristics make AldO industrially relevant, but full biotechnological exploitation of this enzyme is at present restricted by laborious and costly purification steps. To eliminate the need for enzyme purification, this study describes a whole-cell AldO biocatalyst system. To this end, we have directed AldO to the periplasm or cell surface of Escherichia coli. For periplasmic export, AldO was fused to endogenous E. coli signal sequences known to direct their passenger proteins into the SecB, signal recognition particle (SRP), or Twin-arginine translocation (Tat) pathway. In addition, AldO was fused to an ice nucleation protein (INP)-based anchoring motif for surface display. The results show that Tat-exported AldO and INP-surface-displayed AldO are active. The Tat-based system was successfully employed in converting xylitol by whole cells, whereas the use of the INP-based system was most likely restricted by lipopolysaccharide LPS in wild-type cells. It is anticipated that these whole-cell systems will be a valuable tool for further biological and industrial exploitation of AldO and other cofactor-containing enzymes.

Discovery and characterization of a putrescine oxidase from Rhodococcus erythropolis NCIMB 11540

A gene encoding a putrescine oxidase (PuORh, EC 1.4.3.10) was identified from the genome of Rhodococcus erythropolis NCIMB 11540. The gene was cloned in the pBAD vector and overexpressed at high levels in Escherichia coli. The purified enzyme was shown to be a soluble dimeric flavoprotein consisting of subunits of 50 kDa and contains non-covalently bound flavin adenine dinucleotide as a cofactor. From all substrates, the highest catalytic efficiency was found with putrescine (KM=8.2 microM, kcat=26 s(-1)). PuORh accepts longer polyamines, while short diamines and monoamines strongly inhibit activity. PuORh is a reasonably thermostable enzyme with t1/2 at 50 degrees C of 2 h. Based on the crystal structure of human monoamine oxidase B, we constructed a model structure of PuORh, which hinted to a crucial role of Glu324 for substrate binding. Mutation of this residue resulted in a drastic drop (five orders of magnitude) in catalytic efficiency. Interestingly, the mutant enzyme showed activity with monoamines, which are not accepted by wt-PuORh.

Discovery, characterization, and kinetic analysis of an alditol oxidase from Streptomyces coelicolor

A gene encoding an alditol oxidase was found in the genome of Streptomyces coelicolor A3(2). This newly identified oxidase, AldO, was expressed at extremely high levels in Escherichia coli when fused to maltose-binding protein. AldO is a soluble monomeric flavoprotein with subunits of 45.1 kDa, each containing a covalently bound FAD cofactor. From sequence alignments with other flavoprotein oxidases, it was found that AldO contains a conserved histidine (His(46)) that is typically involved in covalent FAD attachment. Covalent FAD binding is not observed in the H46A AldO mutant, confirming its role in covalent attachment of the flavin cofactor. Steady-state kinetic analyses revealed that wild-type AldO is active with several polyols. The alditols xylitol (K(m) = 0.32 mm, k(cat) = 13 s(-1)) and sorbitol (K(m) = 1.4 mm, k(cat) = 17 s(-1)) are the preferred substrates. From pre-steady-state kinetic analyses, using xylitol as substrate, it can be concluded that AldO mainly follows a ternary complex kinetic mechanism. Reduction of the flavin cofactor by xylitol occurs at a relatively high rate (99 s(-1)), after which a second kinetic event is observed, which is proposed to represent ring closure of the formed aldehyde product, yielding the hemiacetal of d-xylose. Reduced AldO readily reacts with molecular oxygen (1.7 x 10(5) m(-1) s(-1)), which confirms that the enzyme represents a true flavoprotein oxidase.

Histaminase PEGylation: Preparation and characterization of a new bioconjugate for therapeutic application

Copper amine oxidase catalyses the oxidative deamination of primary amino groups of several biogenic amines, one of which is histamine, the principal chemical mediator of the first phase of allergic reactions. Looking forward to a possible future therapeutic application of this enzyme in the field of histamine-mediated afflictions, we developed a simple method for the purification of a histaminase from grass pea shoots, a source particularly enriched with the enzyme. Furthermore, in order to improve its therapeutic potential, in particular to reduce the high impurity due to its heterologous source, we conjugated the protein with poly(ethylene glycol) and tested the molecular, immunogenic and pharmacokinetic properties of the native and modified forms. The PEGylated enzyme showed molecular and enzymatic properties similar to those of the unmodified one, but the PEGylation extended the permanence of the injected drug in the body and eliminated its high immunogenic behaviour. The considerable ease of native histaminase production as well as the improved properties after PEGylation, make this engineered plant enzyme a suitable drug candidate for alternative treatment of histamine-mediated affections.

Agmatine oxidation by copper amine oxidase

The product of agmatine oxidation catalyzed by Pisum sativum L. copper amine oxidase has been identified by means of one- and two-dimensional (1)H-NMR spectroscopy to be N-amidino-2-hydroxypyrrolidine. This compound inhibits competitively rat nitric oxide synthase type I and type II (NOS-I and NOS-II, respectively) and bovine trypsin (trypsin) activity, values of Ki being (1.1 +/- 0.1) x 10(-5) m (at pH 7.5 and 37.0 degrees C), (2.1 +/- 0.1) x 10(-5) m (at pH 7.5 and 37.0 degrees C), and (8.9 +/- 0.4) x 10(-5) m (at pH 6.8 and 21.0 degrees C), respectively. Remarkably, the affinity of N-amidino-2-hydroxypyrrolidine for NOS-I, NOS-II and trypsin is significantly higher than that observed for agmatine and clonidine binding. Furthermore, N-amidino-2-hydroxypyrrolidine and agmatine are more efficient than clonidine in displacing [(3)H]clonidine (= 1.0 x 10(-8) m) from specific binding sites in heart rat membranes, values of IC50 being (1.3 +/- 0.4) x 10(-9) m and (2.2 +/- 0.4) x 10(-8) m, respectively (at pH 7.4 and 37.0 degrees C).

Combinatorial synthesis of SSAO inhibitors using sonogashira coupling: SAR of aryl propargylic amines

The structure-activity relationships for semicarbazide-sensitive amine oxidase (SSAO) inhibitors based on arylpropynylamines was investigated using solution-phase combinatorial Sonogashira coupling. The results suggest that binding to the active site occurs by coordination of the amine to the proximal copper(II) and formation of a pi-complex between topaquinone and the electron-rich aryl group of the inhibitor.

Selective inhibition of human mast cell tryptase by gabexate mesylate, an antiproteinase drug

Gabexate mesylate is a non-antigenic synthetic inhibitor of trypsin-like serine proteinases that is therapeutically used in the treatment of pancreatitis and disseminated intravascular coagulation and as a regional anticoagulant for hemodialysis. Considering the structural similarity between gabexate mesylate and arginine-based inhibitors of trypsin-like serine proteinases, the effect of gabexate mesylate on human and bovine mast cell tryptase action was investigated. Values of the inhibition constant (K(i)) for gabexate mesylate binding to human and bovine tryptase were 3.4 x 10(-9) M and 1.8 x 10(-7) M (at pH 7.4 and 37.0 degrees ), respectively. Furthermore, gabexate mesylate inhibited the fibrinogenolytic activity of human tryptase. On the basis of the available x-ray crystal structure of human tryptase, the possible binding mode of gabexate mesylate to human and bovine tryptase was analyzed. Human tryptase inhibition by gabexate mesylate may account for the reported prevention of inflammation, erosion, and ulceration of skin and mucosae.

Inhibition of pig liver and Zea mays L. polyamine oxidase: a comparative study

Polyamine oxidase (PAO) is involved in polyamine metabolism and production of hydrogen peroxide in animal and plants, thus representing a key system in development and programmed cell death. In the present study, the inhibitory effect of amiloride, p-aminobenzamidine, clonidine, 4',6-diamidino-2-phenyl-indole (DAPI), gabexate mesylate, guazatine, and N,N'-bis(2,3-butadienyl)-1,4-butane-diamine (MDL72527) on the catalytic activity of pig liver and Zea mays L. PAO, Lens culinaris L. and Pisum sativum L. and swine kidney copper amine oxidase, bovine trypsin, as well as neuronal constitutive nitric oxide synthase (NOS-I) was investigated. Moreover, agmatine and N(3) -prenylagmatine (G3) were observed to inhibit pig liver and Zea mays L. PAO, bovine trypsin, and NOS-I action, but were substrates for Lens culinaris L., Pisum sativum L. and swine kidney copper amine oxidase. Guazatine and G3 inhibited selectively Zea mays L. PAO with K(i) values of 7.5 x 10(-9) M and 1.5 x 10(-8) M, respectively (at pH 6.5 and 25.0 degrees C). As a whole, the data reported here represent examples of enzyme cross-inhibition, and appear to be relevant in view of the use of cationic L-arginine-and imidazole-based compounds as drugs.

Selective inhibition of nitric oxide synthase type I by clonidine, an anti-hypertensive drug

Clonidine, clinically used in the treatment of hypertension, is a central alpha(2)-adrenergic agonist that reduces blood pressure and slows heart rate by reducing sympathetic stimulation. Considering the structural similarity between clonidine and hydrophobic heterocyclic nitric oxide synthase (NOS) inhibitors, the effect of clonidine on the nitric oxide (NO) pathway was investigated. This was verified by determination of NOS activity in vitro and by analysis of inducible Ca(2+)-independent NOS (NOS-II) mRNA expression and measurement of nitrite levels in rat C6 glioma cells, taken as a cellular model. Clonidine inactivated neuronal Ca(2+)-dependent NOS (NOS-I) competitively without affecting NOS-II and endothelial Ca(2+)-dependent NOS (NOS-III) activity. However, the value of K(i) for clonidine binding to NOS-I depended on tetrahydrobiopterin (BH(4)) concentration, as reported for NOS inhibition by other nitrogen heterocyclic compounds. In particular, the value of K(i) for clonidine binding to NOS-I increased (from [7. 9 +/- 0.4] x 10(-5) M to [8.0 +/- 0.4] x 10(-3) M) as BH(4) concentration was increased (between 3.0 x 10(-7) M and 1.0 x 10(-3) M), at pH 7.5 and 37.0 degrees. In addition, clonidine (1.0 x 10(-4) M) enhanced NOS-II mRNA expression in rat C6 glioma cells, as induced by Escherichia coli lipopolysaccharide (LPS) plus interferon-gamma (IFN-gamma). Finally, clonidine (1.0 x 10(-4) M to 1.0 x 10(-3) M) dose dependently increased the levels of LPS/IFN-gamma-induced nitrites, the breakdown product of NO, in supernatants of rat C6 glioma cells. As reported for other NOS inhibitors, clonidine was also able to regulate NOS-I and NOS-II inversely.

Cross-enzyme inhibition by gabexate mesylate: formulation and reactivity study

Gabexate mesylate (GM; commercialized under the brand name FOY) is a nonantigenic synthetic inhibitor of plasmatic and pancreatic serine proteinases that is used therapeutically in the treatment of pancreatitis and disseminated intravascular coagulation and as a regional anticoagulant for hemodialysis. The inhibitory effect of GM on nitric oxide synthase as well as serine proteinases and swine kidney copper amine oxidase, all acting on cationic substrates, has been investigated. On the basis of the available X-ray crystal structures of the enzymes considered, the possible binding mode(s) of GM has(have) been analyzed. The enzyme cross-inhibition by GM suggests that the use of this drug should be under careful control. With the aim to improve the scarce plasma stability of GM, the positively charged drug has been complexed to the surface of preformed anionic liposomes. The liposome-complexed GM half-life increases about five-fold, indicating the protective effect of liposomes on GM degradation. Moreover, the GM complexation with liposomes does not alter its inhibitory activity on NOS-I and porcine pancreatic trypsin.

Competitive inhibition of Lens culinaris L. copper amine oxidase by amiloride, p-aminobenzamidine, clonidine, 4',6-diamidino-2-phenylindole and gabexate mesylate: a comparative study

The competitive inhibition of Lens culinaris L. copper amine oxidase by amiloride (Ki = 4.1 x 10(-4) M), p-aminobenzamidine (Ki = 6.0 x 10(-4) M), clonidine (Ki = 5.0 x 10(-4) M), 4',6-diamidino-2-phenylindole (DAPI; Ki = 1.9 x 10(-5) M) and gabexate mesylate (Ki = 2.5 x 10(-4) M) has been investigated, at pH 7.0 and 25 degrees C. The affinity of p-aminobenzamidine, clonidine and DAPI for plant and mammalian copper amine oxidase is closely similar. However, values of Ki for amiloride and gabexate mesylate binding to swine kidney copper amine oxidase are lower than those observed for inhibitor binding to Lens culinaris L. cooper amine oxidase. Thus, amiloride and gabexate mesylate may represent useful model compounds for the development of selective inhibitors of mammalian copper amine oxidase, which may be important in view of the potential use of plant copper amine oxidase as drugs.

Effect of gabexate mesylate (FOY), a drug for serine proteinase-mediated diseases, on the nitric oxide pathway

Considering the structural similarity between gabexate mesylate (FOY), a drug for serine proteinase-mediated diseases, and L-arginine, the effect of gabexate mesylate on the nitric oxide (NO) pathway has been investigated. Gabexate mesylate inhibits competitively constitutive and inducible NO synthase (cNOS and iNOS, respectively), with Ki values of 1.0 x 10(-4) M and 5.0 x 10(-3) M, respectively, at pH 7.4 and 37.0 degrees C. However, gabexate mesylate is not an NO precursor. Moreover, like other NOS inhibitors, gabexate mesylate increases iNOS mRNA expression in rat C6 glioma cells, as induced by E. coli lipopolysaccharide plus interferon-gamma. Finally, gabexate mesylate inhibits dose-dependently nitrite production (i.e. NO release) in rat C6 glioma cells, as induced by E. coli lipopolysaccharide plus interferon-gamma. Thus, this drug should be administered under careful control, since enzyme inhibition may occur also in vivo.