Structural insights into the mechanism of amine oxidation by monoamine oxidases A and B

In silico identification of novel and selective monoamine oxidase B inhibitors

Monoamine oxidases (MAO) A and B are flavin adenine dinucleotides containing enzymes bound to the mitochondrial outer membranes of the cells of the brain, liver, intestine, and placenta, as well as platelets. Recently, selective MAO-B inhibitors have received increasing attention due to their neuroprotective properties and the multiple roles they can play in the therapy of neurodegenerative disorders. This study was based on 10 scaffolds that were selected from more than a million lead compounds in the ZINCv12 lead library for their structural and physicochemical properties which inhibit MAO-B. Utilizing ZINC and Accelrys 3.1 fragment-based libraries, which contain about 400 thousand fragments, we generated 200 potential candidates. GOLD, LibDock, and AutoDock 4.02 were used to identify the inhibition constants and their position in the active sites of both MAO isozymes. The dispositions of the candidate molecules within the organism were checked with ADMET PSA 2D (polar surface area) against ADMET AlogP98 (the logarithm of the partition coefficient between n-octanol and water). The MAO-B inhibition activities of the candidates were compared with the properties of rasagiline which is known to be a selective inhibitor of MAO-B.

Flavin-dependent enzymes in cancer prevention

Statistical studies have demonstrated that various agents may reduce the risk of cancer's development. One of them is activity of flavin-dependent enzymes such as flavin-containing monooxygenase (FMO)(GS-OX1), FAD-dependent 5,10-methylenetetrahydrofolate reductase and flavin-dependent monoamine oxidase. In the last decade, many papers concerning their structure, reaction mechanism and role in the cancer prevention were published. In our work, we provide a more in-depth analysis of flavin-dependent enzymes and their contribution to the cancer prevention. We present the actual knowledge about the glucosinolate synthesized by flavin-containing monooxygenase (FMO)(GS-OX1) and its role in cancer prevention, discuss the influence of mutations in FAD-dependent 5,10-methylenetetrahydrofolate reductase on the cancer risk, and describe FAD as an important cofactor for the demethylation of histons. We also present our views on the role of riboflavin supplements in the prevention against cancer.

Rasagiline and selegiline, inhibitors of type B monoamine oxidase, induce type A monoamine oxidase in human SH-SY5Y cells

Type B monoamine oxidase (MAO-B) is proposed to be involved in the pathogenesis of neurodegenerative disorders, such as Parkinson's disease, through oxidative stress and synthesis of neurotoxins. MAO-B inhibitors, rasagiline and selegiline [(-)deprenyl], protect neuronal cells by direct intervention in mitochondrial death signaling and induction of pro-survival Bcl-2 and neurotrophic factors. Recently, type A MAO (MAO-A) was found to mediate the induction of anti-apoptotic Bcl-2 by rasagiline, whereas MAO-A increases in neuronal death and also serves as a target of neurotoxins. These controversial results suggest that MAO-A may play a decisive role in neuronal survival and death. This paper reports that rasagiline and selegiline increased the mRNA, protein and catalytic activity of MAO-A in SH-SY5Y cells. Silencing MAO-A expression with small interfering (si)RNA suppressed rasagiline-dependent MAO-A expression, but MAO-B overexpression in SH-SY5Y cells did not affect, suggesting that MAO-A, not MAO-B, might be associated with MAO-A upregulation. Rasagiline reduced R1, a MAO-A specific repressor, but selegiline did not. Mithramycin-A, an inhibitor of Sp1 binding, and actinomycin-D, a transcriptional inhibitor, reduced the rasagiline-dependent upregulation of MAO-A mRNA, indicating that rasagiline induced MAO-A transcriptionally through R1-Sp1 pathway, whereas selegiline by another non-defined pathway. These results are discussed in relation to the role of MAO-A and these MAO-B inhibitors in neuronal death and neuroprotection.

Mechanistic and structural analyses of the role of His67 in the yeast polyamine oxidase Fms1

The flavoprotein oxidase Fms1 from Saccharomyces cerevisiae catalyzes the oxidation of spermine and N(1)-acetylspermine to spermidine and 3-aminopropanal or N-acetyl-3-aminopropanal. Within the active site of Fms1, His67 is positioned to form hydrogen bonds with the polyamine substrate. This residue is also conserved in other polyamine oxidases. The catalytic properties of H67Q, H67N, and H67A Fms1 have been characterized to evaluate the role of this residue in catalysis. With both spermine and N(1)-acetylspermine as the amine substrate, the value of the first-order rate constant for flavin reduction decreases 2-3 orders of magnitude, with the H67Q mutation having the smallest effect and H67N the largest. The k(cat)/K(O2) value changes very little upon mutation with N(1)-acetylspermine as the amine substrate and decreases only an order of magnitude with spermine. The k(cat)/K(M)-pH profiles with N(1)-acetylspermine are bell-shaped for all the mutants; the similarity to the profile of the wild-type enzyme rules out His67 as being responsible for either of the pK(a) values. The pH profiles for the rate constant for flavin reduction for all the mutant enzymes similarly show the same pK(a) as wild-type Fms1, about ∼7.4; this pK(a) is assigned to the substrate N4. The k(cat)/K(O2)-pH profiles for wild-type Fms1 and the H67A enzyme both show a pK(a) of about ∼6.9; this suggests His67 is not responsible for this pH behavior. With the H67Q, H67N, and H67A enzymes the k(cat) value decreases when a single residue is protonated, as is the case with the wild-type enzyme. The structure of H67Q Fms1 has been determined at a resolution of 2.4 Å. The structure shows that the mutation disrupts a hydrogen bond network in the active site, suggesting that His67 is important both for direct interactions with the substrate and to maintain the overall active site structure.

Mitochondria and reactive oxygen species. Which role in physiology and pathology?

Oxidative stress is among the major causes of toxicity due to interaction of Reactive Oxygen Species (ROS) with cellular macromolecules and structures and interference with signal transduction pathways. The mitochondrial respiratory chain, specially from Complexes I and III, is considered the main origin of ROS particularly under conditions of high membrane potential, but several other sources may be important for ROS generation, such as mitochondrial p66(Shc), monoamine oxidase, α-ketoglutarate dehydogenase, besides redox cycling of redox-active molecules. ROS are able to oxidatively modify lipids, proteins, carbohydrates and nucleic acids in mitochondria and to activate/inactivate signalling pathways by oxidative modification of redox-active factors. Cells are endowed with several defence mechanisms including repair or removal of damaged molecules, and antioxidant systems, either enzymatic or non-enzymatic. Oxidative stress is at the basis of ageing and many pathological disorders, such as ischemic diseases, neurodegenerative diseases, diabetes, and cancer, although the underlying mechanisms are not always completely understood.

Predicting monoamine oxidase inhibitory activity through ligand-based models

The evolution of bio- and cheminformatics associated with the development of specialized software and increasing computer power has produced a great interest in theoretical in silico methods applied in drug rational design. These techniques apply the concept that "similar molecules have similar biological properties" that has been exploited in Medicinal Chemistry for years to design new molecules with desirable pharmacological profiles. Ligand-based methods are not dependent on receptor structural data and take into account two and three-dimensional molecular properties to assess similarity of new compounds in regards to the set of molecules with the biological property under study. Depending on the complexity of the calculation, there are different types of ligand-based methods, such as QSAR (Quantitative Structure- Activity Relationship) with 2D and 3D descriptors, CoMFA (Comparative Molecular Field Analysis) or pharmacophoric approaches. This work provides a description of a series of ligand-based models applied in the prediction of the inhibitory activity of monoamine oxidase (MAO) enzymes. The controlled regulation of the enzymes' function through the use of MAO inhibitors is used as a treatment in many psychiatric and neurological disorders, such as depression, anxiety, Alzheimer's and Parkinson's disease. For this reason, multiple scaffolds, such as substituted coumarins, indolylmethylamine or pyridazine derivatives were synthesized and assayed toward MAO-A and MAO-B inhibition. Our intention is to focus on the description of ligand-based models to provide new insights in the relationship between the MAO inhibitory activity and the molecular structure of the different inhibitors, and further study enzyme selectivity and possible mechanisms of action.

Synthesis and biological assessment of novel 2-thiazolylhydrazones and computational analysis of their recognition by monoamine oxidase B

Monoamine oxidase B (MAO-B) is a promising target for the treatment of neurodegenerative disorders. We report the synthesis and the biological evaluation of halogenated derivatives of 1-aryliden-2-(4-phenylthiazol-2-yl)hydrazines. The fluorinated series shows interesting activity and great selectivity toward the human recombinant MAO-B isoform expressed in baculovirus infected BTI insect cells. The multiple crystal structures alignment of the enzyme highlighted pronounced induced fit (IF) adaptations with respect to bound ligands. Therefore, IF docking (IFD) experiments and molecular dynamic (MD) simulations were carried out to reveal the putative binding mode and to explain the experimentally observed differences in the activity of 1-(aryliden-2-(4-(4-chlorophenyl)thiazol-2-yl)hydrazines. The importance of water molecules within the binding site was also investigated. These are known to play an important role in the binding site cavity and to mediate protein-ligand interactions. Detailed analyses of the trajectories provide insights on the chemical features required for the activity of this scaffold. In particular it was highlighted the importance of fluorine atom interacting with the water close to the cofactor and the influence of steric bulkiness of substituents in the arylidene moiety. Free energy perturbation (FEP) analysis confirmed experimental data. The information we deduced will help to develop novel high-affinity MAO-B inhibitors.

Structures and Mechanism of the Monoamine Oxidase Family

Members of the monoamine oxidase family of flavoproteins catalyze the oxidation of primary and secondary amines, polyamines, amino acids, and methylated lysine side chains in proteins. The enzymes have similar overall structures, with conserved FAD-binding domains and varied substrate-binding sites. Multiple mechanisms have been proposed for the catalytic reactions of these enzymes. The present review compares the structures of different members of the family and the various mechanistic proposals.

Catalytic mechanism investigation of lysine-specific demethylase 1 (LSD1): a computational study

Lysine-specific demethylase 1 (LSD1), the first identified histone demethylase, is a flavin-dependent amine oxidase which specifically demethylates mono- or dimethylated H3K4 and H3K9 via a redox process. It participates in a broad spectrum of biological processes and is of high importance in cell proliferation, adipogenesis, spermatogenesis, chromosome segregation and embryonic development. To date, as a potential drug target for discovering anti-tumor drugs, the medical significance of LSD1 has been greatly appreciated. However, the catalytic mechanism for the rate-limiting reductive half-reaction in demethylation remains controversial. By employing a combined computational approach including molecular modeling, molecular dynamics (MD) simulations and quantum mechanics/molecular mechanics (QM/MM) calculations, the catalytic mechanism of dimethylated H3K4 demethylation by LSD1 was characterized in details. The three-dimensional (3D) model of the complex was composed of LSD1, CoREST, and histone substrate. A 30-ns MD simulation of the model highlights the pivotal role of the conserved Tyr761 and lysine-water-flavin motif in properly orienting flavin adenine dinucleotide (FAD) with respect to substrate. The synergy of the two factors effectively stabilizes the catalytic environment and facilitated the demethylation reaction. On the basis of the reasonable consistence between simulation results and available mutagenesis data, QM/MM strategy was further employed to probe the catalytic mechanism of the reductive half-reaction in demethylation. The characteristics of the demethylation pathway determined by the potential energy surface and charge distribution analysis indicates that this reaction belongs to the direct hydride transfer mechanism. Our study provides insights into the LSD1 mechanism of reductive half-reaction in demethylation and has important implications for the discovery of regulators against LSD1 enzymes.

3-Acetyl-2,5-diaryl-2,3-dihydro-1,3,4-oxadiazoles: a new scaffold for the selective inhibition of monoamine oxidase B

3-Acetyl-2,5-diaryl-2,3-dihydro-1,3,4-oxadiazoles were designed, synthesized, and tested as inhibitors against human monoamine oxidase (MAO) A and B isoforms. Several compounds, obtained as racemates, were identified as selective MAO-B inhibitors. The enantiomers of some derivatives were separated by enantioselective HPLC and tested. The R-enantiomers always showed the highest activity. Docking study and molecular dynamic simulations demonstrated the putative binding mode. We conclude that these 1,3,4-oxadiazoles derivatives are promising reversible and selective MAO-B inhibitors.

²H kinetic isotope effects and pH dependence of catalysis as mechanistic probes of rat monoamine oxidase A: comparisons with the human enzyme

Monoamine oxidase A (MAO A) is a mitochondrial outer membrane-bound flavoenzyme important in the regulation of serotonin and dopamine levels. Because the rat is extensively used as an animal model in drug studies, it is important to understand how rat MAO A behaves in comparison with the more extensively studied human enzyme. For many reversible inhibitors, rat MAO A exhibits K(i) values similar to those of human MAO A. The pH profile of k(cat) for rat MAO A shows a pK(a) of 8.2 ± 0.1 for the benzylamine ES complex and pK(a) values of 7.5 ± 0.1 and 7.6 ± 0.1 for the ES complexes with p-CF(3)-(1)H- and p-CF(3)-(2)H-benzylamine, respectively. In contrast to the human enzyme, the rat enzyme exhibits a single pK(a) value (8.3 ± 0.1) with k(cat)/K(m) for benzylamine versus pH and pK(a) values of 7.8 ± 0.1 and 8.1 ± 0.2 for the ascending limbs, respectively, of k(cat)/K(m) versus pH profiles for p-CF(3)-(1)H- and p-CF(3)-(2)H-benzylamine and 9.3 ± 0.1 and 9.1 ± 0.2 for the descending limbs, respectively. The oxidation of para-substituted benzylamine substrate analogues by rat MAO A has large deuterium kinetic isotope effects on k(cat) and on k(cat)/K(m). These effects are pH-independent and range from 7 to 14, demonstrating a rate-limiting α-C-H bond cleavage step in catalysis. Quantitative structure-activity correlations of log k(cat) with the electronic substituent parameter (σ) at pH 7.5 and 9.0 show a dominant contribution with positive ρ values (1.2-1.3) and a pH-independent negative contribution from the steric term. Quantitative structure-activity relationship analysis of the binding affinities of the para-substituted benzylamine analogues for rat MAO A shows an increased van der Waals volume (V(w)) increases the affinity of the deprotonated amine for the enzyme. These results demonstrate that rat MAO A exhibits functional properties similar but not identical with those of the human enzyme and provide additional support for C-H bond cleavage via a polar nucleophilic mechanism.

Computational investigation on the structure-activity relationship of the biradical mechanism for monoamine oxidase

Although a considerable amount of mechanistic data has accumulated in literature, the detailed mechanism for amine oxidation by monoamine oxidase is still controversial. The single electron transfer mechanism (SET) has been widely discussed, but not completely understood yet. In the present study, the modified SET mechanism, proposed by Silverman et al., was explored by quantum chemical calculations. The ONIOM method was applied with UDFT/B3LYP/6-31 + G(d,p) for the higher layer and with UHF/6-31G(d) for the lower layer. Isoalloxazin heterocyclic ring and benzylamine were employed in the calculations to represent flavin and the substrate, respectively. The substituents CH(3), OH, OCH(3), H, F, Cl, Br, CF(3) and NO(2) were incorporated at the para position of benzylamine to explore structure-activity relationships. The structures of the reactant complex, transition state and product complex were fully optimized. Activation energies and rate constants of all the reactions were calculated. The results obtained from the linear regression analysis showed that electron-donating groups at the para position of benzylamine increase the reaction rate. A linear but inverse correlation between the log of the calculated rate constants (log k) and the electronic parameter of the substituent was observed (R = 0.93). In accordance with this result, a relatively weak inverse correlation between the calculated log k and the experimental log k was obtained (R = 0.78). The results are contrary to the previous kinetic experiments and the computational study on the effect of p-substituents in the flavin reduction of MAO A by p-substituted benzylamine analogs. Therefore, they present negative evidence for the modeled biradical mechanism.

Crystallographic snapshots of the complete reaction cycle of nicotine degradation by an amine oxidase of the monoamine oxidase (MAO) family

FAD-linked oxidases constitute a class of enzymes which catalyze dehydrogenation as a fundamental biochemical reaction, followed by reoxidation of reduced flavin. Here, we present high-resolution crystal structures showing the flavoenzyme 6-hydroxy-l-nicotine oxidase in action. This enzyme was trapped during catalytic degradation of the native substrate in a sequence of discrete reaction states corresponding to the substrate-reduced enzyme, a complex of the enzyme with the intermediate enamine product and formation of the final aminoketone product. The inactive d-stereoisomer binds in mirror symmetry with respect to the catalytic axis, revealing absolute stereospecificity of hydrogen transfer to the flavin. The structural data suggest deprotonation of the substrate when bound at the active site, an overall binary complex mechanism and oxidation by direct hydride transfer. The amine nitrogen has a critical role in the dehydrogenation step and may activate carbocation formation at the α-carbon via delocalization from the lone pair to σ* C(α)-H. Enzymatically assisted hydrolysis of the intermediate product occurs at a remote (P site) cavity. Substrate entry and product exit follow different paths. Structural and kinetic data suggest that substrate can also bind to the reduced enzyme, associated with slower reoxidation as compared to the rate of reoxidation of free enzyme. The results are of general relevance for the mechanisms of flavin amine oxidases.

Catalytic and inhibitor binding properties of zebrafish monoamine oxidase (zMAO): comparisons with human MAO A and MAO B

A comparative investigation of substrate specificity and inhibitor binding properties of recombinant zebrafish (Danio rerio) monoamine oxidase (zMAO) with those of recombinant human monoamine oxidases A and B (hMAO A and hMAO B) is presented. zMAO oxidizes the neurotransmitter amines (serotonin, dopamine and tyramine) with k(cat) values that exceed those of hMAO A or of hMAO B. The enzyme is competitively inhibited by hMAO A selective reversible inhibitors with the exception of d-amphetamine where uncompetitive inhibition is exhibited. The enzyme is unreactive with most MAO B-specific reversible inhibitors with the exception of chlorostyrylcaffeine. zMAO catalyzes the oxidation of para-substituted benzylamine analogs exhibiting (D)k(cat) and (D)(k(cat)/K(m)) values ranging from 2 to 8. Structure-activity correlations show a dependence of log k(cat) with the electronic factor σ(p) with a ρ value of +1.55±0.34; a value close to that for hMAO A but not with MAO B. zMAO differs from hMAO A or hMAO B in benzylamine analog binding correlations where an electronic effect (ρ=+1.29±0.31) is observed. These data demonstrate zMAO exhibits functional properties similar to hMAO A as well as exhibits its own unique behavior. These results should be useful for studies of MAO function in zebrafish models of human disease states.

The butanol fraction of Eclipta prostrata (Linn) increases the formation of brain acetylcholine and decreases oxidative stress in the brain and serum of cesarean-derived rats

Eclipta prostrata has been used as a traditional medicinal plant to prevent dementia and to enhance memory in Asia. Its potential as a nootropic and as an antioxidant have been reported in mice. We hypothesized that Eclipta may affect the formation of neurotransmitters and the inhibition of oxidative stress. Charles River cesarean-derived rats (male, 180 ± 10 g) were fed experimental diets supplemented with 0 mg (control), 25 mg (E25), 50 mg (E50), or 100 mg (E100) of a freeze-dried butanol fraction of E prostrata per kilogram of diet for 6 weeks. The acetylcholine level was significantly increased by 9.6% and 12.1% in the brains of E50 and E100 groups, respectively, as compared with the control group that was fed standard diet alone. The acetylcholine esterase activity was significantly increased by 13.1% and 19.7% in the brains of E50 and E100 groups, respectively, compared with the control group. Monoamine oxidase-B activity was significantly decreased by 10.5% in the brains of the E100 group, and the superoxide radical level was significantly reduced by 9.4% in the serum of the E100 group compared with the control group. Superoxide dismutase activity was significantly increased by 9.6% and 11.6% in the serum of E50 and E100 groups, respectively, compared with the control group. These results clearly demonstrate the effects of E prostrata on the formation of acetylcholine in the brain and the inhibition of oxidative stress in the brain and serum of rats. These findings may have implications for preventing dementia and enhancing memory function in humans.

Behavioral outcomes of monoamine oxidase deficiency: preclinical and clinical evidence

Monoamine oxidase (MAO) isoenzymes A and B are mitochondrial-bound proteins, catalyzing the oxidative deamination of monoamine neurotransmitters as well as xenobiotic amines. Although they derive from a common ancestral progenitor gene, are located at X-chromosome and display 70% structural identity, their substrate preference, regional distribution, and physiological role are divergent. In fact, while MAO-A has high affinity for serotonin and norepinephrine, MAO-B primarily serves the catabolism of 2-phenylethylamine (PEA) and contributes to the degradation of other trace amines and dopamine. Convergent lines of preclinical and clinical evidence indicate that variations in MAO enzymatic activity--due to either genetic or environmental factors--can exert a profound influence on behavioral regulation and play a role in the pathophysiology of a large spectrum of mental and neurodegenerative disorders, ranging from antisocial personality disorder to Parkinson's disease. Over the past few years, numerous advances have been made in our understanding of the phenotypical variations associated with genetic polymorphisms and mutations of the genes encoding for both isoenzymes. In particular, novel findings on the phenotypes of MAO-deficient mice are highlighting novel potential implications of both isoenzymes in a broad spectrum of mental disorders, ranging from autism and anxiety to impulse-control disorders and ADHD. These studies will lay the foundation for future research on the neurobiological and neurochemical bases of these pathological conditions, as well as the role of gene × environment interactions in the vulnerability to several mental disorders.

Mechanistic studies of the yeast polyamine oxidase Fms1: kinetic mechanism, substrate specificity, and pH dependence

The flavoprotein oxidase Fms1 from Saccharomyces cerevisiae catalyzes the oxidation of spermine and N(1)-acetylspermine to yield spermidine and 3-aminopropanal or N-acetyl-3-aminopropanal. The kinetic mechanism of the enzyme has been determined with both substrates. The initial velocity patterns are ping-pong, consistent with reduction being kinetically irreversible. Reduction of Fms1 by either substrate is biphasic. The rate constant for the rapid phase varies with the substrate concentration, with limiting rates for reduction of the enzyme of 126 and 1410 s(-1) and apparent K(d) values of 24.3 and 484 μM for spermine and N(1)-acetylspermine, respectively. The rapid phase is followed by a concentration-independent phase that is slower than turnover. The reaction of the reduced enzyme with oxygen is monophasic, with a rate constant of 402 mM(-1) s(-1) with spermine at 25 °C and 204 mM(-1) s(-1) with N(1)-acetylspermine at 4 °C and pH 9.0. This step is followed by rate-limiting product dissociation. The k(cat)/K(amine)-pH profiles are bell-shaped, with an average pK(a) value of 9.3 with spermine and pK(a) values of 8.3 and 9.6 with N(1)-acetylspermine. Both profiles are consistent with the active forms of substrates having two charged nitrogens. The pH profiles for the rate constant for flavin reduction show pK(a) values of 8.3 and 7.2 for spermine and N(1)-acetylspermine, respectively, for groups that must be unprotonated; these pK(a) values are assigned to the substrate N4. The k(cat)/K(O(2))-pH profiles show pK(a) values of 7.5 for spermine and 6.8 for N(1)-acetylspermine. With both substrates, the k(cat) value decreases when a single residue is protonated.

Histone lysine methylation and demethylation pathways in cancer

The genetic changes leading to the development of human cancer are accompanied by alterations in the structure and modification status of chromatin, which represent powerful regulatory mechanisms for gene expression and genome stability. These epigenetic alterations have sparked interest into deciphering the regulatory pathways and function of post-translational modifications of histones during the initiation and progression of cancer. In this review we describe and summarize the current knowledge of several histone lysine methyltransferase and demethylase pathways relevant to cancer. Mechanistic insight into histone modifications will pave the way for the development and therapeutic application of "epidrugs" in cancer.

A lysine conserved in the monoamine oxidase family is involved in oxidation of the reduced flavin in mouse polyamine oxidase

Lysine 315 of mouse polyamine amine oxidase corresponds to a lysine residue that is conserved in the flavoprotein amine oxidases of the monoamine oxidase structural family. In several structures, this lysine residue forms a hydrogen bond to a water molecule that is hydrogen-bonded to the flavin N(5). Mutation of Lys315 in polyamine oxidase to methionine was previously shown to have no effect on the kinetics of the reductive half-reaction of the enzyme (M. Henderson Pozzi, V. Gawandi, P.F. Fitzpatrick, Biochemistry 48 (2009) 1508-1516). In contrast, the mutation does affect steps in the oxidative half-reaction. The k(cat) value is unaffected by the mutation; this kinetic parameter likely reflects product release. At pH 10, the k(cat)/K(m) value for oxygen is 25-fold lower in the mutant enzyme. The k(cat)/K(O2) value is pH-dependent for the wild-type enzyme, decreasing below a pK(a) of 7.0, while this kinetic parameter for the mutant enzyme is pH-independent. This is consistent with the neutral form of Lys315 being required for more rapid flavin oxidation. The solvent isotope effect on the k(cat)/K(O2) value increases from 1.4 in the wild-type enzyme to 1.9 in the mutant protein, and the solvent inventory changes from linear to bowed. The effects of the mutation can be explained by the lysine orienting the bridging water so that it can accept the proton from the flavin N(5) during flavin oxidation. In the mutant enzyme the lysine amine would be replaced by a water chain.

Mechanistic studies of human spermine oxidase: kinetic mechanism and pH effects

In mammalian cells, the flavoprotein spermine oxidase (SMO) catalyzes the oxidation of spermine to spermidine and 3-aminopropanal. Mechanistic studies have been conducted with the recombinant human enzyme. The initial velocity pattern in which the ratio between the concentrations of spermine and oxygen is kept constant establishes the steady-state kinetic pattern as ping-pong. Reduction of SMO by spermine in the absence of oxygen is biphasic. The rate constant for the rapid phase varies with the substrate concentration, with a limiting value (k(3)) of 49 s(-1) and an apparent K(d) value of 48 microM at pH 8.3. The rate constant for the slow step is independent of the spermine concentration, with a value of 5.5 s(-1), comparable to the k(cat) value of 6.6 s(-1). The kinetics of the oxidative half-reaction depend on the aging time after the spermine and enzyme are mixed in a double-mixing experiment. At an aging time of 6 s, the reaction is monophasic with a second-order rate constant of 4.2 mM(-1) s(-1). At an aging time of 0.3 s, the reaction is biphasic with two second-order constants equal to 4.0 and 40 mM(-1) s(-1). Neither is equal to the k(cat)/K(O(2)) value of 13 mM(-1) s(-1). These results establish the existence of more than one pathway for the reaction of the reduced flavin intermediate with oxygen. The k(cat)/K(M) value for spermine exhibits a bell-shaped pH profile, with an average pK(a) value of 8.3. This profile is consistent with the active form of spermine having three charged nitrogens. The pH profile for k(3) shows a pK(a) value of 7.4 for a group that must be unprotonated. The pK(i)-pH profiles for the competitive inhibitors N,N'-dibenzylbutane-1,4-diamine and spermidine show that the fully protonated forms of the inhibitors and the unprotonated form of an amino acid residue with a pK(a) of approximately 7.4 in the active site are preferred for binding.

Drug interactions

Drugs for allergy are often taken in combination with other drugs, either to treat allergy or other conditions. In common with many pharmaceuticals, most such drugs are subject to metabolism by P450 enzymes and to transmembrane transport. This gives rise to considerable potential for drug-drug interactions, to which must be added consideration of drug-diet interactions. The potential for metabolism-based drug interactions is increasingly being taken into account during drug development, using a variety of in silico and in vitro approaches. Prediction of transporter-based interactions is not as advanced. The clinical importance of a drug interaction will depend upon a number of factors, and it is important to address concerns quantitatively, taking into account the therapeutic index of the compound.

Mechanistic studies of para-substituted N,N'-dibenzyl-1,4-diaminobutanes as substrates for a mammalian polyamine oxidase

The kinetics of oxidation of a series of para-substituted N,N'-dibenzyl-1,4-diaminobutanes by the flavoprotein polyamine oxidase from mouse have been determined to gain insight into the mechanism of amine oxidation by this member of the monoamine oxidase structural family. The k(cat)/K(m) values are maximal at pH 9, consistent with the singly charged substrate being the active form. The rate constant for flavin reduction, k(red), by N,N'-dibenzyl-1,4-diaminobutane decreases about 5-fold below a pK(a) of approximately 8; this is attributed to the need for a neutral nitrogen at the site of oxidation. The k(red) and k(cat) values are comparable for each of the N,N'-dibenzyl-1,4-diaminobutanes, consistent with rate-limiting reduction. The deuterium kinetic isotope effects on k(red) and k(cat) are identical for each of the N,N'-dibenzyl-1,4-diaminobutanes, consistent with rate-limiting cleavage of the substrate CH bond. The k(red) values for seven different para-substituted N,N'-dibenzyl-1,4-diaminobutanes correlate with a combination of the van der Waals volume and sigma value of the substrates, with rho values of -0.59 at pH 8.6 and -0.09 at pH 6.6. These results are consistent with direct transfer of a hydride from the neutral CN bond of the substrate to the flavin as the mechanism of polyamine oxidase.

The emerging therapeutic potential of histone methyltransferase and demethylase inhibitors

Epigenetics is defined as heritable changes to the transcriptome that are independent of changes in the genome. The biochemical modifications that govern epigenetics are DNA methylation and posttranslational histone modifications. Among the histone modifications, acetylation and deacetylation are well characterized, whereas the fields of histone methylation and especially demethylation are still in their infancy. This is particularly true with regard to drug discovery. There is strong evidence that these modifications play an important role in the maintenance of transcription as well as in the development of certain diseases. This article gives an overview of the mechanisms of action of histone methyltransferases and demethylases, their role in the formation of certain diseases, and available inhibitors. Special emphasis is placed on the strategies that led to the first inhibitors which are currently available and the screening approaches that were used in that process.

Potential anticancer application of polyamine oxidation products formed by amine oxidase: a new therapeutic approach

The polyamines spermine, spermidine and putrescine are ubiquitous cell components. These molecules are substrates of a class of enzymes that includes monoamine oxidases, diamine oxidases, polyamine oxidases and copper-containing amine oxidases. Amine oxidases are important because they contribute to regulate levels of mono- and polyamines. In tumors, polyamines and amine oxidases are increased as compared to normal tissues. Cytotoxicity induced by bovine serum amine oxidase (BSAO) and spermine is attributed to H(2)O(2) and aldehydes produced by the reaction. This study demonstrated that multidrug-resistant (MDR) cancer cells (colon adenocarcinoma and melanoma) are significantly more sensitive than the corresponding wild-type (WT) ones to H(2)O(2) and aldehydes, the products of BSAO-catalyzed oxidation of spermine. Transmission electron microscopy (TEM) observations showed major ultrastructural alterations of the mitochondria. These were more pronounced in MDR than in WT cells. Increasing the incubation temperature from 37 to 42 degrees Celsius enhances cytotoxicity in cells exposed to spermine metabolites. The combination BSAO/spermine prevents tumor growth, particularly well if the enzyme has been conjugated to a biocompatible hydrogel polymers. Since both wild-type and MDR cancer cells after pre-treatment with MDL 72527, a lysosomotropic compound, are sensitized to subsequent exposure to BSAO/spermine, it is conceivable that combined treatment with a lysosomotropic compound and BSAO/spermine would be effective against tumor cells. It is of interest to search for such novel compounds, which might be promising for application in a therapeutic setting.

Cyclopiazonic acid biosynthesis of Aspergillus flavus and Aspergillus oryzae

Cyclopiazonic acid (CPA) is an indole-tetramic acid neurotoxin produced by some of the same strains of A. flavus that produce aflatoxins and by some Aspergillus oryzae strains. Despite its discovery 40 years ago, few reviews of its toxicity and biosynthesis have been reported. This review examines what is currently known about the toxicity of CPA to animals and humans, both by itself or in combination with other mycotoxins. The review also discusses CPA biosynthesis and the genetic diversity of CPA production in A. flavus/oryzae populations.

Oxidation of amines by flavoproteins

Many flavoproteins catalyze the oxidation of primary and secondary amines, with the transfer of a hydride equivalent from a carbon-nitrogen bond to the flavin cofactor. Most of these amine oxidases can be classified into two structural families, the D-amino acid oxidase/sarcosine oxidase family and the monoamine oxidase family. This review discusses the present understanding of the mechanisms of amine and amino acid oxidation by flavoproteins, focusing on these two structural families.

Mutagenic probes of the role of Ser209 on the cavity shaping loop of human monoamine oxidase A

The available literature implicating human monoamine oxidase A (MAO A) in apoptotic processes reports levels of MAO A protein that do not correlate with activity, suggesting that unknown mechanisms may be involved in the regulation of catalytic function. Bioinformatic analysis suggests Ser209 as a possible phosphorylation site that may be relevant to catalytic function because it is adjacent to a six-residue loop termed the 'cavity shaping loop' from structural data. To probe the functional role of this site, MAO A Ser209Ala and Ser209Glu mutants were created and investigated. In its membrane-bound form, the MAO A Ser209Glu phosphorylation mimic exhibits catalytic and inhibitor binding properties similar to those of wild-type MAO A. Solubilization in detergent solution and purification of the Ser209Glu mutant results in considerable decreases in these functional parameters. By contrast, the MAO A Ser209Ala mutant exhibits similar catalytic properties to those of wild-type enzyme when purified. Compared to purified wild-type and Ser209Ala MAO A proteins, the Ser209Glu MAO A mutant shows significant differences in covalent flavin fluorescence yield, CD spectra and thermal stability. These structural differences in the purified MAO A Ser209Glu mutant are not exhibited in quantitative structure-activity relationship patterns using a series of para-substituted benzylamine analogs similar to the wild-type enzyme. These data suggest that Ser209 in MAO A does not appear to be the putative phosphorylation site for regulation of MAO A activity and demonstrate that the membrane environment plays a significant role in stabilizing the structure of MAO A and its mutant forms.

ThermoFAD, a Thermofluor-adapted flavin ad hoc detection system for protein folding and ligand binding

In living organisms, genes encoding proteins that contain flavins as a prosthetic group constitute approximately 2-3% of the total. The fluorescence of flavin cofactors in these proteins is a property that is widely employed for biochemical characterisation. Here, we present a modified Thermofluor approach called ThermoFAD (Thermofluor-adapted flavin ad hoc detection system), which simplifies identification of optimal purification and storage conditions as well as high-affinity ligands. In this technique, the flavin cofactor is used as an intrinsic probe to monitor protein folding and stability, taking advantage of the different fluorescent properties of flavin-containing proteins between the folded and denatured state. The main advantage of the method is that it allows a large amount of biochemical data to be obtained using very small amounts of protein sample and standard laboratory equipment. We have explored several cases that demonstrate the reliability and versatility of this technique when applied to globular flavoenzymes, membrane-anchored flavoproteins, and macromolecular complexes. The information gathered from ThermoFAD analysis can be very valuable for any biochemical and biophysical analysis, including crystallisation. The method is likely to be applicable to other classes of proteins that possess endogenous fluorescent cofactors and prosthetic groups.

Kinetic mechanism of pyranose 2-oxidase from trametes multicolor

Pyranose 2-oxidase (P2O) from Trametes multicolor is a flavoprotein oxidase that catalyzes the oxidation of aldopyranoses by molecular oxygen to yield the corresponding 2-keto-aldoses and hydrogen peroxide. P2O is the first enzyme in the class of flavoprotein oxidases, for which a C4a-hydroperoxy-flavin adenine dinucleotide (FAD) intermediate has been detected during the oxidative half-reaction. In this study, the reduction kinetics of P2O by d-glucose and 2-d-d-glucose at pH 7.0 was investigated using stopped-flow techniques. The results indicate that d-glucose binds to the enzyme with a two-step binding process; the first step is the initial complex formation, while the second step is the isomerization to form an active Michaelis complex (E-Fl(ox):G). Interestingly, the complex (E-Fl(ox):G) showed greater absorbance at 395 nm than the oxidized enzyme, and the isomerization process showed a significant inverse isotope effect, implying that the C2-H bond of d-glucose is more rigid in the E-Fl(ox):G complex than in the free form. A large normal primary isotope effect (k(H)/k(D) = 8.84) was detected in the flavin reduction step. Steady-state kinetics at pH 7.0 shows a series of parallel lines. Kinetics of formation and decay of C-4a-hydroperoxy-FAD is the same in absence and presence of 2-keto-d-glucose, implying that the sugar does not bind to P2O during the oxidative half-reaction. This suggests that the kinetic mechanism of P2O is likely to be the ping-pong-type where the sugar product leaves prior to the oxygen reaction. The movement of the active site loop when oxygen is present is proposed to facilitate the release of the sugar product. Correlation between data from pre-steady-state and steady-state kinetics has shown that the overall turnover of the reaction is limited by the steps of flavin reduction and decay of C4a-hydroperoxy-FAD.

pH dependence of a mammalian polyamine oxidase: insights into substrate specificity and the role of lysine 315

Mammalian polyamine oxidases (PAOs) catalyze the oxidation of N1-acetylspermine and N1-acetylspermidine to produce N-acetyl-3-aminopropanaldehyde and spermidine or putrescine. Structurally, PAO is a member of the monoamine oxidase family of flavoproteins. The effects of pH on the kinetic parameters of mouse PAO have been determined to provide insight into the protonation state of the polyamine required for catalysis and the roles of ionizable residues in the active site in amine oxidation. For N1-acetylspermine, N1-acetylspermidine, and spermine, the k(cat)/K(amine)-pH profiles are bell-shaped. In each case, the profile agrees with that expected if the productive form of the substrate has a single positively charged nitrogen. The pK(i)-pH profiles for a series of polyamine analogues are most consistent with the nitrogen at the site of oxidation being neutral and one other nitrogen being positively charged in the reactive form of the substrate. With N1-acetylspermine as the substrate, the value of k(red), the limiting rate constant for flavin reduction, is pH-dependent, decreasing below a pK(a) value of 7.3, again consistent with the requirement for an uncharged nitrogen for substrate oxidation. Lys315 in PAO corresponds to a conserved active site residue found throughout the monoamine oxidase family. Mutation of Lys315 to methionine has no effect on the k(cat)/K(amine) profile for spermine; the k(red) value with N1-acetylspermine is only 1.8-fold lower in the mutant protein, and the pK(a) in the k(red)-pH profile with N1-acetylspermine shifts to 7.8. These results rule out Lys315 as a source of a pK(a) in the k(cat)/K(amine) or k(cat)/k(red) profiles. They also establish that this residue does not play a critical role in amine oxidation by PAO.

A concerted mechanism for berberine bridge enzyme

Berberine bridge enzyme catalyzes the conversion of (S)-reticuline to (S)-scoulerine by formation of a carbon-carbon bond between the N-methyl group and the phenolic ring. We elucidated the structure of berberine bridge enzyme from Eschscholzia californica and determined the kinetic rates for three active site protein variants. Here we propose a catalytic mechanism combining base-catalyzed proton abstraction with concerted carbon-carbon coupling accompanied by hydride transfer from the N-methyl group to the N5 atom of the FAD cofactor.

The structure of monoamine oxidase from Aspergillus niger provides a molecular context for improvements in activity obtained by directed evolution

Monoamine oxidase from Aspergillus niger (MAO-N) is a flavoenzyme that catalyses the oxidative deamination of primary amines. MAO-N has been used as the starting model for a series of directed evolution experiments, resulting in mutants of improved activity and broader substrate specificity, suitable for application in the preparative deracemisation of primary, secondary and tertiary amines when used as part of a chemoenzymatic oxidation-reduction cycle. The structures of a three-point mutant (Asn336Ser/Met348Lys/Ile246Met or MAO-N-D3) and a five-point mutant (Asn336Ser/Met348Lys/Ile246Met/Thr384Asn/Asp385Ser or MAO-N-D5) have been obtained using a multiple-wavelength anomalous diffraction experiment on a selenomethionine derivative of the truncated MAO-N-D5 enzyme. MAO-N exists as a homotetramer with a large channel at its centre and shares some structural features with human MAO B (MAO-B). A hydrophobic cavity extends from the protein surface to the active site, where a non-covalently bound flavin adenine dinucleotide (FAD) sits at the base of an 'aromatic cage,' the sides of which are formed by Trp430 and Phe466. A molecule of l-proline was observed near the FAD, and this ligand superimposed well with isatin, a reversible inhibitor of MAO-B, when the structures of MAO-N proline and MAO-B-isatin were overlaid. Of the mutations that confer the ability to catalyse the oxidation of secondary amines in MAO-N-D3, Asn336Ser reduces steric bulk behind Trp430 of the aromatic cage and Ile246Met confers greater flexibility within the substrate binding site. The two additional mutations, Thr384Asn and Asp385Ser, that occur in the MAO-N-D5 variant, which is able to oxidise tertiary amines, appear to influence the active-site environment remotely through changes in tertiary structure that perturb the side chain of Phe382, again altering the steric and electronic character of the active site near FAD. The possible implications of the change in steric and electronic environment caused by relevant mutations are discussed with respect to the improved catalytic efficiency of the MAO-N variants described in the literature.

Quantitative structure-activity relationship and complex network approach to monoamine oxidase A and B inhibitors

The work provides a new model for the prediction of the MAO-A and -B inhibitor activity by the use of combined complex networks and QSAR methodologies. On the basis of the obtained model, we prepared and assayed 33 coumarin derivatives, and the theoretical prediction was compared with the experimental activity data. The model correctly predicted 27 compounds, and most of the active derivatives showed IC 50 values in the muM-nM range against both the MAO-A and MAO-B isoforms. Compound 14 shows the same MAO-A inhibitory activity (IC 50 = 7.2 nM), as clorgyline used as a reference inhibitor and has the highest MAO-A specificity (1000-fold higher compared to MAO-B). On the other hand, compounds 24 (IC 50 = 1.2 nM) and 28 (IC 50 = 1.5 nM) show higher activity than selegiline (IC 50 = 19.6 nM) and high MAO-B selectivity with 100-fold and 1600-fold inhibition levels, with respect to the MAO-A isoform.

Fluorinated phenylcyclopropylamines. Part 5: Effects of electron-withdrawing or -donating aryl substituents on the inhibition of monoamine oxidases A and B by 2-aryl-2-fluoro-cyclopropylamines

A series of racemic, diastereoisomeric aryl cyclopropylamines substituted with fluorine in the 2-position and electron-donating and electron-withdrawing groups on the aromatic ring have been prepared. These represent analogues of the classic MAO inhibitor tranylcypromine (trans-2-phenylcyclopropylamine, 1). Their activities as inhibitors of recombinant human liver monoamine oxidases A (MAO A) and B (MAO B) were determined. The trans-compounds were low micromolar inhibitors of both MAO A and MAO B with moderate MAO A selectivity while the less active cis-analogues were MAO B selective. In the trans-series, electron-withdrawing para-substituents increased the potency of MAO A inhibition while electron-donating groups such as methyl or methoxy had no influence on this activity. In contrast, aromatic ring substitution in the trans-series had essentially no effect on the inhibition of MAO B. The corresponding cis-compounds were shown to be 10-100 times less active against MAO A, while trans- and cis-compounds were quite similar in terms of inhibition of MAO B. The best MAO A/MAO B selectivity (7:1) in the trans-series was found for trans-2-fluoro-2-(para-trifluoromethylphenyl)cyclopropylamine (7d), while a 1:27 selectivity was found for cis-2-fluoro-2-(para-fluorophenyl)cyclopropylamine (10c). These results are discussed in connection with the pK(a) and logD values, the mechanism of action of tranylcypromines, and the geometry of the active site of the enzymes.

The pH dependence of kinetic isotope effects in monoamine oxidase A indicates stabilization of the neutral amine in the enzyme-substrate complex

A common feature of all the proposed mechanisms for monoamine oxidase is the initiation of catalysis with the deprotonated form of the amine substrate in the enzyme-substrate complex. However, recent steady-state kinetic studies on the pH dependence of monoamine oxidase led to the suggestion that it is the protonated form of the amine substrate that binds to the enzyme. To investigate this further, the pH dependence of monoamine oxidase A was characterized by both steady-state and stopped-flow techniques with protiated and deuterated substrates. For all substrates used, there is a macroscopic ionization in the enzyme-substrate complex attributed to a deprotonation event required for optimal catalysis with a pK(a) of 7.4-8.4. In stopped-flow assays, the pH dependence of the kinetic isotope effect decreases from approximately 13 to 8 with increasing pH, leading to assignment of this catalytically important deprotonation to that of the bound amine substrate. The acid limb of the bell-shaped pH profile for the rate of flavin reduction over the substrate binding constant (k(red)/K(s), reporting on ionizations in the free enzyme and/or free substrate) is due to deprotonation of the free substrate, and the alkaline limb is due to unfavourable deprotonation of an unknown group on the enzyme at high pH. The pK(a) of the free amine is above 9.3 for all substrates, and is greatly perturbed (DeltapK(a) approximately 2) on binding to the enzyme active site. This perturbation of the substrate amine pK(a) on binding to the enzyme has been observed with other amine oxidases, and likely identifies a common mechanism for increasing the effective concentration of the neutral form of the substrate in the enzyme-substrate complex, thus enabling efficient functioning of these enzymes at physiologically relevant pH.