High-level expression of human liver monoamine oxidase B in Pichia pastoris

Bioisosteric replacement based on 1,2,4-oxadiazoles in the discovery of 1H-indazole-bearing neuroprotective MAO B inhibitors

Following a hybridization strategy, a series of 5-substituted-1H-indazoles were designed and evaluated in vitro as inhibitors of human monoamine oxidase (hMAO) A and B. Among structural modifications, the bioisostere-based introduction of 1,2,4-oxadiazole ring returned the most potent and selective human MAO B inhibitor (compound 20, IC₅₀ = 52 nM, SI > 192). The most promising inhibitors were studied in cell-based neuroprotection models of SH-SY5Y and astrocytes line against H₂O₂. Moreover, preliminary drug-like features (aqueous solubility at pH 7.4; hydrolytic stability at acidic and neutral pH) were assessed for selected 1,2,4-oxadiazoles and compared to amide analogues through RP-HPLC methods. Molecular docking simulations highlighted the crucial role of molecular flexibility in providing a better shape complementarity for compound 20 within MAO B enzymatic cleft than rigid analogue 18. Enzymatic kinetics analysis along with thermal stability curves (T_m shift = +2.9 °C) provided clues of a tight-binding mechanism for hMAO B inhibition by 20.

Components of the sympathetic nervous system as targets to modulate inflammation - rheumatoid arthritis synovial fibroblasts as neuron-like cells?

BACKGROUND

Catecholamines are major neurotransmitters of the sympathetic nervous system (SNS) and they are of pivotal importance in regulating numerous physiological and pathological processes. Rheumatoid arthritis (RA) is influenced by the activity of the SNS and its neurotransmitters norepinephrine (NE) and dopamine (DA) and early sympathectomy alleviates experimental arthritis in mice. In contrast, late sympathectomy aggravates RA, since this procedure eliminates anti-inflammatory, tyrosine hydroxylase (TH) positive cells that appear in the course of RA. While it has been shown that B cells can take up, degrade and synthesize catecholamines it is still unclear whether this also applies to synovial fibroblasts, a mesenchymal cell that is actively engaged in propagating inflammation and cartilage destruction in RA. Therefore, this study aims to present a detailed description of the catecholamine pathway and its influence on human RA synovial fibroblasts (RASFs).

RESULTS

RASFs express all catecholamine-related targets including the synthesizing enzymes TH, DOPA decarboxylase, dopamine beta-hydroxylase, and phenylethanolamine N-methyltransferase. Furthermore, vesicular monoamine transporters 1/2 (VMAT1/2), dopamine transporter (DAT) and norepinephrine transporter (NET) were detected. RASFs are also able to degrade catecholamines as they express monoaminoxidase A and B (MAO-A/MAO-B) and catechol-O-methyltransferase (COMT). TNF upregulated VMAT2, MAO-B and NET levels in RASFs. DA, NE and epinephrine (EPI) were produced by RASFs and extracellular levels were augmented by either MAO, COMT, VMAT or DAT/NET inhibition but also by tumor necrosis factor (TNF) stimulation. While exogenous DA decreased interleukin-6 (IL-6) production and cell viability at the highest concentration (100 μM), NE above 1 μM increased IL-6 levels with a concomitant decrease in cell viability. MAO-A and MAO-B inhibition had differential effects on unstimulated and TNF treated RASFs. The MAO-A inhibitor clorgyline fostered IL-6 production in unstimulated but not TNF stimulated RASFs (10 nM-1 μM) while reducing IL-6 at 100 μM with a dose-dependent decrease in cell viability in both groups. The MAO-B inhibitor lazabemide hydrochloride did only modestly decrease cell viability at 100 μM while enhancing IL-6 production in unstimulated RASFs and decreasing IL-6 in TNF stimulated cells.

CONCLUSIONS

RASFs possess a complete and functional catecholamine machinery whose function is altered under inflammatory conditions. Results from this study shed further light on the involvement of sympathetic neurotransmitters in RA pathology and might open therapeutic avenues to counteract inflammation with the MAO enzymes being key candidates.

MAO Visible Spectroscopy for Ligand Interactions, Redox Chemistry, and Kinetics of Irreversible Inhibition

The covalently bound FAD cofactor in monoamine oxidase (MAO) is reduced by the amine substrate and reoxidized by oxygen. Visible spectroscopy provides a convenient tool to study the interaction of ligands and the kinetics of the half-reactions for mechanistic investigations. Equilibrium redox titrations allow measurement of redox potentials, while rapid mixing experiments allow determination of the rate of reduction by different substrates and of covalent adduct formation by irreversible inactivators. Three techniques are described: (1) measuring ligand interactions by alterations in the spectrum, especially at 495 nm; (2) reducing MAO, including the essentials for anaerobic procedures; and (3) studying kinetics of reduction, reoxidation, or inactivation of MAO.

Crystallization of Human Monoamine Oxidase B

The interest in monoamine oxidases A and B (MAO A and B) is due to their central role in regulating the balance of neurotransmitters, both in the central nervous system and in peripheral organs. As validated drug targets for depression and Parkinson's disease, the elucidation of their crystal structures was an essential step to guide drug design investigations. The development of the heterologous expression system of MAO B in Pichia pastoris and the identification of the detergent, buffer, and precipitant conditions allowed to determine the first crystal structure of human MAO B in 2002. A detailed protocol to obtain reproducible MAO B crystals is described.

Radiochemical Assay of Monoamine Oxidase Activity

The activity of monoamine oxidase enzymes may be quantified by measuring the conversion of a radiolabeled amine substrate to a radiolabeled product that occurs during incubation of the substrate with the enzyme in an aqueous buffer. Described herein is an established discontinuous procedure in which separation of the substrate and product is achieved by extracting uncharged aldehydes into an organic solvent, while cationic amines remain in an acidified aqueous layer. Under assay conditions designed to ensure a pseudo-linear catalytic rate for the duration of the incubation, determination of radioactivity in the organic solvent by liquid scintillation counting facilitates estimation of an initial rate for amine turnover.

Purification of MAO A and MAO B from Mammalian Tissue Sources

Procedures are described for the purification of the mitochondrial-bound enzymes human and bovine monoamine oxidases A and B (MAO A and B) from placental and liver tissue sources, respectively. Enzyme purification follows isolation of the mitochondria and preparation of outer membrane particles. The membrane-bound enzymes are solubilized by treatment of membranes with phospholipases and detergent extraction. Functional bovine MAO B is purified by polymer fractionation and differential centrifugation. Functional human MAO A is purified by ion-exchange DEAE-Sepharose chromatography.

Conventional Receptor Radioligand Binding Techniques Applied to the Study of Monoamine Oxidase

Designed to measure binding interactions between small molecules and receptor proteins, radioligand binding approaches may also be applied to interactions between monoamine oxidase (MAO) and its ligands. The technique may be used with tissue homogenates or with mitochondrial membranes and can provide information about binding site density, ligand affinity, binding rate constants, and binding events at sites that do not impact absorbance characteristics of the flavin cofactor and that may not be amenable to spectrophotometric studies. This overview describes the use of a cell harvester in a common filtration approach to measure binding to MAO of radiolabeled substrates, inhibitors, or allosteric ligands in saturation analyses and to take advantage of the principles of competition to obtain quantitative binding data for unlabeled ligands that may bind with much lower affinity. The quality and reproducibility of data are impacted by factors such as choice of ligand concentrations, pipetting technique, graphing and regression approaches, and scintillation counting parameters, and consideration is given to these and other factors that may influence the results.

Purification of Recombinant Eukaryotic MAO A and MAO B Utilizing the Pichia pastoris Expression System

Procedures are described for the heterologous expression and purification of the mitochondrial-bound enzymes human and rat monoamine oxidases A and B and zebrafish MAO in the yeast Pichia pastoris. Enzyme expression is under control of a methanol oxidase promoter and similar procedures have been developed for the preparation of membrane particles and detergent solubilization of the functional enzymes. Similarities and differences are described in the procedures for purification of the respective enzymes using standard column chromatographic techniques to provide enzyme yields in the range of 100-300 mg from 1 L of cell culture.

First purified recombinant CYP75B including transmembrane helix with unexpected high substrate specificity to (2R)-naringenin

Anthochlor pigments (chalcones and aurones) play an important role in yellow flower colourization, the formation of UV-honey guides and show numerous health benefits. The B-ring hydroxylation of chalcones is performed by membrane bound cytochrome P450 enzymes. It was assumed that usual flavonoid 3′-hydroxlases (F3′Hs) are responsible for the 3,4- dihydroxy pattern of chalcones, however, we previously showed that a specialized F3′H, namely chalcone 3-hydroxylase (CH3H), is necessary for the hydroxylation of chalcones. In this study, a sequence encoding membrane bound CH3H from Dahlia variabilis was recombinantly expressed in yeast and a purification procedure was developed. The optimized purification procedure led to an overall recovery of 30% recombinant DvCH3H with a purity of more than 84%. The enzyme was biochemically characterized with regard to its kinetic parameters on various substrates, including racemic naringenin, as well as its enantiomers (2S)-, and (2R)-naringenin, apigenin and kaempferol. We report for the first time the characterization of a purified Cytochrome P450 enzyme from the flavonoid biosynthesis pathway, including the transmembrane helix. Further, we show for the first time that recombinant DvCH3H displays a higher affinity for (2R)-naringenin than for (2S)-naringenin, although (2R)-flavanones are not naturally formed by chalcone isomerase.

Parameters for Irreversible Inactivation of Monoamine Oxidase

The irreversible inhibitors of monoamine oxidases (MAO) slow neurotransmitter metabolism in depression and neurodegenerative diseases. After oxidation by MAO, hydrazines, cyclopropylamines and propargylamines form a covalent adduct with the flavin cofactor. To assist the design of new compounds to combat neurodegeneration, we have updated the kinetic parameters defining the interaction of these established drugs with human MAO-A and MAO-B and analyzed the required features. The K_i values for binding to MAO-A and molecular models show that selectivity is determined by the initial reversible binding. Common to all the irreversible inhibitor classes, the non-covalent 3D-chemical interactions depend on a H-bond donor and hydrophobic-aromatic features within 5.7 angstroms apart and an ionizable amine. Increasing hydrophobic interactions with the aromatic cage through aryl halogenation is important for stabilizing ligands in the binding site for transformation. Good and poor inactivators were investigated using visible spectroscopy and molecular dynamics. The initial binding, close and correctly oriented to the FAD, is important for the oxidation, specifically at the carbon adjacent to the propargyl group. The molecular dynamics study also provides evidence that retention of the allenyl imine product oriented towards FADH⁻ influences the formation of the covalent adduct essential for effective inactivation of MAO.

Exposing the Interplay Between Enzyme Turnover, Protein Dynamics, and the Membrane Environment in Monoamine Oxidase B

There is an increasing realization that structure-based drug design may show improved success by understanding the ensemble of conformations accessible to an enzyme and how the environment affects this ensemble. Human monoamine oxidase B (MAO-B) catalyzes the oxidation of amines and is inhibited for the treatment of both Parkinson's disease and depression. Despite its clinical importance, its catalytic mechanism remains unclear, and routes to drugging this target would be valuable. Evidence of a radical in either the transition state or the resting state of MAO-B is present throughout the literature and is suggested to be a flavin semiquinone, a tyrosyl radical, or both. Here we see evidence of a resting-state flavin semiquinone, via absorption redox studies and electron paramagnetic resonance, suggesting that the anionic semiquinone is biologically relevant. On the basis of enzyme kinetic studies, enzyme variants, and molecular dynamics simulations, we find evidence for the importance of the membrane environment in mediating the activity of MAO-B and that this mediation is related to the protein dynamics of MAO-B. Further, our MD simulations identify a hitherto undescribed entrance for substrate binding, membrane modulated substrate access, and indications for half-site reactivity: only one active site is accessible to binding at a time. Our study combines both experimental and computational evidence to illustrate the subtle interplay between enzyme activity and protein dynamics and the immediate membrane environment. Understanding key biomedical enzymes to this level of detail will be crucial to inform strategies (and binding sites) for rational drug design for these targets.

On the practical aspects of characterising monoamine oxidase inhibition in vitro

The development of novel inhibitors of human monoamine oxidase enzymes with improved pharmacodynamic and pharmacokinetic profiles has, in the past, been hampered by limited access to enzyme, by assay protocols offering limited throughput, and by inappropriate analyses of kinetic data. More recently, high-level expression of human enzymes in yeast has facilitated thorough examinations of steady-state enzyme behaviour that have led to improvements in our understanding of the mathematical underpinnings of kinetic analyses of monoamine oxidases. However, with these improvements have come a realisation that to be useful, more data points across wider concentration ranges are required. In turn, many discontinuous assay approaches, such as those involving radiolabelled substrates or chromatographic separation of product from substrate, have been rendered somewhat obsolete. Justification for the use of a platereader-based approach to assess the effects of novel inhibitors on monamine oxidases is provided, along with details of experimental design optimised to address the unexpectedly complex kinetics followed by these enzymes. Potential sources of error are discussed, and comments provided on techniques that may enhance the quality of experimental data.

The structure of monoamine oxidases: past, present, and future

The first crystal structure of mammalian monoamine oxidases (MAOs) was solved in 2002; almost 65 years after, these FAD-dependent enzymes were discovered and classified as responsible for the oxidation of aromatic neurotransmitters. Both MAO A and MAO B feature a two-domain topology characterized by the Rossmann fold, interacting with dinucleotide cofactors, which is intimately associated to a substrate-binding domain. This globular body is endowed with a C-terminal α-helix that anchors the protein to the outer mitochondrial phospholipid bilayer. As monotopic membrane proteins, the structural elucidation of MAOs was a challenging task that required the screening of different detergent conditions for their purification and crystallization. MAO A and MAO B structures differ both in their oligomerization architecture and in details of their active sites. Purified human MAO B and rat MAO A are dimeric, whereas human MAO A was found to be monomeric, which is believed to result from the detergent treatments used to extract the protein from the membrane. The active site of MAOs consists of a hydrophobic cavity located in front of the flavin cofactor and extending to the protein surface. Some structural features are highly conserved in the two isozymes, such as a Tyr-Tyr aromatic sandwich in front of the flavin ring and a Lys residue hydrogen-bonded to the cofactor N5 atom, whereas a pair of gating residues (Phe208/Ile335 in MAO A; Ile199/Tyr326 in MAO B) specifically determines the different substrate and inhibitor properties of the two enzymes.

Monoamine Oxidases

Monoamine oxidases A and B (MAO A and B) are mammalian flavoenzymes bound to the outer mitochondrial membrane. They were discovered almost a century ago and they have been the subject of many biochemical, structural and pharmacological investigations due to their central role in neurotransmitter metabolism. Currently, the treatment of Parkinson's disease involves the use of selective MAO B inhibitors such as rasagiline and safinamide. MAO inhibition was shown to exert a general neuroprotective effect as a result of the reduction of oxidative stress produced by these enzymes, which seems to be relevant also in non-neuronal contexts. MAOs were successfully expressed as recombinant proteins in Pichia pastoris, which allowed a thorough biochemical and structural characterization. These enzymes are characterized by a globular water-soluble main body that is anchored to the mitochondrial membrane through a C-terminal α-helix, similar to other bitopic membrane proteins. In both MAO A and MAO B the enzyme active site consists of a hydrophobic cavity lined by residues that are conserved in the two isozymes, except for few details that determine substrate and inhibitor specificity. In particular, human MAO B features a dual-cavity active site whose conformation depends on the size of the bound ligand. This article provides a comprehensive and historical review of MAOs and the state-of-the-art of these enzymes as membrane drug targets.

What a Difference a Methyl Group Makes: The Selectivity of Monoamine Oxidase B Towards Histamine and N-Methylhistamine

Monoamine oxidase (MAO) enzymes catalyze the degradation of a very broad range of biogenic and dietary amines including many neurotransmitters in the brain, whose imbalance is extensively linked with the biochemical pathology of various neurological disorders. Although sharing around 70 % sequence identity, both MAO A and B isoforms differ in substrate affinities and inhibitor sensitivities. Inhibitors that act on MAO A are used to treat depression, due to their ability to raise serotonin concentrations, whereas MAO B inhibitors decrease dopamine degradation and improve motor control in patients with Parkinson disease. Despite this functional importance, the factors affecting MAO selectivity are poorly understood. Here, we used a combination of molecular dynamics (MD) simulations, molecular mechanics with Poisson-Boltzmann and surface area solvation (MM-PBSA) binding free energy evaluations, and quantum mechanical (QM) cluster calculations to address the unexpected, yet challenging MAO B selectivity for N-methylhistamine (NMH) over histamine (HIS), differing only in a single methyl group distant from the reactive ethylamino center. This study shows that a dominant selectivity contribution is offered by a lower activation free energy for NMH by 2.6 kcal mol^-1 , in excellent agreement with the experimental ΔΔG^≠_EXP =1.4 kcal mol^-1 , together with a more favorable reaction exergonicity and active-site binding. This study also confirms the hydrophobic nature of the MAO B active site and underlines the important role of Ile199, Leu171, and Leu328 in properly orienting substrates for the reaction.

Crystallization of Membrane Proteins: An Overview

Membrane proteins are crucial components of cellular membranes and are responsible for a variety of physiological functions. The advent of new tools and technologies for structural biology of membrane proteins has led to a significant increase in the number of structures deposited to the Protein Data Bank during the past decade. This new knowledge has expanded our fundamental understanding of their mechanism of function and contributed to the drug-design efforts. In this chapter we discuss current approaches for membrane protein expression, solubilization, crystallization, and data collection. Additionally, we describe the protein quality-control assays that are often instrumental as a guideline for a shorter path toward the structure.

Molecular aspects of monoamine oxidase B

Monoamine oxidases (MAO) influence the monoamine levels in brain by virtue of their role in neurotransmitter breakdown. MAO B is the predominant form in glial cells and in platelets. MAO B structure, function and kinetics are described as a background for the effect of alterations in its activity on behavior. The need to inhibit MAO B to combat decreased brain amines continues to drive the search for new drugs. Reversible and irreversible inhibitors are now designed using data-mining, computational screening, docking and molecular dynamics. Multi-target ligands designed to combat the elevated activity of MAO B in Alzheimer's and Parkinson's Diseases incorporate MAO inhibition (usually irreversible) as well as iron chelation, antioxidant or neuroprotective properties. The main focus of drug design is the catalytic activity of MAO, but the imidazoline I2 site in the entrance cavity of MAO B is also a pharmacological target. Endogenous regulation of MAO B expression is discussed briefly in light of new studies measuring mRNA, protein, or activity in healthy and degenerative samples, including the effect of DNA methylation on the expression. Overall, this review focuses on examples of recent research on the molecular aspects of the expression, activity, and inhibition of MAO B.

The Use of Multiscale Molecular Simulations in Understanding a Relationship between the Structure and Function of Biological Systems of the Brain: The Application to Monoamine Oxidase Enzymes

HIGHLIGHTS Computational techniques provide accurate descriptions of the structure and dynamics of biological systems, contributing to their understanding at an atomic level.Classical MD simulations are a precious computational tool for the processes where no chemical reactions take place.QM calculations provide valuable information about the enzyme activity, being able to distinguish among several mechanistic pathways, provided a carefully selected cluster model of the enzyme is considered.Multiscale QM/MM simulation is the method of choice for the computational treatment of enzyme reactions offering quantitative agreement with experimentally determined reaction parameters.Molecular simulation provide insight into the mechanism of both the catalytic activity and inhibition of monoamine oxidases, thus aiding in the rational design of their inhibitors that are all employed and antidepressants and antiparkinsonian drugs. Aging society and therewith associated neurodegenerative and neuropsychiatric diseases, including depression, Alzheimer's disease, obsessive disorders, and Parkinson's disease, urgently require novel drug candidates. Targets include monoamine oxidases A and B (MAOs), acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and various receptors and transporters. For rational drug design it is particularly important to combine experimental synthetic, kinetic, toxicological, and pharmacological information with structural and computational work. This paper describes the application of various modern computational biochemistry methods in order to improve the understanding of a relationship between the structure and function of large biological systems including ion channels, transporters, receptors, and metabolic enzymes. The methods covered stem from classical molecular dynamics simulations to understand the physical basis and the time evolution of the structures, to combined QM, and QM/MM approaches to probe the chemical mechanisms of enzymatic activities and their inhibition. As an illustrative example, the later will focus on the monoamine oxidase family of enzymes, which catalyze the degradation of amine neurotransmitters in various parts of the brain, the imbalance of which is associated with the development and progression of a range of neurodegenerative disorders. Inhibitors that act mainly on MAO A are used in the treatment of depression, due to their ability to raise serotonin concentrations, while MAO B inhibitors decrease dopamine degradation and improve motor control in patients with Parkinson disease. Our results give strong support that both MAO isoforms, A and B, operate through the hydride transfer mechanism. Relevance of MAO catalyzed reactions and MAO inhibition in the context of neurodegeneration will be discussed.

Why p-OMe- and p-Cl-β-Methylphenethylamines Display Distinct Activities upon MAO-B Binding

Despite their structural and chemical commonalities, p-chloro-β-methylphenethylamine and p-methoxy-β-methylphenethylamine display distinct inhibitory and substrate activities upon MAO-B binding. Density Functional Theory (DFT) quantum chemical calculations reveal that β-methylation and para-substitution underpin the observed activities sustained by calculated transition state energy barriers, attained conformations and key differences in their interactions in the enzyme’s substrate binding site. Although both compounds meet substrate requirements, it is clear that β-methylation along with the physicochemical features of the para-substituents on the aromatic ring determine the activity of these compounds upon binding to the MAO B-isoform. While data for a larger set of compounds might lend generality to our conclusions, our experimental and theoretical results strongly suggest that the contrasting activities displayed depend on the conformations adopted by these compounds when they bind to the enzyme.

Overexpression of membrane proteins from higher eukaryotes in yeasts

Heterologous expression and characterisation of the membrane proteins of higher eukaryotes is of paramount interest in fundamental and applied research. Due to the rather simple and well-established methods for their genetic modification and cultivation, yeast cells are attractive host systems for recombinant protein production. This review provides an overview on the remarkable progress, and discusses pitfalls, in applying various yeast host strains for high-level expression of eukaryotic membrane proteins. In contrast to the cell lines of higher eukaryotes, yeasts permit efficient library screening methods. Modified yeasts are used as high-throughput screening tools for heterologous membrane protein functions or as benchmark for analysing drug-target relationships, e.g., by using yeasts as sensors. Furthermore, yeasts are powerful hosts for revealing interactions stabilising and/or activating membrane proteins. We also discuss the stress responses of yeasts upon heterologous expression of membrane proteins. Through co-expression of chaperones and/or optimising yeast cultivation and expression strategies, yield-optimised hosts have been created for membrane protein crystallography or efficient whole-cell production of fine chemicals.

Monoamine oxidase A and B substrates: probing the pathway for drug development

Drug-discovery and -development efforts focused on the MAOs have increased at an accelerated rate over the past decade. Since the first crystal structure of human MAO-B was solved in 2002, over 40 additional structures have been reported and have helped define new, or confirm speculative, binding modes of inhibitors. The detailed mechanism of the MAO-catalyzed oxidation of amine substrates has not been fully elucidated, but its significance is central in the development of new mechanism-based inactivators. Novel fungal MAO-N variants derived from directed evolution strategies are enabling the production of new chiral amine products. Robust assays have been established for measuring MAO status in tissue and cells, while improved MAO radioligands are being deployed for PET imaging studies. This review will attempt to highlight the more recent and salient aspects of MAO research in drug discovery and development, with emphasis on substrates 'probing the pathway'.

Kinetic and structural analysis of the irreversible inhibition of human monoamine oxidases by ASS234, a multi-target compound designed for use in Alzheimer's disease

Monoamine oxidases (MAO) and cholinesterases are validated targets in the design of drugs for the treatment of Alzheimer's disease. The multi-target compound N-((5-(3-(1-benzylpiperidin-4-yl)propoxy)-1-methyl-1H-indol-2-yl)methyl)-N-methylprop-2-yn-1-amine (ASS234), bearing the MAO-inhibiting propargyl group attached to a donepezil moiety that inhibits cholinesterases, retained activity against human acetyl- and butyryl-cholinesterases. The inhibition of MAO A and MAO B by ASS234 was characterized and compared to other known MAO inhibitors. ASS234 was almost as effective as clorgyline (kinact/KI=3×10(6) min(-1)M(-1)) and was shown by structural studies to form the same N5 covalent adduct with the FAD cofactor.

Monoamine oxidases as sources of oxidants in the heart

Oxidative stress can be generated at several sites within the mitochondria. Among these, monoamine oxidase (MAO) has been described as a prominent source. MAOs are mitochondrial flavoenzymes responsible for the oxidative deamination of catecholamines, serotonin and biogenic amines, and during this process they generate H2O2 and aldehyde intermediates. The role of MAO in cardiovascular pathophysiology has only recently gathered some attention since it has been demonstrated that both H2O2 and aldehydes may target mitochondrial function and consequently affect function and viability of the myocardium. In the present review, we will discuss the role of MAO in catecholamine and serotonin clearance and cycling in relation to cardiac structure and function. The relevant contribution of each MAO isoform (MAO-A or -B) will be discussed in relation to mitochondrial dysfunction and myocardial injury. Finally, we will examine both beneficial effects of their pharmacological or genetic inhibition along with potential adverse effects observed at baseline in MAO knockout mice, as well as the deleterious effects following their over-expression specifically at cardiomyocyte level. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Flavins and flavoproteins: applications in medicine

The potential of flavoproteins as targets of pharmacological treatments is immense. In this review we present an overview of the current research progress on medical interventions based on flavoproteins with a special emphasis on cancer, infectious diseases, and neurological disorders.

Do MAO A and MAO B utilize the same mechanism for the C-H bond cleavage step in catalysis? Evidence suggesting differing mechanisms

The detailed molecular mechanism proposed for the MAO-catalyzed oxidation of amines has been controversial with the basic assumption that both MAO A and MAO B follow the same pathway for the C-H bond cleavage step. Using the mechanistic approach of investigation of electronic effects of various benzylamine ring substituents in experiments at pH 9.0, human MAO A exhibits a kinetic behavior characteristic of an H(+) abstraction, while human MAO B exhibits kinetic properties characteristic of a H(-) abstraction. These results lead to the conclusion that the assumption that MAO A and MAO B follow identical mechanisms is incorrect.

Combining QSAR classification models for predictive modeling of human monoamine oxidase inhibitors

Due to their role in the metabolism of monoamine neurotransmitters, MAO-A and MAO-B present a significant pharmacological interest. For instance the inhibitors of human MAO-B are considered useful tools for the treatment of Parkinson Disease. Therefore, the rational design and synthesis of new MAOs inhibitors is considered of great importance for the development of new and more effective treatments of Parkinson Disease. In this work, Quantitative Structure Activity Relationships (QSAR) has been developed to predict the human MAO inhibitory activity and selectivity. The first step was the selection of a suitable dataset of heterocyclic compounds that include chromones, coumarins, chalcones, thiazolylhydrazones, etc. These compounds were previously synthesized in one of our laboratories, or elsewhere, and their activities measured by the same assays and for the same laboratory staff. Applying linear discriminant analysis to data derived from a variety of molecular representations and feature selection algorithms, reliable QSAR models were built which could be used to predict for test compounds the inhibitory activity and selectivity toward human MAO. This work also showed how several QSAR models can be combined to make better predictions. The final models exhibit significant statistics, interpretability, as well as displaying predictive power on an external validation set made up of chromone derivatives with unknown activity (that are being reported here for first time) synthesized by our group, and coumarins recently reported in the literature.

The 'gating' residues Ile199 and Tyr326 in human monoamine oxidase B function in substrate and inhibitor recognition

The major structural difference between human monoamine oxidases A (MAO A) and B (MAO B) is that MAO A has a monopartite substrate cavity of ~550 Å(3) volume and MAO B contains a dipartite cavity structure with volumes of ~290 Å(3) (entrance cavity) and ~400 Å(3) (substrate cavity). Ile199 and Tyr326 side chains separate these two cavities in MAO B. To probe the function of these gating residues, Ile199Ala and Ile199Ala-Tyr326Ala mutant forms of MAO B were investigated. Structural data on the Ile199Ala MAO B mutant show no alterations in active site geometries compared with wild-type enzyme while the Ile199Ala-Tyr326Ala MAO B mutant exhibits alterations in residues 100-103 which are part of the loop gating the entrance to the active site. Both mutant enzymes exhibit catalytic properties with increased amine K(M) but unaltered k(cat) values. The altered K(M) values on mutation are attributed to the influence of the cavity structure in the binding and subsequent deprotonation of the amine substrate. Both mutant enzymes exhibit weaker binding affinities relative to wild-type enzyme for small reversible inhibitors. Ile199Ala MAO B exhibits an increase in binding affinity for reversible MAO B specific inhibitors which bridge both cavities. The Ile199Ala-Tyr326Ala double mutant exhibits inhibitor binding properties more similar to those of MAO A than to MAO B. These results demonstrate that the bipartite cavity structure in MAO B plays an important role in substrate and inhibitor recognition to distinguish its specificities from those of MAO A and provide insights into specific reversible inhibitor design for these membrane-bound enzymes.

Molecular Insights into Human Monoamine Oxidase B Inhibition by the Glitazone Anti-Diabetes Drugs

The widely employed anti-diabetic drug pioglitazone (Actos) is shown to be a specific and reversible inhibitor of human monoamine oxidase B (MAO B). The crystal structure of the enzyme-inhibitor complex shows the R-enantiomer is bound with the thiazolidinedione ring near the flavin. The molecule occupies both substrate and entrance cavities of the active site establishing non-covalent interactions with the surrounding amino acids. These binding properties differentiate pioglitazone from the clinically used MAO inhibitors, which act through covalent inhibition mechanisms and do not exhibit a high degree of MAO A versus B selectivity. Rosiglitazone (Avandia) and troglitazone, other members of the glitazone class, are less selective in that they are weaker inhibitors of both MAO A and MAO B These results suggest that pioglitazone may have utility as a "re-purposed" neuro-protectant drug in retarding the progression of disease in Parkinson's patients. They also provide new insights for the development of reversible isoenzyme-specific MAO inhibitors.

Nitrogen kinetic isotope effects for the monoamine oxidase B-catalyzed oxidation of benzylamine and (1,1-(2)H2)benzylamine: nitrogen rehybridization and CH bond cleavage are not concerted

Nitrogen kinetic isotope effects for the oxidation of benzylamine and (1,1-(2)H(2))benzylamine by recombinant human monoamine oxidase B show that cleavage of the CH bond is not concerted with rehybridization of the nitrogen atom.

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.

Structural properties of human monoamine oxidases A and B

The structural elucidations of human monoamine oxidases A and B (MAO-A and -B) have provided novel insights into their similarities and differences. Although the enzymes exhibit ∼70% sequence identities, highly conserved chain folds, and are structurally identical in their flavin adenine dinucleotide (FAD)-binding sites, they differ considerably in the structures of their active sites opposite the flavin cofactor. MAO-A has a monopartite cavity of ∼550 ų, and MAO-B exhibits a bipartite cavity structure with an entrance cavity of 290 ų and a substrate cavity of ∼400 ų. Ile199 functions as a conformational "gate" separating the two cavities. Both enzymes are anchored to the outer mitochondrial membrane via C-terminal helical tails. Loop structures are found at the entrances to their active sites at the membrane surface. Although the crystal structure of human MAO-A is monomeric while MAO-B is dimeric, both enzymes are dimeric in their membrane-bound forms. Dimerization may be important for the favorable orientation of the resultant protein dipole moment toward the anionic membrane surface.

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.

Interactions of monoamine oxidases with the antiepileptic drug zonisamide: specificity of inhibition and structure of the human monoamine oxidase B complex

The binding of zonisamide to purified, recombinant monoamine oxidases (MAOs) has been investigated. It is a competitive inhibitor of human MAO B (K(i) = 3.1 ± 0.3 μM), of rat MAO B (K(i) = 2.9 ± 0.5 μM), and of zebrafish MAO (K(i) = 30.8 ± 5.3 μM). No inhibition is observed with purified human or rat MAO A. The 1.8 Å structure of the MAO B complex demonstrates that it binds within the substrate cavity.

Potentiation of ligand binding through cooperative effects in monoamine oxidase B

Crystallographic and biochemical studies have been employed to identify the binding site and mechanism for potentiation of imidazoline binding in human monoamine oxidase B (MAO B). 2-(2-Benzofuranyl)-2-imidazoline (2-BFI) inhibits recombinant human MAO B with a K(i) of 8.3 ± 0.6 μM, whereas tranylcypromine-inhibited MAO B binds 2-BFI with a K(d) of 9 ± 2 nM, representing an increase in binding energy Δ(ΔG) of -3.9 kcal/mol. Crystal structures show the imidazoline ligand bound in a site that is distinct from the substrate-binding cavity. Contributions to account for the increase in binding affinity upon tranylcypromine inhibition include a conformational change in the side chain of Gln(206) and a "closed conformation" of the side chain of Ile(199), forming a hydrophobic "sandwich" with the side chain of Ile(316) on each face of the benzofuran ring of 2-BFI. Data with the I199A mutant of human MAO B and failure to observe a similar binding potentiation with rat MAO B, where Ile(316) is replaced with a Val residue, support an allosteric mechanism where the increased binding affinity of 2-BFI results from a cooperative increase in H-bond strength through formation of a more hydrophobic milieu. These insights should prove valuable in the design of high affinity and specific reversible MAO B inhibitors.

On the formation and nature of the imidazoline I2 binding site on human monoamine oxidase-B

An allosteric binding site with high affinity for imidazoline I(2) ligands has been proposed to exist on monoamine oxidase-B (MAO-B). However, enzyme inhibition only occurs at ligand concentrations far higher than are required to saturate this site. We here confirm previous reports that inactivation of recombinant human MAO-B with tranylcypromine results in the formation of a high affinity I(2) site on the enzyme, measured as an increase in binding of [(3)H]2-BFI. Incubation of MAO-B with 2-phenylethylamine, an endogenous trace amine and MAO-B substrate, resulted in a progressive loss of enzyme activity, increased enzyme mass, distinct spectral changes and, as was observed with tranylcypromine, a parallel increase in high affinity binding of [(3)H]2-BFI. Kinetic studies of the mechanism by which 2-BFI inhibits MAO-B activity suggested binding of 2-BFI, at micromolar concentrations, to a site distinct from the active site on at least two forms of the pure enzyme, probably corresponding to oxidised and reduced enzyme states. Studies with mutant enzymes revealed a pattern of changes consistent with binding of 2-BFI to the substrate entrance channel of human MAO-B. Structural data confirm that high affinity binding of I(2) ligands occurs within the entrance channel of inactive enzyme, while lower affinity binding at the same location in catalytically active enzyme results in mixed inhibition of MAO-B activity. High affinity I(2) sites may form in vivo due to inactivation of a portion of MAO-B during amine oxidation, while the low affinity I(2) site on active enzyme is a target for novel MAO-B inhibitor drugs.

Identification of a hypothetical protein from Podospora anserina as a nitroalkane oxidase

The flavoprotein nitroalkane oxidase (NAO) from Fusarium oxysporum catalyzes the oxidation of primary and secondary nitroalkanes to their respective aldehydes and ketones. Structurally, the enzyme is a member of the acyl-CoA dehydrogenase superfamily. To date no enzymes other than that from F. oxysporum have been annotated as NAOs. To identify additional potential NAOs, the available database was searched for enzymes in which the active site residues Asp402, Arg409, and Ser276 were conserved. Of the several fungal enzymes identified in this fashion, PODANSg2158 from Podospora anserina was selected for expression and characterization. The recombinant enzyme is a flavoprotein with activity on nitroalkanes comparable to the F. oxysporum NAO, although the substrate specificity is somewhat different. Asp399, Arg406, and Ser273 in PODANSg2158 correspond to the active site triad in F. oxysporum NAO. The k(cat)/K(M)-pH profile with nitroethane shows a pK(a) of 5.9 that is assigned to Asp399 as the active site base. Mutation of Asp399 to asparagine decreases the k(cat)/K(M) value for nitroethane over 2 orders of magnitude. The R406K and S373A mutations decrease this kinetic parameter by 64- and 3-fold, respectively. The structure of PODANSg2158 has been determined at a resolution of 2.0 A, confirming its identification as an NAO.

Synthesis of human renalase1 in Escherichia coli and its purification as a FAD-containing holoprotein

Renalase is a protein ubiquitous in vertebrates, which has been proposed to modulate blood pressure and heart rate, and whose downregulation might result in hypertension. Despite its potential relevance for human health, the biochemical characterization of renalase is still lacking, possibly due to difficulties in obtaining it in recombinant form. By expressing two different gene constructs, we found that the major isoform of human renalase, renalase1, is mainly produced in Escherichia coli in inclusion bodies. However, by optimizing the expression conditions, significant amounts of soluble products were obtained. Both soluble renalase forms have been purified to homogeneity exploiting their N-terminal His-tag. Linking of the protein of interest to the SUMO protein did not improve solubility, but yielded untagged renalase1 after proteolytic processing of the fusion product. The two recombinant renalase forms displayed the same molecular properties. They bind equimolar amounts of FAD and appear to be correctly folded by various criteria. The procedures for the production and isolation of recombinant renalase1 here reported are expected to boost the much awaited biochemical studies on this remarkable protein.

2-Arylthiomorpholine derivatives as potent and selective monoamine oxidase B inhibitors

2-Arylthiomorpholine and 2-arylthiomorpholin-5-one derivatives, designed as rigid and/or non-basic phenylethylamine analogues, were evaluated as rat and human monoamine oxidase inhibitors. Molecular docking provided insight into the binding mode of these inhibitors and rationalized their different potencies. Making the phenylethylamine scaffold rigid by fixing the amine chain in an extended six-membered ring conformation increased MAO-B (but not MAO-A) inhibitory activity relative to the more flexible alpha-methylated derivative. The presence of a basic nitrogen atom is not a prerequisite in either MAO-A or MAO-B. The best K(i) values were in the 10(-8)M range, with selectivities towards human MAO-B exceeding 2000-fold.