Demonstration of Isoleucine 199 as a Structural Determinant for the Selective Inhibition of Human Monoamine Oxidase B by Specific Reversible Inhibitors

Structure-activity relationships in the inhibition of monoamine oxidase B by 1-methyl-3-phenylpyrroles

1-Methyl-3-phenyl-3-pyrrolines are structural analogues of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and like MPTP are selective substrates of monoamine oxidase B (MAO-B). As part of an ongoing investigation into the substrate properties of various 1-methyl-3-phenyl-3-pyrrolinyl derivatives, it is shown in the present study that their respective MAO-B catalyzed oxidation products act as reversible competitive inhibitors of the enzyme. The most potent inhibitor among the oxidation products considered was 1-methyl-3-(4-trifluoromethylphenyl)pyrrole with an enzyme-inhibitor dissociation constant (K(i) value) of 1.30 microM. The least potent inhibitor was found to be 1-methyl-3-phenylpyrrole with a K(i) value of 118 microM. The results of an SAR study established that the potency of MAO-B inhibition by the 1-methyl-3-phenylpyrrolyl derivatives examined here is dependent on the Taft steric parameter (E(s)) and Swain-Lupton electronic constant (F) of the substituents attached to C-4 of the phenyl ring. Electron-withdrawing substituents with a large degree of steric bulkiness appear to enhance inhibition potency. Potency was also found to vary with the substituents at C-3, again with E(s) and F being the principal substituent descriptors.

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

Due to their pharmacological importance in the oxidation of amine neurotransmitters, the membrane-bound flavoenzymes monoamine oxidase A and monoamine oxidase B have attracted numerous investigations and, as a result, two different mechanisms; the single electron transfer and the polar nucleophilic mechanisms, have been proposed to describe their catalytic mechanisms. This review compiles the recently available structural data on both enzymes with available mechanistic data as well as current NMR data on flavin systems to provide an integration of the approaches. These conclusions support the proposal that a polar nucleophilic mechanism for amine oxidation is the most consistent mechanistic scheme as compared with the single electron transfer mechanism.

Molecular docking of inhibitors into monoamine oxidase B

Monoamine oxidase B (MAO-B) functions in the deamination of monoamines, including dopamine and norepinephrine. The search for MAO-B inhibitors increased following the discovery that the enzyme may be responsible for generating neurotoxins from various endogenous or exogenous compounds. Computational screening methods aid in the search for new inhibitors, but validation studies for specific software packages and receptors are necessary for effective application of these methods. In this study, DOCK 6.0.0 was used to dock a series of inhibitors to MAO-B. Included were studies of re-docking ligands into MAO-B crystal structures, after which a set of 30 compounds with known inhibition constants for MAO-B were docked, including 15 strong inhibitors and 15 weak inhibitors. Good agreement was observed between the top experimental inhibitors and the top ranked docking results, and key interactions between the ligands and receptor were identified.

Species Differences in the Selective Inhibition of Monoamine Oxidase (1-methyl-2-phenylethyl)hydrazine and its Potentiation by Cyanide

The potentiating effects of cyanide on the inhibition of rat liver mitochondrial monoamine oxidase-A & B and of ox liver mitochondrial MAO-B by pheniprazine [(1-methyl-2-phenylethyl)hydrazine] has been studied. Pheniprazine was shown to behave as a mechanism-based MAO inhibitor. For rat liver MAO-B, the initial non-covalent step was characterized by dissociation constant (K (i)) of 2450 nM and the first-order rate constant (k (+2)) for the covalent adduct formation was 0.16 min(-1). As a reversible inhibitor it was selective towards rat liver MAO-A (K (i) = 420 nM) but the rate of irreversible inhibition of that enzyme was considerably slower (k (+2) = 0.06 min(-1)). MAO-B from ox liver more closely resembled MAO-A from the rat in sensitivity to reversible inhibition by pheniprazine (K (i) = 450 nm) but it was closer to rat liver MAO-B in rate of irreversible inhibition (k (+2) = 0.29 min(-1)). The K (i) values were significantly decreased in the presence of KCN but there was little effect on the k (+2) values. However, sensitivities of the different enzymes to KCN varied widely and considerably higher concentrations of KCN were required for this effect to be apparent with the rat liver mitochondrial MAO-A than with MAO-B from rat and ox liver. The kinetic behaviour of cyanide activation was consistent with partial (non-essential) competitive activation in all cases.

The effects of buffer cations on interactions between mammalian copper-containing amine oxidases and their substrates

We and others have observed that substrates for copper-containing amine oxidases cause substrate inhibition at high concentrations. Through use of a novel "pseudoquantitative" rapid equilibrium approach, kinetic analyses with human and bovine enzymes indicate that these effects are consistent with substrates binding to oxidised and reduced enzyme forms. Small cations compete with binding of substrates to oxidised and reduced enzyme, influencing both substrate turnover and substrate inhibition patterns. Cations reduce affinity of the resting bovine enzyme for spermidine, but not benzylamine, indicating that the predominant effect of cations on substrate oxidation results from binding to an anionic site outside the active site. However, binding of cations to the active site of the reduced form of both enzymes attenuates substrate inhibition with both spermidine and benzylamine. Our observations have significant practical implications for researchers assaying kinetic behaviour of these enzymes, and particularly those developing novel inhibitors of human copper-containing amine oxidases.

Docking of novel reversible monoamine oxidase-B inhibitors: efficient prediction of ligand binding sites and estimation of inhibitors thermodynamic properties

Monoamine oxidase (MAO, EC 1.4.3.4) is a flavoenzyme bound to the mitochondrial outer membranes of the cells, which is responsible for the oxidative deamination of neurotransmitter and dietary amines. It has two distinct isozymic forms, designated MAO-A and MAO-B, each displaying different substrate and inhibitor specificities. They are the well-known target for antidepressant, Parkinson's disease and neuroprotective drugs. Elucidation of the x-ray crystallographic structure of MAO-B has opened the way for molecular modeling studies. In this research 12 reversible and MAO-B selective inhibitors have been docked computationally to the active site of the MAO-B enzyme. AutoDock 3.0.5 was employed to perform the automated molecular docking. The result of docking studies generated thermodynamic properties, such as free energy of bindings (DeltaG(b)) and inhibition constants (K (i)) for the inhibitors. Moreover, 3D pictures of inhibitor-enzyme complexes afforded valuable data regarding the binding orientation of each inhibitor in the active site of MAO-B.

New insights into the structures and functions of human monoamine oxidases A and B

Structural studies on recombinant human monoamine oxidase A (hMAO-A) provides interesting insights on comparison with that determined for human MAO-B (hMAO-B) as well as comparison with that previously published for rat MAO-A. The active site cavity of hMAO-A is monopartite (as with rat MAO-A) while hMAO-B is a bipartite cavity. hMAO-A crystallizes as a monomeric form, in contrast to the dimeric forms exhibited by hMAO-B and rat MAO-A. All of the known MAO structures show nearly identical geometries around the covalent FAD sites. Differences in active site cavity structures occur away from the FAD site through conformational alterations (MAO-A's) and by changes in amino acid residues (hMAO-A and hMAO-B). Differences observed between human and rat MAO-A's raise questions regarding the appropriateness of the rat model in the development of MAO-A specific inhibitors as drugs for eventual human use.

Kinetic properties of recombinant MAO-A on incorporation into phospholipid nanodisks

Recent structural studies of human monoamine oxidase A (MAO-A) suggest the entrance to the active site is positioned near the surface of the mitochondrial outer membrane (Colibus et al., 2005). To determine the influence of the phospholipid bilayer on the structure and catalytic properties of MAO in a defined system, we have incorporated the recombinant protein into phospholipid 'nanodiscs' which have been developed by Stephen G. Sligar's group (Denisov et al., 2004). Purified MAO-A incorporates into pre-formed nanodiscs which are approximately 10 nm in diameter and exhibit the thickness expected for a phospholipid bilayer. Nanodisc assemblies of MAO-A are water-soluble, yield increased enzyme stability relative to detergent solutions, are catalytically active, and reactive with acetylenic inhibitors. As compared to detergent-based systems, the catalytic efficiencies (k (cat)/K (m)) of amine oxidation appear to be greater. Also, nanodisc bound MAO-A binds various inhibitors with K (i) values that are 2-4 fold lower than MAO-A in reduced Triton X-100 solutions. Taken together, these data suggest that the membrane environment affects MAO-A catalytic properties for both substrates and reversible inhibitors.

Selective Inhibitory Activity against MAO and Molecular Modeling Studies of 2-Thiazolylhydrazone Derivatives

A series of 2-thiazolylhydrazone derivatives have been investigated for the ability to inhibit the activity of the A and B isoforms of monoamine oxidase (MAO) selectively. All of the compounds showed high activity against both the MAO-A and the MAO-B isoforms with pKi values ranging between 5.92 and 8.14 for the MAO-A and between 4.69 and 9.09 for the MAO-B isoforms. Both the MAO-A and the MAO-B isoforms, deposited in the Protein Data Bank as model 2BXR and 1GOS, respectively, were considered in a computational study performed with docking techniques on the most active and MAO-B-selective inhibitor, 18.

A peroxidase-coupled continuous absorbance plate-reader assay for flavin monoamine oxidases, copper-containing amine oxidases and related enzymes

This absorbance plate-reader-based assay is suitable for the examination of monoamine oxidase and copper amine oxidase activities versus numerous substrates. The assay is robust, continuous, rapid, highly quantitative, reasonably sensitive, inexpensive and suitable for automation. In the presence of a suitable amine substrate, amine oxidase enzymes generate hydrogen peroxide, which then drives the peroxidase-dependent oxidation of 4-aminoantipyrine. A subsequent interaction with vanillic acid generates stoichiometric amounts of a red quinoneimine dye, the appearance of which is monitored at 498 nm. An alternative procedure in which vanillic acid is replaced by 2,4-dichlorophenol enhances sensitivity but precludes the measurement of monoamine oxidases due to inhibition of these enzymes by dichlorophenol. Some substrates with low redox potentials, such as catecholamines, are not suitable for inclusion in this assay. A researcher familiar with the procedure can manually generate data for 30 full kinetic curves, composed of ten triplicate points, in 8 h.

A synchronized substrate-gating mechanism revealed by cubic-core structure of the bovine branched-chain alpha-ketoacid dehydrogenase complex

The dihydrolipoamide acyltransferase (E2b) component of the branched-chain alpha-ketoacid dehydrogenase complex forms a cubic scaffold that catalyzes acyltransfer from S-acyldihydrolipoamide to CoA to produce acyl-CoA. We have determined the first crystal structures of a mammalian (bovine) E2b core domain with and without a bound CoA or acyl-CoA. These structures reveal both hydrophobic and the previously unreported ionic interactions between two-fold-related trimers that build up the cubic core. The entrance of the dihydrolipoamide-binding site in a 30-A long active-site channel is closed in the apo and acyl-CoA-bound structures. CoA binding to one entrance of the channel promotes a conformational change in the channel, resulting in the opening of the opposite dihydrolipoamide gate. Binding experiments show that the affinity of the E2b core for dihydrolipoamide is markedly increased in the presence of CoA. The result buttresses the model that CoA binding is responsible for the opening of the dihydrolipoamide gate. We suggest that this gating mechanism synchronizes the binding of the two substrates to the active-site channel, which serves as a feed-forward switch to coordinate the E2b-catalyzed acyltransfer reaction.

Synthesis, molecular modeling studies, and selective inhibitory activity against monoamine oxidase of N,N'-bis[2-oxo-2H-benzopyran]-3-carboxamides

A novel series of N,N'-bis[2-oxo-2H-1-benzopyran]-3-carboxamide derivatives have been synthesized and investigated for the ability to inhibit the activity of the A and B isoforms of monoamine oxidase (MAO). Some of the synthesized compounds show good selective inhibitory activity against the MAO-A isoform. Both the MAO-A and -B isoforms, deposited in the Protein Data Bank as the 2BXR and 1GOS models, respectively, were considered in a computational study performed with docking techniques on the most active and selective inhibitors.

The therapeutic potential of monoamine oxidase inhibitors

Monoamine oxidase inhibitors were among the first antidepressants to be discovered and have long been used as such. It now seems that many of these agents might have therapeutic value in several common neurodegenerative conditions, independently of their inhibition of monoamine oxidase activity. However, many claims and some counter-claims have been made about the physiological importance of these enzymes and the potential of their inhibitors. We evaluate these arguments in the light of what we know, and still have to learn, of the structure, function and genetics of the monoamine oxidases and the disparate actions of their inhibitors.

Functional Role of the “Aromatic Cage” in Human Monoamine Oxidase B: Structures and Catalytic Properties of Tyr435 Mutant Proteins,

Current structural results of several flavin-dependent amine oxidizing enzymes including human monoamine oxidases A and B (MAO A and MAO B) show aromatic amino acid residues oriented approximately perpendicular to the flavin ring, suggesting a functional role in catalysis. In the case of human MAO B, two tyrosyl residues (Y398 and Y435) are found in the substrate binding site on the re face of the covalent flavin ring [Binda et al. (2002) J. Biol. Chem. 277, 23973-23976]. To probe the functional significance of this structure, Tyr435 in MAO B was mutated with the amino acids Phe, His, Leu, or Trp, the mutant proteins expressed in Pichia pastoris, and purified to homogeneity. Each mutant protein contains covalent FAD and exhibits a high level of catalytic functionality. No major alterations in active site structures are detected on comparison of their respective crystal structures with that of WT enzyme. The relative k(cat)/K(m) values for each mutant enzyme show Y435 > Y435F = Y435L = Y435H > Y435W. A similar behavior is also observed with the membrane-bound forms of MAO A and MAO B (MAO A Y444 mutant enzymes are found to be unstable on membrane extraction). p-Nitrobenzylamine is found to be a poor substrate while p-nitrophenethylamine is found to be a good substrate for all WT and mutant forms of MAO B. Analysis of these kinetic and structural data suggests the function of the "aromatic cage" in MAO to include a steric role in substrate binding and access to the flavin coenzyme and to increase the nucleophilicity of the substrate amine moiety. These results are consistent with a proposed polar nucleophilic mechanism for catalytic amine oxidation.

Synthesis and Molecular Modelling of Novel Substituted-4,5-dihydro-(1H)-pyrazole Derivatives as Potent and Highly Selective Monoamine Oxidase-A Inhibitors

This report describes novel pyrazoline derivatives investigated for their ability to selectively inhibit the activity of the A and B isoforms of monoamine oxidase. These new synthetic compounds proved to be reversible, potent, and selective inhibitors of monoamine oxidase-A rather than of monoamine oxidase-B, and are promising candidates to further advance drug discovery efforts. The most active compounds show inhibitory activity on monoamine oxidase-A in the 1.0x10(-8)-8.6x10(-9) M range. Moreover, it should be pointed out that for some compounds a high IC50>or=10(-9) M value is associated with a high A-selectivity (Selectivity Index monoamine oxidase-B/monoamine oxidase-A in the 10,000-12,500 range). Further insight to understand enzyme-inhibitor molecular interaction was obtained by docking experiments with the monoamine oxidase-A and monoamine oxidase-B isoforms.

Inhibition of monoamine oxidase B by analogues of the adenosine A2A receptor antagonist (E)-8-(3-chlorostyryl)caffeine (CSC)

The adenosine A2A receptor has emerged as a possible target for the treatment of Parkinson's disease (PD). Evidence suggests that antagonism of the A2A receptor not only improves the symptoms of the disease but may also protect against the underlying degenerative processes. We have recently reported that several known adenosine A2A receptor antagonists (A2A antagonists) also are moderate to very potent inhibitors of monoamine oxidase B (MAO-B). The most potent among these was (E)-8-(3-chlorostyryl)caffeine (CSC), a compound frequently used when examining the in vivo pharmacological effects of A2A antagonists. Since MAO-B inhibitors are also thought to possess antiparkinsonian properties, dual targeting drugs that block both MAO-B and A2A receptors may have enhanced therapeutic potential in the treatment of PD. In this study, we prepared selected analogues of CSC in an attempt to examine specific structural features that may be important for potent MAO-B inhibition. The results of a SAR study established that the potency of MAO-B inhibition by (E)-8-styrylcaffeinyl analogues depends upon the van der Waals volume (V(w)), lipophilicity (pi), and the Hammett constant (sigma(m)) of the substituents attached to C-3 of the phenyl ring of the styryl moiety. Potency also varies with substituents attached to C-4 with bulkiness (V(w)) and lipophilicity (pi) being the principal substituent descriptors.

Docking studies on monoamine oxidase-B inhibitors: Estimation of inhibition constants (Ki) of a series of experimentally tested compounds

Monoamine oxidase (EC1.4.3.4; MAO) is a mitochondrial outer membrane flavoenzyme that catalyzes the oxidation of biogenic amines. It has two distinct isozymic forms designated MAO-A and MAO-B, each displaying different substrate and inhibitor specificities. They are the well-known targets for antidepressant and neuroprotective drugs. Elucidation of the X-ray crystallographic structure of MAO-B has opened the way for molecular modeling studies. A series of experimentally tested (1-10) model compounds has been docked computationally to the active site of the MAO-B enzyme. The AutoDock 3.0.5 program was employed to perform automated molecular docking. The free energies of binding (DeltaG) and inhibition constants (K(i)) of the docked compounds were calculated by the Lamarckian Genetic Algorithm (LGA) of AutoDock 3.0.5. Excellent to good correlations between the calculated and experimental K(i) values were obtained.

Three-dimensional structure of human monoamine oxidase A (MAO A): relation to the structures of rat MAO A and human MAO B

The three-dimensional structure of recombinant human monoamine oxidase A (hMAO A) as its clorgyline-inhibited adduct is described. Although the chain-fold of hMAO A is similar to that of rat MAO A and human MAO B (hMAO B), hMAO A is unique in that it crystallizes as a monomer and exhibits the solution hydrodynamic behavior of a monomeric form rather than the dimeric form of hMAO B and rat MAO A. hMAO A's active site consists of a single hydrophobic cavity of approximately 550 A3, which is smaller than that determined from the structure of deprenyl-inhibited hMAO B (approximately 700 A3) but larger than that of rat MAO A (approximately 450 A3). An important component of the active site structure of hMAO A is the loop conformation of residues 210-216, which differs from that of hMAO B and rat MAO A. The origin of this structural alteration is suggested to result from long-range interactions in the monomeric form of the enzyme. In addition to serving as a basis for the development of hMAO A specific inhibitors, these data support the proposal that hMAO A involves a change from the dimeric to the monomeric form through a Glu-151 --> Lys mutation that is specific of hMAO A [Andrès, A. M., Soldevila, M., Navarro, A., Kidd, K. K., Oliva, B. & Bertranpetit, J. (2004) Hum. Genet. 115, 377-386]. These considerations put into question the use of MAO A from nonhuman sources in drug development for use in humans.