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

Expression of zebrafish (Danio rerio) monoamine oxidase (MAO) in Pichia pastoris: purification and comparison with human MAO A and MAO B

The expression, purification and characterization of zebrafish monoamine oxidase (zMAO) using the methylotropic yeast Pichia pastoris expression system is described. A 1L fermentation culture of Pichia pastoris containing the gene encoding zMAO under control of the methanol oxidase promotor expresses approximately 200mg of zMAO exhibiting 300 U of total activity. The enzyme is found in the mitochondrial fraction of the expression host and is purified in a 30% yield as a homogenous species with a M(r) of approximately 60,000 on SDS-PAGE and a mass of 58,525+/-40 Da from MALDI-TOF measurements. The zMAO preparation contains one mole of covalent flavin cofactor per mole of enzyme and exhibits >80% functionality. The covalent flavin exhibits fluorescence and EPR spectral properties consistent with known properties of 8 alpha-S-cysteinyl FAD. Chemical degradation of the flavin peptide results in the liberation of FAD. zMAO exhibits no immuno-chemical cross-reactivity with polyclonal anti-sera raised against human MAO A. The enzyme preparation exhibits reasonable thermostability up to a temperature of 30 degrees C. Benzylamine is oxidized with a k(cat) value of 4.7+/-0.1 min(-1) (K(m)=82+/-9 microM) and the enzyme oxidizes phenylethylamine with a k(cat) value of 204 min(-1) (K(m)=86+/-13 microM). The K(m) (O(2)) values determined for zMAO using either benzylamine or phenylethylamine as substrates ranges from 108(+/-5) to 140(+/-21)microM. The functional behavior of this teleost MAO relative to human MAO A and MAO B is discussed.

High-level expression and purification of rat monoamine oxidase A (MAO A) in Pichia pastoris: comparison with human MAO A

The high-level heterologous expression in Pichia pastoris, purification and characterization of recombinant membrane-bound rat liver monoamine oxidase A (MAO A) are described. A 1-L culture of cells produces approximately 700 U of rat MAO A activity. The rat MAO A activity is found in outer mitochondrial membrane of the cell. Using a modification of the human MAO A purification procedure, approximately 200mg of recombinant rat MAO A is purified in a 43% yield and exhibits a molecular weight of approximately 60,000 kDa on SDS-PAGE. The purified enzyme contains a covalently bound FAD and forms a N(5) flavocyanine adduct on inhibition by clorgyline. Edman sequencing shows that the amino terminus of rat MAO A is blocked at an N-terminal threonyl residue. The purified rat enzyme exhibits a higher thermal stability than does purified human MAO A. Compared with human MAO A, rat MAO A oxidizes serotonin or kynuramine with twofold higher k(cat)/K(m) values, oxidizes phenethylamine with a 6.7-fold higher catalytic efficiency and benzylamine with a approximately 40-fold higher catalytic efficiency. Although approximately 90% identical in sequence to human MAO A, rat MAO A is a more efficient catalyst for amine neurotransmitter oxidation.

Molecular and mechanistic properties of the membrane-bound mitochondrial monoamine oxidases

The past decade has brought major advances in our knowledge of the structures and mechanisms of MAO A and MAO B, which are pharmacological targets for specific inhibitors. In both enzymes, crystallographic and biochemical data show their respective C-terminal transmembrane helices anchor the enzymes to the outer mitochondrial membrane. Pulsed EPR data show both enzymes are dimeric in their membrane-bound forms with agreement between distances measured in their crystalline forms. Distances measured between active site-directed spin-labels in membrane preparations show excellent agreement with those estimated from crystallographic data. Our knowledge of requirements for development of specific reversible MAO B inhibitors is in a fairly mature status. Less is known regarding the structural requirements for highly specific reversible MAO A inhibitors. In spite of their 70% level of sequence identity and similarities of C(alpha) folds, the two enzymes exhibit significant functional and structural differences that can be exploited in the ultimate goal of the development of highly specific inhibitors. This review summarizes the current structural and mechanistic information available that can be utilized in the development of future highly specific neuroprotectants and cardioprotectants.

Development of spin-labeled pargyline analogues as specific inhibitors of human monoamine oxidases A and B

Three TEMPO-conjugated pargyline analogues (ParSL-1, ParSL-2, and ParSL-3) have been synthesized and their inhibitory properties tested for the two human monoamine oxidase isoforms (hMAOA and hMAOB). The three analogues differ in flexibility and substituent positions (para or meta) of the linkers connecting the TEMPO group to the pargyline phenyl ring. ParSL-1 contains a flexible acetamido (-CH(2)-CO-NH-) linker connecting the two moieties at the para position. In contrast, the TEMPO moieties in ParSL-2 and ParSL-3 are attached with rigid amido (-CO-NH-) linkers to the para or meta positions of the pargyline phenyl ring, respectively. These variations in conformational flexibility and substituent position are shown to have profound effects in tuning the specificities of these analogues toward the two MAO isoforms. ParSL-1 irreversibly inhibits either MAOA and MAOB, ParSL-2 inhibits only MAOB (K(i) = 15 +/- 5 microM), and ParSL-3 is found to be specific for MAOA (K(i) = 268 +/- 72 microM). These results thus provide additional insights into the role of conformational flexibility and structural properties of MAO inhibitors in tuning their isoform specificities. These active site probes have been used to determine the topological orientation of these enzymes in the mitochondrial membrane. Studies with intact mitochondria show MAOA is topologically on the cytosolic face of the outer membrane in human placenta but recombinant MAOA is situated on the opposite inner face in Pichia mitochondria. Recombinant MAOB is found to be situated on the cytosolic face of the outer membrane in Pichia mitochondria.

Inhibition of monoamine oxidase by (E)-styrylisatin analogues

Previous studies have shown that (E)-8-(3-chlorostyryl)caffeine (CSC) is a specific reversible inhibitor of human monoamine oxidase B (MAO-B) and does not bind to human MAO-A. Since the small molecule isatin is a natural reversible inhibitor of both MAO-B and MAO-A, (E)-5-styrylisatin and (E)-6-styrylisatin analogues were synthesized in an attempt to identify inhibitors with enhanced potencies and specificities for MAO-B. The (E)-styrylisatin analogues were found to exhibit higher binding affinities than isatin with the MAO preparations tested. The (E)-5-styrylisatin analogues bound more tightly than the (E)-6 analogue although the latter exhibits the highest MAO-B selectivity. Molecular docking studies with MAO-B indicate that the increased binding affinity exhibited by the (E)-styrylisatin analogues, in comparison to isatin, is best explained by the ability of the styrylisatins to bridge both the entrance cavity and the substrate cavity of the enzyme. Experimental support for this model is shown by the weaker binding of the analogues to the Ile199Ala mutant of human MAO-B. The lower selectivity of the (E)-styrylisatin analogues between MAO-A and MAO-B, in contrast to CSC, is best explained by the differing relative geometries of the aromatic rings for these two classes of inhibitors.

Structural and mechanistic studies of mofegiline inhibition of recombinant human monoamine oxidase B

Mechanistic and structural studies have been carried out to investigate the molecular basis for the irreversible inhibition of human MAO-B by mofegiline. Competitive inhibition with substrate shows an apparent K(i) of 28 nM. Irreversible inhibition of MAO-B occurs with a 1:1 molar stoichiometry with no observable catalytic turnover. The absorption spectral properties of mofegiline inhibited MAO-B show features (lambda(max) approximately 450 nm) unlike those of traditional flavin N(5) or C(4a) adducts. Visible and near-UV circular dichroism spectra of the mofegiline-MAO-B adduct shows a negative peak at 340 nm with an intensity similar to that of N(5) flavocyanine adducts. The X-ray crystal structure of the mofegiline-MAO-B adduct shows a covalent bond between the flavin cofactor N(5) with the distal allylamine carbon atom as well as the absence of the fluorine atom. A mechanism to explain these structural and spectral data is proposed.

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.

Purification and characterization of a copper-containing amine oxidase from Mycobacterium sp. strain JC1 DSM 3803 grown on benzylamine

A bacterial semicarbazide-sensitive amine oxidase (SSAO) was purified and characterized from Mycobacterium sp. strain JC1 DSM 3803 grown on benzylamine. During the purification procedures, the enzyme was tending to aggregate and exhibited heterogeneity in native PAGE. The heterogeneous forms having amine oxidase (AO) activity could be separated by their native molecular weights using gel-filtration chromatography. Most of the AOs behaved as dimers (M(r) 150,000) composed of a 75-kDa subunit, but some aggregated to form tetramers (M(r) 300,000). Besides their native molecular weight, subunit composition and V(max) value, both forms (dimer and tetramer) have almost identical biochemical properties (e.g. subunit size, optimum pH and temperature, activation energy, K(m) value on benzylamine, substrate and inhibitor specificities). When AO activity was observed by activity staining, the best-oxidized substrate was benzylamine, although the AO also oxidized tyramine and histamine. The AO was strongly inhibited by semicarbazide and isoniazid, but KCN did not affect its activity. The purified enzyme was shown to contain 2.39 mol of copper per mole of subunit, but there were no evidences of topaquinone co-factor involvement, when tested by absorption spectrum analysis and redox-cycling staining for quinoprotein detection.

Characterization of detergent purified recombinant rat liver monoamine oxidase B expressed in Pichia pastoris

The high level expression and purification of rat monoamine oxidase B (rMAOB) in the methylotrophic yeast Pichia pastoris is reported. Nearly 100 mg of purified rMAOB is obtained from 130 g (wet weight) of cells (0.5 L of culture). The MALDI-TOF mass spectrum of the purified protein shows a single species with a molecular mass of 59.228 +/- 0.064 kDa, which agrees with the calculated molecular weight of 59.172 kDa for the rMAOB protein sequence assuming one mole of covalent FAD per mole of the enzyme. Consistent with the MALDI-MS data, purified rMAOB shows a single band near 60 kDa in Coomassie-stained SDS-PAGE gel as well as on Western blot analyses performed using antisera raised against human MAOA and BSA-conjugated FAD. A partial amino acid sequence of the purified protein is confirmed to be that of the wild type rMAOB by in-gel trypsin digestion and MALDI-TOF-MS analyses of the liberated peptide fragments. Steady state kinetic data show that purified rMAOB exhibits a K(m)(amine) of 176 +/- 15 microM and a k(cat) of 497 +/- 83 min(-1) for benzylamine oxidation, and a K(m)(O2) of 170 +/- 10 microM. Kinetic parameters obtained for purified rMAOB are compared with those reported earlier for recombinant human liver MAOB expressed in P. pastoris.

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.

Human and rat monoamine oxidase-A are differentially inhibited by (S)-4-alkylthioamphetamine derivatives: insights from molecular modeling studies

Four enantiomerically pure (S)-4-alkylthioamphetamine derivatives were evaluated as monoamine oxidase (MAO) inhibitors using the human and rat isoforms of the enzyme. Molecular dockings were performed in order to gain insights regarding the binding mode of these inhibitors. All compounds were potent and selective MAO-A inhibitors although different rank orders of potencies were observed against the enzymes from different species. This behavior can be rationalized on the basis of different binding modes to each enzyme, as determined in silico. These findings further support the concept that MAO inhibitory activity of novel compounds, determined with enzymes from diverse mammalian species, should be considered with caution if human MAO is the final target to be addressed.

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.

Binding of rasagiline-related inhibitors to human monoamine oxidases: a kinetic and crystallographic analysis

Monoamine oxidases A and B (MAO A and B) catalyze the degradation of neurotransmitters and represent drug targets for the treatment of neurodegenerative disorders. Rasagiline is an irreversible, MAO B-selective inhibitor that has been approved as a novel anti-Parkinson's drug. In this study, we investigate the inhibition of recombinant human MAO A and MAO B by several rasagiline analogues. Different substituents added onto the rasagiline scaffold alter the binding affinity depending on the position on the aminoindan ring and on the size of the substituent. Compounds with a hydroxyl group on either the C4 or the C6 atom inhibit both isozymes, whereas a bulkier substituent such as a carbamate is tolerated only at the C4 position. The 1.7 A crystal structure of MAO B in complex with 4-(N-methyl-N-ethyl-carbamoyloxy)-N-methyl-N-propargyl-1(R)-aminoindan shows that the binding mode is similar to that of rasagiline with the carbamate moiety occupying the entrance cavity space. 1(R)-Aminoindan, the major metabolic product of rasagiline, and its analogues reversibly inhibit both MAO A and MAO B. The crystal structure of N-methyl-1(R)-aminoindan bound to MAO B shows that its aminoindan ring adopts a different orientation compared to that of rasagiline.

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.

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.

Flavoenzyme catalysed oxidation of amines: roles for flavin and protein-based radicals

Amines are a carbon source for the growth of a number of bacterial species and they also play key roles in neurotransmission, cell growth and differentiation, and neoplastic cell proliferation. Enzymes have evolved to catalyse these reactions and these oxidoreductases can be grouped into the flavoprotein and quinoprotein families. The mechanism of amine oxidation catalysed by the quinoprotein amine oxidases is understood reasonably well and occurs through the formation of enzyme–substrate covalent adducts with TPQ (topaquinone), TTQ (tryptophan tryptophylquinone), CTQ (cysteine tryptophylquinone) and LTQ (lysine tyrosyl quinone) redox centres. Oxidation of amines by flavoenzymes is less well understood. The role of protein-based radicals and flavin semiquinone radicals in the oxidation of amines is discussed.

Human recombinant monoamine oxidase B as reliable and efficient enzyme source for inhibitor screening

Interest in inhibitors of monoamine oxidase type B (MAO B) has grown in recent years, due to their therapeutic potential in aging-related neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. This study is devoted to the use of human recombinant MAO B obtained from a Baculovirus expression system (Supersomes MAO B, BD Gentest, MA, USA) as reliable and efficient enzyme source for MAO B inhibitor screening. Comparison of inhibition potencies (pIC50 values) determined with human cloned and human platelet MAO B for the two series of MAO B inhibitors, coumarin and 5H-indeno[1,2-c]pyridazin-5-one derivatives, showed that the difference between pIC50 values obtained with the two enzyme sources was not significant (P>0.05, Student's t-test). Hence, recombinant enzyme is validated as convenient enzyme source for MAO B inhibitor screening.

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

Several reversible inhibitors selective for human monoamine oxidase B (MAO B) that do not inhibit MAO A have been described in the literature. The following compounds: 8-(3-chlorostyryl)caffeine, 1,4-diphenyl-2-butene, and trans,trans-farnesol are shown to inhibit competitively human, horse, rat, and mouse MAO B with K(i) values in the low micromolar range but are without effect on either bovine or sheep MAO B or human MAO A. In contrast, the reversible competitive inhibitor isatin binds to all known MAO B and MAO A with similar affinities. Sequence alignments and the crystal structures of human MAO B in complex with 1,4-diphenyl-2-butene or with trans,trans-farnesol provide molecular insights into these specificities. These inhibitors span the substrate and entrance cavities with the side chain of Ile-199 rotated out of its normal conformation suggesting that Ile-199 is gating the substrate cavity. Ile-199 is conserved in all known MAO B sequences except bovine MAO B, which has Phe in this position (the sequence of sheep MAO B is unknown). Phe is conserved in the analogous position in MAO A sequences. The human MAO B I199F mutant protein of MAO B binds to isatin (K(i) = 3 microM) but not to the three inhibitors listed above. The crystal structure of this mutant demonstrates that the side chain of Phe-199 interferes with the binding of those compounds. This suggests that the Ile-199 "gate" is a determinant for the specificity of these MAO B inhibitors and provides a molecular basis for the development of MAO B-specific reversible inhibitors without interference with MAO A function in neurotransmitter metabolism.

HPLC-based bioactivity profiling of plant extracts: a kinetic assay for the identification of monoamine oxidase-A inhibitors using human recombinant monoamine oxidase-A

An assay for the HPLC-based search for monoamine oxidase-A (MAO-A) inhibitors in plant extracts was established. It combines human recombinant MAO-A, expressed as GST-fusion protein in yeast, with a kinetic measurement of the conversion of kynuramine to 4-hydroxyquinoline. Substrate selectivity and kinetic parameters of the GST-fusion protein were comparable to the wild-type enzyme. The applicability of the assay to HPLC-based activity profiling was tested with plant extracts spiked with small amounts of known MAO inhibitors.

Inactivation of purified human recombinant monoamine oxidases A and B by rasagiline and its analogues

The inactivation of purified human recombinant monoamine oxidases (MAO) A and B by rasagiline [N-propargyl-1(R)-aminoindan] and four of its analogues [N-propargyl-1(S)-aminoindan (S-PAI), 6-hydroxy-N-propargyl-1(R)-aminoindan (R-HPAI), N-methyl-N-propargyl-1(R)-aminoindan (R-MPAI), and 6-(N-methyl-N-ethyl carbamoyloxy)-N-propargyl-1(R)-aminoindan (R-CPAI)] has been investigated. All compounds tested, with the exception of R-CPAI, form stoichiometric N(5) flavocyanine adducts with the FAD moiety of either enzyme. No H(2)O(2) is produced during either MAO A or MAO B inactivation, which demonstrates that covalent addition occurs in a single turnover. Rasagiline has the highest specificity for MAO B, as demonstrated by a 100-fold higher inhibition potency (k(inact)/K(i)) compared to MAO A, with the remaining compounds exhibiting lower isozyme specificities. MAO B and MAO A are more selective for the R-enantiomer (rasagiline) compared to the S-enantiomer (S-PAI) by 2500-fold and 17-fold, respectively. Differences in UV/vis and CD spectral data of the complexes of the studied compounds with both MAO A and MAO B are interpreted in light of crystallographic data of complexes of MAO B with rasagiline and its analogues (Binda, C.; et al. J. Med. Chem. 2004, 47, 1767-1774.

Fluorinated phenylcyclopropylamines. Part 3: Inhibition of monoamine oxidase A and B

Fluorinated phenylcyclopropylamines and alkylamines were examined as inhibitors of recombinant human liver monoamine oxidase A (MAO A) and B (MAO B). For a series of trans- and cis-2-fluoro-2-phenylcyclopropylamine analogues, the presence of fluorine attached to a cyclopropane ring was found to result in an increase in inhibitory activity towards both MAO A and B. In addition, p-substitution of electron-withdrawing groups such as Cl and F in the aromatic ring of the trans-isomers increased the inhibition of both enzymes. (1S,2S)-2-Fluoro-2-phenylcyclopropylamine was a more potent inhibitor of both MAO A and B than was the (1R,2R)-enantiomer, indicating that the presence of fluorine has no influence on the enantioselectivity of MAO inhibition, since a similar effect of stereochemistry has been reported for tranylcypromine. Interestingly, fluorination at the 2-position of 1-phenycyclopropylamine, which is known as a selective inhibitor of MAO B relative to MAO A, reversed the selectivity and resulted in a potent inhibitor selective for MAO A. All inhibitors showed time- and concentration-dependent inhibition for both enzymes, with the exception of trans-2-fluoro-2-phenylcyclopropyl ethylamine, which acts as a competitive and reversible MAO A selective inhibitor.

Characterization and Metabolic Function of a Peroxisomal Sarcosine and Pipecolate Oxidase from Arabidopsis

Sarcosine oxidase (SOX) is known as a peroxisomal enzyme in mammals and as a sarcosine-inducible enzyme in soil bacteria. Its presence in plants was unsuspected until the Arabidopsis genome was found to encode a protein (AtSOX) with approximately 33% sequence identity to mammalian and bacterial SOXs. When overexpressed in Escherichia coli, AtSOX enhanced growth on sarcosine as sole nitrogen source, showing that it has SOX activity in vivo, and the recombinant protein catalyzed the oxidation of sarcosine to glycine, formaldehyde, and H(2) O(2) in vitro. AtSOX also attacked other N-methyl amino acids and, like mammalian SOXs, catalyzed the oxidation of l-pipecolate to Delta(1)-piperideine-6-carboxylate. Like bacterial monomeric SOXs, AtSOX was active as a monomer, contained FAD covalently bound to a cysteine residue near the C terminus, and was not stimulated by tetrahydrofolate. Although AtSOX lacks a typical peroxisome-targeting signal, in vitro assays established that it is imported into peroxisomes. Quantitation of mRNA showed that AtSOX is expressed at a low level throughout the plant and is not sarcosine-inducible. Consistent with a low level of AtSOX expression, Arabidopsis plantlets slowly metabolized supplied [(14)C]sarcosine to glycine and serine. Gas chromatography-mass spectrometry analysis revealed low levels of pipecolate but almost no sarcosine in wild type Arabidopsis and showed that pipecolate but not sarcosine accumulated 6-fold when AtSOX expression was suppressed by RNA interference. Moreover, the pipecolate catabolite alpha-aminoadipate decreased 30-fold in RNA interference plants. These data indicate that pipecolate is the endogenous substrate for SOX in plants and that plants can utilize exogenous sarcosine opportunistically, sarcosine being a common soil metabolite.

Crystal structure of human monoamine oxidase B, a drug target enzyme monotopically inserted into the mitochondrial outer membrane

Monoamine oxidase B (MAO B) is an outer mitochondrial membrane protein that oxidizes arylalkylamine neurotransmitters and has been a valuable drug target for many neurological disorders. The 1.7 angstrom resolution structure of human MAO B shows the enzyme is dimeric with a C-terminal transmembrane helix protruding from each monomer and anchoring the protein to the membrane. This helix departs perpendicularly from the base of the structure in a different way with respect to other monotopic membrane proteins. Several apolar loops exposed on the protein surface are located in proximity of the C-terminal helix, providing additional membrane-binding interactions. One of these loops (residues 99-112) also functions in opening and closing the MAO B active site cavity, which suggests that the membrane may have a role in controlling substrate binding.

Crystal Structures of Monoamine Oxidase B in Complex with Four Inhibitors of the N-Propargylaminoindan Class

Monoamine oxidase B (MAO B) is an outer mitochondrial membrane enzyme that catalyzes the oxidation of arylalkylamine neurotransmitters. The crystal structures of MAO B in complex with four of the N-propargylaminoindan class of MAO covalent inhibitors (rasagiline, N-propargyl-1(S)-aminoindan, 6-hydroxy-N-propargyl-1(R)-aminoindan, and N-methyl-N-propargyl-1(R)-aminoindan) have been determined at a resolution of better than 2.1 A. Rasagiline, 6-hydroxy-N-propargyl-1(R)-aminoindan, and N-methyl-N-propargyl-1(R)-aminoindan adopt essentially the same conformation with the extended propargyl chain covalently bound to the flavin and the indan ring located in the rear of the substrate cavity. N-Propargyl-1(S)-aminoindan binds with the indan ring in a flipped conformation with respect to the other inhibitors, which causes a slight movement of the Tyr326 side chain. Four ordered water molecules are an integral part of the active site and establish H-bond interactions to the inhibitor atoms. These structural studies may guide future drug design to improve selectivity and efficacy by introducing appropriate substituents on the rasagiline molecular scaffold.

Insights into the mode of inhibition of human mitochondrial monoamine oxidase B from high-resolution crystal structures

Monoamine oxidase B (MAO-B) is an outer mitochondrial membrane-bound enzyme that catalyzes the oxidative deamination of arylalkylamine neurotransmitters and has been a target for a number of clinically used drug inhibitors. The 1.7-A structure of the reversible isatin-MAO-B complex has been determined; it forms a basis for the interpretation of the enzyme's structure when bound to either reversible or irreversible inhibitors. 1,4-Diphenyl-2-butene is found to be a reversible MAO-B inhibitor, which occupies both the entrance and substrate cavity space in the enzyme. Comparison of these two structures identifies Ile-199 as a "gate" between the two cavities. Rotation of the side chain allows for either separation or fusion of the two cavities. Inhibition of the enzyme with N-(2-aminoethyl)-p-chlorobenzamide results in the formation of a covalent N(5) flavin adduct with the phenyl ring of the inhibitor occupying a position in the catalytic site overlapping that of isatin. Inhibition of MAO-B with the clinically used trans-2-phenylcyclopropylamine results in the formation of a covalent C(4a) flavin adduct with an opened cyclopropyl ring and the phenyl ring in a parallel orientation to the flavin. The peptide bond between the flavin-substituted Cys-397 and Tyr-398 is in a cis conformation, which allows the proper orientation of the phenolic ring of Tyr-398 in the active site. The flavin ring exists in a twisted nonplanar conformation, which is observed in the oxidized form as well as in both the N(5) and the C(4a) adducts. An immobile water molecule is H-bonded to Lys-296 and to the N(5) of the flavin as observed in other flavin-dependent amine oxidases. The active site cavities are highly apolar; however, hydrophilic areas exist near the flavin and direct the amine moiety of the substrate for binding and catalysis. Small conformational changes are observed on comparison of the different inhibitor-enzyme complexes. Future MAO-B drug design will need to consider "induced fit" contributions as an element in ligand-enzyme interactions.

Structural comparison of human monoamine oxidases A and B: mass spectrometry monitoring of cysteine reactivities

Monoamine oxidases (MAO) A and B are approximately 60-kDa outer mitochondrial membrane flavoenzymes catalyzing the degradation of neurotransmitters and xenobiotic arylalkyl amines. Despite 70% identity of their amino acid sequences, both enzymes exhibit strikingly different properties when exposed to thiol-modifying reagents. Human MAO A and MAO B each contain 9 cysteine residues (7 in conserved sequence locations). MAO A is inactivated by N-ethylmaleimide (NEM) much faster (tau(1/2) = approximately 3 min) than MAO B (tau(1/2) = approximately 8 h). These differences in thiol reactivities are also demonstrated by monitoring the NEM modification stoichiometries by electrospray mass spectrometry. Inactivation of either enzyme with acetylenic inhibitors results in alterations of their thiol reactivities. Cys5 and Cys266 were identified as the only residues modified by biotin-derivatized NEM in clorgyline-inactivated MAO A and pargyline-inactivated MAO B, respectively. The x-ray structure of MAO B (Binda, C., Newton-Vinson, P., Hubalek, F., Edmondson, D. E., and Mattevi, A. (2002) Nat. Struct. Biol. 9, 22-26) shows that Cys5 is located on the surface of the molecule opposite to the membrane-binding region. Cys266 in MAO A is predicted to be located in the same region of the molecule. These thiol residues are also modified by biotin-derivatized NEM in the mitochondrial membrane-bound MAO A and MAO B. This study shows that the MAO A structure is "more flexible" than that of MAO B and that clorgyline and pargyline inactivation of MAO A and B, respectively, increases the structural stability of both enzymes. No evidence is found for the presence of disulfide bonds in either enzyme, contrary to a previous suggestion.

The expression of outer membrane proteins for crystallization

The production of sufficient amounts of chemically and conformationally homogenous protein is a major requirement for successful crystallization and structure determination. With membrane proteins, this constitutes a particular problem because the membrane volume is limited and the organisms are usually very sensitive to changes in membrane properties brought about by massive protein insertion. Moreover, the extraction of membrane proteins from the membrane with detergents is generally a harsh treatment, which gives rise to conformational aberrations. A number of successful procedures for functional expression followed by purification are reviewed here together with nonfunctional expression into inclusion bodies and subsequent (re)folding to produce functional proteins. Most of the data are for prokaryotic outer membrane proteins, but the outer membrane proteins of eukaryotic organelles are also considered as they do show similar features.

Phentermine inhibition of recombinant human liver monoamine oxidases A and B

Recent studies with rat tissue preparations have suggested that the anorectic drug phentermine inhibits serotonin degradation by inhibition of monoamine oxidase (MAO) A with a K(I) value of 85-88 microM, a potency suggested to be similar to that of other reversible MAO inhibitors (Ulus et al., Biochem Pharmacol 2000;59:1611-21). Since there are known differences between rats and humans in substrate and inhibitor specificities of MAOs, the interactions of phentermine with recombinant human purified preparations of MAO A and MAO B were determined. Human MAO A was competitively inhibited by phentermine with a K(I) value of 498+/-60 microM, a value approximately 6-fold weaker than that observed for the rat enzyme. Phentermine was also observed to be a competitive inhibitor of recombinant human liver MAO B with a K(I) value of 375+/-42 microM, a value similar to that observed with the rat enzyme (310-416 microM). In contrast to the behavior with rat tissue preparations, no slow time-dependent behavior was observed for phentermine inhibition of purified soluble human MAO preparations. Difference absorption spectral studies showed similar perturbations of the covalent FAD moieties of both human MAO A and MAO B, which suggests a similar mode of binding in both enzymes. These data suggest that phentermine inhibition of human MAO A (or of MAO B) is too weak to be of pharmacological relevance.

High-level expression of human liver monoamine oxidase A in Pichia pastoris: comparison with the enzyme expressed in Saccharomyces cerevisiae

The high-level expression, purification, and characterization of recombinant membrane-bound human liver monoamine oxidase A (MAO-A) in Pichia pastoris is described. Two liters of fermentation culture produces 1170 units (660 mg) of MAO-A. The enzyme is purified in a 35% yield, is homogeneous on denaturing gel electrophoresis, and exhibits a single species (60,512 +/- 6 Da) on electrospray mass spectrometry. It contains 1 mol of 8alpha-S-cysteinyl FAD/mole of enzyme and exhibits >95% functionality. In contrast, the Saccharomyces cerevisiae-expressed enzyme is partially processed by C-terminal serine removal as demonstrated by mass spectra. The amino termini of both P. pastoris- and S. cerevisiae-expressed MAO-A are acetylated on the N-terminal methionine. The steady-state kinetic properties of P. pastoris-expressed MAO-A are similar to those of S. cerevisiae-expressed MAO-A using the following substrates: phenethylamine, p-CF(3)-benzylamine, dopamine, serotonin, and kynuramine. Reductive titrations demonstrate that the recombinant enzyme is reduced by 1 mol of substrate or dithionite as expected for the two electron equivalents required for flavin reduction. Absorption and EPR spectra show no radical species in the resting enzyme while the anionic flavin radical is formed in 50% yield during the reductive titration with dithionite. These data demonstrate significant advantages in the heterologous expression of human MAO-A in P. pastoris compared with the published S. cerevisiae system in higher expression level (329 mg/L) and in a higher level of homogeneity of the isolated enzyme.