Identification of a region important for human monoamine oxidase B substrate and inhibitor selectivity

Synthesis and pharmacological evaluation of multi-functional homoisoflavonoid derivatives as potent inhibitors of monoamine oxidase B and cholinesterase for the treatment of Alzheimer's disease

A series of homoisoflavonoid derivatives was designed, synthesized and evaluated as potential multi-functional anti-Alzheimer's agents for their inhibitory activity on cholinesterase and monoamine oxidase. Among them, compound 16 showed moderate acetylcholinesterase (AChE) inhibitory activity (eeAChE IC50 = 0.89 ± 0.02 μM; hAChE IC50 = 0.657 ± 0.002 μM) and significant monoamine oxidase B (MAO-B) inhibitory activity (hMAO-B IC50 = 0.0372 ± 0.0002 μM). Kinetic analysis of AChE, MAO-B inhibition and molecular modeling studies revealed that compound 16 is a dual binding site inhibitor of AChE and noncompetitive inhibitor of MAO-B. Furthermore, 16 could penetrate through the blood-brain barrier (BBB) in vitro. Most importantly, oral administration of 16 demonstrated no marked signs of acute toxicity and it could significantly reverse scopolamine-induced memory impairment in mice. These results suggested that compound 16 is a promising multifunctional drug candidate with potential effect for the treatment of Alzheimer's disease.

The monoamine oxidase A inhibitor clorgyline is a broad-spectrum inhibitor of fungal ABC and MFS transporter efflux pump activities which reverses the azole resistance of Candida albicans and Candida glabrata clinical isolates

Resistance to the commonly used azole antifungal fluconazole (FLC) can develop due to overexpression of ATP-binding cassette (ABC) and major facilitator superfamily (MFS) plasma membrane transporters. An approach to overcoming this resistance is to identify inhibitors of these efflux pumps. We have developed a pump assay suitable for high-throughput screening (HTS) that uses recombinant Saccharomyces cerevisiae strains hyperexpressing individual transporters from the opportunistic fungal pathogen Candida albicans. The recombinant strains possess greater resistance to azoles and other pump substrates than the parental host strain. A flow cytometry-based HTS, which measured increased intracellular retention of the fluorescent pump substrate rhodamine 6G (R6G) within yeast cells, was used to screen the Prestwick Chemical Library (PCL) of 1,200 marketed drugs. Nine compounds were identified as hits, and the monoamine oxidase A inhibitor (MAOI) clorgyline was identified as an inhibitor of two C. albicans ABC efflux pumps, CaCdr1p and CaCdr2p. Secondary in vitro assays confirmed inhibition of pump-mediated efflux by clorgyline. Clorgyline also reversed the FLC resistance of S. cerevisiae strains expressing other individual fungal ABC transporters (Candida glabrata Cdr1p or Candida krusei Abc1p) or the C. albicans MFS transporter Mdr1p. Recombinant strains were also chemosensitized by clorgyline to other azoles (itraconazole and miconazole). Importantly, clorgyline showed synergy with FLC against FLC-resistant C. albicans clinical isolates and a C. glabrata strain and inhibited R6G efflux from a FLC-resistant C. albicans clinical isolate. Clorgyline is a novel broad-spectrum inhibitor of two classes of fungal efflux pumps that acts synergistically with azoles against azole-resistant C. albicans and C. glabrata strains.

Molecular characteristics of a single and novel form of carp (Cyprinus carpio) monoamine oxidase

Two mammalian monoamine oxidases (MAO), MAO-A and MAO-B, are similar in primary structures but have unique substrate/inhibitor selectivities. Carp (Cyprinus carpio) contains a MAO enzyme (C-MAO) with properties different from MAO-A and MAO-B. To determine the molecular characteristics of C-MAO and its phylogenetic relationship with other fish and mammalian MAOs, the primary structure of C-MAO was estimated. The putative C-MAO cDNA encodes 526 amino acids with 59.001 Da, and the deduced amino acid sequence showed as much as 68.9% homology with some mammalian MAO-A proteins, 69.8% homology with some mammalian MAO-B proteins, and as much as 92.4% homology with some fish MAOs. Comparison of two regions in the polypeptide sequence of C-MAO determining possible substrate/inhibitor preferences of MAO-A and MAO-B showed both 79.5% homologies.

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.

Liquid chromatographic and tandem mass spectrometric assay for evaluation of in vivo inhibition of rat brain monoamine oxidases (MAO) A and B following a single dose of MAO inhibitors: application of biomarkers in drug discovery

A simple and selective assay for the evaluation of in vivo inhibition of rat brain monoamine oxidases (MAO) A and B following a single dose of MAO inhibitors was developed through the simultaneous determination of endogenous 5-hydroxy tryptamine, 5-hydroxyindole-3-acetic acid (5-HIAA), tryptophane, and 2-phenethylamine (PEA) in rat brain using liquid chromatography-tandem mass spectrometry (LC/MS/MS). These analytes were separated on a Zorbax SB-C18 column using a gradient elution with acetonitrile and 0.2% formic acid and detected on an electrospray ionization mass spectrometer in positive-ion multiple-reaction-monitoring mode. The susceptibility and variability of these analytes as potential biomarkers in response to MAO inhibition in vivo were evaluated after application to three MAO inhibitors, tranylcypromine, clorgyline, and pargyline. A dramatic increase (about 40-fold) in PEA brain level and a decrease in 5-HIAA by more than 90% were observed after administration of 15 mg/kg of the nonselective MAO inhibitor tranylcypromine. As expected, the brain level of PEA escalated to about 6-fold, while the 5-HIAA level remained unchanged following a dose of the MAO B inhibitor pargyline at 2mg/kg. In contrast, the brain level of 5-HIAA reduced by approximately 53%, but the PEA level was unaffected following the same dose of the MAO A inhibitor clorgyline. The results indicated that 5-HIAA and PEA were susceptible and effective biomarkers in the rat brain in response to MAO A and B inhibition, respectively. The LC/MS/MS method is useful not only for the determination of inhibitory potency but also for the differentiation of the selectivity of a MAO inhibitor against rat brain MAO A and B in vivo.

Structure of rat monoamine oxidase A and its specific recognitions for substrates and inhibitors

Monoamine oxidase (MAO), a mitochondrial outer membrane enzyme, catalyzes the degradation of neurotransmitters in the central nervous system and is the target for anti-depression drug design. Two subtypes of MAO, MAOA and MAOB, are similar in primary sequences but have unique substrate and inhibitor specificities. The structures of human MAOB complexed with various inhibitors were reported early. To understand the mechanisms of specific substrate and inhibitor recognitions of MAOA and MAOB, we have determined the crystal structure of rat MAOA complexed with the specific inhibitor, clorgyline, at 3.2A resolution. The comparison of the structures between MAOA and MAOB clearly explains the specificity of clorgyline for MAOA inhibition. The fitting of serotonin into the binding pockets of MAOs demonstrates that MAOB Tyr326 would block access of the 5-hydroxy group of serotonin into the enzyme. These results will lead to further understanding of the MAOA function and to new anti-depression drug design. This study also presents that MAOA has a transmembrane helix at the C-terminal region. This is the first crystal structure of membrane protein with an isolated transmembrane helix.

An in vitro interethnic comparison of monoamine oxidase activities between Japanese and Caucasian livers using rizatriptan, a serotonin receptor 1B/1D agonist, as a model drug

AIMS

Monoamine oxidase (MAO) is located in human liver, and catalyses the oxidative deamination step of many xenobiotics. However, whether there exists an interethnic difference in MAO activities has, to our knowledge, not been clarified. We aimed to assess the MAO type A (MAO-A) involvement in the metabolic pathway of rizatriptan (RIZ), an antimigraine 5-hydroxytryptamine (5-HT)1B/1D agonist, and the interethnic difference in MAO activities between Caucasians and Japanese using RIZ as a model drug in in vitro experiments.

METHODS

Oxidative deaminase activities were determined with the subcellular fractions of Japanese livers and the microsomal fraction of Caucasian livers using RIZ, 5-HT (MAO-A substrate) and 2-phenylethylamine (PEA) (MAO-B substrate) as substrates.

RESULTS

The oxidative deaminase activities of RIZ vs. 5-HT were highly (r = 0.87 and 0.96, P < 0.001) correlated with each other in both the microsomal and mitochondrial fractions of Japanese livers. Subsequent results were obtained from in vitro experiments using liver microsomes based upon these findings. The oxidative deaminase activities of RIZ were inhibited completely by the nanomolar-order concentration of clorgyline and Ro 41-1049 (MAO-A selective inhibitors), but not by that of Ro 16-6491 (MAO-B selective inhibitor). The majority of the mean Michaelis-Menten values for three substrates toward MAO obtained from six Japanese and six Caucasian liver microsomes reached no significant differences between the two ethnic groups. The mean microsomal oxidative deaminase activities assessed in 18 Japanese and 20 Caucasian livers using the three substrates also showed no significant differences between the two ethnic groups.

CONCLUSIONS

RIZ is mainly metabolized by MAO-A and the in vitro oxidative deaminase activities mediated via MAO-A and -B do not appear to differ between Japanese and Caucasians.

DNA sequence analysis of monoamine oxidase B gene coding and promoter regions in Parkinson's disease cases and unrelated controls

The allele G of the intron 13 G/A polymorphism of the monoamine oxidase B gene (MAO-B) has been associated with Parkinson's disease (PD) in several studies. Apart from a potential direct effect on splicing processes, the association of this intronic polymorphism with PD is due possibly to linkage disequilibrium with other mutations in the coding or promoter regions of the gene. We addressed this latter hypothesis by determining the DNA sequence of the entire MAO-B coding region comprising 15 exons and partial intronic sequences flanking each exon, in 33 cases with idiopathic PD and 38 unrelated controls. The promoter region of MAO-B gene up to base -1,369 from ATG (start point of mRNA translation) was also sequenced to identify variants with potential functional effects on gene transcription. In the promoter region, a new polymorphism consisting of a C to T single base change was detected in position -1,114 from ATG, with an allelic frequency of 3.5%, but it was not associated with PD risk. No commonly occurring (>10%) polymorphisms were found in the exons or the intronic sequences flanking the exons, although several rare variants were detected in the coding and promoter regions.

Differential substrate specificity of monoamine oxidase in the rat heart and renal cortex

Although it is known that substrate specificities differ with species and within each species with the tissues, in the rat heart no natural substrate was found for MAO-B. beta-phenylethylamine (beta-PEA) has always been considered the "endogenous" substrate of MAO B. We thought worthwide to evaluate the effect of Ro 41-1049 and lazabemide, both members of a class of highly selective, mechanism-based and reversible inhibitors for MAO-A and MAO B, respectively on the metabolization of beta-PEA by the rat heart. Also the lack of molecular data on rat heart MAOs, prompted us to better characterize rat heart MAOs, both kinetically and using molecular biology techniques. K(m) values for deamination of beta-PEA in the rat heart were 13-fold those in the kidney, by contrast, K(m) values for deamination of 5-HT were quite similar in both tissues. Unexpectedly, the selective MAO-A inhibitor Ro 41-1049 was by far the most potent inhibitor of beta-PEA (20 microM) deamination in the rat heart, while clorgyline, another MAO A inhibitor, and lazabemide, a MAO B inhibitor, had intermediate efficacy; selegiline was found unable to inhibit deamination of beta-PEA. In the rat renal cortex lazabemide and selegiline both inhibited beta-PEA deamination. The reduction of beta-PEA concentration to just 200 nM, the use of heart membranes instead of tissue homogenates or the use of heart membranes pre-treated with 1% digitonine failed to change this pattern of inhibition. Semicarbazide was found not to alter deamination of beta-PEA. Western blot showed the presence of both isoforms (55 kd and 61 kd) in the renal cortex. In the heart there was a predominance of the A form, the B form being undetected. The RT-PCR products for both MAO-A and MAO-B, were found to have the expected sizes. In conclusion, we found mRNA for MAO-B but were unable to detect the protein itself or its activity when using beta-PEA as the substrate.

Inhibition by Zn(2+) of A-form monoamine oxidase in monkey brain mitochondria

The effects of ZnSO(4) on mitochondrial monoamine oxidase (MAO) activity in monkey brain were compared with those in rat and rabbit, in vitro. After preincubation at 25 degrees C for 20 min with 1 microM ZnSO(4), MAO-A activity in monkey brain was about 50% using serotonin (5-HT) as a substrate, and the inhibition was proportional to the concentration of ZnSO(4). However, ZnSO(4) had no effect on MAO-B activity in monkey brain using beta-phenylethylamine (beta-PEA) as a substrate. The inhibition by ZnSO(4) of MAO-A activity was competitive and reversible. CdSO(4) also inhibits MAO-A, but not MAO-B in monkey brain mitochondria. ZnSO(4) did not inhibit either MAO-A or MAO-B activity in rat and rabbit brain mitochondria. These results indicate that the inhibiting action of Zn(2+) differs depending on animal species. In monkey brain mitochondria, MAO-A was highly sensitive to Zn(2+) and MAO-B was less sensitive. These results also suggest that Zn(2+) may regulate the level of catecholamine content in monkey brain.

Effects of zinc ion on type A monoamine oxidase in monkey brain mitochondria

The effects of ZnSO(4) on types A and B monoamine oxidase (MAO) isozymes in monkey brain mitochondria were investigated, in vitro. Type A MAO activity in monkey brain decreased to about 50% with 1 microM ZnSO(4) using serotonin as a substrate, and this inhibition was proportional to the concentration of ZnSO(4). ZnSO(4) had no effect, however, on type B MAO activity in monkey brain using beta-phenylethylamine as a substrate. The inhibition by ZnSO(4) of type A MAO activity was competitive and reversible. ZnSO(4) did not inhibit either type A or type B MAO activity in rat brain mitochondria. Almost similar results were also obtained when ZnCl(2) was used, in vitro. These results indicate that the inhibiting action of zinc ion differs depending on animal species and organ. Type A MAO in monkey brain mitochondria was highly sensitive to zinc ion, while type B activity was less sensitive.

Substrate and inhibitor specificities for human monoamine oxidase A and B are influenced by a single amino acid

Monoamine oxidase (MAO) is responsible for the oxidation of biogenic and dietary amines. It exists as two isoforms, A and B, which have a 70% amino acid identity and different substrate and inhibitor specificities. This study reports the identification of residues responsible for conferring this specificity in human MAO A and B. Using site-directed mutagenesis we reciprocally interchanged three pairs of corresponding nonconserved amino acids within the central portion of human MAO. Mutant MAO A-I335Y became like MAO B, which exhibits a higher preference for beta-phenylethylamine than for the MAO A preferred substrate serotonin (5-hydroxytryptamine), and became more sensitive to deprenyl (MAO B-specific inhibitor) than to clorgyline (MAO A-specific inhibitor). The reciprocal mutant MAO B-Y326I exhibited an increased preference for 5-hydroxytryptamine, a decreased preference for beta-phenylethylamine, and, similar to MAO A, was more sensitive to clorgyline than to deprenyl. These mutants also showed a distinct shift in sensitivity for the MAO A- and B-selective inhibitors Ro 41-1049 and Ro 16-6491. Mutant pair MAO A-T245I and MAO B-I236T and mutant pair MAO A-D328G and MAO B-G319D reduced catalytic activity but did not alter specificity. Our results indicate that Ile-335 in MAO A and Tyr-326 in MAO B play a critical role in determining substrate and inhibitor specificities in human MAO A and B.

Molecular characterization of monoamine oxidases A and B

Monoamine oxidase A and B (MAO A and B) are the major neurotransmitter-degrading enzymes in the central nervous system and in peripheral tissues. MAO A and B cDNAs from human, rat, and bovine species have been cloned and their deduced amino acid sequences compared. Comparison of A and B forms of the enzyme shows approximately 70% sequence identity, whereas comparison of the A or B forms across species reveals a higher sequence identity of 87%. Within these sequences, several functional regions have been identified that contain crucial amino acid residues participating in flavin adenine dinucleotide (FAD) or substrate binding. These include a dinucleotide-binding site, a second FAD-binding site, a fingerprint site, the FAD covalent-binding site, an active site, and the membrane-anchoring site. The specific residues that play a role in FAD or substrate binding were identified by comparing sequences in wild-type and variants of MAO with those in soluble flavoproteins of known structures. The genes that encode MAO A and B are closely aligned on the X chromosome (Xp11.23), and have identical exon-intron organization. Immunocytochemical localization studies of MAO A and B in primate brain showed distribution in distinct neurons with diverse physiological functions. A defective MAO A gene has been reported to associate with abnormal aggressive behavior. A deleterious role played by MAO B is the activation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a proneurotoxin that can cause a parkinsonian syndrome in mammals. Deprenyl, an inhibitor of MAO B, has been used for the treatment of early-stage Parkinson's disease and provides protection of neurons from age-related decay.

Purification, cloning, and three-dimensional structure prediction of Micrococcus luteus FAD-containing tyramine oxidase

The FAD-containing tyramine oxidase enzyme and gene from the Gram (+) bacterium Micrococcus luteus were isolated, and computer prediction was used to propose a preliminary 3D model of the protein. A 2.8-kb Sau3AI fragment containing the structural gene of tyramine oxidase was cloned from a M. luteus genomic DNA library. The 1332 bp gene encodes a protein of 443 amino acids, with a calculated molecular mass of 49.1 kDa. The enzyme was found to be a homodimer with a molecular weight of 49,000. It oxidizes tyramine, adrenaline, 3-hydroxytyramine, dopamine, and noradrenaline, and was reversibly inhibited by FAD-containing monoamine oxidase A and B specific inhibitors. Sequence comparison show that tyramine oxidase is smaller than other FAD-amine oxidases but that it contains well-conserved amino acid residues reported in all other FAD-amine oxidases. A hypothetical three-dimensional structure of tyramine oxidase has also been proposed based on secondary structure predictions, threading, and comparative modeling.

A reversible monoamine oxidase A inhibitor, befloxatone: structural approach of its mechanism of action

Experimental and theoretical physico-chemical methods were used to investigate the interaction between several reversible monoamine oxidase A inhibitors in the oxazolidinone series and the active site of the enzyme. Phenyloxazolidinones include toloxatone and analogues, among which befloxatone was selected as drug candidate for the treatment of depression. Identification of the forces responsible for the crystal cohesion of befloxatone reveals functional groups that could interact with monoamine oxidase. Calculation of electronic properties of those compounds using ab initio molecular orbital methods lead to a description of the mode of interaction between befloxatone and the cofactor of the enzyme. Electronic absorption spectroscopy measurements confirm the hypothesis of a privileged interaction of phenyloxazolidinone-type inhibitors with the flavin cofactor of MAO. Additional sites of interaction with the protein core of MAO A are also examined with regard to the primary structure of the enzyme. As a result of this work, a model is proposed for the reversible inhibition of MAO A by befloxatone via long distance, reversible interactions with the flavin adenine dinucleotide (FAD) cofactor of the enzyme and with specific amino acids of the active site. This model is partially corroborated by experimental evidence and should be helpful in designing new potent inhibitors of monoamine oxidase.

Monoamine oxidase: from genes to behavior

Cloning of MAO (monoamine oxidase) A and B has demonstrated unequivocally that these enzymes are made up of different polypeptides, and our understanding of MAO structure, regulation, and function has been significantly advanced by studies using their cDNA. MAO A and B genes are located on the X-chromosome (Xp11.23) and comprise 15 exons with identical intron-exon organization, which suggests that they are derived from the same ancestral gene. MAO A and B knock-out mice exhibit distinct differences in neurotransmitter metabolism and behavior. MAO A knock-out mice have elevated brain levels of serotonin, norephinephrine, and dopamine and manifest aggressive behavior similar to human males with a deletion of MAO A. In contrast, MAO B knock-out mice do not exhibit aggression and only levels of phenylethylamine are increased. Mice lacking MAO B are resistant to the Parkinsongenic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine. Both MAO A and B knock-out mice show increased reactivity to stress. These knock-out mice are valuable models for investigating the role of monoamines in psychoses and neurodegenerative and stress-related disorders.

First partial three-dimensional model of human monoamine oxidase A

A survey of the major known structural aspects of monoamine oxidase (MAO) is given and a first partial model of human MAO A is presented. This 3D model has been established using secondary structure predictions and fold recognition methods. It shows two alpha/beta domains (the FAD-binding N-terminal and central domains) and an alpha+beta domain. The C-terminal region is predicted to be responsible for anchoring the protein into the mitochondrial membrane and was not modeled. The covalent binding of the flavin cofactor to a cysteine residue is well predicted. The model is validated with experimental data from the literature and should be useful in designing new experimental studies (site-directed mutagenesis, chemical modification, specific antibodies). This first step towards the 3D structure of monoamine oxidase should contribute to a better understanding of the mechanisms of action and inhibition of this drug target in the treatment of clinical depression.

Determination of regions important for monoamine oxidase (MAO) A and B substrate and inhibitor selectivities

MAO-A and -B are defined by their substrate and inhibitor preferences. To determine which regions of the isoenzymes confer these preferences, we have constructed six chimeric MAO enzymes by reciprocally exchanging corresponding N-terminal, C-terminal, and internal segments of MAO-A and -B then determined the catalytic properties of these chimeric enzymes. N-terminal chimerics A45B and B36A were made by exchanging amino acid segments 1-45 and 1-36 of MAO-A and -B respectively. C-terminal chimerics A402B and B393A were made by exchanging amino acid segments 403-527 and 394-520 of MAO-A and -B respectively, and internal chimerics AB161-375A and BA152-366B were made by exchanging amino acid segments 161-375 and 152-366 of MAO-A and -B respectively. The enzymatic properties observed for the chimerics suggest that the exchanged internal regions but not the N- or C-terminal regions confer substrate and inhibitor preferences.

A key amino acid responsible for substrate selectivity of monoamine oxidase A and B

Monoamine oxidase (MAO) oxidizes biologically important amines including neurotransmitters and plays a central role in the regulation of intracellular level of these amines. Two distinct forms of MAO (MAO A and MAO B) were defined based on differences in substrate and inhibitor specificities. We earlier reported that the region between about residues 120 and 220 of rat MAO is responsible for determination of the substrate selectivity of MAO A and B (Tsugeno, Y. Hirashiki, I., Ogata, F., and Ito, A. (1995) J. Biochem. (Tokyo) 118, 974-980). To determine the essential amino acids in this region that participate in substrate recognition, a series of mutant enzymes in which amino acid residues that are conserved among various species but are different between the two forms of the enzyme were replaced with the corresponding amino acids of the counterpart and were engineered from the cDNAs of rat liver MAO A and B, and affinities for several substrates were examined. A single mutation in which Phe-208 in MAO A was substituted by the corresponding residue of Ile in MAO B was sufficient to convert the A-type substrate selectivity, and the reverse was exactly the case. Phe at this position was replaceable with Tyr for the A-type specificity and Ile was replaceable with Val and Ala for the B-type. Thus, aromatic and aliphatic residues seem to contribute to render substrate selectivity of MAO A and MAO B, respectively.