The covalently-bound flavin of hepatic monoamine oxidase. 2. Identification and properties of cysteinyl riboflavin

Protective effects of niacin following high fat rich diet: an in-vivo and in-silico study

Niacin had long been understood as an antioxidant. There were reports that high fat diet (HFD) may cause psychological and physical impairments. The present study was aimed to experience the effect of Niacin on % growth rate, cumulative food intake, motor activity and anxiety profile, redox status, 5-HT metabolism and brain histopathology in rats. Rats were administered with Niacin at a dose of 50 mg/ml/kg body weight for 4 weeks following normal diet (ND) and HFD. Behavioral tests were performed after 4 weeks. Animals were sacrificed to collect brain samples. Biochemical, neurochemical and histopathological studies were performed. HFD increased food intake and body weight. The exploratory activity was reduced and anxiety like behavior was observed in HFD treated animals. Activity of antioxidant enzymes was decreased while oxidative stress marker and serotonin metabolism in the brain of rat were increased in HFD treated animals than ND fed rats. Morphology of the brain was also altered by HFD administration. Conversely, Niacin treated animals decreased food intake and % growth rate, increased exploratory activity, produced anxiolytic effects, decreased oxidative stress and increased antioxidant enzyme and 5-HT levels following HFD. Morphology of brain is also normalized by the treatment of Niacin following HFD. In-silico studies showed that Niacin has a potential binding affinity with degradative enzyme of 5-HT i.e. monoamine oxidase (MAO) A and B with an energy of ~ - 4.5 and - 5.0 kcal/mol respectively. In conclusion, the present study showed that Niacin enhanced motor activity, produced anxiolytic effect, and reduced oxidative stress, appetite, growth rate, increased antioxidant enzymes and normalized serotonin system and brain morphology following HFD intake. In-silico studies suggested that increase 5-HT was associated with the binding of MAO with Niacin subsequentially an inhibition of the degradation of monoamine. It is suggested that Niacin has a great antioxidant potential and could be a good therapy for the treatment of HFD induced obesity.

Roasted cashew (Anacardium occidentale L.) nut-enhanced diet forestalls cisplatin-initiated brain harm in rats

The incessant dose constraining symptom of the chemotherapeutic agent, cisplatin is neurotoxicity. This examination tried to explore the neuroprotective impact of roasted cashew nut-enhanced diet against brain deficits related with treatment with cisplatin. Rats were separated in to six groups: Control, CIS (cisplatin [7 mg/kg body weight, i.p]), CIS +10% CN (cisplatin plus 10% roasted cashew nut), CIS +20% CN (cisplatin plus 20% roasted cashew nut), 10% CN (10% roasted cashew nut) and 20% CN (20% roasted cashew nut) for 28 days. Key enzymes associated with brain function, including cholinesterases (AChE and BChE), monoaminergic enzyme (MAO), arginase, and adenosine deaminase (ADA), were investigated after the treatment. The following oxidative stress indicators were also measured in the rat brain: glutathione-S-transferase (GST), glutathione peroxidase (GPx), total antioxidant capacity (TAC), total thiol (T-SH), non-protein thiol (NPSH), thiobarbituric acid reactive substances (TBARS), reactive oxygen species (ROS), nitric oxide (NO), superoxide dismutase (SOD). Our outcomes demonstrated that roasted cashew nut enhanced diet showed inhibitory impact on activities of AChE, BChE, ADA, MAO and arginase in cisplatin-induced rats. The roasted cashew nut supplemented diet also boosted redox equilibrium and displayed protection against cispaltin-induced oxidative damage to rats' brains by an increase in SOD, CAT, GST and GPx activities, TAC, T-SH, NPSH and NO levels as well as a considerable drop in ROS and RBARS levels. Roasted cashew nut enhanced diet additionally forestalled neuronal degeneration in rat brain. Thus, roasted cashew nuts could be used as a nutraceutical or functional food to treat cisplatin-induced neurotoxicity.

Practical applications

The results show that increasing roasted cashew nut consumption can significantly improve antioxidant status, reduce lipid peroxidation, and suppress cholinesterase, adenosine deaminase, monoamine oxidase, and arginase activities in the brain under cisplatin-induced circumstances.

Integrated Binary QSAR-Driven Virtual Screening and In Vitro Studies for Finding Novel hMAO-B-Selective Inhibitors

The increased activity of monoamine oxidase (MAO) enzymes may lead to serious consequences since they reduce the level of neurotransmitters and are associated with severe neurodegenerative diseases. The inhibition of this enzyme, especially the B isoform, plays a vital role in the treatment of Parkinson's disease (PD). This study is aimed to find novel human MAO-B (hMAO-B) selective inhibitors. A total of 256.750 compounds from the Otava small molecules database were virtually screened gradually by employing several screening techniques for this purpose. Initially, a high-throughput virtual screening (HTVS) method was employed, and 10% of the molecules having high docking scores were subjected to binary QSAR models for further screening of their therapeutic activities against PD, Alzheimer's disease (AD), and depression as well as for their toxicity and pharmacokinetic properties. Then, enzyme selectivity of the ligands towards the A and B forms that passed through all the filters were studied using the induced-fit docking method and molecular dynamics simulations. At the end of this exhaustive research, we identified two hit molecules ligand 3 (Otava ID: 7131545) and ligand 4 (Otava ID: 7566820). Based on the in vitro results, these two compounds (ligands 3 and 4) together with ligands 1 and 2 found in our previous study showed activity at the nanomolar (nM) level, and the results indicated that these four ligands inhibit hMAO-B better than the FDA-approved drug selegiline.

Molecular cloning, sequence analysis, and tissue distribution of marmoset monoamine oxidases A and B

The common marmoset (Callithrix jacchus), a New World primate, is currently attracting much attention as a nonhuman primate model for pharmacological and pharmacokinetic studies in preclinical research. In this study, we newly isolated the cDNAs of marmoset monoamine oxidase A (MAO-A) and MAO-B from liver and brain, respectively. MAO-A and MAO-B cDNAs, respectively, contained open reading frames of 527 and 520 amino acids and were approximately 92% and 95% identical to their human orthologs. Marmoset MAOs were phylogenetically closer to primate MAOs, including human MAOs, than to pig, dog, or rodent MAOs. The genomic and gene structures of marmoset MAOs were similar to those of humans. Among the five marmoset tissue types analyzed, the expression levels of MAO-A mRNA were relatively abundant in lung, liver, kidney, and small intestine, whereas the expression levels of MAO-B mRNA were relatively abundant in brain, liver, kidney, and small intestine; these tissue distributions are similar to those of human MAOs. These results suggest that MAO-A and MAO-B are similar at a molecular level in marmosets and humans.

TV 3326 for Alzheimer's dementia: a novel multimodal ChE and MAO inhibitors to mitigate Alzheimer's-like neuropathology

OBJECTIVES

Alzheimer's disease (AD) is one of the most prevalent neurodegenerative disorders and a well-recognized cause of dementia with ageing. In this review, we have represented the ChE and MAO inhibitory potential of TV 3326 against AD based on current scientific evidence.

KEY FINDINGS

The aetiology of AD is quite complex and not completely understood. However, it has been observed that AD involves the deposition of abnormal amyloid beta (Aβ), along with hyperphosphorylation of tau, oxidative stress, low acetylcholine (ACh) level and biometal dyshomeostasis. Due to the complex nature of AD aetiology, active research is required in the areas of development of multitarget drugs with 2 or more complementary biological functions, as they might represent significant progress in the AD treatment. Interestingly, it has been found that TV 3326 (i.e. ladostigil) is regarded as a novel therapeutic agent since it has the potential to cause inhibition of monoamine oxidase (MAO) A and B, and acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) in the brain. Furthermore, it has the capacity to reverse memory impairments, which further suggests the ability of this drug to elevate cholinergic activity in the brain.

SUMMARY

TV 3326 can avert oxidative-nitrative stress and gliosis. It has also been confirmed that TV 3326 contains neuroprotective and anti-apoptotic properties. Therefore, this distinctive combined inhibition of ChE and MAO along with its neuroprotective property makes TV 3326 a useful drug in the treatment of AD.

Structure-Based Specific Detection and Inhibition of Monoamine Oxidases and Their Applications in Central Nervous System Diseases

Monoamine oxidases (MAOs) are the enzymes that catalyze the oxidation of monoamines, such as dopamine, norepinephrine, and serotonin, which serve as key neurotransmitters in the central nervous system (CNS). MAOs play important roles in maintaining the homeostasis of monoamines, and the aberrant expression or activation of MAOs underlies the pathogenesis of monoamine neurotransmitter disorders, including neuropsychiatric and neurodegenerative diseases. Clearly, detecting and inhibiting the activities of MAOs is of great value for the diagnosis and therapeutics of these diseases. Accordingly, many specific detection probes and inhibitors have been developed and substantially contributed to basic and clinical studies of these diseases. In this review, progress in the detecting and inhibiting of MAOs and their applications in mechanism exploration and treatment of neurotransmitter-related disorders is summarized. Notably, how the detection probes and inhibitors of MAOs were developed has been specifically addressed. It is hoped that this review will benefit the design of more effective and sensitive probes and inhibitors for MAOs, and eventually the treatment of monoamine neurotransmitter disorders.

Proposing Novel MAO-B Hit Inhibitors Using Multidimensional Molecular Modeling Approaches and Application of Binary QSAR Models for Prediction of Their Therapeutic Activity, Pharmacokinetic and Toxicity Properties

Monoamine oxidase (MAO) enzymes MAO-A and MAO-B play a critical role in the metabolism of monoamine neurotransmitters. Hence, MAO inhibitors are very important for the treatment of several neurodegenerative diseases such as Parkinson's disease (PD), Alzheimer's disease (AD), and amyotrophic lateral sclerosis (ALS). In this study, 256 750 molecules from Otava Green Chemical Collection were virtually screened for their binding activities as MAO-B inhibitors. Two hit molecules were identified after applying different filters such as high docking scores and selectivity to MAO-B, desired pharmacokinetic profile predictions with binary quantitative structure-activity relationship (QSAR) models. Therapeutic activity prediction as well as pharmacokinetic and toxicity profiles were investigated using MetaCore/MetaDrug platform which is based on a manually curated database of molecular interactions, molecular pathways, gene-disease associations, chemical metabolism, and toxicity information. Particular therapeutic activity and toxic effect predictions are based on the ChemTree ability to correlate structural descriptors to that property using recursive partitioning algorithm. Molecular dynamics (MD) simulations were also performed to make more detailed assessments beyond docking studies. All these calculations were made not only to determine if studied molecules possess the potential to be a MAO-B inhibitor but also to find out whether they carry MAO-B selectivity versus MAO-A. The evaluation of docking results and pharmacokinetic profile predictions together with the MD simulations enabled us to identify one hit molecule (ligand 1, Otava ID: 3463218) which displayed higher selectivity toward MAO-B than a positive control selegiline which is a commercially used drug for PD therapeutic purposes.

A review on flavonoid-based scaffolds as multi-target-directed ligands (MTDLs) for Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is a multifactorial neurodegenerative disease. The target enzymes inhibition including cholinesterase, beta-secretase, monoamine oxidase and inhibition of amyloid-β aggregation as well as oxidative stress and metal chelation play an important role in the pathogenesis of AD. Chroman-4-one scaffold with benzo-γ-pyrone network is a privileged structure in organic synthesis and drug design. A large number of research has been carried out on modified naturally occurring chromanone scaffolds and/or synthesized new analogues, to obtain effective drugs for AD management. The present review summarizes aspects related to the multi-target-directed ligands (MTDLs) strategy in enzyme targets modulation performed with natural and synthesized chroman-4-one-based structures to look at their potential in the management of multifactorial Alzheimer's disease.

Effect of caffeine, caffeic acid and their various combinations on enzymes of cholinergic, monoaminergic and purinergic systems critical to neurodegeneration in rat brain-In vitro

Caffeine and caffeic acid are two bioactive compounds that are present in plant foods and are major constituent of coffee, cocoa, tea, cola drinks and chocolate. Although not structurally related, caffeine and caffeic acid has been reported to elicit neuroprotective properties. However, their different proportional distribution in food sources and possible effect of such interactions are not often taken into consideration. Therefore, in this study, we investigated the effect of caffeine, caffeic acid and their various combinations on activities of some enzymes [acetylcholinesterase (AChE), monoamine oxidase (MAO) ecto-nucleoside triphosphate diphosphohydrolase (E-NTPase), ecto-5¹-nucleotidase (E-NTDase) and Na⁺/K⁺ ATPase relevant to neurodegeneration in vitro in rat brain. The stock concentration of caffeine and caffiec acid and their various proportional combinations were prepared and their interactions with the activities of these enzymes were assessed (in vitro) in different brain structures. The Fe²⁺ and Cu²⁺ chelating abilities of the samples were also investigated. The results revealed that caffeine, caffeic acid and their various combinations exhibited inhibitory effect on activities of AChE, MAO, E-NTPase and E-NTDase, but stimulatory effect on Na⁺/K⁺ ATPase activity. The combinations also exhibited Fe²⁺ and Cu²⁺ chelating abilities. Considering the various combinations, a higher caffeine to caffeic acid ratio produced significantly highest enzyme modulatory effects; these were significantly lower to the effect of caffeine alone but significantly higher than the effect of caffeic acid alone. These findings may provide new insight into the effect of proportional combination of these bioactive compounds as obtained in many foods especially with respect to their neuroprotective effects.

A review on tacrine-based scaffolds as multi-target drugs (MTDLs) for Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial neurological disorder among elderly people and combinatorial factors such as genetic, lifestyle, and environmental are involved in onset and disease progression. It has been demonstrated that loss of cholinergic transmission is one of the most significant causes of AD. One strategy currently being investigated for the development of new therapeutics relates to the enhancement of cholinergic system through several ways. At this juncture, anticholinesterase inhibitors have absorbed lots of attention and different marketed drugs such as donepezil, rivastigmine, tacrine, and galantamine have been developed. 9-Amino-1,2,3,4-tetrahydroacridine known as tacrine was introduced in 1945 as an efficient anticholinesterase agent. The mechanism of action of tacrine was proved to inhibit the metabolism of acetylcholine and therefore extending its activity and raising levels in the cerebral cortex. However, extensive use of tacrine was limited since it showed various side effects and toxicity. Thus, lots of efforts were carried out to prepare tacrine analogues to overcome the related adverse effects. This review describes differently synthesized tacrine-based scaffolds as cholinesterase inhibitors to manage Alzheimer's disease (AD).

Dual functional cholinesterase and MAO inhibitors for the treatment of Alzheimer's disease: synthesis, pharmacological analysis and molecular modeling of homoisoflavonoid derivatives

Because of the complexity of Alzheimer's disease (AD), the multi-target-directed ligand (MTDL) strategy is expected to provide superior effects for the treatment of AD, instead of the classic one-drug-one-target strategy. In this context, we focused on the design, synthesis and evaluation of homoisoflavonoid derivatives as dual acetyl cholinesterase (AChE) and monoamine oxidase (MAO-B) inhibitors. Among all the synthesized compounds, compound 10 provided a desired balance of AChE and hMAO-B inhibition activities, with IC50 value of 3.94 and 3.44 μM, respectively. Further studies revealed that compound 10 was a mixed-type inhibitor of AChE and an irreversible inhibitor of hMAO-B, which was also confirmed by molecular modeling studies. Taken together, the data indicated that 10 was a promising dual functional agent for the treatment of AD.

Novel monoamine oxidase inhibitors: a patent review (2012 - 2014)

INTRODUCTION

Monoamine oxidase (MAO) inhibitors, despite the initial pharmacological interest, are used in clinic for their antidepressant effect and in the management of Parkinson symptoms, due to the established neuroprotective action. Efficacy and tolerability emerged from large-scale and randomized clinical trials.

AREAS COVERED

Thirty-six patents range from April 2012 to September 2014. The number of chemotypes with inhibitory effects on MAO is truly high (40 synthetic compounds, 22 natural products and 6 plant extracts reported and licensed), and the present review is comprehensive of all compounds, which have been patented for their relevance to clinical medicine in this period range (27 patents). Moreover, some of the collected patents deal with new formulations of compounds endowed with MAO inhibitory properties (two patents) and new therapeutic options/drug associations for already known MAO inhibitors (seven patents).

EXPERT OPINION

The patents reported in this review showed that the interest in this field is constant and mainly devoted to the study of selective MAO-B inhibitors, used as drugs for the treatment of neurological disorders. The development of novel human MAO inhibitors took advantage of the discovery of new therapeutic targets (cancer, hair loss, muscle dystrophies, cocaine addiction and inflammation), the recognized role of MAOs as molecular biomarkers and their activity in other tissues.

Separation and structure of the prosthetic group of the blue fluorescence protein from the bioluminescent bacterium Photobacterium phosphoreum

The highly fluorescent prosthetic group of the blue fluorescence protein purified from the bioluminescent bacterium Photobacterium phosphoreum has been dissociated and separated from its apoprotein by affinity chromatography on Cibacron Blue-Sepharose. It has been identified as 6,7-dimethyl-8-(1'-D-ribityl)lumazine by several methods of characterization, all of which gave results identical to those for an authentic sample. In neutral solution, absorption maxima are at 407, 275 (shoulder), and 256 nm, with a single fluorescence maximum at 491 nm. The proton magnetic resonance spectrum exhibits a singlet at 2.66 ppm corresponding to the methyl substituted at the 6 position of lumazine and a multiplet centered at 3.85 ppm corresponding to the C-2'-5' protons of the ribityl group. A Raman spectrum was obtained by the technique of coherent anti-Stokes Raman scattering and the R(F) values by paper chromatography were determined in four solvent systems. The isolated compound was readily transformed into riboflavin by riboflavin synthetase. Fifty grams (dry weight) of P. phosphoreum contains at least 20 mg of this lumazine derivative, an amount comparable to that found in other microorganisms classified as riboflavin overproducers. The overproduction of this lumazine in this case apparently has to do with its function in the generation of bioluminescence.

Structure of human monoamine oxidase A at 2.2-A resolution: the control of opening the entry for substrates/inhibitors

The mitochondrial outer membrane-anchored monoamine oxidase (MAO) is a biochemically important flavoenzyme that catalyzes the deamination of biogenic and xenobiotic amines. Its two subtypes, MAOA and MAOB, are linked to several psychiatric disorders and therefore are interesting targets for drug design. To understand the relationship between structure and function of this enzyme, we extended our previous low-resolution rat MAOA structure to the high-resolution wild-type and G110A mutant human MAOA structures at 2.2 and 2.17 A, respectively. The high-resolution MAOA structures are similar to those of rat MAOA and human MAOB, but different from the known structure of human MAOA [De Colibus L, et al. (2005) Proc Natl Acad Sci USA 102:12684-12689], specifically regarding residues 108-118 and 210-216, which surround the substrate/inhibitor cavity. The results confirm that the inhibitor selectivity of MAOA and MAOB is caused by the structural differences arising from Ile-335 in MAOA vs. Tyr-326 in MAOB. The structures exhibit a C-terminal transmembrane helix with clear electron density, as is also seen in rat MAOA. Mutations on one residue of loop 108-118, G110, which is far from the active center but close to the membrane surface, cause the solubilized enzyme to undergo a dramatic drop in activity, but have less effect when the enzyme is anchored in the membrane. These results suggest that the flexibility of loop 108-118, facilitated by anchoring the enzyme into the membrane, is essential for controlling substrate access to the active site. We report on the observation of the structure-function relationship between a transmembrane helical anchor and an extra-membrane domain.

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.

Molecular characterization of NikD, a new flavoenzyme important in the biosynthesis of nikkomycin antibiotics

Nikkomycin antibiotics are potent inhibitors of chitin synthase, effective as therapeutic antifungal agents in humans and easily degradable insecticides in agriculture. NikD is a novel flavoprotein that catalyzes the oxidation of Delta(1)- or Delta(2)-piperideine-2-carboxylate, a key step in the biosynthesis of nikkomycin antibiotics. The resulting dihydropicolinate product may be further oxidized by nikD or converted to picolinate in a nonenzymic reaction. Saturated nitrogen heterocycles (L-pipecolate, L-proline) and 3,4-dehydro-L-proline act as alternate substrates. The ability of nikD to oxidize 3,4-dehydro-L-proline, but not 1-cyclohexenoate, suggests that the enzyme is specific for the oxidation of a carbon-nitrogen bond. An equivalent reaction is possible with the enamine (Delta(2)), but not the imine (Delta(1)), form of the natural piperideine-2-carboxylate substrate. Apparent steady-state kinetic parameters for the reaction of nikD with Delta(1)- or Delta(2)-piperideine-2-carboxylate (k(cat) = 64 min(-1); K(m) = 5.2 microM) or 3,4-dehydro-L-proline (k(cat) = 18 min(-1); K(m) = 13 mM) were determined in air-saturated buffer by measuring hydrogen peroxide formation in a coupled assay. NikD appears to be a new member of the monomeric sarcosine oxidase (MSOX) family of amine oxidizing enzymes. The enzyme contains 1 mol of flavin adenine dinucleotide (FAD) covalently linked to Cys321. The covalent flavin attachment site and two residues that bind substrate carboxylate in MSOX are conserved in nikD. NikD, however, exhibits an unusual long-wavelength absorption band, attributed to charge-transfer interaction between FAD and an ionizable (pK(a) = 7.3) active-site residue. Similar long-wavelength absorption bands have been observed for flavoproteins containing an active site cysteine or cysteine sulfenic acid. Interestingly, Cys273 in nikD aligns with an active-site histidine in MSOX (His269) that is, otherwise, a highly conserved residue within the MSOX family.

The covalent FAD of monoamine oxidase: structural and functional role and mechanism of the flavinylation reaction

The family of flavoenzymes in which the flavin coenzyme redox cofactor is covalently attached to the protein through an amino acid side chain is covered in this review. Flavin-protein covalent linkages have been shown to exist through each of five known linkages: (a) 8alpha-N(3)-histidyl, (b) 8alpha-N(1)-histidyl, (c) 8alpha-S-cysteinyl, (d) 8alpha-O-tyrosyl, or (e) 6-S-cysteinyl with the flavin existing at either the flavin mononucleotide or flavin adenine dinucleotide (FAD) levels. This class of enzymes is widely distributed in diverse biological systems and catalyzes a variety of enzymatic reactions. Current knowledge on the mechanism of covalent flavin attachment is discussed based on studies on the 8alpha-S-cysteinylFAD of monoamine oxidases A and B, as well as studies on other flavoenzymes. The evidence supports an autocatalytic quinone-methide mechanism of protein flavinylation. Proposals to explain the structural and mechanistic advantages of a covalent flavin linkage in flavoenzymes are presented. It is concluded that multiple factors are involved and include: (a) stabilization of the apoenzyme structure, (b) steric alignment of the cofactor in the active site to facilitate catalysis, and (c) modulation of the redox potential of the covalent flavin through electronic effects of 8alpha-substitution.

Effects of carboxyl-terminal truncations on the activity and solubility of human monoamine oxidase B

Monoamine oxidase is an outer mitochondrial membrane protein that catalyzes the deamination of a number of neurotransmitters and dietary amines. To determine the roles of the carboxyl-terminal amino acids on the activity and solubility of human monoamine oxidase (MAO B), 10 sequential mutants were made with stop codons at amino acid positions 511, 504, 498, 492, 486, 481, 476, 467, 417, and 397, respectively. All truncated mutants were expressed in Sf21 insect cells using baculovirus, and the enzyme kinetic parameters were determined. Truncations at amino acid positions 511, 504, and 498 slightly decreased MAO B catalytic activity and had no significant changes on deprenyl inhibition. Further deletions up to amino acid 417 decreased the specific activity 10--100-fold without significant changes of the K(m) for phenylethylamine or dopamine or the IC(50) for deprenyl and clorgyline. The truncation mutant C397, which lacks covalently attached FAD, was inactive. Progressive carboxyl-terminal truncations up to position 481 were correlated with increased solubility of MAO B mutants. 47% of the activity of the truncated C481 was found in the 105,000 x g supernatant in the absence of detergent. However, further truncated mutants, C476, C467, and C417, remained associated with the membrane fraction. In contrast to crude homogenate, the water-soluble C481 mutant was rapidly inactivated at 4 degrees C and 37 degrees C, which indicates that the membrane environment is required for the stability of MAO B. Expression of the green fluorescent protein-MAO B C481 fusion protein revealed that this mutant was located in the cytoplasm, whereas its counterpart in MAO A, truncated mutant C490, was located on the mitochondria. These results suggest that the carboxyl-terminal amino acid residues 417--520 of MAO B are not directly involved in the active site but are required for maintaining the appropriate conformation and interaction with the outer mitochondrial membrane. The different solubilities of the various carboxyl-terminal truncation mutants indicate that the interaction of MAO B with mitochondrial membrane is not simply anchoring through the carboxyl-terminal hydrophobic tail. Further, our results suggest that the carboxyl-terminal of MAO A and B plays different roles in mitochondrial attachment.

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

The high-level heterologous expression, purification, and characterization of the mitochondrial outer membrane enzyme human liver monoamine oxidase B (MAO B) using the methylotrophic yeast Pichia pastoris expression system are described. A 2-L culture of P. pastoris expresses approximately 1700 U of MAO B activity, with the recombinant enzyme associated tightly with the membrane fraction of the cell lysate. By a modification of the published procedure for purification of bovine liver MAO B [Salach, J. I. (1979) Arch. Biochem. Biophys. 192, 128-137], recombinant human liver MAO B is purified in a 34% yield ( approximately 200 mg from 2 L of cell culture). The isolated enzyme exhibits an M(r) of approximately 60, 000 on SDS-PAGE and 59,474 from electrospray mass spectrometry measurements, which is in good agreement with the mass predicted from the gene sequence and inclusion of the covalent FAD. One mole of covalent FAD per mole of MAO B is present in the purified enzyme and is bound by an 8alpha-S-cysteinyl(397) linkage, as identified by electrospray mass spectrometry of the isolated tryptic/chymotryptic flavin peptide. Recombinant human liver MAO B and bovine liver MAO B are shown to be acetylated at the seryl residues at their respective amino termini. The benzylamine oxidase activity of recombinant MAO B ranges from 3.0 to 3.4 U/mg and steady-state kinetic parameters for this enzyme preparation compare well with those published for the bovine liver enzyme: k(cat) = 600 min(-1), K(m)(benzylamine) = 0.50 mM, and K(m)(O(2)) = 0.33 mM. Kinetic isotope effect parameters using [alpha,alpha-(2)H(2)]benzylamine are also similar to those found for the bovine enzyme. Recombinant MAO B exhibits a (D)k(cat) = 4.7, a (D)[k(cat)/K(m)(benzylamine)] = 4.5, and a (D)[k(cat)/K(m)(O(2))] = 1.0. In contrast to bovine liver MAO B, no evidence was found for the presence of any anionic flavin radical either by UV-vis or by EPR spectroscopy in the resting form of the enzyme. These data demonstrate the successful heterologous expression of a functional, membrane-bound MAO B, which will permit a number of mutagenesis studies as structural and mechanistic probes not previously possible.

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.

Riboflavin and riboflavin-derived cofactors in adolescent girls with anorexia nervosa

BACKGROUND

Thyroid hormones, riboflavin, riboflavin cofactors, and organic acids were assessed in girls with anorexia nervosa.

OBJECTIVE

The objective was to examine the effect of malnutrition and low thyroid hormone concentrations on erythrocyte and plasma riboflavin metabolism and their relation with urinary organic acid excretion.

DESIGN

Seventeen adolescent girls with anorexia nervosa [body mass index (BMI; in kg/m2): 14.8 +/- 2.2] and 17 age-matched, healthy girls (control subjects; BMI: 20.5 +/- 2.2) took part in the feeding study. Erythrocyte and plasma riboflavin as well as riboflavin cofactors (flavin mononucleotide and flavin adenine dinucleotide) were assessed by HPLC, whereas urinary organic acids were assessed by gas chromatography-mass spectrometry.

RESULTS

Anorectic patients who began a feeding program had higher erythrocyte riboflavin (3.5 +/- 2.2 compared with <0.1 nmol/mol hemoglobin; P < 0.001), lower plasma flavin adenine dinucleotide (57.8 +/- 18.5 compared with 78.5 +/- 54.3 nmol/L; P < 0.05), and higher urinary ethylmalonic acid (7.12 +/- 4.39 compared with 1.3 +/- 2.8 micromol/mmol creatinine; P < 0.001) and isovalerylglycine (7.65 +/- 4.78 compared with 3.8 +/- 0.9 micromol/mmol creatinine; P < 0.05) concentrations than did control subjects. Triiodothyronine concentrations were low and negatively correlated with plasma riboflavin concentrations (r = -0.69, P < 0.01). Not all patients showed improvements in these biochemical indexes after 30 d of refeeding.

CONCLUSIONS

The low triiodothyronine concentrations observed in anorexia nervosa could alter the extent of riboflavin conversion into cofactors, thus leading to high erythrocyte riboflavin concentrations, low plasma flavin adenine dinucleotide concentrations, and high rates of ethylmalonic acid and isovalerylglycine excretion.

Arginine-42 and threonine-45 are required for FAD incorporation and catalytic activity in human monoamine oxidase B

Monoamine oxidase B (MAO B) is an integral protein of the outer mitochondrial membrane that is involved in the deamination of vasoactive and neuroactive amines. The oxidation of these amine substrates requires the cofactor FAD, which is covalently bound to Cys-397 of human MAO B. Previously, Glu-34 and Tyr-44 of MAO B have been identified as residues which engage in noncovalent interactions with FAD that are required for subsequent covalent FAD binding and generation of catalytic activity. In this study, we have identified two additional residues, Arg-42 and Thr-45, which form noncovalent contacts with FAD that are prerequisite steps to the covalent attachment of FAD. Arg-42 and Thr-45, along with Tyr-44, comprise part of a highly conserved flavin binding sequence, RXY(T,S), that is found in other flavoproteins, several of which have well-defined X-ray crystal structures. We tested the roles of Arg-42 and Thr-45 in MAO B by constructing mutant MAO B cDNAs which encode amino acid substitutions at these residues and expressed the variant proteins in COS-7 cells. Substitution of Arg-42 or Thr-45 with alanine resulted in complete loss of MAO B activity and FAD incorporation. However, conservative substitutions of Arg-42 with lysine or Thr-45 with serine resulted in MAO B variants that retain both partial activity and partial FAD incorporation. These results indicate that Arg-42 and Thr-45 form critical noncovalent interactions with FAD that are required for the subsequent activation of MAO B by covalent coupling of FAD.

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.

Covalent attachment of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) to enzymes: the current state of affairs

The first identified covalent flavoprotein, a component of mammalian succinate dehydrogenase, was reported 42 years ago. Since that time, more than 20 covalent flavoenzymes have been described, each possessing one of five modes of FAD or FMN linkage to protein. Despite the early identification of covalent flavoproteins, the mechanisms of covalent bond formation and the roles of the covalent links are only recently being appreciated. The main focus of this review is, therefore, one of mechanism and function, in addition to surveying the types of linkage observed and the methods employed for their identification. Case studies are presented for a variety of covalent flavoenzymes, from which general findings are beginning to emerge.

Sarcosine oxidase contains a novel covalently bound FMN

Sarcosine oxidase from Corynebacterium sp. P-1 is a heterotetrameric protein containing three different enzymes: noncovalent FAD, noncovalent NAD+, and covalently bound flavin which is released as 8 alpha-(N3-histidyl)riboflavin upon complete hydrolysis of the protein. The following results show that the covalent flavin is not at the FAD level, as previously proposed, but it is rather as 8 alpha-(N3- histidyl)FMN coenzyme. First, no AMP is released when the protein moiety is treated with phosphodiesterase or subjected to mild acid hydrolysis. The enzyme contains a total of 5 mol of phosphate. Only one phosphate is covalently bound. The other four phosphates are noncovalent and attributed to noncovalently bound FAD and NAD+. The 31P NMR spectrum of native enzyme exhibits resonances due to a single phosphate monoester an two pyrophosphates. Only a resonance due to phosphate monoester is observed after removal of the noncovalent cofactors and proteolytic digestion of the protein moiety. The 8 alpha-(N3-histidyl)FMN found in corynebacterial sarcosine oxidase represents a novel type of covalent flavin. Studies with sarcosine oxidases from Arthrobacter sp. and Pseudomonas sp. show that these heterotetrameric enzymes also contain covalently bound FMN plus noncovalently bound FAD and NAD+, similar to corynebacterial sarcosine oxidase. In contrast, two monomeric sarcosine oxidases (from Bacillus sp. and an unidentified microorganism) were found to contain only covalently bound FAD.

Flavinylation of monoamine oxidase B

Monoamine oxidase B (MAO B) catalyzes the oxidative deamination of biogenic and xenobiotic amines. The oxidative step is coupled to the reduction of an obligatory cofactor, FAD, which is covalently linked to the enzyme at Cys397. In this study, we developed a novel riboflavin-depleted (Rib-) COS-7 cell line to study the flavinylation of MAO B. ApoMAO B can be obtained by expressing MAO B cDNA in these cells. We found that MAO B is expressed equally in the presence or absence of FAD and that apoMAO B can be inserted into the outer mitochondrial membrane. Flavinylation of MAO B was achieved by introducing MAO B cDNA and different flavin derivatives simultaneously into Rib- COS-7 cells via electroporation. Since the addition of riboflavin, FMN, or FAD resulted in equal levels of MAO B activity, we conclude that the flavin which initially binds to apoMAO B is FAD. In our previous work, we used site-directed mutagenesis to show that Glu34 in the dinucleotide-binding motif of MAO B is essential for MAO B activity, and we postulated that this residue is involved in FAD binding. In this study, we tested the role of residue 34 in flavin binding by expressing wild-type or mutant MAO B cDNA in Rib- COS-7 cells with the addition of [14C]FAD. We found that Glu34 is essential for both FAD binding and catalytic activity. Thus, FAD binds to MAO B in a dual manner at Glu34 noncovalently and Cys397 covalently. We conclude that Glu34 is critical for the initial non-covalent binding of FAD and is instrumental in delivering FAD to the covalent attachment site at Cys397.

Catalytically active monoamine oxidase type A from human liver expressed in Saccharomyces cerevisiae contains covalent FAD

Monoamine oxidase type A from human liver cDNA was expressed in Saccharomyces cerevisiae. This enzyme's properties with respect to Km and Ki values for kynuramine and amphetamine, respectively, were similar to values for human placental enzyme. As expected, clorgyline inhibited the yeast enzyme at lower concentrations than deprenyl. Interestingly, the FAD cofactor was covalently attached and fluorescence properties of the enzyme bound prosthetic group indicate that it is attached to a cysteine residue, the same linkage observed in other monoamine oxidases. The yield of expressed enzyme is about 15 mg/l of culture with an A600 of 15. It is suggested that covalent flavin attachment proceeds by an autoflavination mechanism.

Biochemistry and genetics of monoamine oxidase

This chapter reviews the two mitochondrial flavin containing isozymes of monoamine oxidase. Section 1, "Biochemistry" discusses assays, substrates and inhibitors, phylogenic and tissue distribution, interactions with lipids, nutritional studies, protein structure, kinetic and chemical mechanistic proposals, and biosynthesis. Section 2, "Inheritance" discusses possible genes involved in expression, genetic studies of platelet MAO-B and fibroblast MAO-A, and chromosomal location. Section 3, "Molecular Genetics" reviews the cloning of their cDNAs, their intra- and interspecies homology and structural inferences made from deduced amino acid sequences. Section 4, "Regulation" gives an overview of levels in development and aging, and effect of drugs. The final section 5, "Role in Human Disease" discusses physiological function and effects of altered levels in humans and animal models including complete absence due to a submicroscopic chromosomal deletion in several human patients.

Human monoamine oxidase gene (MAOA): chromosome position (Xp21-p11) and DNA polymorphism

An essentially full-length cDNA clone for the human enzyme monoamine oxidase type A (MAO-A) has been used to determine the chromosomal location of a gene encoding it. This enzyme is important in the degradative metabolism of biogenic amines throughout the body and is located in the outer mitochondrial membrane of many cell types. Southern blot analysis of PstI-digested human DNA revealed multiple fragments that hybridized to this probe. Using rodent-human somatic cell hybrids containing all or part of the human X chromosome, we have mapped these fragments to the region Xp21-p11. A restriction fragment length polymorphism (RFLP) for this MAOA gene was identified and used to evaluate linkage distances between this locus and several other loci on Xp. The MAOA locus lies between DXS14 and OTC, about 29 cM from the former.

Platelet monoamine oxidase B: use and misuse

The human platelet in addition to having serotonin (5-HT) receptors, uptake carriers (receptor) and transmitter storage vesicles, primarily possesses mitochondrial monoamine oxidase (MAO) type B. Similar to the major form of MAO in the human brain, this enzyme actively oxidizes A-B and B substrates (tyramine, dopamine, phenylethylamine) as well as the novel secondary amine anticonvulsant, milacemide and dopaminergic neurotoxin, MPTP. 5-HT oxidation is hardly affected by the platelet enzyme and MAO inhibitors have no net effect on its accumulation. MAO-B is selectively inhibited by 1-deprenyl and thus the platelet enzyme may be useful to monitor the anti-Parkinson activity of such drugs, as related to their ability to inhibit brain MAO-B. The oxidation of the anticonvulsant, milacemide, to glycine in vitro and in vivo by MAO-B, may herald new prospects for the development of inert prodrugs capable of being metabolized to neuroactive substances by MAO-B. The plasma levels of their metabolites may be an index of MAO-B activity found in the platelet and brain.

Photoaffinity labeling of beef liver monoamine oxidase-B by 4-fluoro-3-nitrophenyl azide

4-Fluoro-3-nitrophenyl azide (FNPA) competitively inhibited beef liver monoamine oxidase-B (MAO-B) in the dark (Ki = 2.8 microM). Upon irradiation in the presence of FNPA, a concentration-dependent photoinactivation of MAO-B was observed. The kinetic analysis showed that the photoinactivation of MAO-B resulted in a decrease in Vmax but no change in Km. This result suggests that an irreversible linkage may be formed between the enzyme and the photolyzed FNPA. When [3H]FNPA was photoirradiated with the purified MAO-B, a single radioactive band associated with MAO-B was observed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The photo-dependent incorporation could be protected by phenylethylamine, the substrate for MAO-B, in a concentration-dependent manner. Complete tryptic-chymotryptic digestion of [3H]FNPA-labeled MAO-B resulted in three radioactive peaks on Sephadex G-25 column chromatography. With the same digestion and separation procedures, only one major radioactive peak was observed for the [3H]pargyline-labeled MAO-B, and its elution volume was different from that of [3H]FNPA-labeled peptides. These results suggest that, upon photolysis, FNPA may incorporate into a region in the active site of MAO-B which may be different from the pargyline binding site--the FAD prosthetic group of the enzyme.

Bacterial sarcosine oxidase: comparison of two multisubunit enzymes containing both covalent and noncovalent flavin

Sarcosine oxidase was purified to homogeneity from Corynebacterium sp. P-1, a soil organism isolated by a serial enrichment technique. The enzyme contains 1 mol of noncovalently bound flavin [flavin adenine dinucleotide (FAD)] plus 1 mol of covalently bound flavin [8 alpha-(N3-histidyl)-FAD] per mole of enzyme (Mr 168,000). The two flavins appear to have different roles in catalysis. The enzyme has an unusual subunit composition, containing four dissimilar subunits (Mr 100,000, 42,000, 20,000, and 6000). The same subunits are detected in Western blot analysis of cell extracts prepared in the presence of trichloroacetic acid, indicating that the subunits are a genuine property of the enzyme as it exists in vivo. The presence of both covalent and noncovalent flavin in a single enzyme is extremely unusual and has previously been observed only with a sarcosine oxidase from a soil Corynebacterium isolated in Japan. The enzymes exhibit many similarities but are distinguishable in electrophoretic studies. Immunologically, the enzymes are cross-reactive but not identical. The results indicate that the synthesis of a sarcosine oxidase containing both covalent and noncovalent flavin is not a particularly unusual event in corynebacteria.

Effects of pH and exocyclic substitution on flavosemiquinone reactivity with redox proteins and inorganic oxidants

The effect of pH on the reaction of free flavosemiquinone analogs generated by laser-flash photolysis with oxidized Chromatium vinosum high-potential iron-sulfur protein, other iron-containing redox proteins, and nonbiological one-electron oxidants has been investigated. The results demonstrate that the second-order rate constant for the oxidation of lumiflavin flavosemiquinone increases dramatically with increasing pH for the redox proteins and some of the other oxidants. The pH-rate constant profiles for the redox proteins closely follow the ionization of the proton at the N-5 position of the neutral lumiflavin flavosemiquinone, suggesting a higher intrinsic reactivity for the anionic lumiflavin flavosemiquinone. This increased reactivity apparently results from changes in the redox potential and in the electron spin density distribution between the two protonic forms of the semiquinone. Similar pH dependencies are observed for a number of other flavin structural analogs, yielding estimates of the N-5 pK values for these analogs. The data are consistent with the involvement of both the N-5-dimethylbenzene ring portion and the C-4a position of the flavin macrocycle in flavosemiquinone oxidation by one-electron oxidants.

4-Fluoro-3-nitrophenyl azide, a selective photoaffinity label for type B monoamine oxidase

The effects of 4-fluoro-3-nitrophenyl azide (FNPA) on types A and B monoamine oxidase in rat brain cortex were studied using serotonin and phenylethylamine as substrates respectively. FNPA competitively inhibited the oxidative deamination of both serotonin (Ki = 3 microM) and phenylethylamine (Ki = 0.78 microM) in the dark. Upon photoirradiation in the presence of FNPA, a photodependent inhibition of type B MAO activity resulted. This photodependent inhibition was apparently irreversible since there was no recovery of activity upon washing of the photolyzed FNPA-enzyme mixture. Additional evidence for the photoinduced covalent binding of FNPA to type B MAO is that non-competitive inhibition kinetics resulted after photolysis. The specificity of the photodependent incorporation of FNPA to type B MAO was shown by the protective effect of phenylethylamine and by decreased [3H]pargyline labeling after the enzyme was photolyzed with FNPA. Under the same experimental conditions, only minimal photodependent inhibition of type A MAO by FNPA was found. The observed difference in the efficiencies of the photodependent inactivation of the two types of MAO by FNPA suggests that there is a conformational or a structural difference in the active sites of the two types of MAO. The active site of type B MAO could be characterized by utilizing FNPA as a photoaffinity labeling probe.