Monoamine oxidase B explained

Monoamine oxidase B (MAO-B) is an enzyme that in humans is encoded by the MAOB gene.

The protein encoded by this gene belongs to the flavin monoamine oxidase family. It is an enzyme located in the outer mitochondrial membrane. It catalyzes the oxidative deamination of biogenic and xenobiotic amines and plays an important role in the catabolism of neuroactive and vasoactive amines in the central nervous system and peripheral tissues. This protein preferentially degrades benzylamine and phenethylamine.[1] Similar to monoamine oxidase A (MAO-A), MAO-B is also involved in the catabolism of dopamine.[2]

Structure and function

MAO-B has a hydrophobic bipartite elongated cavity that (for the "open" conformation) occupies a combined volume close to 700 Å3. hMAO-A has a single cavity that exhibits a rounder shape and is larger in volume than the "substrate cavity" of hMAO-B.[3]

The first cavity of hMAO-B has been termed the entrance cavity (290 Å3), the second substrate cavity or active site cavity (~390 Å3) – between both an isoleucine199 side-chain serves as a gate. Depending on the substrate or bound inhibitor, it can exist in either an open or a closed form, which has been shown to be important in defining the inhibitor specificity of hMAO-B. At the end of the substrate cavity is the FAD cofactor with sites for favorable amine binding about the flavin involving two nearly parallel tyrosyl (398 and 435) residues that form what has been termed an aromatic cage.[3]

Like MAO-A, MAO-B catalyzes O2-dependent oxidation of primary arylalkyl amines, the initial step in the breakdown of these molecules. The products are the corresponding aldehyde, hydrogen peroxide, and ammonia:

Amine + + → Aldehyde + +

This reaction is believed to occur in three steps. First, the amine is oxidized to the corresponding imine, with reduction of the FAD cofactor to FADH2. Second, O2 accepts two electrons and two protons from FADH2, forming and regenerating FAD. Third, the imine is hydrolyzed by water, forming ammonia and the aldehyde.[4]

Differences between MAO-A and MAO-B

MAO-A generally metabolizes tyramine, norepinephrine, serotonin, and dopamine (and other less clinically relevant chemicals). In contrast, MAO-B metabolizes dopamine and β-phenethylamine, as well as other less clinically relevant chemicals. The differences between the substrate selectivity of the two enzymes are utilized clinically when treating specific disorders; MAO-A inhibitors have been typically used in the treatment of depression, whereas MAO-B inhibitors are typically used in the treatment of Parkinson's disease.[5] [6] Concurrent use of MAO-A inhibitors with sympathomimetic drugs can induce a hypertensive crisis as a result of excessive norepinephrine.[7] Likewise, the consumption of tyramine-containing substances, such as cheese, whilst using MAO-A inhibitors also carries the risk of hypertensive crisis. Selective MAO-B inhibitors bypass this problem by preferentially inhibiting MAO-B, which allows tyramine to be metabolized freely by MAO-A in the gastrointestinal tract.

In 2021, it was discovered that MAO-A completely or almost completely mediates striatal dopamine catabolism in the rodent brain and that MAO-B is not importantly involved.[8] [9] In contrast, MAO-B appears to mediate γ-aminobutyric acid (GABA) synthesis from putrescine in the striatum, a minor and alternative metabolic pathway of GABA synthesis, and this synthesized GABA in turn inhibits dopaminergic neurons in this brain area.[10] MAO-B specifically mediates the transformations of putrescine into γ-aminobutyraldehyde (GABAL or GABA aldehyde) and N-acetylputrescine into N-acetyl-γ-aminobutyraldehyde (N-acetyl-GABAL or N-acetyl-GABA aldehyde).[11] These findings may warrant a rethinking of the actions of MAO-B inhibitors in the treatment of Parkinson's disease.

Roles in disease and aging

Alzheimer's disease (AD) and Parkinson's disease (PD) are both associated with elevated levels of MAO-B in the brain.[12] [13] The normal activity of MAO-B creates reactive oxygen species, which directly damage cells.[14] MAO-B levels have been found to increase with age, suggesting a role in natural age related cognitive decline and the increased likelihood of developing neurological diseases later in life.[15] More active polymorphisms of the MAO-B gene have been linked to negative emotionality, and suspected as an underlying factor in depression.[16] Activity of MAO-B has also been shown to play a role in stress-induced cardiac damage.[17] [18] Over-expression and increased levels of MAO-B in the brain have also been linked to the accumulation of amyloid β-peptides (), through mechanisms of the amyloid precursor protein secretase, γ-secretase, responsible for the development of plaques, observed in Alzheimer's and Parkinson's patients. Evidence suggests that siRNA silencing of MAO-B, or inhibition of MAO-B through MAO-B inhibitors (Selegline, Rasagiline), slows the progression, improves and reverses the symptoms, associated with AD and PD, including the reduction of plaques in the brain.[19] [20]

Animal models

Transgenic mice that are unable to produce MAO-B are shown to be resistant to a mouse model of Parkinson's disease.[21] [22] [23] They also demonstrate increased responsiveness to stress (as with MAO-A knockout mice)[24] and increased β-PEA.[24] In addition, they exhibit behavioral disinhibition and reduced anxiety-like behaviors.[25]

Treatment with selegiline, an MAO-B inhibitor, in rats has been shown to prevent many age-related biological changes, such as optic nerve degeneration, and extend average lifespan by up to 39%.[26] [27] However, subsequent research suggests that the anti-aging effects of selegiline in animals are due to its catecholaminergic activity enhancer actions rather than MAO-B inhibition.[28]

Effects of deficiency in humans

While people lacking the gene for MAO-A display intellectual disabilities and behavioral abnormalities, people lacking the gene for MAO-B display no abnormalities except elevated phenethylamine levels in urine.[29] [30] Newer research indicates the importance of phenethylamine and other trace amines, which are now known to regulate catecholamine and serotonin neurotransmission through the same receptor as amphetamine, TAAR1.[31]

The prophylactic use of MAO-B inhibitors to slow natural human aging in otherwise healthy individuals has been proposed, but remains a highly controversial topic.[32] [33]

Selective inhibitors

Species-dependent divergences may hamper the extrapolation of inhibitor potencies.[34]

Reversible

Natural

Synthetic

Irreversible (covalent)

See also

Notes and References

  1. Web site: Entrez Gene: MAOB monoamine oxidase B.
  2. Tan YY, Jenner P, Chen SD . Monoamine Oxidase-B Inhibitors for the Treatment of Parkinson's Disease: Past, Present, and Future . Journal of Parkinson's Disease . 12 . 2 . 477–493 . 2022 . 34957948 . 8925102 . 10.3233/JPD-212976 . There are two MAO isoenzymes: MAO-A and MAO-B. MAO-A is mainly distributed in the gastrointestinal tract, platelets, and heart, and can promote the metabolism of tyramine-containing substances in food so avoiding hypertensive crises caused by the accumulation of tyramine (“cheese reaction”). MAO-A also exists in catecholaminergic neurons, such as dopaminergic neurons in SN, norepinephrine neurons in locus coeruleus, etc. [18]. MAO-B is mainly distributed in platelets and glial cells, and total MAO activity within the brain is composed of approximately 20% MAO-A and 80% MAO-B [19–22]. Both MAO-A and MAO-B regulate the amine neurotransmitters, including dopamine. MAO-A metabolizes dopamine in presynaptic neurons, while MAO-B metabolizes dopamine released to synaptic cleft and taken up by glial cells. The number of glial cells was shown to increase with age, and in neurodegenerative diseases, as expected, the activity of MAO-B also increased [23–25]. MAO-B inhibitors inhibit MAO-B activity in the brain, block dopamine catabolism, enhance dopamine signaling, and selectively enhance dopamine levels at synaptic cleft [21]. .
  3. Edmondson DE, Binda C, Mattevi A . Structural insights into the mechanism of amine oxidation by monoamine oxidases A and B . Arch. Biochem. Biophys. . 464 . 2 . 269–76 . August 2007 . 17573034 . 1993809 . 10.1016/j.abb.2007.05.006 .
  4. Binda C, Mattevi A, Edmondson DE . Structure-function relationships in flavoenzyme-dependent amine oxidations: A comparison of polyamine oxidase and monoamine oxidase . Journal of Biological Chemistry . 277 . 27 . 23973–23976 . July 5, 2002 . 10.1074/jbc.R200005200 . 12015330 . free .
  5. Nolen WA, Hoencamp E, Bouvy PF, Haffmans PM . Reversible monoamine oxidase-A inhibitors in resistant major depression . Clin Neuropharmacol . 16 . Suppl 2 . S69–76 . 1993 . 8313400 .
  6. Riederer P, Laux G . MAO-inhibitors in Parkinson's Disease . Exp Neurobiol . 20 . 1 . 1–17 . March 2011 . 22110357 . 3213739 . 10.5607/en.2011.20.1.1 .
  7. Calvi A, Fischetti I, Verzicco I, Belvederi Murri M, Zanetidou S, Volpi R, Coghi P, Tedeschi S, Amore M, Cabassi A . 6 . Antidepressant Drugs Effects on Blood Pressure . Frontiers in Cardiovascular Medicine . 8 . 704281 . 2021 . 10.3389/fcvm.2021.704281 . free . 34414219 . 8370473 . The risk of developing the “cheese reaction” during treatment with MAOIs depends on the concurrent consumption of meals containing tyramine or sympathomimetic drugs (Table 3). Tyramine is normally metabolized by MAO-A located on the gut wall and by MAO-B in the liver; if MAO-A is inhibited, the bioavailability of tyramine is increased, which leads to an excess in NE, resulting in a hypertensive crisis (55, 217). Currently, they are not first-line antidepressant medications, and their use is limited to treatment-resistant or atypical depression. ... Selegiline is a selective MAO-B at low doses and a non-selective MAOI at higher doses; it also induces dopaminergic activity at low doses. This different action, depending on the dose, implies different use: low doses (up to 10 mg/day) for Parkinson's disease and higher doses as antidepressant treatment (Table 1) (55). ... Higher doses of oral and transdermal selegiline have been linked to a major frequency of orthostatic hypotension (227). No hypertensive crisis was reported with patch administration, but a small portion of patients with preexisting hypertension showed a worse BP control (224). .
  8. Nam MH, Sa M, Ju YH, Park MG, Lee CJ . Revisiting the Role of Astrocytic MAOB in Parkinson's Disease . Int J Mol Sci . 23 . 8 . April 2022 . 4453 . 35457272 . 9028367 . 10.3390/ijms23084453 . free .
  9. Cho HU, Kim S, Sim J, Yang S, An H, Nam MH, Jang DP, Lee CJ . Redefining differential roles of MAO-A in dopamine degradation and MAO-B in tonic GABA synthesis . Exp Mol Med . 53 . 7 . 1148–1158 . July 2021 . 34244591 . 8333267 . 10.1038/s12276-021-00646-3 .
  10. Watanabe M, Maemura K, Kanbara K, Tamayama T, Hayasaki H . GABA and GABA receptors in the central nervous system and other organs . Int Rev Cytol . International Review of Cytology . 213 . 1–47 . 2002 . 11837891 . 10.1016/s0074-7696(02)13011-7 . 978-0-12-364617-0 .
  11. Seiler N . Catabolism of polyamines . Amino Acids . 26 . 3 . 217–233 . June 2004 . 15221502. 10.1007/s00726-004-0070-z .
  12. Saura J, Luque JM, Cesura AM, Da Prada M, Chan-Palay V, Huber G, Löffler J, Richards JG . 6 . Increased monoamine oxidase B activity in plaque-associated astrocytes of Alzheimer brains revealed by quantitative enzyme radioautography . Neuroscience . 62 . 1 . 15–30 . September 1994 . 7816197 . 10.1016/0306-4522(94)90311-5 . 38740469 .
  13. Mallajosyula JK, Chinta SJ, Rajagopalan S, Nicholls DG, Andersen JK . Metabolic control analysis in a cellular model of elevated MAO-B: relevance to Parkinson's disease . Neurotoxicity Research . 16 . 3 . 186–193 . October 2009 . 19526285 . 2727365 . 10.1007/s12640-009-9032-2 .
  14. Book: Nagatsu T, Sawada M . Molecular mechanism of the relation of monoamine oxidase B and its inhibitors to Parkinson's disease: Possible implications of glial cells . Oxidative Stress and Neuroprotection . Journal of Neural Transmission. Supplementum . 71 . 71 . 53–65 . 2006 . 17447416 . 10.1007/978-3-211-33328-0_7 . 978-3-211-33327-3 .
  15. Kumar MJ, Andersen JK . Perspectives on MAO-B in aging and neurological disease: where do we go from here? . Molecular Neurobiology . 30 . 1 . 77–89 . August 2004 . 15247489 . 10.1385/MN:30:1:077 . 19776473 .
  16. Dlugos AM, Palmer AA, de Wit H . Negative emotionality: monoamine oxidase B gene variants modulate personality traits in healthy humans . Journal of Neural Transmission . 116 . 10 . 1323–1334 . October 2009 . 19657584 . 3653168 . 10.1007/s00702-009-0281-2 .
  17. Kaludercic N, Carpi A, Menabò R, Di Lisa F, Paolocci N . Monoamine oxidases (MAO) in the pathogenesis of heart failure and ischemia/reperfusion injury . Biochimica et Biophysica Acta (BBA) - Molecular Cell Research . 1813 . 7 . 1323–1332 . July 2011 . 20869994 . 3030628 . 10.1016/j.bbamcr.2010.09.010 .
  18. Kaludercic N, Carpi A, Nagayama T, Sivakumaran V, Zhu G, Lai EW, Bedja D, De Mario A, Chen K, Gabrielson KL, Lindsey ML, Pacak K, Takimoto E, Shih JC, Kass DA, Di Lisa F, Paolocci N . 6 . Monoamine oxidase B prompts mitochondrial and cardiac dysfunction in pressure overloaded hearts . Antioxidants & Redox Signaling . 20 . 2 . 267–280 . January 2014 . 23581564 . 3887464 . 10.1089/ars.2012.4616 .
  19. Schedin-Weiss S, Inoue M, Hromadkova L, Teranishi Y, Yamamoto NG, Wiehager B, Bogdanovic N, Winblad B, Sandebring-Matton A, Frykman S, Tjernberg LO . 6 . Monoamine oxidase B is elevated in Alzheimer disease neurons, is associated with γ-secretase and regulates neuronal amyloid β-peptide levels . Alzheimer's Research & Therapy . 9 . 1 . 57 . August 2017 . 28764767 . 5540560 . 10.1186/s13195-017-0279-1 . free .
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  21. Shih JC, Chen K . MAO-A and -B gene knock-out mice exhibit distinctly different behavior . Neurobiology (Bp) . 7 . 2 . 235–46 . 1999 . 10591056 .
  22. Grimsby J, Toth M, Chen K, Kumazawa T, Klaidman L, Adams JD, Karoum F, Gal J, Shih JC . Increased stress response and beta-phenylethylamine in MAOB-deficient mice. . Nature Genetics . 17 . 2 . 206–10 . October 1997 . 9326944 . 10.1038/ng1097-206 . 31804364 .
  23. Jean Chen Shih . Shih JC, Chen K, Ridd MJ . Monoamine oxidase: from genes to behavior. . Annual Review of Neuroscience . 22 . 197–217 . 1999 . 10202537 . 2844879 . 10.1146/annurev.neuro.22.1.197 .
  24. Shih JC . Cloning, after cloning, knock-out mice, and physiological functions of MAO A and B. . Neurotoxicology . 25 . 1–2 . 21–30 . January 2004 . 14697877 . 10.1016/s0161-813x(03)00112-8 . 2004NeuTx..25...21S .
  25. Bortolato M, Godar SC, Davarian S, Chen K, Shih JC . Behavioral disinhibition and reduced anxiety-like behaviors in monoamine oxidase B-deficient mice. . Neuropsychopharmacology . 34 . 13 . 2746–57 . December 2009 . 19710633 . 2783894 . 10.1038/npp.2009.118 .
  26. Nebbioso M, Pascarella A, Cavallotti C, Pescosolido N . Monoamine oxidase enzymes and oxidative stress in the rat optic nerve: age-related changes . International Journal of Experimental Pathology . 93 . 6 . 401–5 . December 2012 . 23082958 . 3521895 . 10.1111/j.1365-2613.2012.00832.x .
  27. Kitani K, Kanai S, Sato Y, Ohta M, Ivy GO, Carrillo MC . Chronic treatment of (-)deprenyl prolongs the life span of male Fischer 344 rats. Further evidence . Life Sci. . 52 . 3 . 281–8 . 1993 . 8423709 . 10.1016/0024-3205(93)90219-S .
  28. Miklya I . The significance of selegiline/(-)-deprenyl after 50 years in research and therapy (1965-2015) . Mol Psychiatry . 21 . 11 . 1499–1503 . November 2016 . 27480491 . 10.1038/mp.2016.127 .
  29. Bortolato M, Shih JC . Behavioral outcomes of monoamine oxidase deficiency: preclinical and clinical evidence . International Review of Neurobiology . 100 . 13–42 . 2011 . 21971001 . 3371272 . 10.1016/B978-0-12-386467-3.00002-9 . 978-0-12-386467-3 . To the best of our knowledge, there have been no reports of clinical conditions characterized by selective MAO-B deficiency. However, in few cases of atypical ND with MAO-B deletion, the latter deficit was reported to result in increased urinary excretion of PEA, but no overt behavioral abnormalities or cognitive deficits (Berger et al., 1992; Lenders et al., 1996). .
  30. Bortolato M, Floris G, Shih JC . From aggression to autism: new perspectives on the behavioral sequelae of monoamine oxidase deficiency . Journal of Neural Transmission . 125 . 11 . 1589–1599 . November 2018 . 29748850 . 6215718 . 10.1007/s00702-018-1888-y . In striking contrast with the evidence on MAOA deficiency, the clinical consequences of low MAO B activity remain partially elusive. Indeed, the only cases with a documented loss-of-function mutation were described in atypical Norrie disease patients, harboring deletions of both the ND gene as well as the (adjacent) MAOB gene (Lenders et al., 1996). These patients did not exhibit any overt psychopathological alterations, pointing to a lack of overt clinical sequelae of MAOB deficiency (Lenders et al., 1996). ... The behavioral sequelae of MAO B deficiency are unlikely to be reflective of early neurodevelopmental problems (given the lower expression of this enzyme in perinatal stages), but may instead reflect tonic enhancements of PEA and/or other MAO B substrates. PEA is a trace amine that has been involved in several neuropsychiatric disorders (Beckmann et al., 1983; Szymanski et al., 1987; O’Reilly et al., 1991; Berry, 2007). The effects of PEA are not fully clear, but its chemical similarity with d-amphetamine (in which a methyl group is substituted at the α-carbon) underlines the possibility that this molecule may serve as a facilitator of catecholamine and serotonin release. On the other hand, the identification of TAAR1 as the endogenous receptor for PEA, as well as other monoamines metabolized by MAO B (such as tyramine and 3-iodothyronamine), calls into question whether the effects of PEA may result from a combination of different mechanisms. .
  31. Miller GM . The emerging role of trace amine-associated receptor 1 in the functional regulation of monoamine transporters and dopaminergic activity . J. Neurochem. . 116 . 2 . 164–176 . January 2011 . 21073468 . 3005101 . 10.1111/j.1471-4159.2010.07109.x .
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