Monoamine oxidase

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Monoamine oxidase (MAO), an integral mitochondrial flavoprotein, can be found anchored to the outer membrane of mitochondria in neurones, glia and other cells through a transmembrane helix located within the carboxyl-terminal domain (Siegel et al., 2015)[1]. It plays an important role in metabolising and thus inactivating monoamine neurotransmitters including serotonin (5-HT) as well as catecholamines such as noradrenaline (NA) and dopamine (DA). Two isoenzymes of MAO – MAOA and MAOB – have been identified in human, both having different substrate preference, inhibitor sensitivity and distribution in tissue and brain (Tong et al., 2015)[2].

Two isoforms of monoamine oxidase (MAO) in human - MAO-A and MAO-B

5-HT is preferably metabolised by MAOA whereas catecholamines are metabolised by MAOB. However, 5-HT and catecholamines that are stored in vesicles are be protected from action of MAO. Analyses and assays have also showed that more MAOB than MAOA are observed in human brain (Shih, Chen & Ridd, 1999; Siegel et al., 2015)[3][1].

MAOA and MAOB are encoded by two independent genes that are located on the X-chromosome and they have 70% homology (Wu et al., 2009)[4]. Thus, MAOA and MAOB are not the same protein that are modified differently but made up of two different polypeptides (Shih, Chen & Ridd, 1999)[3].


  1. 1.0 1.1 Siegel, G. J., Agranoff, B. W., Albers, R. W., Fisher, S. K. and Uhler, M. D. (1999) Basic Neurochemistry. [Online] Philadelphia: Lippincott-Raven. Available from: [Accessed 18/10/2015]
  2. Tong, J., Meyer, J.H., Furukawa, Y., Boileau, I., Chang, L. J., Wilson, A. A., Houle, S. and Kish, S. J. (2013) Distribution of monoamine oxidase proteins in human brain: implications for brain imaging studies. Journal of Cerebral Blood Flow and Metabolism. [Online] 33(6). p. 863-871. Available from: [Accessed 18/10/2015]
  3. 3.0 3.1 Shih, J. C., Chen, K. and Ridd. M. J. (1999) Monoamine Oxidase: From Genes to Behavior. Annu Rev Neurosci. [Online] 22. p. 197-217. Available from: [Accessed 18/10/2015]
  4. Wu, J. B., Chen, K., Li, Y., Lau, Y. F. C. and Shih, J. C. (2009) Regulation of monoamine oxidase A by the SRY gene on the Y chromosome. The FASEB Journal. [Online] 23(11). p. 4029-4038. Available from: [Accessed 18/10/2015]
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