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Ribbon diagram of a monomer of human MAO-A, with FAD and clorgiline bound, oriented as if attached to the outer membrane of a mitochondrion. From PDB 2BXS.
monoamine oxidase A
Identifiers
Symbol	MAOA
Entrez	4128
HUGO	6833
OMIM	309850
RefSeq	NM_000240
UniProt	P21397
Other data
EC number	1.4.3.4
Locus	Chr. X p11.4-p11.3

Cartoon diagram of human MAO-B. From PDB 1GOS.
monoamine oxidase B
Identifiers
Symbol	MAOB
Entrez	4129
HUGO	6834
OMIM	309860
RefSeq	NM_000898
UniProt	P27338
Other data
EC number	1.4.3.4
Locus	Chr. X p11.4-p11.3

Monoamine oxidases (singular abbreviation MAO) (EC 1.4.3.4) are enzymes that catalyze the oxidation of monoamines. They are found bound to the outer membrane of mitochondria in most cell types in the body. The enzyme was discovered by Mary Hare in the liver, and received the name of tyramine oxidase.^[1] They belong to protein family of flavin containing amine oxidoreductases.

Contents 1 Locations of MAO-A and MAO-B 2 Function 3 Subtype Specificities 4 Disorders resulting from MAO dysfunction 5 Genetics 6 See also 7 References 8 External links

In humans there are two types of MAO: MAO-A and MAO-B.

• Both are found in neurons and astroglia.
• Outside the central nervous system:
  • MAO-A is also found in the liver, gastrointestinal tract and placenta.
  • MAO-B is mostly found in blood platelets.

Monoamine oxidases catalyze the oxidative deamination of monoamines. Oxygen is used to remove an amine group from a molecule, resulting in the corresponding aldehyde and ammonia. The general form of the catalyzed reaction (with R denoting an arbitrary group) is

     H                      H
   R-C-NH2 + O2 + H2O  →  R-C=O + NH3 + H2O2 
     H

Monoamine oxidases contain the covalently-bound cofactor FAD and are thus classified as flavoproteins.

MAO-A is particularly important in the catabolism of monoamines ingested in food. Both MAOs are also vital to the inactivation of monoaminergic neurotransmitters, for which they display different specificities.

• Serotonin, norepinephrine (noradrenaline), and epinephrine (adrenaline) are mainly broken down by MAO-A.
• Phenethylamine is broken down by MAO-B.
• Both forms break down dopamine.

Because of the vital role that MAOs play in the inactivation of neurotransmitters, MAO dysfunction (too much/too little MAO activity) is thought to be responsible for a number of neurological disorders. For example, unusually high or low levels of MAOs in the body have been associated with depression, substance abuse, attention deficit disorder, and irregular sexual maturation. Monoamine oxidase inhibitors are one of the major classes of drug prescribed for the treatment of depression, although they are last line treatment due to risk of the drug's interaction with diet or other drugs. Excessive levels of catecholamines (epinephrine, norepinephrine, and dopamine) may lead to a hypertensive crisis, and excessive levels of serotonin may lead to serotonin syndrome.

PET research has shown that MAO is also heavily depleted by use of tobacco cigarettes.^[2]

The genes encoding MAO-A and MAO-B are located side-by-side on the short arm of the X chromosome, and have about 70% sequence similarity. Rare mutations in the gene are associated with Brunner syndrome.

A study reported in Science in August 2002 concluded that maltreated children with a low-activity polymorphism in the promoter region of the MAO-A gene were more likely to develop antisocial conduct disorders than maltreated children with the high-activity variant.^[3] The suggested mechanism for this effect is the decreased ability of those with low MAO-A activity to quickly degrade norepinephrine, the synaptic neurotransmitter involved in sympathetic arousal and rage. This is alleged to provide direct support for the idea that genetic susceptibility to disease is not determined at birth, but varies with exposure to environmental influences.

Research also uncovered a possible link between predisposition to novelty seeking and a genotype of the MAO-A gene.^[4]

In 2006, a New Zealand researcher, Dr Rod Lea said that a particular variant (or genotype) was over-represented in Māori, a Warrior gene. This supported earlier studies finding different proportions of variants in different ethnic groups. This is the case for many genetic variants, with 33% White/Non-Hispanic, 61% Asian/Pacific Islanders having the low-activity MAO-A promoter variant.^[5]

• Genetics and violence
• Neophilia
• Warrior gene
• Brunner syndrome

• MAO-B Structure at eurekalert.org
• Calculated orientations of Monoamine oxidases in membrane
• Monoamine oxidase (MAO) at bmc.uu.se
• Slides showing the effects of tobacco smoking on MAO at nida.nih.gov
• Foods to avoid when taking MAO inhibitors at lycaeum.org
• Information Hyperlinked Over Proteins -- MAO-A

v • d • e CH-NH2 oxidoreductases (EC 1.4) - primarily amino acid oxidoreductases
1.4.1 - NAD/NADP acceptor	Glutamate dehydrogenase (GLUD1)
1.4.3 - oxygen acceptor	D-amino acid oxidase - Amine oxidase - Lysyl oxidase - Monoamine oxidase
1.4.4 - disulfide acceptor	Glycine decarboxylase complex
1.4.99 - other acceptors	D-amino acid dehydrogenase - Amine dehydrogenase

v • d • e Mitochondrial enzymes and transporters
Outer membrane	Long fatty acyl CoA synthetase - Kynureninase - Monoamine oxidase
Intermembrane space	Adenylate kinase - Creatine kinase
Inner membrane	Coenzyme Q - cytochrome c reductase -Cytochrome c - Dihydroorotate dehydrogenase -Glutamate aspartate transporter - Glycerol-3-phosphate dehydrogenase - NADH dehydrogenase - Succinate dehydrogenase
Matrix	Aconitase - Aspartate transaminase - ALDH2 - Carbamoyl phosphate synthase - Citrate synthase - Fumarase - Glutamate dehydrogenase - Isocitrate dehydrogenase - Malate dehydrogenase - Ornithine transcarbamoylase - Oxoglutarate dehydrogenase - Pyruvate dehydrogenase complex - Succinyl coenzyme A synthetase
Mitochondrial DNA	Complex I (7 units) - Complex III (1 unit) - Complex IV (3 units) - ATP synthase (2 units)