|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
|monoamine oxidase A|
|Locus||Chr. X p11.4-p11.3|
|monoamine oxidase B|
|Locus||Chr. X p11.4-p11.3|
L-Monoamine oxidases (MAO) (EC 22.214.171.124) are a family of 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 originally discovered by Mary Bernheim in the liver and was named tyramine oxidase. They belong to the protein family of flavin-containing amine oxidoreductases.
Subtypes and tissue distribution
- Both are found in neurons and astroglia.
- Outside the central nervous system:
They are well known enzymes in pharmacology, since they are the substrate for the action of a number of monoamine oxidase inhibitor drugs. 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, melatonin, noradrenaline, and adrenaline are mainly broken down by MAO-A.
- Phenethylamine and benzylamine are mainly broken down by MAO-B.
- Both forms break down dopamine, tyramine, and tryptamine equally.
Specific reactions catalyzed by MAO include:
- Adrenaline or noradrenaline to 3,4-Dihydroxymandelic acid
- Metanephrine or normetanephrine to vanillylmandelic acid (VMA)
- Dopamine to dihydroxyphenylacetic acid
- 3-Methoxytyramine to homovanillic acid
Because of the vital role that MAOs play in the inactivation of neurotransmitters, MAO dysfunction (too much or too little MAO activity) is thought to be responsible for a number of psychiatric and neurological disorders. For example, unusually high or low levels of MAOs in the body have been associated with schizophrenia, depression, attention deficit disorder, substance abuse, migraines, and irregular sexual maturation. Monoamine oxidase inhibitors are one of the major classes of drug prescribed for the treatment of depression, although they are often 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.
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 based on the Dunedin cohort 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. Out of the 442 total males in the study (maltreated or not), 37% had the low activity variant. Of the 13 maltreated males with low MAO-A activity, 11 had been assessed as exhibiting adolescent conduct disorder and 4 were convicted for violent offenses. 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 argued to provide direct support for the idea that genetic susceptibility to disease is not determined at birth, but varies with exposure to environmental influences. However, most individuals with conduct disorder or convictions did not have low activity of MAO-A; maltreatment was found to have caused stronger predisposition for antisocial behavior than differences in MAO-A activity.
The claim that an interaction between low MAO-A activity and maltreatment would cause anti-social behavior has been criticized since the predisposition towards anti-social behavior could equally well have been caused by other genes inherited from abusive parents.
A particular variant (or genotype), dubbed "warrior gene" in the popular press, was over-represented in Māori. 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.
Unlike many other enzymes, MAO-B activity is increased during aging in the brain of humans and other mammals. Increased MAO-B activity was also found in the pineal gland of aging rats. This may contribute to lowered levels of monoamines in aged brain and pineal gland.
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- 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