Monoamine oxidase (MAO) is an enzyme that breaks down key brain chemicals, including serotonin, dopamine, and norepinephrine. It does this through a chemical reaction called oxidative deamination, stripping an amine group from these neurotransmitters and rendering them inactive. This process is essential for keeping brain signaling in balance, but MAO also plays important roles outside the brain, including neutralizing potentially dangerous compounds in food before they reach your bloodstream.
How MAO Breaks Down Neurotransmitters
MAO sits anchored to the outer membrane of mitochondria, the energy-producing structures inside your cells. This positioning is not incidental. Being embedded in the membrane actually affects the enzyme’s efficiency, controlling how easily neurotransmitters can access its active site. When a neurotransmitter like dopamine or serotonin drifts into MAO’s active site, the enzyme removes its amine group and produces several byproducts: an aldehyde, hydrogen peroxide, and either ammonia or a substituted amine depending on the original molecule.
Those byproducts aren’t harmless. Hydrogen peroxide and ammonia are toxic at high concentrations and can cause oxidative stress and even neuronal cell death. Under normal conditions, other enzymes step in to process these byproducts quickly. Aldehyde dehydrogenase converts the aldehyde into an acid, while aldehyde reductase can turn it into an alcohol or glycol instead. This chain of reactions keeps the toxic intermediates from accumulating.
Two Forms With Different Jobs
Your body produces two forms of this enzyme, MAO-A and MAO-B, and they have overlapping but distinct roles. MAO-A primarily handles serotonin, norepinephrine, and epinephrine, though it also acts on dopamine. MAO-B has a particular affinity for dopamine and a compound called phenylethylamine. Both forms can break down tyramine, a substance found in aged and fermented foods.
Their activity levels also change differently over a lifetime. MAO-A stays relatively stable across the lifespan. MAO-B, on the other hand, increases with age, which means dopamine gets broken down faster as you get older. This age-related increase in MAO-B activity is one reason dopamine levels naturally decline over time.
Protecting You From Dietary Tyramine
One of MAO’s lesser-known but critical roles happens in your gut and liver. Foods like aged cheese, cured meats, and fermented products contain tyramine, a compound that can spike blood pressure if it enters the bloodstream in large amounts. Under normal circumstances, this never becomes a problem. MAO-A in the intestinal wall breaks down a large proportion of tyramine before it’s even absorbed, and whatever slips through gets caught by MAO in the liver during what’s called first-pass clearance.
This system is remarkably effective. A healthy person can tolerate anywhere from 800 to 2,000 milligrams of tyramine in a single meal without any issue, precisely because MAO intercepts it. The danger arises only when someone takes a medication that blocks MAO activity, removing this safety net. Without functioning MAO in the gut and liver, even a moderate amount of aged cheese can flood the bloodstream with tyramine, triggering a sudden and potentially dangerous spike in blood pressure. This is the origin of the well-known dietary restrictions associated with certain antidepressants.
The Connection to Depression and Parkinson’s
Because MAO controls how quickly your brain clears serotonin, dopamine, and norepinephrine, it has a direct relationship to conditions where those chemicals are out of balance. In depression, one therapeutic strategy is to slow MAO down so that more of these neurotransmitters remain available in the brain. This is the logic behind MAO inhibitor medications, one of the oldest classes of antidepressants.
The link to Parkinson’s disease is especially strong for MAO-B. Parkinson’s involves the progressive loss of dopamine-producing neurons, leading to tremor, stiffness, and difficulty initiating movement. Since MAO-B is the primary enzyme degrading dopamine in the brain, and its activity increases with age, blocking it can help preserve whatever dopamine the remaining neurons still produce. MAO-B inhibition also reduces the oxidative stress generated by the breakdown reaction itself, which may offer some neuroprotective benefit beyond simply boosting dopamine levels.
What Happens When MAO Is Missing
Rare genetic mutations can eliminate MAO-A activity entirely, causing a condition known as Brunner syndrome. Without this enzyme, serotonin, norepinephrine, and other neurotransmitters build up in the brain to abnormal levels. The consequences are visible from early childhood: mild intellectual disability, difficulty controlling impulses, and aggressive or violent outbursts that go beyond typical childhood behavior.
Children with this deficiency often show features that overlap with autism spectrum disorder and ADHD, including obsessive behaviors, trouble forming friendships, and problems with attention. Sleep disturbances like difficulty falling asleep or night terrors are common. Some affected individuals also experience physical symptoms, including episodes of skin flushing, sweating, headaches, and diarrhea, likely driven by the excess neurotransmitter activity affecting the body as well as the brain.
Even in people without a complete deficiency, natural genetic variation influences MAO-A levels. The gene has versions (alleles) that produce more or less of the enzyme depending on how many times a specific DNA sequence is repeated. Versions with 3.5 or four repeats produce more MAO-A, while versions with only two or three repeats produce less. These differences in enzyme activity have been studied in the context of impulse control and stress response, though the effects in the general population are far subtler than in full deficiency.
The Balancing Act
MAO is essentially a cleanup enzyme, and the brain depends on it to prevent neurotransmitter signals from lingering too long. Too much MAO activity strips away dopamine, serotonin, and norepinephrine too quickly, contributing to conditions marked by low mood or motor decline. Too little MAO lets these chemicals accumulate, impairing impulse control and creating oxidative stress from the byproducts that do get produced. The enzyme’s location on the mitochondrial membrane, its two distinct forms, and its presence in both the brain and the gut all reflect how deeply integrated it is into the body’s chemical regulation. It’s not just a brain enzyme. It’s a system-wide gatekeeper for some of the most powerful signaling molecules in your body.