MAO-B

MAO-B (monoamine oxidase B) is one of two forms of the enzyme monoamine oxidase, the other being MAO-A. Its primary job is to break down certain monoamines — most notably dopamine and phenethylamine — once they have done their signalling work in the brain and body.

The two isoforms share a basic function but differ in their substrate preferences and distribution. MAO-B is concentrated in the brain, liver, and blood platelets. Because it governs dopamine metabolism, it has become a major target in the treatment of Parkinson's disease, where dopamine-producing neurons are progressively lost.

How it works · its role

MAO-B sits on the outer membrane of mitochondria inside cells. When dopamine or phenethylamine enters the cell, MAO-B oxidises it — stripping electrons from the molecule — converting it into an inactive aldehyde that is then cleared from the body.

This oxidative step is a normal part of monoamine housekeeping. Drugs that selectively block MAO-B prevent this breakdown, allowing dopamine to accumulate. Selective MAO-B inhibitors such as selegiline and rasagiline are used clinically for exactly this reason: they raise dopamine tone without, at therapeutic doses, significantly disrupting serotonin or norepinephrine metabolism — the province of MAO-A.

Relevance to substances & effects

MAO-B is most directly relevant to substances that work through dopamine. Selective MAO-B inhibitors are used in Parkinson's disease and are sometimes combined with levodopa to extend its effect. They also slow the metabolism of any dopaminergic substance taken alongside them.

Phenethylamines — a broad class that includes amphetamines and a number of psychoactive research chemicals — are natural MAO-B substrates. When MAO-B is inhibited, their effects may be prolonged or amplified because they are not cleared as quickly.

Tobacco smoke contains compounds that inhibit both MAO-A and MAO-B, a pharmacological detail thought to contribute to nicotine's rewarding effects and to the increased dopamine activity associated with smoking. This inhibition may partly explain why tobacco use alters sensitivity to other substances.

At higher doses, the selectivity of MAO-B inhibitors breaks down and they begin to inhibit MAO-A as well. At that point the interaction risks associated with non-selective MAOIs — including dangerous reactions with serotonergic or tyramine-rich foods — become relevant.