Which MAO enzyme is known for breaking down serotonin and norepinephrine?

The correct choice is based on the specific roles of monoamine oxidase (MAO) enzymes in the metabolism of neurotransmitters. MAO-A is particularly responsible for the breakdown of serotonin and norepinephrine. This enzyme plays a crucial role in the regulation of mood and emotional response by degrading these neurotransmitters in the synaptic cleft after they have been released.

By breaking down serotonin, which is essential for mood regulation, and norepinephrine, which impacts arousal and alertness, MAO-A helps to maintain balance in neurotransmitter levels. Dysregulation or inhibition of this enzyme is also a target for certain antidepressants, as blocking MAO-A can lead to increased availability of these important neurotransmitters, thereby alleviating depressive symptoms.

In contrast, the other options specify different forms of the MAO enzyme. For instance, MAO-B primarily breaks down phenylethylamine and plays a lesser role in serotonin and norepinephrine metabolism. Options C and D refer to less-researched forms (MAO-C and MAO-D), which do not actively participate in metabolizing serotonin and norepinephrine like MAO-A does. Therefore, the focus on MAO-A as the enzyme involved in the breakdown of serotonin