 Dopamine is broken down into inactive metabolites by a set of enzymes—monoamine oxidase (MAO), catechol-"O"-methyl transferase (COMT), and aldehyde dehydrogenase (ALDH), acting in sequence.Both isoforms of monoamine oxidase, MAO-A and MAO-B, effectively metabolize dopamine.Different breakdown pathways exist but the main end-product is homovanillic acid (HVA), which has no known biological activity.