Catecholamines (norepinephrine (NE), epinephrine (E) and dopamine (DA)) play important roles as neurotransmitters or hormones, and are known to be metabolized by catechol−O−methyltransferase (COMT) and monoamine oxidase. As shown in Figure 1, catecholamines are metabolized into their respective 3−O− methyl metabolites (normetanephrine (NMN), metanephrine (MN) and 3−methoxytyramine (3−MT)) by CO...