<unk>  is involved in the inactivation of the<unk> amine neurotransmitters (dopamine, epinephrine, and norepinephrine). The enzyme introduces a methyl group to the<unk> amine, which is donated by S-adenosyl methionine (SAM). Any compound having a<unk>  structure, like<unk> estrogens and<unk> -containing flavonoids, are substrates of<unk> . Levodopa, a precursor of<unk> amines, is an important substrate of<unk> .<unk>  inhibitors, like entacapone, save levodopa from<unk>  and prolong the action of levodopa. Entacapone is a widely used adjunct drug of levodopa therapy. When given with an inhibitor of dopa decarboxylase (carbidopa or benserazide), levodopa is optimally saved. This "triple therapy" is becoming a standard in the treatment of Parkinson's disease. Specific reactions catalyzed by<unk>  include: * Dopamine → 3-Methoxytyramine * DOPAC → HVA (homovanillic acid) * Norepinephrine → Normetanephrine * Epinephrine → Metanephrine * Dihydroxyphenylethylene glycol (DOPEG) → Methoxyhydroxyphenylglycol (MOPEG) * 3,4-Dihydroxymandelic acid (DOMA) → Vanillylmandelic acid (VMA) In the brain,<unk> -dependent dopamine degradation is of particular importance in brain regions with low expression of the presynaptic dopamine transporter (DAT), such as the prefrontal cortex. (In the PFC, dopamine is also removed by presynaptic norepinephrine transporters (NET) and degraded by monoamine oxidase.) Controversy exists about the predominance and orientation of membrane bound<unk>  in the CNS, that is, whether this<unk>  process is active intracellularly in postsynaptic neurons and glia, or oriented outward on the membrane, acting extracellularly on synaptic and extrasynaptic dopamine. Soluble<unk>  can also be found extracellularly, although extracellular<unk>  plays a less significant role in the CNS than it does peripherally. Despite its importance in neurons,<unk>  is actually primarily expressed in the liver.
Catechol-O-methyltransferase
Phenylalanine hydroxylase
Dopamine beta-hydroxylase
Tyrosine hydroxylase
Aromatic L-amino acid decarboxylase
Monoamine oxidase B
Monoamine oxidase
Monoamine oxidase A