The synthesis of norepinephrine depends on the presence of tyrosine, an amino acid found in proteins such as meat, nuts, and eggs. Dairy products such as cheese also contain high amounts of tyrosine (the amino acid is named for "tyros", the Greek word for cheese). However, the body can synthesise tyrosine from phenylalanine, an essential amino acid. Tyrosine is the precursor to dopamine, which in turn is a precursor to epinephrine and norepinephrine.
Serotonin, a neurotransmitter that is in many ways the opposite of the catecholamines, is also directly synthesized from an amino acid (tryptophan). However, tryptophan has a somewhat different process of degradation. When serotonin is catabolized in the body, it does not break down into useful substrates in the way that dopamine is further degraded into epinephrine and norepinephrine. Instead, it breaks down into 5-hydroxyindoleacetic acid (5-HIA), an organic acid that may be harmful in high amounts. Tryptophan can further be catabolized into kynurenate, quinolinate, and picolinate, harmful substances that are generally regarded as markers of bodily inflammation.
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