Serotonin is synthesized from the amino acid tryptophan using two processes similar to converting tyrosine to dopamine. The very first process, which generates 5-hydroxytryptophan, is mediated by tryptophan hydroxylase, which is located exclusively in serotonin-generating cells and is the rate-limiting phase (Hernández-Hernández et al., 2019). Aromatic l-amino acid decarboxylase and 5-hydroxytryptophan decarboxylase accelerate the transformation to serotonin. Serotonin synthesis starts with modifying L-tryptophan to 5-hydroxytryptophan by the enzyme L-tryptophan hydroxylase, which is identified in cytoplasmic and granular brain cell components. This enzyme is required for serotonin production in the same way as norepinephrine and dopamine production. Adrenergic and dopaminergic synapses are regulated by the activity of a comparable enzyme, L-tyrosine hydroxylase, to change L-tyrosine to L-dihydroxyphenylalanine (Hernández-Hernández et al., 2019). Certain TPH regulators, including α-propyldopacetamide, are also suppressors of tyrosine hydroxylase, while others, such as p-chlorophenyl alanine, are more specific for TPH.
In the conversion of the precursor substance tyrosine to dopamine, the following process occurs: the rate-limiting biomolecule in catecholamine production is tyrosine hydroxylase (TyrH). Saranya et al. (2021) enumerate that tyrosine hydroxylase stimulates tyrosine’s hydrogenation to L-DOPA. The catecholamines dopamine, epinephrine, and norepinephrine are the chain’s constituents involved in the peripheral nervous system as endocrine glands (Saranya et al., 2021). Phenylalanine hydroxylase catalyzes the conversion of phenylalanine to tyrosine, while tyrosine hydroxylase catalyzes the conversion of tyrosine to L-DOPA (Saranya et al., 2021). Aromatic amino acid deaminase converts DOPA to dopamine.
Lastly, the following mechanisms take place in the formation of norepinephrine from tyrosine: norepinephrine is the primary messenger for postganglionic autonomic ventricular neurons. It is generated within the nerve fiber, held in blisters, and then discharged by the neuron in response to the passage of a signaling cascade down the nerves (Płonka et al., 2022). The following information, as illustrated herein, pertains to the production of norepinephrine. First, tyrosine is carried into the sympathetic nerve end via the amino acid tyrosine. Second, tyrosine is transformed into DOPA by the enzyme tyrosine hydroxylase, which is the rate-limiting phase in the biosynthesis of norepinephrine (Płonka et al., 2022). Lastly, DOPA is then decarboxylated to become dopamine. As a result, dopamine is carried into vesicles and transformed by dopamine-hydroxylase (DBH) to norepinephrine (NE).