L-tyrosine by the enzyme tyrosine hydroxylase (TH). It is also the precursor for the monoamine or catecholamine neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline). Dopamine is formed by the decarboxylation of L-DOPA.
L-DOPA can be directly metabolized by catechol-O-methyl transferase (COMT) to 3-O-methyldopa (3-OMD), and then further to vanillactic acid (VLA). This metabolic pathway is non-existent in the healthy body, but becomes important after peripheral L-DOPA administration in patients with Parkinson's Disease or in the rare cases of patients with aromatic L-amino acid decarboxylase (AADC) enzyme deficiency.
The prefix L- references its property of levorotation (compared with dextrorotation or D-DOPA).
L-Phenylalanine, L-tyrosine, and L-DOPA, are all precursors to the biological pigment melanin. The enzyme tyrosinase catalyzes the oxidation of L-DOPA to the reactive intermediate dopaquinone, which reacts further, eventually leading to melanin oligomers.