Figure 1.

The role of aromatic amino acids (AAA) in the organism and induction of tyrosine derivatives (3,4-dihydroxyphenylalanine (DOPA) and 2,5-dityrosine) by reactive oxygen species. Phenylalanine (Phe) is converted to tyrosine (Tyr) in the liver and kidneys with the participation of phenylalanine hydroxylase (PAH). Tyr is used for the synthesis of catecholamines (dopamine, epinephrine, and norepinephrine) in the adrenal medulla and for the synthesis of pheo- and eumelanins in skin melanocytes, the first stage of which is the formation of L-DOPA. An elevated L-DOPA/L-Tyr ratio occurs in patients with malignant melanoma and local lymph node metastases. Tyr is also the starting compound for the synthesis of thyroid hormones—thyroxine (T4) and tri-iodothyronine (T3). In addition, Tyr derivatives such as 3-Cl-D-Tyr or 3-N-D-Tyr may be formed under oxidative, nitrifying, and chlorinating stress conditions. Tyr catalyzes the reaction that converts Trp to kynurenine (KYN). IDO (indoleamine-2,3-dioxygenase) and TDO (tryptophan-2,3-dioxygenase) are induction enzymes that are activated under stressful situations, among others under the proinflammatory cytokines.