Tyrosine

Tyrosine is an aromatic, non-essential amino acid in humans, represented by the symbol Tyr (Y). It is classified as a proteinogenic amino acid but is not required in the diet for healthy individuals because it can be synthesized from phenylalanine via the enzyme phenylalanine hydroxylase. In phenylketonuria, phenylalanine cannot be efficiently converted to tyrosine, making tyrosine conditionally essential.

Tyrosine plays multiple roles in metabolism and physiology. It is incorporated into body proteins and serves as a precursor for several important compounds. Through the action of tyrosine hydroxylase, it is converted to the catecholamines dopamine, norepinephrine, and epinephrine. Via thyroid peroxidase-mediated iodination of tyrosine in the thyroid gland, it gives rise to the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Tyrosine is also used in melanin synthesis in pigment cells.

Metabolism of tyrosine occurs mainly in the liver. It is degraded through transamination to p-hydroxyphenylpyruvate, later processed to fumarate and acetoacetate, which enter the citric acid cycle or ketone body formation. The pathway involves enzymes such as tyrosine aminotransferase and p-hydroxyphenylpyruvate dioxygenase.

Dietary sources and intake: Tyrosine is abundant in high-protein foods such as meat, fish, dairy products, eggs, soy, legumes, and nuts. In a typical diet, tyrosine needs are met by protein intake; there is no separate recommended daily allowance for tyrosine, though it becomes essential in phenylalanine hydroxylase deficiency.

Supplementation and safety: Tyrosine is available as a dietary supplement, sometimes marketed for cognitive support, but evidence is mixed. High doses can cause nausea, headaches, or heartburn and may interact with levodopa or monoamine oxidase inhibitors. Individuals with phenylketonuria or other metabolic disorders should seek medical advice.