Background and Relevant Pharmacokinetics
Tyrosine is a conditionally essential amino acid. It can be taken in through the diet or synthesised in the body from phenylalanine, except in phenyketonurics. L-Tyrosine is absorbed in the small intestine by active transport and transported to the liver where it is involved in a number of biochemical reactions, such as protein synthesis and oxidative catabolic reactions. L-Tyrosine that is not metabolised in the liver is distributed via systemic circulation to various tissues in the body where it is involved in the synthesis of a number of catecholamines and hormones.
There is some question as to tyrosine‘s ability to cross the blood-brain barrier and this may explain some of the negative results demonstrated in clinical trials. A rat study has demonstrated that combining the essential fatty acid, alpha-linolenic acid, with L-tyrosine, via a special bond, produces an active biological molecule with potent dopaminergic activity, suggesting that the alpha-linolenic acid may play a dual role as a carrier for tyrosine and a membrane-and receptor-improving agent.
Tyrosine has been used by the military in the USA and the Netherlands to counter the stressful effects of cold, prolonged and excessive physical activity. It also appears to improve cognition and performance in soldiers under psychologically stressful conditions and has been scientifically shown to improve physical and mental endurance.
Good dietary sources include soy products, chicken, fish, almonds, avocados, bananas, dairy products, meat, eggs, nuts, beans, oats, wheat, lima beans, pumpkin seeds, sesame seeds and fermented foods such as yoghurt and miso.
Deficiency Signs and Symptoms
The following have been associated with low levels of tyrosine:
• low blood pressure
• low body temperature
• restless legs syndrome
Tyrosine: Main Actions
NEUROTRANSMITTER AND HORMONE PRODUCTION
Many of the pharmacological actions of tyrosine relate to its role as a precursor for a number of neurotransmitters and hormones.
Elevating tyrosine concentrations in brain catecholamine neurons (particularly dopamine and noradrenaline neurons) can stimulate neurotransmitter production in actively firing neurones but not in those that are quiescent or firing slowly.
It plays an essential role in the body as a precursor to the catecholamine neurotransmitters as illustrated below.
Phenylalanine -» Tyrosine -» L-Dopa -» Dopamine -» Noradrenaline -» Adrenaline
Folate, vitamins B3, B6, B12 and C, iron, copper and other nutrients are required for the metabolism of tyrosine to catecholamines.
As tyrosine is a precursor for the synthesis of thyroid hormones it is involved in the regulation of basal metabolic rate, oxygen use, cellular metabolism, growth and development.
Tyrosine undergoes iodination to form T1 (mono-iodotyrosine), a second iodination produces T2 (di-iodotyrosine) and these combine to produce the active thyroid hormones known as T3 (tri-iodothyronine) and T4 (tetra-iodothyronine or thyroxine).
Tyrosine is also involved in the production of other compounds such as melanin, and some types of oestrogen.
Because tyrosine binds unstable molecules that can potentially cause damage to the cells and tissues, it is considered an antioxidant.