Abstract
L-Tyrosine occupies a central position in the synthesis of several biologically active molecules, including the catecholamines, thyroxine and melanin. Its conversion to L-DOPA by tyrosine hydroxylase (TH) is the rate-limiting step in the generation of dopamine, epinephrine and norepinephrine, in the adrenal medulla, sympathetic nerve cells and certain neurones of the brain. Under conditions of stress or neuronal stimulation TH activity increases and becomes responsive to increased levels of tyrosine. This has led to studies investigating the potential usefulness of supplementary tyrosine for treating diseases in which catecholamine deficiency may be a feature, such as Parkinson's disease and some types of depression. The usual doses employed have been 30 to 100 mg kg-1 daily and in preliminary, uncontrolled studies results were encouraging. Tyrosine has also been investigated for its potential use in counteracting performance decrement in military sustained operations, and the effects of stress due to cold, hypoxia, noise or extended wakefulness. The effectiveness of short-term administration at doses of 100 to 150 mg kg-1 has been demonstrated in controlled trials. Other potential uses include a possible role in facilitating melanogenesis. Very few adverse reactions have been reported in the studies described. However, a report prepared for the US FDA in 1992 on the safety of amino acids as dietary supplements referred to toxicological findings in rats fed high doses of tyrosine, as well as reviewing data in humans. It identified a number of population groups likely to be at increased risk of adverse effects. Finally, the problems of funding definitive research to clarify the benefit/risk of such compounds are discussed.