Changes in catecholamine excretion after short-term tyrosine ingestion in normally fed human subjects

The effects of ingesting the aromatic amino acid L-tyrosine on excretion of unconjugated catecholamines (dopamine, norepinephrine, and epinephrine) and tyrosine were studied. (Tyrosine is the circulating precursor for the catecholamines, but only a small fraction of the tyrosine in the body is utilized for catecholamine synthesis.) In 10 of 11 normal volunteer subjects, ingestion of 100 mg/kg tyrosine (in three divided doses, preceding each meal, between 8 AM and 5 PM) for 1 day increased the 24-h excretions of total catecholamines by 25%. Only 0.42% of the tyrosine dose was excreted unchanged, but this was sufficient to increase urinary tyrosine by 138%. Both tyrosine and catecholamine excretions varied diurnally; 60% or more of the total output occurred during the day. Since urinary catecholamines reflect molecules synthesized outside the central nervous system, these findings indicate that tyrosine administration can accelerate catecholamine synthesis in the human sympathoadrenal system, probably by enhancing saturation of tyrosine hydroxylase. Therefore, tyrosine may be useful therapeutically in diseases characterized by peripheral catecholamine deficiencies.