Increased vigilance and dopamine synthesis by large doses of tyrosine or phenylalanine restriction in phenylketonuria.

In a group of 9 patients with classical phenylketonuria (PKU) aged 15-24 years we examined the effect of phenylalanine restricted diet on vigilance, as judged by the continuous visual reaction times, and neurotransmitter synthesis, as judged by cerebrospinal fluid (CSF) homovanillic acid (HVA) and 5-hydroxyindole acetic acid (5-HIAA) levels. HVA and 5-HIAA levels decreased significantly with increase in plasma phenylalanine concentration on free diet (p less than 0.01 and p less than 0.0005 respectively). Vigilance improved on phenylalanine restricted diet in 6 of the 7 patients with abnormally long reaction times on free diet. Addition of tyrosine (160 mg/kg/24 h) to the free diet resulted in an increased CSF in the six patients examined. In 14 patients on free diet supplemented with tyrosine, an improvement in vigilance (reaction times at the 90 percentile) was seen in all 12 patients with values longer than the normal mean (264 msec) (p less than 0.001). Tyrosine treatment may be a therapeutical alternative when phenylalanine restriction is impractical.