Phenylalanine and tyrosine metabolism in Parkinson's disease treated with levodopa

Plasma levels of phenylalanine and tyrosine were measured before and after oral administration of either phenylalanine (phenylalanine tolerance test and phenylalanine conversion test) or tyrosine (tyrosine tolerance test) in patients with Parkinson's disease before and during L‐dihydroxyphenylalanine (levodopa) therapy. In the untreated parkinsonian patients there was no abnormality in the phenylalanine tolerance test, tyrosine tolerance test, or conversion of phenylalanine to tyrosine. The fasting plasma phenylalanine and tyrosine were normal in Parkinson's disease and were not altered by levodopa therapy. When the last dose of levodopa (1 Gm.) was administered half an hour before the phenylalanine tolerance tests, phenylalanine conversion tests, or tyrosine tolerance tests, the plasma levels of these amino acids were found to be lower at 0.5, 1, and 2 hours. Twelve hours after the last dose of levodopa, the tolerance and conversion tests were normal. Levodopa treatment did not alter the concentration of phenylalanine or tyrosine in the cerebrospinal fluid (esP) of parkinsonian patients. The competition of levodopa with phenylalanine and tyrosine for intestinal transport pathways is probably the maior effect of levodopa therapy on the metabolism of these amino acids.