Phenylalanine loading test in Dopa-responsive dystonia
(DRD due to GTPCH deficiency)
and other forms of tetrahydrobiopterin deficiency without hyperphenylalaninemia (SR deficiency)
05 November 2004
100 mg/kg (oral load)
1 ml plasma (Heparin or EDTA) 0, 1, 2, and 4 hrs after administration of Phe, keep frozen at -20°C
Phenylalanine, tyrosine, neopterin, and biopterin
Phenylalanine levels peak at 2 hrs and are markedly elevated at 4 and 6 hrs compared to controls. Tyrosine levels do not increase at all.
The slower clearance rate of phenylalanine from the plasma and inadequate production of tyrosine indicate a defect in liver PAH activity in patients with DRD.
Similar Plasma Phe/Tyr profiles can be expected in patients with other forms of BH4 deficiency.
Figure Phe, Phe/Tyr, and biopterin
Hyland K, Fryburg JS, Wilson WG, Bebin EM, Arnold LA, Gunasekera RS, Jacobson RD, Rostruffner E, Trugman JM. Oral phenylalanine loading in Dopa-responsive dystonia - a possible diagnostic test. Neurology 1997 48, 1290-1297.