Phenylalanine
loading test in Dopa-responsive dystonia
(DRD due to GTPCH deficiency)
and other forms
of tetrahydrobiopterin deficiency without hyperphenylalaninemia
(SR deficiency)
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Dosage
100 mg/kg (oral load)
Sample collection
1 ml plasma (Heparin or EDTA) 0, 1,
2, and 4 hrs after administration of Phe, keep frozen
at -20°C
Measurements
Phenylalanine, tyrosine, neopterin, and biopterin
Interpretation
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 5 - Preload : Phe, Phe/Tyr, and biopterin
Reference
- 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.
- Blau N, Bonafé L, Thöny
B. Tetrahydrobiopterin deficiencies without hyperphenylalaninemia:
Diagnosis and
genetics of Dopa-responsive dystonia and sepiapterin
reductase deficiency. Mol Genet Metab 2001:in press.
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