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Related Experiment Videos

Tetrahydrobiopterin loading test in hyperphenylalaninemia.

A Ponzone1, O Guardamagna, S Ferraris

  • 1Institute of Pediatric Clinic, University of Torino, Italy.

Pediatric Research
|November 1, 1991
PubMed
Summary

Tetrahydrobiopterin deficiency in newborns requires cofactor substitution therapy, not just diet. A high-dose tetrahydrobiopterin loading test effectively identifies these cases, distinguishing them from phenylketonuria for timely treatment.

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Area of Science:

  • Biochemistry
  • Genetics
  • Pediatrics

Background:

  • Primary hyperphenylalaninemia can stem from phenylalanine hydroxylase deficiency or its cofactor, tetrahydrobiopterin (BH4) deficiency.
  • BH4-deficient patients do not respond to phenylalanine-restricted diets and require specific substitution therapy.
  • Existing diagnostic methods for BH4 deficiency, such as urinary pterin or enzyme assays, have limited availability.

Purpose of the Study:

  • To reevaluate the tetrahydrobiopterin (BH4) loading test for diagnosing BH4 deficiency in newborns with hyperphenylalaninemia.
  • To determine if increased cofactor dosage could overcome previous limitations of the BH4 loading test.

Main Methods:

  • Administered synthetic tetrahydrobiopterin (BH4) orally (7.5 and 20 mg/kg) and intravenously (2 mg/kg) to 15 patients.

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  • Patients included those with phenylketonuria, 6-pyruvoyl tetrahydropterin synthase deficiency, and dihydropteridine reductase deficiency.
  • Monitored serum phenylalanine concentration changes following cofactor administration.

    • Main Results:

    • Tetrahydrobiopterin-deficient patients showed a marked decrease in serum phenylalanine below baseline after a 20 mg/kg oral BH4 dose.
    • This response was significant enough to be detectable on Guthrie cards.
    • Phenylketonuria patients did not exhibit a similar response.

    Conclusions:

    • The high-dose oral tetrahydrobiopterin (BH4) loading test is a simple and effective screening method for BH4 deficiency in hyperphenylalaninemia.
    • This method is valuable when enzyme or pterin studies are not readily available.
    • Accurate diagnosis through this test enables appropriate substitution therapy for BH4-deficient patients.