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Summary
Classic phenylketonuria (PKU) is caused by a deficiency in phenylalanine hydroxylase (PAH). Studies show a mutant PAH enzyme, not a regulatory mutation, is responsible for low activity in PKU patients.
Area of Science:
- Biochemistry
- Genetics
- Metabolic Disorders
Background:
- Phenylalanine to tyrosine conversion is vital in mammals.
- This process is enzyme-catalyzed, involving phenylalanine hydroxylase (PAH).
- Classic phenylketonuria (PKU) is a disorder linked to this pathway.
Purpose of the Study:
- To identify the molecular basis of enzyme deficiency in classic PKU.
- To investigate the role of phenylalanine hydroxylase (PAH) in PKU pathogenesis.
- To differentiate between regulatory and structural mutations in PAH.
Main Methods:
- Assay of enzyme system components in liver biopsy samples from PKU patients.
- Immunotitration using a specific antiserum to phenylalanine hydroxylase (PAH).
- Sensitive enzyme activity assays to quantify residual PAH function.
Main Results:
- All components of the phenylalanine to tyrosine conversion system were present except PAH.
- A low level (0.27%) of phenylalanine hydroxylase (PAH) activity was detected in PKU liver samples.
- Evidence suggests a mutant PAH enzyme form, rather than low normal enzyme levels, causes reduced activity.
Conclusions:
- Phenylketonuria (PKU) is caused by a deficiency in phenylalanine hydroxylase (PAH).
- The low PAH activity in PKU results from a mutation in the gene coding for the enzyme's structure.
- These findings exclude deletion mutations and regulatory gene mutations as causes of classic PKU.