PKU is an inborn error of protein metabolism due to decrease of phenylalanine hydroxilase, or the tetrahydrobiopterin cofactor, causing an impaired ability to metabolize the essential amino acid phenylalanine.
The Phenylketonuria is present when plasma phenylalanine levels exceed 20mg/dl (1200mmol/L).
Phenylalanine hydroxilase deficiency is inherited in an autosomal recessive manner.
A small percentage of children with elevated phenylalanine levels exhibit normal phenylalanine hydroxilase but have a deficiency in synthesis or recycling of the enzyme’s cofactor, tetrahydrobiopterin, this condition is termed Malignant Phenylketonuria.
The biopterin cofactor is also required for hydroxylation of tyrosine, a precursor of dopamine, and tryptophan, a precursor of serotonin.
The patients with biopterin cofactor deficiency have more significant neurological problems that are not fully corrected by dietary phenylalanine reduction.
Almost individuals with PKU appear normal at birth. If newborn screening fails, progressive developmental delay is the most common presentation.
Other finding in untreated children in later infancy and childhood may include vomiting, musty body odor, eczema, seizures, self-mutilation, fair coloring as a result of tyrosine deficiency, and several behavioral disorders.
Laboratory studies: Screening for Phenylketonuria within 2-3 days after birth.
Treatment: Dietary restriction of phenylalanine with tyrosine supplementation.
Most of US facilities recommend that phenylalanine levels be maintained from 2-6 mg/dl (120-360mmol/L).
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