The most frequent defects in amino acid metabolism involve
the amino acids phenylalanine and tyrosine. Numerous enzymes
are required to convert phenylalanine into a variety of biochemical
products. The metabolism of phenylalanine and the various metabolic
blocks are illustrated in graphic on the left where for simplicity,
many intermediate steps have been omitted. The circled letters
indicate enzyme defects which will of course disrupt the reactions
which follow it.
A. Phenylketonuria (PKU)
If the enzyme which converts phenylalanine into tyrosine is
then phenylalanine undergoes a transamination reaction to make
phenylpyruvic acid instead.
Phenylpyruvic acid can be found in the urine when excessive
levels have built up. If untreated, mental retardation effects
are evident by the first year along with other symptoms which
include: unusual irritability, epileptic seizures and skin lesions.
Since the defect is found in 1 in 20,000 births, a general screening
program (PKU Test) has been mandated in many states and is routinely
made several days after the birth of an infant. The defective
metabolism can only be controlled by a diet containing small
amounts of phenylalanine.
Link to Chime: Phenylalanine
Hydroxylase - Department of Biochemistry,The University of