Phenylketonuria (PKU) is associated with deficient activity of phenylalanine hydroxylase (PAH). This results in an accumulation of phenylalanine and decrease in tyrosine, altering the normal ratio of the two amino acids.
Phenylalanine hydroxylase (PAH) converts phenylalanine to tyrosine. With decreased PAH activity there is:
(1) an accumulation of phenylalanine
(2) decreased conversion to tyrosine
(3) increased conversion of phenylalanine to phenylpyruvic acid which is degraded to phenylacetic and phenyllactic acid (phenylketones)
Parameters:
(1) serum phenylalanine in µmol/L
(2) serum tyrosine in µmol/L
ratio of phenylalanine to tyrosine =
= (serum phenylalanine) / (serum tyrosine)
Interpretation:
• Normally the ratio is < 1.
• In untreated phenylketonuria the ratio is > 3. An increased ratio and persistently elevated serum phenylalanine concentration help to identify a patient who may have phenylketonuria.
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