Atypical phenylketonuric heterozygote

A child with an elevated serum level of phenylalanine, a typical phenylketonuria phenylalanine tolerance test, an absence of increased urinary concentrations of phenylketones, ortho- and parahydroxyphenyl acids, and phenolic acids, as well as no abnormality in urinary excretion of 5-hydroxyindoleacetic acid or in indole-3-acetic acid is reported. It appears that this child has marked limitation in phenylalanine transaminase activity and, as a consequence, the characteristic associated biochemical abnormalities related to limited activity of this enzyme were not expressed and could not be evoked.