A male infant is diagnosed with phenylketonuria (PKU) on newborn screening. Which of the following amino acids must be present in his diet to prevent a negative nitrogen balance?

Question

Quizplus · Accepted Answer

In PKU, the enzyme phenylalanine hydroxylase is deficient, preventing the conversion of phenylalanine to tyrosine. Tyrosine becomes an essential amino acid in this condition and must be included in the diet to prevent a negative nitrogen balance.  A deficiency in phenylalanine hydroxylase (PAH) is the cause of PKU. Tyrosine is produced in cells by hydroxylating the essential amino acid phenylalanine and this reaction is catalyzed by PAH. Half of the phenylalanine required goes into the production of tyrosine; if the diet is rich in tyrosine itself, the requirements for phenylalanine are reduced by about 50%.

A Male Infant Is Diagnosed with Phenylketonuria (PKU) on Newborn