Tetrahydrobiopterin is a cofactor for phenyalanine hydroxylase (and also tryptophan and tyrosine hydroxylases). Raised blood phenyalanine found through newborn screening can be due to either a cofactor or an enzyme deficiency. All new PKU patients need a urine pterin assay to determine which of the above applies as the treatment differs (supplied cofactor if this is deficient; phenyalanine reduced diet if the enzyme is deficient). In this case the test is funded by the Newborn Metabolic Screening Programme.
Specialist Biochemistry
UPTE
Tetrahydrobiopterin
5820