Pterins and Dihydropteridine Reductase

The cofactor tetrahydrobiopterin (BH4) is essential for the function of various enzymes, such as phenylalanine, tyrosine, and tryptophan hydroxylases, all isoforms of NO synthase, and the glyceryl-ether mono-oxygenase enzyme. The synthesis of BH4 involves three distinct enzymes: GTP cyclohydrolase I (GTPCH), 6-pyruvoyl-tetrahydropterin synthase (PTPS), and sepiapterin reductase (SR). Additionally, BH4 can be regenerated through the actions of two enzymes: pterin-4a-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR). Variants in genes coding for these enzymes, except for sepiapterin reductase, can lead to BH4 deficiency and result in hyperphenylalaninemia. Therefore, it is recommended to screen all newborns with elevated plasma phenylalanine levels for BH4 deficiency. This chapter provides a detailed overview of the most commonly used analytical methods for quantifying pterins in urine, CSF, plasma, or DBS, as well as an enzymatic assay for measuring DHPR activity in DBS. Reference ranges are included for all methods; however, some adjustments may be necessary before implementing them in other laboratories.