<unk>  can be caused by a deficiency of the enzyme dihydrobiopterin reductase (DHPR), whose activity is needed to replenish quinonoid-dihydrobiopterin back into its<unk>  form. Those with this deficiency may produce sufficient levels of the enzyme phenylalanine hydroxylase (PAH) but, since<unk>  is a cofactor for PAH activity, deficient dihydrobiopterin reductase renders any PAH produced unable to use phenylalanine to produce tyrosine.<unk>  is a cofactor in the production of L-DOPA from tyrosine and 5-hydroxy-L-tryptophan from tryptophan, which must be supplemented as treatment in addition to the supplements for classical PKU. Other underlying causes of<unk>  are: * 6-Pyruvoyltetrahydropterin synthase (PTPS) deficiency * Autosomal recessive guanosine triphosphate cyclohydrolase I (GTPCH) deficiency * Pterin-4alpha-carbinolamine dehydratase (PCD) deficiency
Tetrahydrobiopterin deficiency
Cerebral folate deficiency
Cytochrome b5 deficiency
Adenine phosphoribosyltransferase deficiency
Molybdenum cofactor deficiency
Methylenetetrahydrofolate reductase deficiency
Dihydropyrimidine dehydrogenase deficiency
Biotin deficiency