Tetrahydrobiopterin or BH4 is a cofactor in the synthesis of nitric oxide. It is also essential in the conversion of phenylalanine to tyrosine by the enzyme phenylalanine-4-hydroxylase; the conversion of tyrosine to L-dopa by the enzyme tyrosine hydroxylase; and conversion of tryptophan to 5-hydroxytryptophan via tryptophan hydroxylase. A defect in BH4 production and/or a defect in the enzyme dihydropteridine reductase (DHPR) causes phenylketonuria type IV, as well as dopa-responsive dystonias. -- Wikipedia. [HMDB]