Dihydrobiopterin is a compound produced in the synthesis of dopamine, norepinephrine and epinephrine through production of the intermediate 3,4-dihydroxy phenylalanine, also known as dopa. Phenylalanine is converted into dopa using tetrahydrobiopterin releasing dihydrobiopterin and water, which is then converted back into tetrahydrobiopterin using the NADPH-dependent enzyme dihydrobiopterin reductase. This enzyme is sometimes defective in patients with phenylketonuria, and is treated in the same fashion, with tyrosine supplements and a controlled diet which is lacking in phenylalanine.^[1]