Dihydrobiopterin reductase is an enzyme that catalyzes the reversible formation of tetrahydrobiopterin from dihydrobiopterine using NADPH. A deficiency of this enzyme can result in malignant hyperphenylalaninemia.
Dihydrobiopterin is a compound produced in the synthesis of dopamine, norepinephrine and epinephrine through production of the intermediate L-3,4-dihydroxyphenylalanine, also known as L-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.