Dihydrobiopterin, also known as BH2, 7,8-dihydrobiopterin, L-erythro-7,8-dihydrobiopterin, quinonoid dihydrobiopterin or q-BH2, belongs to the class of organic compounds known as biopterins and derivatives. These are coenzymes containing a 2-amino-pteridine-4-one derivative. Dihydrobiopterin is also classified as a pteridine. Pteridines are aromatic compounds composed of fused pyrimidine and pyrazine rings. Dihydrobiopterin is produced during the synthesis of neurotransmitters L-DOPA, dopamine, norepinephrine and epinephrine. It is restored to the required cofactor tetrahydrobiopterin via the NADPH-dependant reduction of dihydrobiopterin reductase. Dihydrobiopterin can also be converted to tetrahydrobiopterin by nitric oxide synthase (NOS) which is catalyzed by the flavoprotein "diaphorase" activity of NOS. This activity is located on the reductase (C-terminal) domain of NOS, whereas the high affinity tetrahydrobiopterin site involved in NOS activation is located on the oxygenase (N-terminal) domain (PMID: 8626754). Sepiapterin reductase (SPR) is another enzyme that plays a role in the production of dihydrobiopterin. SPR catalyzes the reduction of sepiapterin to dihydrobiopterin (BH2), the precursor for tetrahydrobiopterin (BH4). BH4 is a cofactor critical for nitric oxide biosynthesis and alkylglycerol and aromatic amino acid metabolism (PMID: 25550200). Dihydrobiopterin is known to be synthesized in several parts of the body, including the pineal gland. Dihydrobiopterin exists in all eukaryotes, ranging from yeast to humans. In humans, dihydrobiopterin is involved in several metabolic disorders including dihydropteridine reductase (DHPR) deficiency. DHPR deficiency is a severe form of hyperphenylalaninemia (HPA) due to impaired regeneration of tetrahydrobiopterin (BH4) leading to decreased levels of neurotransmitters (dopamine, serotonin) and folate in cerebrospinal fluid, and causing neurological symptoms such as psychomotor delay, hypotonia, seizures, abnormal movements, hypersalivation, and swallowing difficulties. Dihydrobiopterin is also associated with another metabolic disorder known as sepiapterin reductase deficiency (SRD). Sepiapterin reductase catalyzes the (NADP-dependent) reduction of carbonyl derivatives, including pteridines, and plays an important role in tetrahydrobiopterin biosynthesis. Low dihydrofolate reductase activity in the brain leads to the accumulation of dihydrobiopterin, which in turn, inhibits tyrosine and tryptophan hydroxylases. This uncouples neuronal nitric oxide synthase, leading to neurotransmitter deficiencies and neuronal cell death. SRD is characterized by low cerebrospinal fluid neurotransmitter levels and the presence of elevated cerebrospinal fluid dihydrobiopterin. SRD is characterized by motor delay, axial hypotonia, language delay, diurnal fluctuation of symptoms, dystonia, weakness, oculogyric crises, dysarthria, parkinsonian signs and hyperreflexia.