Sepiapterin, also known as 2-amino-6-lactoyl-7,8-dihydropteridin-4(3H)-one, belongs to the class of organic compounds known as pterins and derivatives. These are polycyclic aromatic compounds containing a pterin moiety, which consist of a pteridine ring bearing a ketone and an amine group to form 2-aminopteridin-4(3H)-one. Sepiapterin is also classified as a member of the pteridine class of organic chemicals. It is a yellow fluorescing pigment. Sepiapterin is an intermediate in the salvage pathway of tetrahydrobiopterin (BH(4)). More specifically, sepiapterin can be metabolized into tetrahydrobiopterin via the BH(4) salvage pathway. Tetrahydrobiopterin is an essential cofactor in humans for breakdown of phenylalanine and a catalyst of the metabolism of phenylalanine, tyrosine, and tryptophan to the neurotransmitters dopamine and serotonin. A deficiency of tetrahydrobiopterin can cause toxic buildup of phenylalanine (phenylketonuria) as well as deficiencies of dopamine, norepinephrine, and epinephrine, leading to dystonia and other neurological illnesses. Sepiapterin accumulates in the brain of patients with sepiapterin reductase (SR) deficiency, an inborn error of metabolism. Sepiapterin reductase deficiency is a condition characterized by movement problems, most often a pattern of involuntary, sustained muscle contractions known as dystonia. Other movement problems can include muscle stiffness (spasticity), tremors, problems with coordination and balance (ataxia), and involuntary jerking movements (chorea). People with sepiapterin reductase deficiency can experience episodes called oculogyric crises. These episodes involve abnormal rotation of the eyeballs; extreme irritability and agitation; and pain, muscle spasms, and uncontrolled movements, especially of the head and neck. Movement abnormalities are often worse late in the day. Most affected individuals have delayed development of motor skills such as sitting and crawling, and they typically are not able to walk unassisted. The problems with movement tend to worsen over time. Within humans, sepiapterin participates in a number of enzymatic reactions. In particular, sepiapterin can be converted into 7,8-dihydroneopterin; which is mediated by the enzyme sepiapterin reductase. In addition, sepiapterin can be converted into 7,8-dihydroneopterin through its interaction with the enzyme carbonyl reductase [NADPH] 1.