Tetrahydropterin formation and recycling

The diagram illustrates how tetrahydropterin is formed from guanosine triphosphate (GTP) utilized in the hydroxylation reactions and recycled. Metabolism of tetrahydrobiopterin (BH4), cofactor of tyrosine hydroxylase (TYH), of tryptophan hydroxylase (TPH), of phenylalanine hydroxylase (PAH) and of nitric oxide synthase (NOS). Biosynthesis of BH4: GTP cyclohydrolase I (GTPCH) converts GTP to dihydroneopterin triphosphate (NH2TP), 6-pyruvoly-tetrahydropterin synthase (PTPS) converts NH2TP to 6-pyruvoly-tetrahydropterin (PTP), and sepiapterin reductase (SR) catalyzes the final two steps of reduction of PTP to BH4. HO-BH4, product of the hydoxylation reactions, is dehydrated to the quinonoid dihydrobiopterin (BH2) by pterin-4 alpha-carbinolamine dehydratase (PCD) and subsequently reduced back to BH4 by dihydropteridine reductase (DHPR). BH4, BH2, biopterin, neopterin, sepiapterin, and primapterin are metabolites detectable in different body fluids, especially CSF, and used as makers for disorders of BH4 metabolism. (Contributed by Dr. Georg F Hoffmann.)