Pterins: Compounds based on 2-amino-4-hydroxypteridine.Dihydropteridine Reductase: An enzyme that catalyzes the reduction of 6,7-dihydropteridine to 5,6,7,8-tetrahydropteridine in the presence of NADP+. Defects in the enzyme are a cause of PHENYLKETONURIA II. Formerly listed as EC 22.214.171.124.Pteridines: Compounds based on pyrazino[2,3-d]pyrimidine which is a pyrimidine fused to a pyrazine, containing four NITROGEN atoms.Biopterin: A natural product that has been considered as a growth factor for some insects.GTP Cyclohydrolase: (GTP cyclohydrolase I) or GTP 7,8-8,9-dihydrolase (pyrophosphate-forming) (GTP cyclohydrolase II). An enzyme group that hydrolyzes the imidazole ring of GTP, releasing carbon-8 as formate. Two C-N bonds are hydrolyzed and the pentase unit is isomerized. This is the first step in the synthesis of folic acid from GTP. EC 126.96.36.199 (GTP cyclohydrolase I) and EC 188.8.131.52 (GTP cyclohydrolase II).