Positively regulates nitric oxide synthesis in umbilical vein endothelial cells (HUVECs). May be involved in dopamine synthesis. May modify pain sensitivity and persistence. Isoform GCH-1 is the functional enzyme, the potential function of the enzymatically inactive isoforms remains unknown. {ECO:0000269|PubMed:12176133, ECO:0000269|PubMed:16338639, ECO:0000269|PubMed:17057711, ECO:0000269|PubMed:8068008, ECO:0000269|PubMed:9445252}. This is the function of GCH1 (GTP cyclohydrolase 1, ENSG00000131979).