Tetrahydrobiopterin serves well-characterized cofactor functions for hydroxylating aromatic amino acids and ether lipids and for formation of nitric oxide (NO) from L-arginine. Formation of NO involves two cycles of oxidation of Tetrahydrobiopterin to its radical with subsequent rehydroxylation into Tetrahydrobiopterin, one for reduction of the heme-bound arginine-Fe(II)O2 complex of NO synthase (NOS), the other for reduction of the N-hydroxy-L-arginine-Fe(II)O2 complex. Tetrahydrobiopterin-dependent glyceryl ether monooxygenase (EC 1.14.16.5) is found not only in liver and the gastrointestinal tract but also in brain and other organs (this enzyme plays an essential role in conjugation with the cleavage enzyme in the regulation of cellular levels of -alkyl moieties in glycerolipids). Tetrahydrobiopterin is essential for the enzymatic reaction of tyrosine 3-monooxygenase (EC 1.14.16.2) for the first step in the biosynthesis of catecholamines such as norepinephrine, epinephrine and dopamine. Limited Tetrahydrobiopterin availability not only decreases formation of NO but also causes NOS-derived superoxide/hydrogen peroxide production leading to formation of peroxynitrite as well as S-nitrosoglutathione. As a consequence of its oxygen-activating potential, Tetrahydrobiopterin is also subject to autoxidation in a free radical chain reaction in leading to formation of superoxide and finally to hydrogen peroxide. On the other hand, Tetrahydrobiopterin, like other H4-pterins, can scavenge reactive oxygen species and peroxynitrite. Thus, Tetrahydrobiopterin may have opposing effects in various biological systems depending on whether its cofactor roles outweigh its chemical reactivity or vice versa. Sepiapterin reductase (EC 1.1.1.153) catalyzes the reduction of tetrahydro-sepiapterin to tetrahydrobiopterin -the terminal step in this biosynthetic pathway for tetrahydrobiopterin. This reaction is N-acetyl-serotonin-sensitive and can completely inhibit tetrahydrobiopterin synthesis. (PMID: 3881214, 17303893, 3756924, 15223071).