tetrahydrobiopterin

BH4 is synthesized from GTP and serves as a cofactor for four key enzymes: phenylalanine hydroxylase (converts phenylalanine to tyrosine), tyrosine hydroxylase (converts tyrosine to L-DOPA, the rate-limiting step in catecholamine synthesis), tryptophan hydroxylase (converts tryptophan to 5-HTP, precursor of serotonin), and nitric oxide synthase (produces NO from arginine). During these reactions, BH4 is oxidized to dihydrobiopterin (BH2) and must be regenerated by dihydropteridine reductase (DHPR) using NADH. Deficiencies in BH4 synthesis (e.g., GTP cyclohydrolase or 6-pyruvoyltetrahydropterin synthase deficiency) or in DHPR lead to malignant hyperphenylalaninemia—elevated phenylalanine levels combined with neurotransmitter deficiencies causing progressive neurologic deterioration, seizures, and developmental delay despite dietary phenylalanine control. This distinguishes it from classic PKU (due to phenylalanine hydroxylase deficiency), which responds to dietary restriction. Treatment includes BH4 supplementation (sapropterin) for responsive cases, neurotransmitter precursor replacement (L-DOPA, 5-HTP), and folinic acid. Clinically, sapropterin (a synthetic BH4 analog) is FDA-approved for BH4-responsive PKU to enhance residual PAH enzyme activity. BH4 also plays a role in endothelial function via eNOS; its oxidation to BH2 under oxidative stress leads to eNOS uncoupling, contributing to endothelial dysfunction seen in atherosclerosis and hypertension—a concept tested in vascular biology contexts.