Tetrahydrobiopterin (
BH4,
THB, trade name
Kuvan) or
sapropterin (
INN) is a naturally occurring essential
cofactor of the three
aromatic amino acid hydroxylase enzymes, used in the degradation of amino acid
phenylalanine and in the
biosynthesis of the
neurotransmitters serotonin (5-hydroxytryptamine, 5-HT),
melatonin,
dopamine,
norepinephrine (noradrenaline),
epinephrine (adrenaline), and is a cofactor for the production of
nitric oxide (NO) by the nitric oxide synthases. Chemically, its structure is that of a reduced
pteridine derivative.