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n. A naturally-occurring nutrient and essential cofactor of the three aromatic amino acid hydroxylase enzymes, used in the biosynthesis of the neurotransmitters serotonin, melatonin, dopamine, norepinephrine, epinephrine, and nitric oxide.

"Dudley and his colleagues knew that -- in blood vessels -- the problem was depletion of a chemical called tetrahydrobiopterin, or BH4, which is needed for the tissues to make NO."

sapropterin