Tetrahydrobiopterin (THB)

Tetrahydrobiopterinis a naturally occurring nutrient and essential cofactor of the three aromatic amino acid hydroxylase enzymes, used in the biosynthesis of the neurotransmitters serotonin (5-hydroxytryptamine (5-HT)), melatonin, dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline), and nitric oxide (NO).

A deficit in THB biosynthesis and/or regeneration results in phenylketonuria (PKU) from excess L-phenylalanine concentrations or hyperphenylalaninemia (HPA), as well as monoamine and nitric oxide neurotransmitter deficiency or chemical imbalance. The chronic presence of PKU can result in severe brain damage, including symptoms of mental retardation, microcephaly, speech impediments such as stuttering, slurring, and lisps, seizures or convulsions, and behavioral abnormalities, among other effects.