Tetrahydrobiopterin (BH4) has been also known as sapropterin. BH4 is an essential cofactor of the three aromatic amino acid hydroxylase enzymes which has used in the degradation of amino acid phenylalanine and in the biosynthesis of the neurotransmitters serotonin (5-HT), melatonin, norepinephrine, dopamine, epinephrine (adrenaline). BH4 is a cofactor of nitric oxide synthases [2].
L-Biopterin was reduced to BH4 via thioredoxin reductase followed by dihydropteridine reductase or reduced glutathione. L-Biopterin was extremely toxic to human melanocytes in culture with the IC50 of 0.2 µM after 48 hrs. L-Biopterin was rarely existed under physiological conditions except in the