L-Biopterin is the oxidized form of BH4, a NOS cofactor. L-Biopterin can be reduced to BH4 via thioredoxin reductase followed by dihydropteridine reductase or reduced glutathione. It is extremely toxic to human melanocytes in culture (IC50 = 0.2 µM after 48 hrs). L-Biopterin is rarely found under physiological conditions except in the epidermis of patients with the depigmentation disorder Vitiligo.