6-biopterin is extremely cytotoxic to human melanocytes under in vitro conditions. Thioredoxin reductase has the capacity to reduce 6-biopterin to q-BH2 where further reduction to 6-BH4 follows via dihydropteridine reductase or reduced glutathione. (6R)5,6,7,8 tetrahydrobiopterin undergoes redox-cycling by its oxidation to quinonoid dihydrobiopterin and to 6-biopterin through consecutive two electron oxidation reactions.