(6S)-5,6,7,8-tetrahydro-L-Biopterin is a diastereomer of (6R)-5,6,7,8-tetrahydro-L-biopterin , a cofactor for tyrosine, tryptophan, and phenylalanine hydroxylases and nitric oxide synthase (NOS).^1,2,3,4

1.Ishii, A., Hagihara, M., Matsuura, S., et al.Effect of (6R)- and (6S)-tetrahydrobiopterin on L-3,4-dihydroxyphenylalanine (DOPA) formation in NRK fibroblasts transfected with human tyrosine hydroxylase type 2 cDNANeurochem. Int.17(4)625-632(1990) 2.Kappock, T.J., and Caradonna, J.P.Pterin-dependent amino acid hydroxylasesChem. Rev.96(7)2659-2756(1996) 3.Mayer, B., and Werner, E.R.In search of a function for tetrahydrobiopterin in the biosynthesis of nitric oxideN.-S. Arch. Pharmacol.351(5)453-463(1995) 4.Kraft, V.A.N., Bezjian, C.T., Pfeiffer, S., et al.GTP cyclohydrolase 1/tetrahydrobiopterin counteract ferroptosis through lipid remodelingACS Cent. Sci.6(1)41-53(2020)