- Tetrahydrobiopterin biosynthetic activities in human macrophages, fibroblasts, THP-1, and T 24 cells. GTP-cyclohydrolase I is stimulated by interferon-gamma, and 6-pyruvoyl tetrahydropterin synthase and sepiapterin reductase are constitutively present. Werner, E.R., Werner-Felmayer, G., Fuchs, D., Hausen, A., Reibnegger, G., Yim, J.J., Pfleiderer, W., Wachter, H. J. Biol. Chem. (1990)