Tetrahydrobiopterin, cytokines, and nitric oxide synthesis

Nitric oxide synthases require a surprisingly rich selection of cofactors to perform the conversion of L-arginine to citrulline and nitric oxide (NO): NADPH, FAD, FMN, heme and tetrahydrobiopterin. In a previous minireview in this journal we summarized work concerning the induction of tetrahydrobiopterin biosynthesis by cytokines, which yields increased intracellular tetrahydrobiopterin concentrations supporting NO formation by intact cells (P.S.E.B.M. 203:1-12). The present review updates work on the induction of tetrahydrobiopterin biosynthesis by cytokines, and summarizes recent advances in research of tetrahydrobiopterin dependence of the NO synthase reaction. Studies using recombinant NO synthases and site-directed mutations thereof have localized several amino acids critical for tetrahydrobiopterin binding, which are discussed in reference to the recently published crystal structure of the dimer of the oxygenase domain of murine inducible NO synthase with substrate and pterin. Allosteric actions of tetrahydrobiopterin on NO synthases are stabilization of dimers, stabilization of a conformation with high-spin heme iron, and support of binding of the substrate L-arginine. Since the 4-amino analog of tetrahydrobiopterin, which is a dihydropteridine reductase inhibitor, supports these allosteric actions but inhibits the enzyme activity, tetrahydrobiopterin appears to play a redox-active role in stimulating the NO synthase reaction in addition to its allosteric actions on NO synthases. Amelioration of endothelial dysfunction by tetrahydrobiopterin in animal models and in humans in vivo has been observed. It remains to be investigated, however, to what extent the role of tetrahydrobiopterin as cofactor of NO synthases contributes to these in vivo effects of tetrahydrobiopterin.