2-Thiouracil is a selective inhibitor of neuronal nitric oxide synthase antagonising tetrahydrobiopterin-dependent enzyme activation and dimerisation.

2-Thiouracil (TU), an established antithyroid drug and melanoma-seeker, was found to selectively inhibit neuronal nitric oxide synthase (nNOS) in a competitive manner (K-i = 20 muM), being inactive on the other NOS isoforms, The drug apparently interfered with the substrate- and tetrahydrobiopterin (BH4)-binding to the enzyme. It caused a 60% inhibition of H2O2 production in the absence of L-arginine and BH4, and antagonised BH4-induced dimerisation of nNOS, but did not affect cytochrome c reduction. These results open new perspectives in the understanding of the antithyroid action of TU and provide a new lead structure for the development of selective nNOS inhibitors to elucidate the interdependence of the substrate and pteridine sites and to modulate pathologically aberrant NO formation.