Inhibition of nitric oxide synthase isoforms by tris-malonyl-C(60)-fullerene adducts

C(3)-tris-malonyl-C(60)-fullerene and D(3)-tris-malonyl-C(60)-fullerene derivatives inhibit citrulline and NO formation by all three nitric oxide synthase isoforms in a manner fully reversible by dilution. The inhibition of citrulline formation by C(3)-tris-malonyl-C(60)-fullerene occurs with IC(50) values of 24, 17, and 123 microM for the neuronal, endothelial, and inducible nitric oxide synthase (NOS) isoforms, respectively. As measured at 100 microM l-arginine, neuronal NOS-catalyzed nitric oxide formation was inhibited 50% at a concentration of 25 microM C(3)-tris-malonyl-C(60)-fullerene. This inhibition was a multisite, positively cooperative inhibition with a Hill coefficient of 2.0. C(3)-tris-malonyl-C(60)-fullerene inhibited the arginine-independent NADPH-oxidase activity of nNOS with an IC(50) value of 22 microM but had no effects on its cytochrome c reductase activity at concentrations as high as 300 microM. The inhibition of nNOS activity by C(3)-tris-malonyl-C(60)-fullerene reduced the maximal velocity of product formation but did not alter the EC(50) value for activation by calmodulin. C(3)-tris-malonyl-C(60)-fullerene reduced the maximal velocity of citrulline formation by inducible NOS without altering the K(m) for l-arginine substrate or the EC(50) value for tetrahydrobiopterin cofactor. As measured by sucrose density gradient centrifugation, fully inhibitory concentrations of C(3)-tris-malonyl-C(60)-fullerene did not produce a dissociation of nNOS dimers into monomers. These observations are consistent with the proposal that C(3)-tris-malonyl-C(60)-fullerene inhibits the inter-subunit transfer of electrons, presumably by a reversible distortion of the dimer interface.