Suramin Discriminates between the Calmodulin‐Binding Sites of Neuronal and Inducible Nitric Oxide Synthase

Nitric oxide synthase (NOS) continues to be at the center of several studies as it is a formidable target for drug design. Nitric oxide (·NO), the product of the NOS reaction, is linked to a number of pathological conditions. Suramin is a polysulfonated naphthylurea that has been in use clinically for the treatment of trypanosomiasis and onchocerciasis. It has been shown to exhibit a broad range of biological actions in vivo and in vitro, and specifically has been shown to discriminate among calmodulin‐binding sites on proteins. In the current study, suramin was shown to inhibit both neuronal NOS and inducible NOS, however, it was found to have a greater effect on inducible NOS. By Dixon analysis it was demonstrated that there is approximately a sevenfold difference in the inhibitory constants of suramin for the neuronal and the inducible NOS, 13 μM and 2 μM, respectively. Dixon analysis further demonstrated that suramin is a non‐competitive inhibitor of the neuronal NOS, but an uncompetitive inhibitor of the inducible NOS. Further kinetic analysis also showed that the IC 50 for the inducible NOS , in the absence of EDTA ranges between 20 μM and 40 μM, but in the presence of 1 mM EDTA, it is 2 μM. The IC 50 for the neuronal NOS was found to be between 60 μM and 100 μM. These data suggest that suramin distinguishes between the inducible and the neuronal NOS. Western analysis further suggests that suramin's distinction of the NOS forms may occur through its interaction at the calmodulin‐binding sites. Docking studies are being conducted to assess the interactions between suramin and NOS, and will be presented. These results point to a potentially new target site on NOS for drug design.