The effect of divalent cations on neuronal nitric oxide synthase activity

Neuronal nitric oxide synthase (NOS I) is a Ca²⁺/calmodulin-binding enzyme that generates nitric oxide (NO•) and L-citrulline from the oxidation of L-arginine, and superoxide (O₂•^-) from the one-electron reduction of oxygen (O₂). Nitric oxide in particular has been implicated in many physiological processes, including vasodilator tone, hypertension, and the development and properties of neuronal function. Unlike Ca²⁺, which is tightly regulated in the cell, many other divalent cations are unfettered and can compete for the four Ca²⁺ binding sites on calmodulin. The results presented in this article survey the effects of various divalent metal ions on NOS I-mediated catalysis. As in the case of Ca²⁺, we demonstrate that Ni²⁺, Ba²⁺, and Mn²⁺ can activate NOS I to metabolize L-arginine to L-citrulline and NO•, and afford O₂•^- in the absence of L-arginine. In contrast, Cd²⁺ did not activate NOS I to produce either NO• or O₂•^-, and the combination of Ca²⁺ and either Cd²⁺, Ni²⁺, or Mn²⁺ inhibited enzyme activity. These interactions may initiate cellular toxicity by negatively affecting NOS I activity through production of NO•, O₂^- and products derived from these free radicals.