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 (O-2.(-)) from the one-electron reduction of oxygen (O-2). 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 O-2.(-) in the absence of L-arginine. In contrast, Cd2+ did not activate NOS I to produce either NO. or O-2.(-), 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., O-2.(-) and products derived from these free radicals.