Ni2+, a double-acting inhibitor of neuronal nitric oxide synthase interfering with L-arginine binding and Ca2+/calmodulin-dependent enzyme activation

Ni²⁺, a toxic and carcinogenic pollutant and one of the leading causes of contact dermatitis, is shown to inhibit neuronal nitric oxide synthase (nNOS) in a competitive, reversible manner with respect to the substrate L-arginine (K_i = 30 ± 4 μM). The IC₅₀ values were dependent on calmodulin (CAM) concentration, but proved independent of Ca²⁺, tetrahydrobiopterin (BH₄) and other essential cofactors. Ni²⁺ also inhibited CAM-dependent cytochrome c reduction, NADPH oxidation, and H₂O₂ production by nNOS. Overall, the action profile of Ni²⁺ was suggestive of an unusual, double-acting inhibitor of nNOS affecting L-arginine-binding and Ca²⁺/CAM-dependent enzyme activation.