The Activation of Neuronal Nitric-Oxide Synthase by Various Divalent Cations

Abstract

Nitric-oxide synthase (NOS; EC 1.14.13.39) catalyzes the oxidation ofl-arginine to nitric oxide (NO^⋅) andl-citrulline via the intermediateN^ω-hydroxy-l-arginine. Of the three distinct isoforms of NOS that have been characterized, the constitutive neuronal NOS (NOS I) generates NO^⋅associated with long-term potentiation (LTP) and early brain development. All of the NOS isoforms contain an N-terminal oxidase and a C-terminal reductase domain connected by a Ca²⁺/calmodulin binding region. To activate NOS I, Ca²⁺ has to bind to calmodulin, allowing electron transport through both domains. Calcium ions are tightly regulated in cells. However, a number of other metal ions that bind and activate calmodulin may also activate NOS I. One such metal ion may be Pb²⁺, which is associated with neurobehavioral and psychological alterations, including the inhibition of LTP. The effect of various divalent cations on NOS I activity was tested, and the results presented herein demonstrate that Pb²⁺ and Sr²⁺ can activate NOS I to a level similar to that found for Ca²⁺. Finally, there is a synergy between Pb²⁺ and Ca²⁺ resulting in maximal activation of NOS I using minimal concentrations of both metal ions.