Electrochemical investigation of the role of reducing agents in copper-catalyzed nitric oxide release from S-nitrosoglutathione

Studies of nitric oxide (NO) release from S-nitrosoglutathione (GSNO) decomposition by Cu2+ in the presence of reducing agents were performed using a nickel porphyrin and Nafion-coated microsensor in order to compare the efficiency of sodium hydrosulfite (Na2S2O4) and sodium borohydride (NaBH4) to that of the most abundant endogenous reducer, glutathione (GSH). When it was mixed to Cu(NO3)(2) and added to equimolar concentration of GSNO, each reducing agent caused a NO release (measured in terms of oxidation current) but only NaBH4 induced a proportional rise if its concentration doubled and that of Cu2+ remained constant. For Na2S2O4, there was a mild increase and for GSH, no change. Furthermore, when Cu2+ concentrations ranging from 0.5 to 5 mu M were mixed with 2 mu M reducing agent and added to 2 mu M GSNO the NO oxidation current linearly increased with NaBH4 and was constant with Na2S2O4, Concerning GSH, Cu2+ dose-dependently increased the NO release from GSNO only if the Cu2+-to-reducer ratio was < 1. However, GSH formed the catalytic species Cu+ even in excess Of Cu2+ and GSNO as indicated by suppression of the Cu2+/GSH-induced NO release when the Cu+ chelator neocuproine was added to GSNO. This work shows that, among the 3 reducing agents, only NaBH4 allows Cu2+ to dose-dependently increase the NO release from GSNO for Cu2+-to-reducer ratios ranging from 0.25 to 2.5. Despite this good effectiveness, excess of NaBH4 compared to both Cu2+ and GSNO seems to be required for optimal NO release.