Nitric oxide (NO)-releasing pathway of FK409 in the presence of sulfhydryl-bearing compounds.

We have recently reported that degradation of FK409 with generation of NO is spontaneous and is accelerated in the presence of sulfhydryl-bearing compounds, such as L-cysteine (Cys) and glutathione (GSH). The purpose of the present study is to investigate the NO-releasing pathway of FK409 in the presence of sulfhydryl-bearing compounds.The degradation process of FK409 in the presence of Cys or GSH was investigated by means of 1H-nuclear magnetic resonance (NMR) spectroscopy and high-performance liquid chromatography (HPLC).The degradation of FK409 in the presence of Cys was dependent on concentration of Cys, and showed pH-dependency, accelerating with an increase in pH. The 1H-NMR spectra of FK409 with Cys suggested that time-dependent elimination of the hydrogen atom at the alpha-position of the nitro moiety (5-position) was accelerated by Cys in weakly alkaline solution. Cys and GSH were transformed readily, concomitant with FK409 degradation, to give their oxidized forms and probably S-nitrosothiols.The effect of sulfhydryl-bearing compounds on FK409 degradation is due to the acceleration of deprotonation of the hydrogen atom at the 5-position by thiolate anion as well as hydroxyl ion. Sulfhydryl-bearing compounds reacted with the released NO resulting in formation of disulfides via intermediate S-nitrosothiols.