The role of thiol and nitrosothiol compounds in the nitric oxide-forming reactions of the iron-N-methyl-D-glucamine dithiocarbamate complex

Abstract

The object of the present study is to investigate whether the physiologically dominant thiol compounds such as GSH and cysteine or their nitrosothiol compounds affect the formation of the iron-N-methyl-D-glucamine dithiocarbamate [(MGD)2Fe2+]nitric oxide complex. The present study provided experimental evidence that physiological concentrations of GSH (approx. 5 mM) and L-cysteine (approx, 0.5 mM) accelerated the formation of the (MGD)2Fe2+-NO complex from nitrite by two and three times respectively. The rate constants for the reduction of (MGD)3Fe3+ to (MGD)2Fe2+ by GSH and cysteine were calculated as 1.3 and 2.0 × 102 M-1 · s-1 respectively. Furthermore, depletion of GSH was demonstrated in PC12 cells, and thiol compounds enhanced the formation of reactive oxygen species by the (MGD)2Fe2+ complex by accelerating its redox turnover. The main effect of the physiological concentration of thiols was the reduction of (MGD)3Fe3+. S-nitrosoglutathione spontaneously reacted with (MGD)2Fe2+ to produce the (MGD)2 Fe2+-NO complex with a 1 : 2 stoichiometry. In fact, (MGD)2Fe2+ was as good an indicator of nitrosothiols as it was of NO itself. The present study elucidates the difficulties of utilizing the (MGD)2Fe2+ complex for the quantification of NO in biological samples, especially in vivo.