The reaction between GSNO and H₂S: On the generation of NO, HNO and N₂O

Several recent reports suggest that HNO may be produced endogenously by reaction of H₂S and S-nitrosoglutathione (GSNO). This hypothesis was tested using deoxymyoglobin (MbFe^II) to trap the expected HNO released from the target reaction, which should generate the stable HNO adduct, HNO-Mb, under anaerobic conditions. Under numerous experimental conditions, the sole globin product was NO-Mb, as characterized by absorbance, EPR, and NMR spectroscopies. Analogous reactions of GSNO with other biological reductants such as ascorbic acid, dithiothreitol, glutathione, and dithionite also yielded NO-Mb as the sole globin product; however, whereas analogous reduction of GSNO using NaBH₄ generates HNO-Mb in high yield. Quantitative GC/MS analyses of reactions of GS¹⁵NO with H₂S showed that the main reaction product was ¹⁵NO, with ¹⁵N₂ produced at a comparable level to ¹⁵N₂O. Overall yield of N₂O is unchanged by the presence of MbFe^II, discounting the intermediacy of either NO or HNO in its formation. Taken together, these results argue against the generation of free HNO as a major pathway in the reactions of GSNO with H₂S, and instead imply some as yet uncharacterized intermediates generate the nitrogenic gases.