OP28 Is hydrogen sulfide involved in mediating functional effects of dietary nitrate ingestion in humans?

Abstract

Leafy green vegetables, such as lettuce, contain relatively high amounts of inorganic nitrate. When such vegetables are eaten, nitrate is converted to nitrite by bacteria present in the human mouth. Nitrate/nitrite are absorbed into the circulation from the gastrointestinal system, or further reduced to NO and S-nitrosothiols. Oral nitrate lowers diastolic/systolic blood pressure and the oxygen-cost of exercise, presumably via NO. This involves the reduction of nitrite and S-nitrosothiols to NO, perhaps involving the xanthine oxidase-catalysed reduction of nitrite. However, other mammalian mechanisms of nitrite reduction have been suggested, and this is an area of controversy. Hydrogen sulfide (H2S) is a gasotransmitter which exerts physiological effects similar to NO. We examined whether H2S affects NO production from nitrite and S-nitrosothiols in simple chemical systems, and in cell culture. Using electron paramagnetic resonance (EPR) spin trapping and chemiluminescence we observed that, in the presence of H2S, both nitrite and S-nitrosoalbumin (nitrosated at the Cys-34 residue of isolated human albumin) were able to generate NO. EPR spectra of spin-trapped NO showed that the mixing of nitrite with NaSH (H2S donor) resulted in increased NO formation compared to nitrite alone, at pH 7.4. Chemiluminescence measurements confirmed that H2S was able to reduce nitrite to NO and that the reaction was thiol-dependent, being inhibited by N-ethylmaleimide. In the presence of NaSH, or the slow-releasing H2S donor GYY4137, S-nitrosothiol levels (as determined by chemiluminescence) in S-nitrosoalbumin were significantly decreased. In human aortic smooth muscle cells, the addition of nitrite in the presence of slow releasing H2S donors or endogenous H2S caused an increase in cGMP production. This study suggests a possible H2S-mediated pathway for the formation of NO after the ingestion of dietary inorganic nitrate.