In Vivo Studies on the Interaction Between Orally Administered Nitrite and Omeprazole: Beyond Proton-Catalyzed S-Nitrosation.

Inorganic nitrite contributes to the nitrosation of biomolecules and exerts antioxidant effects. The proton pump inhibitor omeprazole has pro-oxidant effects, inhibits the formation of nitroso species in the stomach, and abrogates the blood pressure-lowering effects of orally administered nitrite. Here, we examine whether a two-week treatment with nitrite leads to tissue nitrosation that scales with local thiol concentrations and whether oral nitrite treatment can prevent the pro-oxidant effects of omeprazole. Male Sprague-Dawley rats received daily doses of omeprazole 10 mg/kg i.p. (or vehicle) and sodium nitrite 15 mg/kg by gavage (or water) for 14 days. Animals were euthanized 6 h after the last nitrite dose, and blood and tissues (brain, heart, and liver) were collected for biochemical analyses. We found that nitrite treatment increased liver nitrite and total nitroso species (RxNO) concentrations approximately eight-fold (with minor increases in other organs), and omeprazole treatment attenuated these effects. Nitrite treatment selectively elevated non-protein thiol concentrations in the liver, but not in animals also receiving omeprazole. Tissue thiol elevation was associated with increased nitrosation of hepatic proteins, which was prevented by omeprazole. Nitrite upregulated mRNA expression of microsomal glutathione S-transferase-1 (Mgst1) and decreased superoxide and hydrogen peroxide production, especially in rats co-treated with omeprazole. While omeprazole increased liver xanthine oxidoreductase (XOR), nitrite treatment attenuated this effect. These results demonstrate that oral nitrite treatment robustly elevates nitrite and RxNO concentrations in the liver, and these effects are associated with increased hepatic glutathione production and an upregulation of Mgst1 expression, counteracting the pro-oxidant effects induced by omeprazole.