Glutathione regulates nitric oxide synthase in cultured hepatocytes

Objective The authors determine the relationship between glutathione and nitric oxide (NO) synthesis in cultured hepatocytes. Summary Background Data Glutathione is a cofactor for a number of enzymes, and its presence is essential for maximal enzyme activity by the inducible macrophage nitric oxide synthase (iNOS), which produces the reactive nitric oxide radical. Hepatocytes contain substantial quantities of glutathione, and this important tripeptide is decreased in hepatocytes stressed by ischemia/reperfusion or endotoxemia. Endotoxemia also induces the synthesis of inflammatory cytokines that result in the production of nitric oxide from hepatocytes by iNOS, suggesting that hepatocytes may be attempting to synthesize nitric oxide at times when intracellular glutathione is reduced. Methods Hepatocytes were cultured with buthionine sulfoximine and 1,3-bis(chloroethyl)-1-nitrosourea (BCNU) to inhibit glutathione. After exposure to cytokines, NO synthesis was assessed by supernatant nitrite levels, cytosolic INOS enzyme activity, and iNOS mRNA levels. Results Inhibition of glutathione synthesis with buthionine sulfoximine or inhibition of glutathione reductase activity with BCNU inhibited nitrite synthesis. Both buthionine sulfoximine and BCNU inhibited the induction of iNOS mRNA, as detected by Northern blot analysis. Exogenous glutathione increased cytokine-stimulated INOS induction, overcame the inhibitory effects of BCNU, and increased nitrite production by intact hepatocytes, induced hepatocyte cytosol, and partially purified hepatocyte iNOS. Conclusions In cultured hepatocytes, adequate glutathione levels are required for optimal nitric oxide synthesis. This finding is predominantly due to an effect on INOS mRNA levels, although glutathione also participates in the regulation of iNOS enzyme activity.