In vivo regulation by glutathione of methionine adenosyltransferase S-nitrosylation in rat liver

Background/Aims: Ethanol consumption and pathological conditions such as ci rrhosis lead to a reduction of hepatic glutathione, Hepatic methionine aden osyltransferase, the enzyme that synthesizes S-adenosylmethionine, the majo r methylating agent, is regulated in vivo by glutathione levels. We have pr eviously shown that nitric oxide inactivates methionine adenosyltransferase in vivo by S-nitrosylation, In this study, we aimed to investigate the reg ulation by glutathione of methionine adenosyltransferase S-nitrosylation in rat liver. Methods: Rat hepatocytes and whole animals were treated with buthionine sul foximine, an inhibitor of glutathione synthesis, and methionine adenosyltra nsferase S-nitrosylation and activity were determined, Results: In hepatocytes, buthionine sulfoximine led to the S-nitrosylation and inactivation of methionine adenosyltransferase. Restoring glutathione l evels in hepatocytes treated with buthionine sulfoximine, by the addition o f glutathione monoethyl ester, a permeable derivative of glutathione, led t o the denitrosylation and reactivation of methionine adenosyltransferase, I n whole animals, buthionine sulfoximine led also to methionine adenosyltran sferase S-nitrosylation and inactivation. S-Nitrosylation and inactivation of methionine adenosyltransferase induced by buthionine sulfoximine in whol e animals was prevented by glutathione monoethyl ester, Conclusions: These results indicate that in vivo hepatic methionine adenosy ltransferase exists in two forms in equilibrium, nitrosylated (inactive) an d denitrosylated (active), which are regulated by both the cellular levels of nitric oxide and glutathione.