NITRIC-OXIDE INACTIVATES RAT HEPATIC METHIONINE ADENOSYLTRANSFERASE IN-VIVO BY S-NITROSYLATION

We investigated the mechanism of nitric oxide (NO) action on hepatic m ethionine adenosyltransferase (MAT) activity using S-nitrosoglutathion e (GSNO) as NO donor. Hepatic MAT plays an essential role in the metab olism of methionine, converting this amino acid into S-adenosylmethion ine, Hepatic MAT exists in two oligomeric states: as a tetramer (MAT I ) and as a dimer (MAT III) of the same subunit, This subunit contains 10 cysteine residues, In MAT I, S-nitrosylation of 1 thiol residue per subunit was associated with a marked inactivation of the enzyme (abou t 70%) that was reversed by glutathione (GSH). In MAT III, S-nitrosyla tion of 3 thiol residues per subunit led to a similar inactivation of the enzyme, which was also reversed by GSH, Incubation of isolated rat hepatocytes with S-nitrosoglutathione monoethyl ester (EGSNO), a NO d onor permeable through the cellular membrane, induced a dose-dependent inactivation of MAT that was reversed by removing the NO donor from t he cell suspension. MAT, purified from isolated rat hepatocytes, conta ined S-nitrosothiol groups and the addition of increasing concentratio ns of EGSNO to the hepatocyte suspension led to a progressive S-nitros ylation of the enzyme. Removal of the NO donor from the incubation med ia resulted in loss of most NO groups associated to the enzyme. Finall y, induction in rats of the production of NO, by the administration of bacterial lipopolysaccharide (LPS), induced a fivefold increase in th e S-nitrosylation of hepatic MAT, which led to a marked inactivation o f the enzyme. Thus, the activity of liver MAT appears to be regulated in vivo by S-nitrosylation.