CARBON-DIOXIDE STIMULATES PEROXYNITRITE-MEDIATED NITRATION OF TYROSINE RESIDUES AND INHIBITS OXIDATION OF METHIONINE RESIDUES OF GLUTAMINE-SYNTHETASE - BOTH MODIFICATIONS MIMIC EFFECTS OF ADENYLYLATION

The activity of glutamine synthetase (EC 6.3.1.2) from Escherichia col i is regulated by the cyclic adenylylation and deadenylylation of Tyr- 397 in each of the enzyme's 12 identical subunits. The nitration of Ty r-397 or of the nearby Tyr-326 by peroxynitrite can convert the unaden ylylated enzyme to a form exhibiting regulatory characteristics simila r to the form obtained by adenylylation. The adenylylated conformation can also be elicited by the oxidation of surface-exposed methionine r esidues to methionine sulfoxide. However, the nitration of tyrosine re sidues and the oxidation of methionine residues are oppositely directe d by the presence and absence of CO2. At physiological concentrations of CO2, pH 7.4, nitration occurs but oxidation of methionine residues is inhibited. Conversely, in the absence of CO2 methionine oxidation i s stimulated and nitration of tyrosine is prevented, It was further es tablished that adenylylation of Tyr-397 precludes its nitration by per oxynitrite. Furthermore, nitration of Tyr-326 together with either nit ration or adenylylation of Tyr-397 leads to inactivation of the enzyme . These results demonstrate that CO2 can alter the course of peroxynit rite-dependent reactions and serve notice that (i) the reactions have physiological significance only if they are shown to occur at physiolo gical concentrations of CO2 and physiological pH; and (ii) the peroxyn itrite-dependent nitration of tyrosine residues or the oxidation of me thionine residues of metabolically regulated proteins can seriously co mpromise their biological function.