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Treatment of unadenylylated glutamine synthetase from Escherichia coli with tetranitromethane or with N-acetylimidazole produces alterations in catalytic parameters that are similar to alterations caused by the physiologically important process of adenylylation. All three modification reactions lead to a change in divalent ion requirement for biosynthetic activity; the unmodified enzyme requires Mg(2+) for activity, whereas the modified enzymes exhibit increased activity with Mn(2+). The gamma-glutamyl transferase activity of the modified enzyme is more sensitive to feedback inhibition by tryptophan, histidine, CTP, and AMP, and to inhibition by Mg(2+) or to inactivation by 5 M urea. Finally, the pH optimum for the unmodified enzyme is 7.9, while the modified enzymes are more active at pH 6.8. Since treatment of the enzyme with N-acetylimidazole results in a decrease in absorbancy at 278 mmu and treatment with tetranitromethane causes an increase in absorbancy at 428 mmu, the effects of these reagents are probably due to modification of certain tyrosyl groups on the enzyme. However, other evidence indicates that the tyrosyl residues which are susceptible to adenylylation in the adenylyltransferase-catalyzed reaction are not involved in the acetylation or nitration reactions.

Ability of Nonenzymic Nitration or Acetylation of E. coli Glutamine Synthetase to Produce Effects Analogous to Enzymic Adenylylation