Inactivation of Escherichia coli glutamine synthetase by thiourea trioxide

Incubation of Escherichia coli glutamine synthetase with thiourea trioxide resulted in partial inactivation of the enzyme. This reagent specifically modifies lysine residues to form homoarginine. By amino acid analysis 2.3 ± 0.3 residues of homoarginine are produced per enzyme subunit after treatment with thiourea trioxide. Previously we determined that thiourea dioxide totally inactivated glutamine synthetase and modified both lysine and histidine residues (J. Colanduoni and J. J. Villafranca (1985) J. Biol. Chem. 260, 15,042–15,050). Thiourea trioxide reacted with the same lysine residues of glutamine synthetase as thiourea dioxide. The K_m values for the thiourea trioxide modified enzyme were determined and are 210 ± 30 μm and 10 ± 1 mm for ATP and glutamate, respectively. Both values are about threefold higher than for native enzyme assayed under the same conditions. Fluorescence titrations of native and thiourea trioxide labeled enzyme showed that ATP binding was virtually unchanged by the modification while glutamate and methionine sulfoximine bound about twofold more weakly to the modified enzyme.