GLUTAMINE SYNTHETASE. V. DEPENDENCE OF ITS SULFHYDRYL REQUIREMENT ON ORGANIC LIGANDS AND METAL IONS.

A partially purified preparation of glutamine synthetase (l-glutamate: ammonia ligase (ADP), EC 6.3.1.2) has been obtained from rat liver. The enzyme preparation showed no activity in the absence of Mg2+; Mn2+ could not replace Mg2+. Mn2+ exhibited a slight activation of the enzyme at low Mg2+ concentrations, but a pronounced inhibition at high Mg2+ concentrations. The inhibition by a number of metal ions including Fe2+, Fe3+, Co2+, Cu+, Zn2+, Cd2+, and Hg2+ depended on the concentration of a thiol. The enzyme showed a partial requirement for a thiol, which could be abolished by certain organic ligands. Among them the most effective were EDTA, hydroxyethyl-ethylenediaminetriacetate, nitrilotriacetate, and 1,10-phenanthroline.