Purification and properties of glutamine synthetase from the cellular slime mould Dictyostelium discoideum

Glutamine synthetase (GS) from the cellular slime mould Dictyostelium discoideum was purified to apparent electrophoretic homogeneity with a final yield of 21.7%. The native enzyme appeared to be a GS-II type enzyme. SDS-PAGE of the final preparation revealed a single band of 43.5 kDa. The enzyme has a native molecular mass of 376 kDa, determined using Superose 6, indicating that the enzyme is likely to be an octamer of identical subunits. Dictyostelium discoideum GS has an optimal temperature of 42.5 degrees C, although it is thermolabile in the absence of L-glutamate and (or) Mg(2+)-ATP. The enzyme exhibits a K(m) for L-glutamate, ATP, and NH4Cl of 2.18, 0.18, and 0.11 mM, respectively, in the L-glutamine synthetic reaction with an optimal pH of 7.9. GS from D. discoideum does not appear to be significantly inhibited by various end products of L-glutamine metabolism, although it is potently inhibited by methionine sulphoximine. These properties are those expected for an enzyme for which the primary function is the assimilation of ammonia.