Thermal stability of poplar GS holoenzymes. Samples (50 μg) of purified GS2, GS1.1, GS1.2 and GS1.3 native isoenzymes were incubated at different temperatures: ◆ 37°C; ∎ 42°C; ▲ 50°C; ● 60°C. At the indicated periods of incubation (0, 5, 10, 15 and 20 min) samples were removed from the bath and stored on ice until GS activity was determined. The maximum level (100%) of activity for GS1.1, GS1.2, GS1.3 and GS2 was 38 nkatal, 56nkatal, 48 nkatal and 25 nkatal, respectively. Values are the mean ± SD of at least three independent determinations.