Purified glutamine synthetase (GS) from the leaves of a rainforest tree, Bauerella simplicifolia exhibited Mg2+-dependent activity which was completely inhibited by 5 mM MnSO4. No shift in the pH optimum for the Mg2+-dependent enzyme was observed with Mn2+. AMP, ADP, NO3− and NO2− also inhibited GS activity, although at low concentrations NO3− and NO2− stimulated GS. Using the Mg2+-dependent assay, the Km for NH4+ was about 25 × 10−4 M. The Km for glutamate was 3 mM whilst the Km for ATP was lower at 0·5 mM.
With the exception of Bauerella simplicifolia and Argyrodendron trifoliolatum, GS activity in leaves of a variety of rainforest plants was very low or essentially zero.