Inhibition of plant glutamine synthetases by substituted phosphinothricins

Glutamine synthetase (GS) utilizes various substituted glutamic acids as substrates. We have used this information to design herbicidal alpha- and gamma-substituted analogs of phosphinothricin (l-2-amino-4-(hydroxymethylphosphinyl)butanoic acid, PPT), a naturally occurring GS inhibitor and a potent herbicide. The substituted phosphinothricins inhibit cytosolic sorghum GS(1) and chloroplastic GS(2) competitively versusl-glutamate, with K(i) values in the low micromolar range. At higher concentrations, these inhibitors inactivate glutamine synthetase, while dilution restores activity through enzyme-inhibitor dissociation. Herbicidal phosphinothricins exhibit low K(i) values and slow enzyme turnover, as described by reactivation characteristics. Both the GS(1) and GS(2) isoforms of plant glutamine synthetase are similarly inhibited by the phosphinothricins, consistent with the broad-spectrum herbicidal activity observed for PPT itself as well as other active compounds in this series.