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Metabolism and Enzymology

Inhibition of Plant Glutamine Synthetases by Substituted Phosphinothricins

Monsanto Agricultural Company, A Unit of Monsanto Company, St. Louis, Missouri 63198

Glutamine synthetase (GS) utilizes various substituted glutamic acids as substrates. We have used this information to design herbicidal - and -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 GS₁ and chloroplastic GS₂ 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 GS₁ and GS₂ 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.