N-pyridyl-aminomethylene-bisphosphonic acids inhibit the first enzyme in the shikimate pathway, 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase

A series of seven structurally diverse N-substituted aminomethylene bisphosphonic acids exhibiting remarkable herbicidal activity was found to inhibit the activity of the first enzyme in the prechorismate pathway, 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase, partially purified from Nicotiana plumbaginifolia suspension cultured cells. At millimolar concentrations all compounds inhibited the Co2+-dependent, cytosol-localized enzyme form. However, the addition of excess divalent cations to the reaction mixture was able to completely relieve their effect, suggesting that the chelating properties of these compounds could account for their inhibitory effect. In contrast, only five compounds of seven reduced significantly the activity of the plastidial and Mn2+-stimulated isoform. The inhibition brought about by N-2-(6-methyl-pyridyl)- and N-2-(5-chloro-pyridyl)-aminomethylene bisphosphonic acid could not be relieved by raising the manganese concentration in the assay mixture. A kinetic analysis showed that the latter compound inhibits enzyme activity uncompetitively with respect to phosphoeno/pyruvate, competitively with respect to the other substrate, erythrose-4-phosphate. As manganese is a V-max stimulator of the plastidial enzyme, this ruled out the possibility of an inhibition simply based upon metal chelation. (C) 1996 Academic Press