N-PYRIDYL-AMINOMETHYLENE-BISPHOSPHONIC ACIDS INHIBIT THE FIRST ENZYMEIN THE SHIKIMATE PATHWAY, 3-DEOXY-D-ARABINO-HEPTULOSONATE-7-PHOSPHATESYNTHASE

A series of seven structurally diverse N-substituted aminomethylene bi sphosphonic acids exhibiting remarkable herbicidal activity was found to inhibit the activity of the first enzyme in the prechorismate pathw ay, 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase, partially pu rified from Nicotiana plumbaginifolia suspension cultured cells. At mi llimolar concentrations all compounds inhibited the Co2+-dependent, cy tosol-localized enzyme form. However, the addition of excess divalent cations to the reaction mixture was able to completely relieve their e ffect, suggesting that the chelating properties of these compounds cou ld account for their inhibitory effect. In contrast, only five compoun ds of seven reduced significantly the activity of the plastidial and M n2+-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 a ssay mixture. A kinetic analysis showed that the latter compound inhib its enzyme activity uncompetitively with respect to phosphoeno/pyruvat e, competitively with respect to the other substrate, erythrose-4-phos phate. As manganese is a V-max stimulator of the plastidial enzyme, th is ruled out the possibility of an inhibition simply based upon metal chelation. (C) 1996 Academic Press