Binding affinities of peroxidizing herbicides to protoporphyrinogen oxidase from corn
1995
Nicolaus, B. | Johansen, J.N. | Boger, P.
The binding of the 14C-labeled herbicide thidiazimin (SN 124 085) to membrane-bound protoporphyrinogen oxidase (protox) from corn (Zea mays) and its displacement by the diphenyl ethers acifluorfen (AF), acifluorfen methyl, and oxyfluorfen were examined and compared to their inhibitory activity on protox. SN 124 085 is a potent inhibitor of protox comparable to oxyfluorfen and acifluorfen methyl, all with I50 values in the nanomolar range. SN 124 085 is a competitive inhibitor with respect to the substrate protoporphyrinogen having an inhibition constant (Ki) of 0.25 nM. Using 14C-labeled SN 124 085, a binding constant Kb of 0.26 nM was obtained. The ability of acifluorfen, acifluorfen methyl, and oxyfluorfen to strictly compete with SN 124 085 was used to examine the binding affinity of the diphenyl ether-type herbicides to protox. All three inhibitors displaced SN 124 085 competitively, acifluorfen methyl being most effective. Secondary plots of the binding of SN 124 085 with different concentrations of the competing nonlabeled inhibitor present yielded the binding constant of the latter. While the potent herbicides acifluorfen methyl and oxyfluorfen showed Kb constants below 1 nM, the binding affinity of AF was found to be 10-fold lower, according to its lower inhibitory activity. Binding constants of all inhibitors were found to be similar to their inhibition constant, demonstrating displacement studies as a reliable technique to determine the binding affinity of protox inhibitors.
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