Synthesis and thylakoid membrane binding of the radioactively labeled herbicide dinoseb

Abstract

2,4-Dinitro-6-isobutylphenol (i-dinoseb), an isomer of the phenolic herbicide dinoseb and equally active as an inhibitor of photosynthetic electron transport and photophosphorylation, has been synthesized, 3H-labeled with a specific activity of 490 mCi/mmol. Its binding to broken chloroplasts is strongly pH dependent and biphasic representing a high- and a low-affinity binding site. For specific binding of i-dinoseb a binding constant K b = 6.9 × 10 −8 M has been determined. The number of binding sites corresponds to one molecule i-dinoseb per 830 molecules of chlorophyll, i.e., one molecule per two electron transport chains. i-Dinoseb can be displaced from the thylakoid membrane by DCMU-type inhibitors, and inhibitory uncouplers, but not by DBMIB-type inhibitors and uncouplers of oxidative phosphorylation. An extensive analysis of displacement by DCMU-type and phenolic herbicides indicates that DCMU-type herbicides interfere noncompetetively but phenolic herbicides interfere competetively with the specific binding of i-Dinoseb. It is concluded, therefore, that the binding sites of both types of herbicides are not identical although they are located on the same protein. The specific binding constant of i-dinoseb does not change in trypsin-treated chloroplasts whereas the number of binding sites is slightly reduced.