Dinoseb inhibition

The distinctly different behavior of the phenol-type herbicides following trypsin treatment suggests that different determinants within the PS II protein complex establish the "domains" that regulate the binding properties of these inhibitors. In spite of the fact that phenol-type herbicides will displace bound radiolabeled herbicides such as diuron, these inhibitors show noncompetitive inhibition (29, 30). At present, there are three lines of evidence which favor TH e involvement of two domains within the PS II complex that participate in creating the binding sites for these herbicides (a) isolated PS II particles can be selectively depleted of a polypeptide with parallel loss of atrazine sensitivity, but not dinoseb inhibition activity (33) (b) in resistant weed biotypes, chloroplast membranes that exhibit extreme triazine resistance have increased sensitivity to the phenol-type herbicides (13) and (c) experiments with azido (photoaffinity) derivatives of phenol and triazine herbicides result in the covalent labeling of different PS II polypeptides (, 31). 

The phenolic derivatives indicated in Figure 8.1 are also bound to the same binding niche on PS II as the triazines (Oettmeier, 1992). However, they have a somewhat different inhibition pattern than the classical family of PS II herbicides (e.g., triazines and ureas) and, therefore, were regarded as a separate family with a somewhat different mode of action (Van Rensen et al., 1978 Trebst and Draber, 1986). It is now clear that they just orient somewhat differently in the same binding niche, as discussed below. Although the phenolics are photosynthesis inhibitors, dinoseb and the halogenated benzonitriles also inhibit respiration. 

Dinitro-6-sec-butylphenol (the pesticide Dinoseb), p-nitrophenol, and o-nitrophenol inhibit the metabolism of 2,4-DNP (Eiseman etal. 1974). O-nitrophenol is a competitive inhibitor and is metabolized by the same enzyme as 2,4-DNP. 2,4-Dinitro-6-sec-butylphenol and p-nitrophenol are non-competitive inhibitors of 2,4-DNP metabolism. If an individual is exposed simultaneously to... 

Environmental pollution by toxic chemicals has become one of the worlds most serious problems. Among the most widespread pesticides is photosynthesis inhibiting herbicides, such as atrazine, metribuzin, diuron, bromacil, ioxynil and dinoseb. They all beloi to different families but have a common mode of action binding specifically to the chloroplast D1 protein with subsequent intetmption of the electron and proton flow through Photosystem II. The goal of this chapter is to evaluate the possibility of application of the natural receptor properties of D1 protein in various biosensor systems for herbicide detection. 

The above results imply that DCMU-type inhibitors (Group 1), have one site of action on or near the B-protein" complex that is located upon the external surface of the photosynthetic membrane. They specifically bind (non-specific binding is not taken into account) to this protein. On the other hand, ioxynil and i-dinoseb (Group 2) seem to affect another site of action on the O2 evolving system. This is located presumably on the inside of the thylakoid membrane. These two inhibitors do not lose their inhibitory potency towards electron transport because a part of their activity lies in an area that is not easily accessible to trypsin. The Group 2 inhibitors also inhibit silicomolybdate-mediated O2 evolution (data not shown). This reaction is essentially insensitive to DCMU (18) and DCMU-like inhibitors (, 16). 

Figure 1. Representative traces of absorbance changes that show inhibition by dinoseb of valinomycin-induced swelling of intact spinach chloroplasts (B), spinach thylakoids (C), and mung bean mitochondria (D) suspended in isotonic KSCN. The model system is presented diagrammatically in A. Swelling was initiated by the addition of 0.1 /xM valinomycin (VAL),...

Dose/response curves were developed from traces such as those shown in Figure 2 for dinoseb. For comparative purposes, the concentration of compound required to induce an increase in the swelling rate of 0,02 A in 1 min relative to the no-herbicide controls is shown in Table V for chloroplasts, thylakoids, and mitochondria, Thylakoids did not swell as extensively as the other two organelles, consequently, the values are for a change of 0,01 A, The relative order of activity shown by the compounds is similar to that given in Table IV for inhibition of valinomycin-induced swelling in the three organelles. 

Our results on inhibition of herbicide binding activity agree with the involvement of the Qb site in photoinhibition. In fact the binding curves correlates with the loss of electron transport activity in the first 30 min. The best correlation was observed with the herbicide loxynil. Atrazine and Dinoseb showed a residual binding even when Di protein was not detectable by immunoblotting (data not shown) indicating non-specific binding or the involvement of other proteins in their association with thylakoids. This is in accordance with previous observations (3). 

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