Thiamethoxam binding

Neonicotinoids do not act as a homogenous class of insecticides. Radioligand receptor binding assays revealed two classes of neonicotinoids described here as competitive and non-competitive , respectively, relative to [ H]imidacloprid. Differences in affinity, mode of displacement, number of binding sites and temperature sensitivity suggest that thiamethoxam binds in a way unique among the commercial neonicotinoids. Metabolic transformation is not relevant for its insecticidal effects. 

Thiamethoxam Binding is Sensitive to Temperature Under In Vitro Conditions... 

Binding experiments revealed unusual properties of thiamethoxam at the target site (77). The optimal assay conditions to demonstrate thiamethoxam binding to aphid membranes are different and more strictly deflned than those for imidacloprid. Highest specific binding of thiamethoxam is observed at low assay temperature (2 C) with freshly prep ed membranes. Furthermore, preparations stored frozen prior to the assay are as good as fresh ones for studies with imidacloprid but not with thiamethoxam. 

Our studies with aphids clearly suggest that thiamethoxam, like the other examined neonicotoinoids, binds to nicotinic receptors. However, there are clear differences to the other commercial neonicotinoids as documented by a kinetic analysis of competition experiments. While thiamethoxam binds to receptors with nanomolar affinity, micromolar concentrations are required to displace imidacloprid. Further, the interaction between the two compounds is noncompetitive meaning that binding of thiamethoxam reduces the binding capacity of the receptor preparation for imidacloprid but not its affinity. Thiamethoxam shares this unusual mode of inhibition with other neonicotinoids (not commercialized) also featured by an W-Methyl group in die pharmacophore. In the competitive mode, displayed by the other commercial neonicotinoids, the capacity is unchanged, while the affinity is reduced. 

The toxicokinetics of the thianicotinyl thiamethoxam is similar to that of imidacloprid. When applied orally to rats, goats, or chickens, thiamethoxam is rapidly and almost quantitatively absorbed. Its excretion, predominantly in urine, is fast. Accumulation in tissues is negligible. Thiamethoxam itself does not bind strongly to the neonicotinoid binding site of the nicotinic acetylcholine receptor but it is reported to be converted to clothianidin, a neonicotinoid with high affinity for the insect receptors, in insects and plants (Figure 6). It is possible that this activation proceeds via formation of an N-desmethyl thiamethoxam intermediate, another... 

One notable omission is the five-membered thiamethoxam (10) (Chapter 29.2.3), showing binding affinities up to 10000-fofd fess than other neonicotinoids, using housefly head membrane preparations. This fow affinity may be attributed to its proneonicotinoid structure, as it was shown to be activated to the open-chain dothianidin (12) (Chapter 29.2.1) in plants and insects [135]. The latter exhibits high activity as agonist on isolated neurons at concentrations as low as 30 mM. 

Saturation binding studies revealed the following data for affinity (Kj) and binding capacity (Bmax) for thiamethoxam 13 and imidadoprid 8 with fresh membranes from M. persicae assayed at 2 °C ... 

Non-spedfic binding was fairly low with both radioligands, typically around 10% with [ H]thiamethoxam (13) and 5% with [ H]imidadoprid (8). 

In summary, varied and minor structural differences in neonicotinoid molecules may confer diversity in their binding modes, depending upon insect spedes and may explain the unique receptor binding behavior of thiamethoxam (13) [56-58] as well as of dinotefiiran [64]. 

Biochemical experiments with several neonicotinoids on insect membranes showed that both, thiamethoxam and imidacloprid bind to the nicotinic acetylcholine receptor. Imidacloprid however inhibits the binding of thiamethoxam, while not competing for the same binding site. Thiamethoxam and other equally non-competitive neonicotinoids, which only served as research tools, share as a common structural element the N-methyl group at position 5 of the 1,3,5-oxadiazinane ring. 

Thiamethoxam High-Affinity Binding and Unusual Mode of Interference with Other Neonicotinoids at Aphid Membranes... 

At temperatures k20°C, binding capacity for thiamethoxam was significantly lower by 40-60% compared to 2°C. This was typically not the case with imidacloprid though variation was observed. It should be noted that non-specific binding was not affected by temperature with both compounds. 

For A. craccivora, specific binding of [ H]thiamethoxam was similarly sensitive to temperature and membrane quality as in M. persicae IT). The data for affinity and capacity at 2 C were about 90 nM and 1000 finol/mg protein. Higher values for A. craccivora compared to M. persicae were also obtained with imidacloprid 8, IS). 

In conclusion, temperature affects binding of thiamethoxam as well as imidacloprid in a reversible way, though the former compound is clearly stronger affected. This also suggests that the binding modes or sites of the two neonicotinoids at the target receptor are obviously not identical. 

Thiamethoxam Like Imidacloprid, Binds to a Nicotinic Receptor Site... 

As previously demonstrated (75), thiamethoxam, like the other examined neonicotinoids and known natural nicotinic ligands, competes with the binding... 

Like other insecticides, thiamethoxam is transformed in the insect, crop, soil and other compartments to variable degrees to yield products that may not or may be active in their own right. An example for the latter case m the neonicotinoid class is imidacloprid, which is metabolized via hydroxylated intermediates to an olefin product that is more active than the parent compound by one order of magnitude in aphids screens and in receptor binding 20. 21). 

As effects like those documented for thiamethoxam have not yet been described in the receptor binding field, the results, though based on thorough and self-critical experimentation, may be questioned. In our view, these novel results necessarily call for novel interpretations. A most evident conclusion from the present results is that binding of thiamethoxam has a different impact (as measurable in binding assays) on the receptor complex as binding of other, competitive neonicotinoids. Whether this different mode of interaction is due... 

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