Thiamethoxam membranes

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 ... 

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... 

Figure 1. Scatchard representations of the effects of (A) thiamethoxam (TMX) and (B) acetamiprid (ACT) on /Hfimidacloprid displacement from membranes ofM. persicae as examples for the non-competitive (A) and competitive (B) modes of displacement or inhibition.

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. 

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). 

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