Target site resistance

Salgado, V.L. (1999). Resistant target sites and insecticide discovery. In G.T. Brooks and T.R. Roberts (Eds.) Pesticide Chemistry and Bioscience—The Food-Environment Challenge. Cambridge, Royal Society of Chemistry 236-246. 

In the presence of a continued selection pressure, metabolic resistance may facilitate the evolution of other defenses such as target site resistance, reported for the OPs and carbamates 6 and 10 years after metabolic resistance. Target site resistance to OPs and carbamates resides in modified forms of acetyl-cholinesterases (AChEs) with reduced affinity for the insecticides. AChE-based target site resistance does not necessarily confer cross resistance to all other OPs and carbamates and may be unstable in the absence of a selection pressure. 

However, I5g measurements do not provide information about the biochemical characteristics of the enzyme-inhibitor interaction and what specific feature of the interaction may have changed and so caused the resistance. Also, I5g measurements need to be done with pooled tissues of several to many insects, depending on their size. To study the frequency of the resistant target site in a population, measurements must be done in individual insects. This can and has been done in house flies (58. 6J.), an armyworm (59), and plant hoppers and leaf hoppers (62.). Detailed inhibition kinetics studies will reveal if a mutation has occurred (58). A mutation causing the enzyme to have decreased affinity for the inhibitor is most clearly reflected in Kinverse relationship between and kj, kj is smaller in resistant insects with insensitive AChE. This is illustrated in Table 2. 

From the mechanistic point of view three basic principles of microbial resistance to drugs are known inactivation of the drug, alteration of the target, and reduced drug accumulation at the target site. However, several variations on these themes are known. 

Two mechanisms are operating alone or in concert to minimize the antibiotic concentration at the intracellular target site Downregulation of the expression of the pore proteins, also called porins, and upregulation of one or a set of several unspecific efflux pumps. However, the impact of these mechanisms on the resistance is low, since due to the essential function of porins for uptake of nutrients their reduction is limited and to avoid disturbances of membrane integrity due to extensive oveiproduction of mdr efflux pumps these are subjected a strict regulation. 

A common means that causes resistance is the inactivation of the antibiotic before it reaches the target site. Antibiotics can be either enzymatically cleaved or modified. In both cases the antibiotic loses its capacity to bind to its target. 

The second general mechanism to cause resistance to antibacterial agents is to prevent the drug from reaching its target site. This is either achieved by altered rates of entry (reduced uptake) or by the active removal of the dtug (active efflux) [4]. 

Bacterial resistance can be caused by actively pumping antibiotics out of the cell and therefore decreasing the concentration at the target site. Drug efflux systems in bacteria are classified into four major groups based on their sequence homologies and functional similarities (Table 3). 

Lambert PA (2005) Bacterial resistance to antibiotics modified target sites. Adv Drug Deliv Rev 29 1471-1485... 

Resistance to DDT has been developed in many insect species. Although there are some cases of metabolic resistance (e.g., strains high in DDT dehydrochlorinase activity), particular interest has been focused on kdr and super kdr mechanisms based upon aberrant forms of the sodium channel—the principal target for DDT. There are many examples of insects developing resistance to dieldrin. The best-known mechanism is the production of mutant forms of the target site (GABA receptor), which are insensitive to the insecticide. 

Indeed, the resistance problem has sometimes been severe enough to threaten a loss of control over the pest. A study of a number of resistant strains from the field has revealed two major types of resistance mechanism. Some individuals possess aberrant forms of the target site, the Na" channel. At least two forms are known that confer either kdr (<100-fold) or super kdr (>100-fold) resistance, which is the consequence of the presence of insensitive forms of the Na+ channel protein (McCaffery 1998, and Chapter 4, Section 4.4 of this book). 

These mechanisms of resistance rely on reducing or preventing access of antibiotic to their target sites, but other mechanisms of resistance involving the target sites themselves can be considered. Alterations in the target site which reduce the binding of... 

Acquired resistance to /Mactam antibiotics can occur by three different mechanisms inactivation of the antibiotic, alteration of the target site and reduced permeability (Sanders 1992 Georgopapadakou 1993). 

Bloomquist JR (1993) Toxicology, mode of action and target site-mediated resistance to insecticides acting on chloride channels. Comp Biochem Physiol C 106 301-314... 

Work in our laboratory on various parameters in R and S fish has investigated the factor(s) responsible for resistance. The results have indicated that resistance is multifactorial, involving a barrier to insecticide penetration, insecticide storage, insecticide metabolism, and an apparent "insensitivity" at the target site to the toxic effects of the insecticide. The present report concentrates on two of these factors insecticide disposition and metabolism. 

The uptake and distribution of organochlorine insecticides has been studied under a variety of conditions. Although the results indicate that further study is needed on a characterization of extraneous factors that affect disposition, the studies clearly demonstrate the presence of a membrane barrier to insecticide penetration in the R population. This membrane barrier would aid in the protection of target sites in the R fish from the insecticide. This barrier is felt to be an important factor in resistance to organochlorine insecticides in mosquitofish. 

Barriers to insecticide penetration undoubtedly contribute to chlorinated alicyclic resistance. However, we are led to conclude that these extremely high levels of resistance are the result of a postulated insensitivity of the target site which allows these fish to tolerate elevated internal levels of these toxicants. 

We have, therefore, been able to indirectly assess the importance of three factors involved in chlorinated alicyclic insecticide resistance in mosquitofish disposition, metabolism and target site sensitivity. In a highly polluted environment in which mosquitofish have been placed under severe selective pressures by chronic exposure to insecticides, the system of metabolism appears to be of little significance in resistance the... 

Hepatic mixed-function oxidase activities demonstrated seasonal trends, with higher specific activities in the cold weather months in both populations with few differences in enzyme activities or cytochrome levels between the two populations. Metabolism of aldrin, dieldrin and DDT was similar between the two populations. R fish have larger relative liver size and, therefore, a greater potential for xenobiotic metabolism. However, biotransformation appears to be of minor importance in chlorinated alicyclic insecticide resistance in mosquitofish barriers to penetration appear to be of greater importance and an implied target site insensitivity appears to be the most important factor in resistance. 

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