Common mechanism group

Cumulative Assessment Group (CAG) A subset of chemicals selected from a common mechanism group for inclusion in a refined quantifative estimate of risk. The chemicals in the CAG, as well as their pathways/routes and pesticide uses, are judged to have a hazard and exposure potential that could result in the expression of a cumulative risk. Thus, negligible contributors are not included in quantifying the risk (USEPA, 2002). 

In the alcohol oxidations, the sulfonium intermediate (2, nucleophile = R2C(OH)) loses a proton and dimethyl sulfide to give the carbonyl compound (42). The most common mechanism for the decomposition of (2) is attack by a mild base to remove a proton from one of the methyl groups. Subsequent cycHc coUapse leads to the carbonyl compound and dimethyl sulfide (eq. 9) ... 

The major mechanism of resistance to chloramphenicol is mediated by the chloramphenicol acetyltransferases (CAT enzymes) which transfer one or two acetyl groups to one molecule of chloramphenicol. While the CAT enzymes share a common mechanism, different molecular classes can be discriminated. The corresponding genes are frequently located on integron-like structures and are widely distributed among Gramnegative and - positive bacteria. 

This chapter has three parts. In the first part, we look at the structure and properties of some of the common functional groups and describe some of the characteristic mechanisms by which the groups react. Then we examine how functional groups are used to create modern polymers. As in so many spheres, nature has preceded chemists explorations. In the final part of the chapter, we see some examples of how functional groups in nature sustain us, feed us, and replicate our genetic material. 

It has come to be suggested that coplanar congeners as a group express toxicity through a common mechanism interaction with the cytosolic Ah receptor (Safe 1990 Ahlborg et al. 1994) (Environmental Health Criteria 140). Although the full... 

These three classes of antibiotics are often grouped together because of their similar mode of action. They share a common mechanism of resistance, but there are some mechanisms specific to each group (Leclerq Courvalin 1991). 

Even though all OP insecticides have a common mechanism of action, differences occur among individual compounds. OP insecticides can be grouped into direct and indirect ACHE inhibitors. Direct inhibitors are effective without any metabolic modification, while indirect inhibitors require biotransformation to be effective. Moreover, some OP pesticides inhibit ACHE more than PCHE, while others do the opposite. For example, malathion, diazinon, and dichlorvos are earlier inhibitors of PCHE than of ACHE. In these cases, PCHE is a more sensitive indicator of exposure, even though it is not correlated with symptoms or signs of toxicity. 

Under the Food Quality Protection Act (FQPA), the U.S. EPA evaluates the potential for people to be exposed to more than one pesticide at a time from a group of chemicals with an identified common mechanism of toxicity. As part of the examinations, to clarify whether some or all of the pyrethroids share a common mechanism of toxicity, a comparative FOB (functional observational battery) studies with 12 pyrethroids were carried out under standardized conditions [15]. The FOB was evaluated at peak effect time following oral administration of non-lethal doses of pyrethroids to rats using com oil as vehicle. Four principal components were observed in the FOB data [22], Two of these components described behaviors associated with CS syndrome (lower body temperature, excessive salivation, impaired mobility) and the others described behaviors associated with the T syndrome (elevated body temperature, tremor myoclonus). From the analysis, pyrethroids can be divided into two main groups (Type I T syndrome and Type II CS syndrome) and a third group (Mixed Type) that did not induce a clear typical response. Five other pyrethroids were also classified by an FOB study conducted in the same manner [16]. The results of these classifications are shown in Table 1. The FOB results for all non-cyano pyrethroids were classified as T syndrome, and the results of four ot-cyano pyrethroids were classified as CS syndrome however, three of the ot-cyano pyrethroids, esfenvalerate, cyphenothrin, and fenpropathrin, were classified as Mixed Type. 

However, one of the most common mechanisms is undoubtedly proton transfer but whereas in alkene polymerizations this reaction leaves a terminal double bond, in arylene polymerizations these are generally not found. Instead the terminal group is usually a substituted indane formed by an internal Friedel-Crafts alkylation [8, 21, 23], e.g., for a-methyl styrene ... 

An ILSI Working Group (Mileson et al. 1998) has also addressed the common mechanism issue. The ILSI Working Group concluded that a common mechanism might exist if two compounds ... 

The TEF approach normalizes exposures to common mechanism chemicals with different potencies to yield a total equivalent exposure (TEQ) to one of the chemicals, the index compound. TEFs are derived as the ratio of the POD of the index compound to that of each member in the group. The exposure to each chemical is then multiplied by the respective TEF value to express exposure in terms of the index compound. Summation of these values result in the total combined exposure (TEQ) expressed in terms of the index compound. 

---