Species Differences in Detoxification Enzyme Activity

A CA epoxidase perhaps identical to the precocene epoxidase biosynthesizes Insect juvenile hormones (JH) from the analogous inactive oleflnic precursor, and the enzyme activity appears higher in precocene-sensitive species (32). Subsequent detoxification of JH occurs primarily by EHs and esterases in peripheral tissues, and preliminary information does not indicate major differences for JH degradation routes between chewing and sucking herbivores, or insect carnivores (33,34). More study of the role of detoxification in regulating the action of JH in target tissues is required. 

It is not easy to predict the role of age or sex in the activity of detoxification enzymes. In some species, these enzymes are almost as active at birth as in mature animals, whereas in others there is a large difference. Likewise, males are sometimes more active in detoxification, whereas in other cases, there is no sex difference. Therefore, it is important to consider such variables in dosage-mortality experiments, as discussed earlier in Chapter 5. 

In various species of bacteria several different types of non-assimilatory nitrite reductases are found. Escherichia coli has a cytoplasmic NAD(P)H-dependent enzyme whose role seems to be detoxification of nitrite. This type of enzyme, coded for by the nirB gene, also contains siroheme as the redox active catalytic center (Cole, 1988). Additionally in E. coli, and expressed under different conditions to the cytoplasmic enzyme, is a periplasmic nitrite reductase that catalyses formation of ammonia from nitrite (Cole, 1988). This enzyme has five c-type (Figure 1) hemes per polypeptide chain one of these hemes, the catalytic site, has the unique CXXCK sequence as its attachment site (Einsle et al., 1999). Electrons reach this type of nitrite reductase, which is fairly widely distributed amongst the microbial world, from the cytoplasmic membrane electron transfer chain. The exact electron donor partner from such chains for this type of nitrite reductase is unknown (Berks et al., 1995). 

Different to the cases described above, the herbicide propanil is detoxified in rice and weed species by the action of an aryl acylamidase (aryl-acylamine amido-hydrolase). High activity of this enzyme in rice confers crop tolerance. In Colombia, a biotype of Echinochloa colona was found that is resistant to propanil. Enzyme tests with extracts from this biotype revealed an about three-fold higher activity of aryl acylamidase in the resistant than in a susceptible biotype. It was concluded that resistance of the E. colona biotype is based on enhanced propanil detoxification [74]. 

It is well known that different species show different de-epoxidation efficiencies, but this capability in humans is quite low. In particular, from the data reported in the literature, it could be argued that the pre-exposure of the microbiota to DON is a key factor for inducing the appearance of the bacterial detoxification activity, either through the induction of the expression of particular enzymes and/or the selection of particular detoxifying bacterial species. According to very recent studies, experiments conducted with human feces from five volunteers showed that only one spontaneously possessed bacteria able to transform DON in DOM-1. 

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