Metabolic activating enzymes, animal species

Animal Species Used as Sources of Metabolic Activating Enzymes... 

When metabolic activation is used, S9 mix should not exceed 1-10 percent of the culture medium by volume. It has been shown that the S9 mix is clastogenic in CHO cells and mouse lymphoma cells (Cifone et al., 1987 Kirkland et al., 1989) but not in human lymphocytes, where blood components can inactivate active oxygen species which could cause chromosome damage. When S9 mix from animals treated with other enzyme-inducing agents such as phenobarbitone/beta-naphtho-flavone, is used, clastogenesis may be minimized (Kirkland et al., 1989). 

Table II summarizes the results together with the detailed experimental conditions. As is evident, metabolic activities were detectable in these 3 aquatic species, but the rate was far lower as compared with mammalian hepatic enzume preparations, and the oxidative activities in snail were particularly low although the possibility was not ruled out of the presence of inhibitors of mixed-function oxidases in the fractions. The O-demethylation reaction proceeds extremely slowly in the enzyme preparation of aquatic animals, at less than one hundredth that of mammals.

Excised nasal mucosae obtained from various animal species are tools frequently used to study nasal transport and metabolism ([53], Chap. 4). Maintaining the viability of the excised nasal tissues during the experimental period is crucial. Most studies were performed with epithelia excised from rabbits, bovine, sheep, and dogs tissues [54-57], This excised nasal tissue model has been shown to be well suited for studies on nasal permeation and metabolism of drugs. However, species differences in the activity of various enzymes found in human versus these animal nasal mucosae have become an important issue. 

The induction of the CYPs has been demonstrated in many different species including humans, and in various different tissues as well as the liver. Induction usually results from repeated or chronic exposure, although the extent of exposure is variable. The result of induction is an increase in the amount of an enzyme induction requires de novo protein synthesis, and therefore an increase in the apparent metabolic activity of a tissue in vitro or animal in vivo. Consequently, inhibitors of protein synthesis, such as cycloheximide, inhibit induction. It is a reversible cellular response to exposure to a substance. Thus, it can be shown in isolated cells, such as hamster fetal cells in culture, that exposure to benzo[a]anthracene induces aryl hydrocarbon hydroxylase (AHH) activity (CYP1A1). 

Paracetamol is a widely used analgesic, which causes liver necrosis and sometimes renal failure after overdoses in many species. The half-life is increased after overdoses because of impaired conjugation of the drug. Toxicity is due to metabolic activation and is increased in patients or animals exposed to microsomal enzyme inducers. The reactive metabolite (NAPQI) reacts with GSH, but depletes it after an excessive dose and then binds to liver protein. Cellular target proteins for the reactive metabolite of paracetamol have been detected, some of which are enzymes that are inhibited. Therefore, a number of events occur during which ATP is depleted, Ca levels are deranged, and massive chemical stress switches on the stress response. 

Although OPPs and carbamates exhibit very similar modes of action in various animal species, i.e, acetylcholinesterase inhibition in the CNS with resulting paralysis—there is an important difference between the two classes of pesticides. Carbamates do not require metabolic conversion prior to exhibiting their toxicity. Furthermore the enzyme activity may at times be rapidly regenerated by reversal of inhibition. The kinetics of the inhibition (carbamoylation) reaction have been well studied in it electrophilic carbamoyl moieties form covalent bonds with enzyme esteratic sites. This is followed by carbamate transfer of an acidic group to the site to yield the acetylated enzyme complex (ref. 176). 

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