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presence of NADPH and oxygen, proceeds. It appears probable that the metabolite in question is a carbine formed spontaneously by elimination of water following hydroxylation of the methylene carbon by the cytochrome (see Figure 7.8 for metabolism of methylenedioxyphenyl compounds). Piperonyl butoxide inhibits the in vitro metabolism of many substrates of the monooxygenase system, including aldrin, ethylmorphine, aniline, and aminopyrine, as well as carbaryl, biphenyl, hexobarbital, and p-nitroanisole among many others. Although most of the studies carried out on piperonyl butoxide have involved rat or mouse liver microsomes, they have also been carried out on pig, rabbit, and carp liver microsomes, and in various preparations from house ies, cockroaches, and other insects. Certain classes of monooxygenase inhibitors, in addition to methylenedioxyphenyl compounds, are now known to form metabolite inhibitory complexes, including amphetamine and its derivatives and SKF-525A and its derivatives. The inhibition of the carboxylesterase that hydrolyzes malathion by organophosphorus compounds, such as EPN is a further example of xenobiotic interaction resulting from irreversible inhibition. In this case the enzyme is phosphorylated by the inhibitor. Another class of irreversible inhibitors of toxicological signi cance consists of those compounds that bring about the destruction of the xenobiotic-metabolizing enzymes, hence the designation suicide substrates. The drug allylisopropylacetamide (Figure 9.6), as well as other allyl compounds, has long been known to cause the breakdown of P450 and the resultant release of heme. More recently the hepatocarcinogen vinyl chloride has also been shown to have a similar effect, probably also mediated through the generation of a highly reactive intermediate (see Figure 8.2). Much information has accumulated since the mid-1970s on the mode of action of the hepatotoxicant carbon tetrachloride, which effects a number of irreversible changes in both liver proteins and lipids, such changes being generated by reactive intermediates formed during its metabolism (Figure 8.3). The less speci c disruptors of protein structure, such as urea, detergents, strong acids, and so on, are probably of signi cance only in vitro experiments.
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Synergism and Potentiation. The terms synergism and potentiation have been used and de ned in various ways, but in any case, they involve a toxicity that is greater when two compounds are given simultaneously or sequentially than would be expected from a consideration of the toxicities of the compounds given alone. Some toxicologists have used the term synergism for cases that t this de nition, but only when one compound is toxic alone whereas the other has little or no intrinsic toxicity. For example, the nontoxic synergist, piperonyl butoxide is often included in pesticide formulations because of its ability to signi cantly increase the toxicity of the active pesticide ingredient by inhibiting its detoxication in the target species. The term potentiation is then reserved for those cases where both compounds have appreciable intrinsic toxicity, such as in the case of malathion and EPN. Malathion has a low mammalian toxicity due primarily to its rapid hydrolysis by a carboxylesterase. EPN (Figure 9.6) another organophosphate insecticide, causes a dramatic increase in malathion toxicity to mammals at dose levels, which, given alone, cause essentially no inhibition of acetylcholinesterase. The increase in toxicity as a result of coadministration of these two toxicants is the result of the ability of EPN, at low concentrations, to inhibit the carboxylesterase responsible for malathion degradation. Unfortunately, the terms synergist and potentiation have often been used by some toxicologists in precisely the opposite manner. Historically, the term synergist has
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been used by pharmacologists to refer to simple additive toxicity and potentiation either as a synonym or for examples of greater than additive toxicity or ef cacy. In an attempt to make uniform the use of these terms, it is suggested that insofar as toxic effects are concerned, the terms be used according to the following: Both synergism and potentiation involve toxicity greater than would be expected from the toxicities of the compounds administered separately, but in the case of synergism one compound has little or no intrinsic toxicity when administered alone, whereas in the case of potentiation both compounds have appreciable toxicity when administered alone. It is further suggested that no special term is needed for simple additive toxicity of two or more compounds.
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Antagonism. In toxicology, antagonism may be de ned as that situation where the toxicity of two or more compounds administered together or sequentially is less than would be expected from a consideration of their toxicities when administered individually. Strictly speaking, this de nition includes those cases in which the lowered toxicity results from induction of detoxifying enzymes (this situation is considered separately in Section 9.5.2). Apart from the convenience of treating such antagonistic phenomena together with the other aspects of induction, they are frequently considered separately because of the signi cant time that must elapse between treatment with the inducer and subsequent treatment with the toxicant. The reduction of hexobarbital sleeping time and the reduction of zoxazolamine paralysis time by prior treatment with phenobarbital to induce drug metabolizing enzymes are obvious examples of such induction effects at the acute level of drug action, whereas protection from the carcinogenic action of benzo(a)pyrene, a atoxin B1, and diethylnitrosamine by phenobarbital treatment are examples of inductive effects at the level of chronic toxicity. In the latter case the P450 isozymes induced by phenobarbital metabolize the chemical to less toxic metabolites. Antagonism not involving induction is a phenomenon often seen at a marginal level of detection and is consequently both dif cult to explain and of marginal signi cance. In addition several different types of antagonism of importance to toxicology that do not involve xenobiotic metabolism are known but are not appropriate for discussion in this chapter. They include competition for receptor sites, such as the competition between CO and O2 in CO poisoning or situations where one toxicant combines nonenzymatically with another to reduce its toxic effects, such as in the chelation of metal ions. Physiological antagonism, in which two agonists act on the same physiological system but produce opposite effects, is also of importance.
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