Interactions Involving Xenobiotic Metabolizing Enzymes

Although examples are known in which synergistic interactions take place at the receptor site, the majority of such interactions appear to involve the inhibition of xenobiotic-metabolizing enzymes. Two examples involve the insecticide synergists, particularly the methylenedioxyphenyl synergists, and the potentiation of the insecticide malathion by a large number of other organophosphate compounds. 

Humans are exposed continuously and unavoidably to a myriad of potentially toxic chemicals that are inherently lipophilic and, consequently, very difficult to excrete. To effect their elimination, the human body has developed appropriate enzyme systems that can transform metabolically these chemicals to hydrophilic, readily excretable, metabolites. This biotransformation process occurs in two distinct phases. Phase I and Phase II, and involves several enzyme systems, the most important being the cytochromes P450. The expression of these enzyme systems is regulated genetically but can be modulated also other factors, such as exposure to chemicals that can either increase or impair activity. Paradoxically, the same xenobiotic-metabolizing enzyme systems also can convert biologically inactive chemicals to highly reactive intermediates that interact with vital cellular macromolecules and elicit various forms of toxicity. Thus, xenobiotic metabolism does not always lead to deactivation but can result also in metabolic activation with deleterious consequences. 

This phytochemical has been found to interact with microsomal xenobiotic-metabolizing enzymes in rodents. Capsaicin has been proposed to inactivate cytochrome P-450 HEl by irreversibly binding to the active sites of the enzyme [135]. Besides cytochrome P-450 HEl, other isoforms of the P-450 super family are also reported to be inhibited by capsaicin. The inhibition by capsaicin of microsomal monooxygenases involved in carcinogen activation implies its chemopreventive potential. 

In vitro Effects. In vitro measurement of the effect of one xenobiotic on the metabolism of another is by far the most common type of investigation of interactions involving inhibition. Although it is the most useful method for the study of inhibitory mechanisms, particularly when purified enzymes are used, it is of limited utility in assessing the toxicological implications for the intact animal. The principal reason for this is that in vitro measurement does not assess the effects of factors that affect absorption, distribution, and prior metabolism, all of which occur before the inhibitory event under consideration. 

Xenobiotics can be absorbed across the cellular barriers and may be biologically active and possibly toxic to the cell. Metabolism of these molecules by enzymes to hydrophilic metabolites is a prerequisite for their eventual elimination from the body. However, in some cases bioactivation may also occur, and such metabolites may be toxic. Xenobiotic metabolism has therefore been widely studied since the early 1800s [1]. The parent molecule and the products of metabolic pathways may also be involved in drug interactions where they... 

No examples of the use of deuterium and tritium NMR in xenobiotic metabolism were found. Their use in biosynthetic studies has been reviewed by Garson and Staunton (31). Sensitivity problems exist with deuterium, but should not be a problem with tritium since it is the most sensitive nucleus available (1.21 x proton) and because of negligible tritium backgrounds. Tritium NMR may be useful in the studies of xenoblotic-enzyme interactions as shown by Scott et al. (32). Hazards due to the use of radioactivity should be minimal because 1 mCi of activity should provide sufficient material for many experiments. However, isotope effects may be a problem if the metabolic reaction directly involves the tritium (or deuterium) atom because Isotopes of hydrogen can greatly affect enzymic reaction rates. Also, lability may be a problem as Bakke and Feil have found with CD3SO compounds, where exchange was too rapid to permit metabolism studies (W). 

In contrast, selective inhibition of enzyme activity involves highly specific interactions between the protein and chemical groups on the xenobiotic. An excellent example of this type of inhibition is seen in the toxic effect of fluoroacetate, which is used as a rodenticide. Although fluoroacetate is not directly toxic, it is metabolized to fluoroacetyl-CoA, which enters the citric acid cycle due to its structural similarity to acetyl-CoA (Scheme 3.5). Within the cycle, fluoroacetyl-CoA combines with oxalo-acetate to form fluorocitrate, which inhibits the next enzyme, aconitase, in the cycle [42]. The enzyme is unable to catalyze the dehydration to cis-aconitate, as a consequence of the stronger C-F bond compared with the C-H bond. Therefore, fluorocitrate acts as a pseudosubstrate, which blocks the citric acid cycle and, subsequently, impairs ATP synthesis. 

Another form of chemical interaction, resulting from inhibition in vivo, that can then be demonstrated in vitro involves those xenobiotics that function by causing destruction of the enzyme in question, so-called suicide substrates. Exposure of rats to vinyl chloride results in a loss of cytochrome P450 and a corresponding reduction in the capacity of microsomes subsequently isolated to metabolize foreign compounds. Allyl isopropy-lacetamide and other allyl compounds have long been known to have a similar effect. 

In the early 1960s, during investigations on the N-demethylation of aminoazo dyes, it was observed that pretreatment of mammals with the substrate or, more remarkably, with other xenobiotics, caused an increase in the ability of the animal to metabolize these dyes. It was subsequently shown that this effect was due to an increase in the microsomal enzymes involved. A symposium in 1965 and a landmark review by Conney in 1967 established the importance of induction in xenobiotic interactions. Since then, it has become clear that this phenomenon is widespread and nonspecific. Several hundred compounds of diverse chemical structure have been shown to induce monooxygenases and other enzymes. These compounds include drugs, insecticides, polycyclic hydrocarbons, and many others the only obvious common denominator... 

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