Xenobiotic metabolizing enzymes inhibitors

Many differences in overall toxicity between males and females of various species are known (Table 9.1). Although it is not always known whether metabolism is the only or even the most important factor, such differences may be due to gender-related differences in metabolism. Hexobarbital is metabolized faster by male rats thus female rats have longer sleeping times. Parathion is activated to the cholinesterase inhibitor paraoxon more rapidly in female than in male rats, and thus is more toxic to females. Presumably many of the gender-related differences, as with the developmental differences, are related to quantitative or qualitative differences in the isozymes of the xenobiotic-metabolizing enzymes that exist in multiple forms, but this aspect has not been investigated extensively. 

As previously indicated, inhibition of xenobiotic-metabolizing enzymes can cause either an increase or a decrease in toxicity. Several well-known inhibitors of such enzymes are shown in Figure 9.6 and are discussed in this section. Inhibitory effects can be demonstrated in a number of ways at different organizational levels. 

Figure 9.6 Some common inhibitors of xenobiotic-metabolizing enzymes.

Because exogenous chemicals can be inducers and/or inhibitors of the xenobiotic-metabolizing enzymes of which they are substrates such chemicals may interact to bring about toxic sequelae different from those that might be expected from any of them administered alone. 

Age, Sex, Inducers and Inhibitors of Metabolism. The ability of an organ to activate a specific toxin is one explanation of organ-selective toxicity. Factors such as age, sex, circadian rhythms, nutritional status, and exposure to chemicals are known to affect xenobiotic metabolizing enzymes, and therefore might affect organ-specific toxicity of metabolically activated toxins. Several of these factors have striking effects on the organ-specific toxicity produced by IPO. 

Masubuchi N, Hakusui H, Okazaki O (1997) Effects of pantoprazole on xenobiotic metabolizing enzymes in rat liver microsomes a comparison with other proton pump inhibitors. 

A new tool for computational ADME/Tox called MetaDrug includes a manually annotated Oracle database of human drug metabolism information including xenobiotic reactions, enzyme substrates, and enzyme inhibitors with kinetic data. The MetaDrug database has been used to predict some of the major metabolic pathways and identify the involvement of P450s [78]. This database has enabled the generation of over 80 key metabolic... 

GUO Z, SMITH T J, WANG E, SADRIEH N, MA Q, THOMAS P E and YANG C S (1992) Effects of phenethyl isothiocyanate, a carcinogenesis inhibitor, on xenobiotic-metabohzing enzymes and nitrosamine metabolism in rats . Carcinogenesis, 13 2205-10. 

Flavonoids, especially flavones and flavonols, also directly bind to several CYP isoforms (lAl, 1A2, IBl, 3A4) involved in xenobiotics metabolism and inhibit enzyme activity. Structure-activity relationships show rather high isoform selectivities depending on the flavonoid substitution pattern and contrasted inhibition mechanisms. For instance, inhibition by flavonoids of 7-methoxyresorufin O-demethylation in microsomes enriched in CYP lAl and 1A2 reveals that galangin (3,5,7-trihydroxyflavone) is a mixed inhibitor of CYP 1A2 (.ST = 8 nM) and a five times less potent inhibitor of CYP 1A1. By contrast, 7-hydroxy flavone is a competitive inhibitor of CYP lAl (Aii = 15 nM) and a six times less potent inhibitor of CYP 1A2. In addition, fairly selective inhibition of CYP IBl (specifically detected in cancer cells) by some flavonoids has been reported. For example, 5,7-dihydroxy-4 -methoxyflavone inhibits IBl, 1 Al, and 1A2 with IC50 values of 7, 80, and 80 nM, respectively. ... 

Uncompetitive inhibition has seldom been reported in studies of xenobiotic metabolism. It occurs when an inhibitor interacts with an enzyme-substrate complex but cannot interact with free enzyme. Both Km and Vmax change by the same ratio, giving rise to a family of parallel lines in a Lineweaver-Burke plot. 

These reactive metabolites can bind covalently to cellular macromolecules such as nucleic acids, proteins, cofactors, lipids, and polysaccharides, thereby changing their biologic properties. The liver is particularly vulnerable to toxicity produced by reactive metabolites because it is the major site of xenobiotic metabolism. Most activation reactions are catalyzed by the cytochrome P450 enzymes, and agents that induce these enzymes, such as phenobarbital and 3-methylcholanthrene, often increase toxicity. Conversely, inhibitors of cytochrome P450, such as SKF-525A and piperonyl butoxide, frequently decrease toxicity. 

Enzymes are extremely important because they must function properly to enable essential metabolic processes to occur in cells. Substances that interfere with the proper action of enzymes obviously have the potential to be toxic. Many xenobiotics that adversely affect enzymes are enzyme inhibitors, which slow down or stop enzymes from performing their normal functions as biochemical catalysts. Stimulation of the body to make enzymes that serve particular purposes, a process called enzyme induction, is also important in toxicology. 

As previously mentioned, many of the enzymes involved in xenobiotic metabolism are inducible. Inducibility allows for more enzymatic activity, thereby ensuring an adequate detoxication response however, it also provides a mechanism whereby an activation pathway may be increased. This occurs in the example given earlier of the combined effects of ethanol and acetaminophen. When CYP2E1 is induced by ethanol prior to administration of acetaminophen, subsequent activation of acetaminophen to NAPQI is prevalent however, without induction by ethanol, CYP2E1 is not the predominant enzyme for metabolizing acetaminophen, and detoxication is favored. Interestingly, simultaneous administration decreases the toxicity of acetaminophen because both are substrates for 2E1 ethanol acts as a competitive inhibitor, thereby blocking the activation of acetaminophen. 

Some neurons are more sensitive than others to the effects of a variety of toxicants that is, they display a selective vulnerability to neurotoxicants. For example, mitochondrial respiratory complex inhibitors such as cyanide and 3-nitropropionic acid are toxic to all cell types yet within the CNS, neurons in the basal ganglia (a group of regions that collectively control motor behavior) appear to be particularly sensitive to these agents. In most cases, selective vulnerability to neurotoxicants arises because of a unique combination of factors that predispose a cell type or region to particular insults. These factors may include the presence of certain ion channels, receptors or uptake sites, the activity level of xenobiotic metabolizing or antioxidant enzymes, the expression profile of neurotrophic factors or their receptors, and so on. Three CNS sites highly vulnerable to neurotoxicant effects are described separately below. 

The epoxidase catalyzing the 10,11-epoxidation of methyl farnesoate in homogenates of corpora allata from Locusta miqratoria has been studied in detail and has been shown to be a cytochrome P-450-mediated monooxygenase associated with the microsomal fraction. The enzyme is strictly dependent on NADPH and requires oxygen (36). It is sensitive to inhibition by carbon monoxide (36) and to offier compounds such as methyl enedioxyphenyl compounds and imidazoles that are well established inhibitors of the cytochrome P-450-mediated monooxygenases involved in xenobiotic metabolism (37-39). 

Piperonyl butoxide, isoniazid, and SKF 525A and related chemicals are inhibitors of various xenobiot-ic-metabolizing enzymes. For instance, piperonyl butoxide increases the toxicity of pyrethrum (an insecticide) by inhibiting MFO activity in insects that detoxifies this agent. Isoniazid, when taken along with phenytoin, lengthens the plasma half-life of the antiepileptic drug and increases its toxicity. Iproniazid inhibits monoamine oxidase and increases the cardiovascular effects of tyramine, which is found in cheese and which is normally readily metabolized by the oxidase. 

A preliminary report by Obach and Van Vunakis (1990) claimed that cotinine could also be formed by a microsomal nicotinamide adenine dinucleotide (NAD) -dependent dehydrogenase (abbreviated MND). Inhibitor studies suggested that MND is not a typical aldehyde dehydrogenase. The presence of this activity in rabbit microsomes was confirmed in the authors laboratories (Flammang 1994). The rate of cotinine production from the -iminium by this route was found to be comparable to the rate of conversion of nicotine to the this enzyme might play in xenobiotic metabolism in general. It is expected that the substrate-structure dependence of MND will be quite different from that of cytosolic AO. 

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