Xenobiotic oxidation

Mueller and Miller (33) and Brodie et al. (34) were the first to show that enzymes in the microsomal fraction of rat liver could effectively oxidize xenobiotics. Comparable enzymes (aryl hydrocarbon monooxygenases) were later reported in the hepatic tissues of fresh water and marine fish by Creaven et al. (35) and Buhler and Rasmusson (36). Reconstituted hepatic microsomal systems require cytochrome P-450 for monooxygenase activity in both mammals (37) and fish (38,39). Bend et al. 

Systems reconstituted from purified CYP, NADPH-cytochrome P450 reductase and phosphatidylchloline will, in the presence of NADPH and 02, oxidize xenobiotics such as benzphetamine, often at rates comparable to microsomes. Although systems reconstituted from this minimal number of components are enzymatically active, other microsomal components, such as cytochrome bs, may facilitate activity either in vivo or in vitro or may even be essential for the oxidation of certain substrates. 

A number of other enzymes, such as monoamine oxidase, alcohol dehydrogenase and xanthine oxidase, are also involved in drug metabolism. These enzymes tend to be more specific, oxidizing xenobiotics related to the normal substrate for the enzyme. 

Several unique features of the catalytic cycle of the FMOs are important for understanding the mechanism by which they oxidize xenobiotics. The catalytic mechanism for the FMO has been shown to involve the formation of an enzyme bound 4a-hydroperoxyl-flavin (Figure 10.6) in an NAD PH and 02 dependent reaction. Reduction of the flavin by NAD PH occurs before binding of oxygen can occur, and activation of oxygen by the enzyme occurs in the absence of substrate by oxidizing NADPH to form NADP and peroxide. Finally, addition of the substrate to the peroxyflavin complex is the last step prior to oxygenation. This is in contrast to the CYP catalytic cycle in which the substrate binds to the oxidized enzyme which is subsequently reduced. 

Aldehyde oxidases (AO) are also molybdenum-containing enzymes that, like XO, exist as homodimers of 300 Kdaltons. It is presumed that they behave mechanistically similarly to XO. Both AO and XO can mediate reductive reactions through the transfer of electrons from FADH2 to oxidized xenobiotic. For example, zonisamide can be reduced by AO to 2-sulfamoylacetylphenol. 

Franklin, M.R. (1972). Inhibition of hepatic oxidative xenobiotic metabolism by piperonyl butoxide. Biochem. Pharmacol. 21, 3287-3299. 

Besides the monooxygenases discussed above, a number of other oxidoreductases can oxidize xenobiotics. These enzymes are mostly but not exclusively nonmicrosomal, being present in the cytosol or mitochondria of the liver and extrahepatic tissues. The list includes alcohol dehydrogenases, aldehyde dehydrogenases, dihydrodiol dehydrogenases, haemoglobin, monoamine oxidases, xanthine oxidase and aldehyde oxidase. Some of these enzyme systems are discussed below. 

Bioactivation to a free radical intermediate has been implicated in the teratological mechanism for a number of xenobiotics, including phenytoin and structurally-related AEDs, benzo[a]pyrene, thalidomide, methamphetamine, valproic acid, and cyclophosphamide (Fantel 1996 Wells et al. 2009 Wells and Winn 1996). Unlike in the case of most CYPs, the embryo-fetus has relatively high activities of PHSs and lipoxygenases (LPOs), which via intrinsic or associated hydroperoxidase activity can oxidize xenobiotics to free radical intermediates (Fig. 10) (Wells et al. 2009). These xenobiotic free radical intermediates can in some cases react with double bonds in cellular macromolecules to form covalent adducts, or more often react directly or indirectly with molecular oxygen to initiate the formation of potentially teratogenic reactive oxygen species (ROS). 

Fig. 10 Bioactivation of xenobiotics via the prostaglandin H synthase (PHS) and lipoxygenase (LPO) pathways-postuiated role in teratogenesis. The hydroperoxidase component of embryonic and fetal PHSs, and hydroperoxidases associated with LPOs, can oxidize xenobiotics to free radical intermediates that initiate the formation of reactive oxygen species causing oxidative stress (modified from Yu and Wells 1995)...

Phase I reactions include oxidation, reduction, hydrolysis, epoxide hydration, and dehydrohalogenation reactions. The cytochrome P450 oxygenase system (CYP450) is the most important one in the metabolism of foreign chemicals. These enzymes that oxidize xenobiotics are widely distributed in the body. They are found in high... 

Cellular defense mechanisms against toxins (A multistep mechanism for elimination of toxic metabolites and xenobiotics. It involves various transport, oxidation, and conjugation steps.) are usually divided into several steps as it is visualized on Fig. 3. Organic anion transporting proteins (OATPs) are responsible for the cellular uptake of endogenous compounds and... 

The numerous biotransformations catalyzed by cytochrome P450 enzymes include aromatic and aliphatic hydroxylations, epoxidations of olefinic and aromatic structures, oxidations and oxidative dealkylations of heteroatoms and as well as some reductive reactions. Cytochromes P450 of higher animals may be classified into two broad categories depending on whether their substrates are primarily endogenous or xenobiotic substances. Thus, CYP enzymes of families 1-3 catalyze... 

In phase 1, the pollutant is converted into a more water-soluble metabolites, by oxidation, hydrolysis, hydration, or reduction. Usually, phase 1 metabolism introduces one or more hydroxyl groups. In phase 2, a water-soluble endogenous species (usually an anion) is attached to the metabolite— very commonly through a hydroxyl group introduced during phase 1. Although this scheme describes the course of most biotransformations of lipophilic xenobiotics, there can be departures from it. 

Apart from monooxygenases, other enzymes concerned wih xenobiotic metabolism may also be induced. Some examples are given in Table 2.5. Induction of glucuronyl transferases is a common response and is associated with phenobarbital-type induction of CYP family 2. Glutathione transferase induction is also associated with this. A variety of compounds, including epoxides such as stilbene oxide and... 

Monooxygenases (MOs) Enzyme systems of the endoplasmic reticulum of many cell types, which can catalyze the oxidation of a great diversity of lipophilic xenobiotics, are particularly well developed in hepatocytes. Forms of cytochrome P450 constitute the catalytic centers of monooxygenases. 

The first is cell injury (cytotoxicity), which can be severe enough to result in cell death. There are many mechanisms by which xenobiotics injure cells. The one considered here is covalent binding to cell macromol-ecules of reactive species of xenobiotics produced by metabolism. These macromolecular targets include DNA, RNA, and protein. If the macromolecule to which the reactive xenobiotic binds is essential for short-term cell survival, eg, a protein or enzyme involved in some critical cellular function such as oxidative phosphorylation or regulation of the permeability of the plasma membrane, then severe effects on cellular function could become evident quite rapidly. 

These have already been noted in the context of hydroxyl radical-initiated oxidations, and reference should be made to an extensive review by Worobey (1989) that covers a wider range of abiotic oxidations. Some have attracted interest in the context of the destruction of xenobiotics, and reference has already been made to photochemically induced oxidations. 

Guengerich P (1990) Enzymatic oxidation of xenobiotic chemicals. CritRev Biochem Mol Biol 25 97-153. 

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