Glutathione conjugates discussion

Together with glutathione conjugation, hydration is a major pathway in the inactivation and detoxification of arene oxides. Exceptions to this rule will be treated when discussing polycyclic aromatic hydrocarbons. Arene oxides are good substrates for microsomal EH, as evidenced in Table 10.1, where hydration of selected arene oxides, alkene oxides, and cy-cloalkene oxides by purified rat liver epoxide hydrolase is compared. The hy- ... 

The further metabolism of suitably stable epoxides may occur, with the formation of dihydrodiols as discussed later. Dihydrodiols may also be further metabolized to catechols. Other products of aromatic hydroxylation via epoxidation are glutathione conjugates. These may be formed by enzymic or nonenzymic means or both, depending on the reactivity of the epoxide in question. 

Another example of a glutathione conjugate responsible for toxicity is the industrial chemical hexachlorobutadiene discussed in chapter 7. The diglutathione conjugate of bromobenzene is believed to be involved in the nephrotoxicity after further metabolic activation (chap. 7, Fig. 7.31). 

Methyl sulfones are ultimate mammalian metabolites of PCBs produced by a series of reactions involving arene oxides and glutathione conjugates and have been synthesized to enable their conclusive identification and quantification (Bergman and Wach-meister 1978 Haraguchi et al. 1987). This is discussed further in Section 7.5.2. 

The first two of these are discussed in Chapter 4 and there are specific examples in Chapter 7. The products are either excreted directly into the bile or further metabolized and excreted into the urine as cysteine or /V-acetylcysteine conjugates. There are, however, examples of glutathione conjugates being involved in toxicity as indicated in Chapters 4 and 7. 

Traditionally, the hydroxylation of aromatic compounds by CYP450 has been considered to be mediated by an arene oxide (epoxide) intermediate followed by the NIH shift," as discussed previously (29,30,31) (Fig. 10.6). The formation of phenols and fhe isolation of urinary dihydrodiols, catechols, and glutathione conjugates (mercapturic acid derivatives) implicates arene oxides as intermediates in the metabolism of benzene and substituted benzenes in mammalian systems. The arene oxides... 

The tetrachloroethylene metabolite, jV-acetyl-5 -(trichlorovinyl)cysteine (TCVC), produced from glutathione conjugation, has been measured in rat urine using a negative ion chemical ionization gas chromatographic/tandem mass spectrometric method (Bartels 1994). The detection limit of this method was reported as 0.3 ng TCVC/mL of rat urine. As discussed in Section 2.3.3, glutathione conjugation of tetrachloroethylene has not been identified in hiunans. 

In vivo intermediate in mammals by the detection of two of its derivatives,, the glutathione (GSH) conjugate and its further metabolites formed by an initial carbamylation reaction (W) (see below) and 2-chloroacrolein detected in the microsome-NADPH system and derived from the rearrangement-elimination reaction sequence discussed above (6). Sulfallate also yields 2-chloroacrolein in the microsome-NADPH system, presumably by -CH2 hydroxylation (22) on analogy with the metabolism of EPTC shown previously. 

Oxygen is normally readily available to all reasonably well-perfused tissues, but deep inside organs such as the liver, especially the centrilobular area (see chap. 6), there will be a reduction in the oxygen concentration. This is clearly important when both oxidative and reductive pathways are available for a particular substrate. Therefore, as conditions in a particular tissue become more anaerobic, reductive pathways will become more important. This is well illustrated by the metabolism of halo thane where, in the rat, hypoxia will increase reductive metabolism and hepa to toxicity (see chap. 7). Glutathione is an extremely important cofactor, involved in both protection and conjugation. It may be depleted by both of these processes, or under certain circumstances, such as hereditary glucose-6-phosphate deficiency in man, supply may be reduced (see chap. 5). This will clearly influence toxicity, and there are a number of examples discussed in chapter 7 in which it is important. 

As already discussed in chapter 4, reactive intermediates can react with reduced GSH either by a direct chemical reaction or by a GSH transferase-mediated reaction. If excessive, these reactions can deplete the cellular GSH. Also, reactive metabolites can oxidize GSH and other thiol groups such as those in proteins and thereby cause a change in thiol status. When the rate of oxidation of GSH exceeds the capacity of GSH reductase, then oxidized glutathione (GSSG) is actively transported out of the cell and thereby lost. Thus, reduced GSH may be removed reversibly by oxidation or formation of mixed disulfides with proteins and irreversibly by conjugation or loss of the oxidized form from the cell. Thus, after exposure of cells to quinones such as menadione, which cause oxidative stress, GSH conjugates, mixed disulfides, and GSSG are formed, all of which will reduce the cellular GSH level. 

The basis of the selectivity and differences in type of activity of the triazines has been discussed thoroughly conjugation with glutathione, via displacement of the chlorine. 

Mechanisms such as conjugation of the reactive chemical with glutathione are protective mechanisms that exist within the cell for the rapid removal and inactivation of many potentially toxic compounds. Because of these interactions, cellular toxicity is a function of the balance between the rate of formation of reactive metabolites and the rate of their removal. Examples of these interactions are presented in the following discussions of specific hepatotoxicants. 

Endogenous substances other than metallothionein may be involved in minimizing the effects of heavy metals and excreting them from the body. Hepatic (liver) glutathione, discussed as a phase II conjugating agent in Section 7.4, plays a role in the excretion of several metals in bile. These include the essential metals copper and zinc toxic cadmium, mercury(II), and lead(II) ions and organometallic methyl mercury. 

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