Glutathione transferases reactions

Figure 7.19 Glutathione transferase reaction and formation of mercapturic acids.

Glutathione Transferase Reactions Glutathione (GSH), a tripep-tide, comprised of glutamic acid, cysteine and glycine, is a common constituent of most cells. Note that the peptide bond of the glutamic involves the y- rather... 

More general cases are encountered in the metabolism of a variety of ha-log enated hydrocarbon solvents and insecticides [58]. Examples include the dehydrochlorination of 1,1,2,2-tetrachloroethane to trichloroethylene in the mouse, and of DDT (l,l,l-trichloro-2,2-bis(4-chlorophenyl)ethane) to DDE (l,l-dichloro-2,2-bis(4-chlorophenyl)ethene) [58][77]. Glutathione transferases may be involved in some of these reactions. 

For example ca. 4 pmole of parathion can be metabolized (to a 4-nitrcphenol hydrolysis prodix t) in one hour by a suspension of 10 cells (Shen and Dcwd unpublished data). Dechlorination of 1-chloro 2 4-dinitrdDenzene, by a cell-free extract (1000 g supernatant) occurred at a rate of 20 nmole/min/mg protein (Shen and DcMd unpublished data). This dechlorination reaction appears to be performed by a glutathione transferase-like enzyme, since the reaction is glutathione dependent. Both a-glucosidase and 3-glucosidase activity have also been detected (Shen and Dcwd uipublished data). 

This reaction is mediated by an enzyme called a glutathione transferase, which facilitates the encounter of GSH and the compound it is attacking (Mannervik, 1985 Vuilleumier, 1997). Formation of the GSH adduct (i.e., the compound formed when the two reactants are attached to one another) permits the attack of water on the previously chlorinated carbon and since the resulting intermediate is not particularly stable in this case, it decomposes, releasing formaldehyde and regenerating glutathione in a reaction much like the dehydration of gem-diols ... 

Multiple forms of GST have been demonstrated in the liver of many mammalian species multiple forms also occur in insects. Most GSTs are soluble dimeric proteins with molecular weights ranging between 45,000 and 50,000 daltons. All forms appear to be nonspecific with respect to the reaction types described, although the kinetic constants for particular substrates vary from one form to another. They are usually named from their chromatographic behavior. At least two are membrane-bound glutathione transferases, one of which is involved in metabolism of xenobiotics and is designated... 

Phase II Reactions. As with phase I reactions, phase II reactions usually depend on several enzymes with different cofactors and different prosthetic groups and, frequently, different endogenous cosubstrates. All of these many components can depend on nutritional requirements, including vitamins, minerals, amino acids, and others. Mercapturic acid formation can be cited to illustrate the principles involved. The formation of mercapturic acids starts with the formation of glutathione conjugates, reactions catalyzed by the glutathione -transferases. 

Fig. 6. (Opposite page) A high-throughput production method for screening proteins from cDNA libraries. Authentic (A) and glutathione -transferase (GST)-fused (G) proteins in the reaction mixtures after a semi-automated polymerase chain reaction/ transcription and translation from 54 different cDNAs separated by sodium dodecyl sul-fide-polyaciylamide gel electrophoresis and stained with Coomassie Brilliant Blue. T and S, respectively, mark total translation product and the supernatant fraction after centrifugation at 30,000g for 15 min.

The epoxidation of aldrin to dieldrin is an example of the metabolic formation of a stable epoxide (Figure 10.1A), while the oxidation of naphthalene was one of the earliest understood examples of an epoxide (arene oxide) as an intermediate in aromatic hydroxylation (Figure 10.1B). The arene oxide can rearrange nonenzy-matically to yield predominantly 1-naphthol, can interact with the enzyme epoxide hydrolase to yield the dihydrodiol, or can interact with glutathione -transferase to yield the glutathione conjugate that ultimately is metabolized to a mercapturic acid. This reaction is also of importance in the metabolism of the insecticide carbaryl, which contains the naphthalene nucleus. 

Protein cross-links may be also produced in reaction of 4-hydroxynonenal with lysine, histidine, serine, and cysteine residues, primarily via Michael addition (J5, R7, U8). These reactions occur spontaneously, but also may be catalyzed by certain glutatione 5-transferases. The glutathione transferase A4-4, which unlike other alpha-class glutathione transferases, shows high catalytic activity toward lipid peroxidation products such as 4-hydroxynon-2-enal, is the key enzyme for these reactions (B31). Products of protein coupling with aldehydes secondary to lipid peroxidation have a specific fluorescence, which can herald the protein oxidative modification process (CIO). 

The presence of the combined alleles Ml and Tl, which mark deficiencies in glutathione-.S -transferase genes, increases susceptibility to tacrine hepatotoxicity (5). It would be interesting to use this molecular epidemiological approach to identify the role of combinations of glutathione- -transferase genotypes in other adverse drug reactions. 

Glutathione transferases catalyze a substitution reaction at electrophilic centers of molecules. They are also binding proteins in analogy with the E4 esterase a mammalian form, called ligandin, binds with high affinity to a broad spectrum of compounds but does not catalyze the subsequent substitution reaction (38). The role of the transferases in the catalytic reaction is thought to be to provide close proximity between the xenobiotic and the reduced glutathione anion, GS-. 

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