GSH conjugation

The effects of changing Jt-conjugation at the 4-position on both the rate of isomerization of the initially formed o-quinones to QMs and the reactivity of the quinoids formed from 4-propylcatechol, 2,3-dihydroxy-5,6,7,8-tetrahydronaphtha-lene (2-THNC), hydroxychavicol, and 4-cinnamylcatechol were studied (Fig. 10.6).9 These catechols were selectively oxidized to the corresponding o-quinones or QMs and trapped with GSH. Microsomal incubations with the parent catechols produced only o-quinone-GSH conjugates. However, if GSH was added after an initial incubation period both o-quinone- and QM-GSH conjugates were observed. The results indicate that the extended Jt-conjugation at the para position enhances the rate... 

Toremifene also undergoes oxidative metabolism to form a QM.59 The 4-hydro-xytoremifene QM has a half-life of lh at physiological pH and temperature (Table 10.2), while its half-life in the presence of GSH is approximately 6 min.59 The 4-hydroxytoremifene QM reacts with two molecules of GSH and loses chlorine to yield the corresponding di-GSH conjugate (Scheme 10.9). This reaction mechanism likely involves an electrophilic episulfonium ion intermediate, which could contribute to the potential cytotoxicity of toremifene. 

Screening is usually carried out with liver microsomes from humans, rats, mice, dogs and monkeys and liver S9 fraction from aroclor 1254-induced rats. The incubation is typically mn with a volume of 0.2-1. OmL in a microcentrifuge or a glass tube. Different incubation conditions are used for CYP and UGT reactions. The incubation mixture for formation of oxidative metabolites and/or GSH conjugates contains ... 

Microbial oxidation of drug substrates occurs in a similar fashion to mammalian oxidative biotransformation. In contrast, microbial cultures rarely catalyze conjugations comparable to those in mammalian system (glucuronidation, sulfation and GSH conjugation). It is thus not surprising that microbial bioreactors are mainly used in the synthesis of oxidative metabolites. 

Cysteine conjugation Mercapturic acid (from GSH conjugation) r-ch=ch2> R-CH2-CH2-Cys +121 (119) +161... 

FIGURE 7.16 Examples of electrophiles that form glutathione (GSH) conjugates. 

The herbicide alachlor (4.146, Fig. 4.7) also displayed species-dependent toxicity, since it induced nasal tumors in rats but not in mice. Its metabolic scheme in rats and mice (Fig. 4.7) shows that alachlor can be transformed into 2,6-diethylaniline (4.149) by two different pathways, one of which proceeds via formation of 4.147. The other pathway implies glutathione (GSH) conjugation, followed by /3-lyase-mediated liberation of the thiol, followed by S-methylation to produce the methylsulfide 4.148. The two secondary amides 4.147 and 4.148 were hydrolyzed by microsomal arylamidases, but alachlor itself was not a substrate for this enzyme. The hydrolytic product 2,6-diethylaniline (4.149) was oxidized in nasal tissues to the electrophilic quinonimine metabolite 4.150, which can bind covalently to proteins. Aryl-... 

Plant. In plants, alachlor is absorbed, translocated, and transformed into glutathione (GSH) conjugates (Breaux et ah, 1987 O Connell et al, 1988). Four hours after treating corn with C-labeled alachlor, 46% was converted to GSH conjugates and 42% was unreacted alachlor (O Connell et ah, 1988). 

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. 

Carbamylation Reactions. -Alkyl, -benzyl and -chloroallyl thiocarbamates do not readily react with GSH. In contrast, their sulfoxide derivatives ( and 6,) are very effective carbamylating agents for many thiols including GSH (19, ). The GSH conjugates formed vivo via 3 and 6 are quickly cleaved, acetylated and further metabolized as follows (19-21. 23. 24). 

In spite of the Importance of the Initial GSH conjugtlon reaction, very little Information Is present In the literature regarding the catabolism of GSH conjugates of pesticides In higher plants. The catabolism of the GSH conjugate of atrazlne In sorghum has been studied (Figure 1), but there appear to be few similar reports. 

Because of the Importance of GSH conjugation In pesticide metabolism and the diversity of the plant kingdom. It seemed desirable to study the catabolism of another GSH conjugate In several plant species. Pentachloronltrobenzene-UL- C,... 

ClPCNB (Figure 2), was chosen as the primary biochemical probe to extend studies on GSH conjugate catabolism In higher plants for the following reasons ... 

The study of several GSH conjugates was possible because several sites on the PCNB nucleus were susceptible to attack by GSH. 

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