Metabolism of Natural and Xenobiotic Substrates by the Plant Glutathione S-Transferase Superfamily
Glutathione transferases, also referred to as glutathione S-transferases (GSTs; EC 2. 5.1.18), were first described in animals in the 1960s (Booth et al. 1961), where they were determined to have important roles in catalysing the conjugation of drugs with
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2.1 Plant Glutathione Transferase (GST) Superfarnily 2.1.1 Discovery
Glutathione transferases, also referred to as glutathione S-transferases (GSTs; EC 2. 5.1.18), were first described in animals in the 1960s (Booth et al. 1961), where they were determined to have important roles in catalysing the conjugation of drugs with the tripeptide glutathione (y-glutamyl-cysteinylglycine). In this detoxification reaction the cysteinyl SH-group of the glutathione serves as a nucleophile, which attacks an electrophilic cent re in the drug to give rise to the S-glutathionylated derivative via substitution, or, more rarely, addition reactions (Fig. 2.1). The discovery of plant GSTs soon foHowed that of their mammalian counterparts. The identification of a glutathioneconjugated metabolite of atrazine in maize (see Fig. 2.1A; Shimabukuro and Swanson 1969) quickly led to the partial purification and characterisation of a maize GST that catalysed this reaction (Frear and Swanson 1970). These early studies clearly identified GSTs as a primary determinant of atrazine selectivity, with ce real crops containing much greater herbicide-detoxifying GST activity than competing weeds. Interests in the role of GSTs in herbicide selectivity in crops and weeds then became a major driver for further work on these enzymes for much of the next 30 years. GSTs involved in herbicide metabolism have since been purified and then cloned from major crops (maize, soybean, wheat and rice) as weH as problem weeds (reviewed by Edwards and Dixon 2000). It has also been recognised that GST activities toward xenobiotics appear to be ubiquitous in both lower and higher plants (Pflugmacher et al. 2000) as weH as in algae (Tang et al. 1998). Although most work has concentrated on the involvement of GSTs in herbicide selectivity, GSTs active in conjugating model substrates such as 1-chloro-2,4-dinitrobenzene (CDNB) without specific roIes in herbicide detoxification in the field have been purified to homogeneity from a diversity of plants including sugar Ecological Studies, Vol. 170 H. Sandermann (Ed.) Molecular Ecotoxicology of Plants © Springer-Verlag Berlin Heidelberg 2004
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R. Edwards and D. P. Dixon
A
HCI
GSH
Atrazine
CI
B
SG
CI CI CI CI
+
GSH
CI
CI
CI OH
CI
Tridiphane
CI
CI
CI
Fig.2.1. Detoxification of xenobiotics by GSTs mediated by catalysing their conjugation with glutathione by substitution (A) or addition (B) reactions. A shows the glutathione conjugation of the chloro-s-triazine herbicide atrazine, while B shows the two possible conjugation reactions seen with the atrazine synergist tridiphane. (Lamoureux and Rusness 1986)
cane (Singhal et al. 1991), Norway spruce (Schröder and Berkau 1993), pumpkin (Fujita et al. 1995), broccoli (Lopez et al. 1994) and peas (Edwards 1996). In all these studies, the importance of these enzymes in xenobiotic detoxification has been to the fore, and this preoccupation with the roles of GSTs in foreign compound metabolism has detracted from our understanding of the roles of these enzymes in both co
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