mitochondria (Pemble et al., 1996). GST classes are abbreviated in Roman
capitals and individual members are designated with a number. There may be
more than one enzyme within a class (Table 3.7). GSTs are soluble cytosolic
proteins existing as homo- or heterodimers and each monomer weighs approxi-
mately 25,000 Da for example, GSTA1 has 221 amino acids, MW = 25,500 Da.
Heterodimers are named according to their subunit composition, for example,
GSTA1-2. The amino acid sequence identity within each class is greater than 50%.
3.3.3 Localization and Expression
Soluble glutathione-S-transferases are found in most cells in the body. They are
cytosolic enzymes, and all of the GSTs have been crystallized. In the liver, they
represent a large percentage of the cytosolic protein (up to 5%) and may play a role
in shuttling highly lipophilic molecules such as bilirubin within the cell. A trivial
name for this function is ligandin. The alpha class is expressed highly in liver,
intestine, testes,kidney, and adrenals. GSTA1release into the plasma is a highly
sensitive marker of liver toxicity. GSTM2-2 is found primarily in skeletal muscle.
GSTM3 is in brain, lung, testes, GSTM4 in lymphoblasts, and GSTM5 in brain.
GSTP1-1 is widely expressed in all tissues except the liver. It is the major form in
fetal liver. GSTT enzymes are also widely expressed. Human sigma class enzymes
are expressed most highly in macrophages, placenta, and adipose tissue. There are
also membrane bound GSTsa microsomal GST (MGST1) conjugates chlor-
odinitrobenzene (CDNB) like most other GSTs (Morgenstem, 2005).
3.3.4 Reactions Catalyzed by GSTs
Glutathione-S-transferases catalyze the reaction of glutathione with electro-
philes, for example, epoxides, quinones, quinonemethides, to form glutathione
conjugates by activating the thiol of glutathione into the more reactive thiolate
anion within the active site. This greatly enhances the nucleophilicity of the
sulfur atom and significantly enhances reaction rates. Reactive electrophilic
metabolites of drugs, often formed by the cytochrome P450 system, are
common substrates for the GSTs. GSTs also catalyze SN2 displacement
reactions of halogen atoms with glutathione, for example, conjugation of
chlorodinitrobenzene (CDNB) or dichloronitrobenzene (DCNB), two com-
monly used general substrates. In addition, several anticancer alkylating agents
such as nitrogen mustards, busulfan, melphalan, and cyclophosphamide are
also substrates for these enzymes. The newly identified zeta class enzyme
(GSTZ1) functions as a maleylacetoacetate isomerase, a biochemical step in the
catabolism of tyrosine, by activating haloacetic acids (Board and Ander, 2005;
Tong et al., 1998). Two typical reactions are shown in Fig. 3.9.
3.3.5 Regulation of GSTs
GST alpha, mu, and pi expression is increased when cells are exposed to
electrophiles (e.g., diethymaleate, paraquat) that generate reactive oxygen
GLUTATHIONE-S-TRANSFERASES 73