Abstract

Introduction

Materials & Methods

Results

Discussion & Conclusion

References

Discussion
Board

The Glutathione-S-Transferases (GSTs; E.C. 2.5.1.18) are a practically ubiquitous multigene enzyme superfamily responsible for detoxification of xenobiotics in aerobic organisms. Endogenous substrates include the toxic products of tissue damage, including the hydroperoxide products of oxidative damage, and aromatic xenobiotics. GSTs are catalysts of reactions in which reduced glutathione (GSH) acts as a nucleophile, conjugating to and facilitating removal or reduction of the second substrate. In all organisms shown to have GST activity, multiple forms of the enzyme (which may or may not be tissue-specific) have been discovered. Subtypes of GST have been grouped into classes on the basis of isoelectric point, substrate and inhibitor properties, antibody recognition, and N-terminal amino acid sequence; the main cytosolic classes are Alpha, Mu and Pi (previously known as basic, near-neutral and acidic, on the basis of isoelectric point)(Mannervik et al., 1992). This nomenclature system was developed for rat tissue but class homologies have been noted in other mammalian, bacterial and amphibian sources. GSTs are dimeric, forming homodimers or heterodimers (heterodimers only form within a class). Alpha class GST exhibits the highest hydroperoxidase (i.e. antioxidant) activity of the GSTs; Mu or Pi classes show low to negligible activity.

GST activity may be transcriptionally regulated by xenobiotics or environmental stresses such as heat or pollution (Makar et al., 1994). Studies from this laboratory have shown changes in GST activity in animals which endure natural environmental stresses (Pannunzio and Storey, 1998; Willmore and Storey, 1997; Hermes-Lima and Storey, 1993). The spadefoot toad of the southwestern United States (Scaphiopus couchii) withstands desert dryness and desiccation by burrowing beneath the soil and entering a state of metabolic depression known as estivation. GST and other enzymes with antioxidant function were surveyed in tissues of awake and estivating toads, and it was found that estivation was accompanied by an overall reduction of antioxidant metabolite and enzyme defences, resulting in oxidative damage(Grundy and Storey, 1998). GST activity was found to be reduced by close to 50% in liver and leg muscle of estivating toads (Grundy and Storey, 1998). To determine whether reduced GST activity was due to reduced levels of the same isozyme, or to alterations in the activity of the enzyme itself, we purified and partially characterized GST from toad liver and muscle. For comparison, GST was also isolated from the liver and leg muscle of a non-estivator, the leopard frog, Rana pipiens. Preliminary classification of purified frog and toad GSTs was done on the basis of activity (i.e. presence of peroxidase activity might indicate Alpha class), and inhibition with cibacron blue, rose bengal and s-hexylglutathione, as per Tahir et al., (1985).

Back to the top.