Enzymology at the dimer interface of cytosolic glutathione S-transferases

Abstract

The glutathione S-transferases (GSTs) represent a family of cytosolic enzymes whose primary function is the detoxification of electrophilic chemical species of endogenous and exogenous origin. As described in the introductory chapter (Chapter 1) these enzymes exist as homodimers and exhibit a broad, solvent-accessible cleft above the dimer interface. This interface region represents an area of active research in GST enzymology, as it is purported to be a ligand binding site, and it lies directly between the two active sites of the GST dimer. We exploited this geometry to design bifunctional inhibitors of this enzyme which simultaneously occupy both active sites by spanning the intersubunit cleft (Chapter 2). Such inhibitors may be desirable as adjuvants to cytotoxic tumor chemotherapy, as GSTs are believed to play a key role in acquired resistance to these important drugs. We also prepared chemically modified GST in which the chemical environment of the intersubunit cleft is altered (Chapter 3). The results of these studies suggest that many ligands which have been reported by other researchers to bind in the intersubunit cleft may actually bind at the active site. Finally, Chapter 4 describes an unusual property of the GST cofactor, glutathione, which was discovered during the above studies---in polar organic solvents, glutathione disulfide appears to self-assemble to form extensive intramolecular arrays which tangle and result in gelation of the solvent. This self-assembly property is highly specific and depends on the unusual geometry of the tripeptide glutathione.