To cope with various endogenous toxin and xenobiotics nature has equipped the organisms with a proper protection system. Glutathione transferases (GSTs) are important components of the cellular defense against oxidative stress. These proteins appear to be suited for different tasks. Based on catalytic activity of GSTs with monochlorobimane (MCB), a screening method was developed for identification of active GSTs in bacterial colonies and for characterization of combinatorial GST libraries. Solvent viscosity effects on kcat and kcat/Km on wild-type human GST A1-1 and phenylalanine-220 mutants indicate a physical step being the rate-limiting step in the catalytic mechanism. Three residues that were under evolutionary selection pressure were...