Dissociation and unfolding of Pi-class glutathione transferase: evidence for a monomeric inactive intermediate

The dissociation and unfolding of the homodimeric glutathione transferase (GST) Pi from human placenta, using different physicochemical denaturants, have been investigated at equilibrium. The protein transitions were followed by monitoring loss of activity, intrinsic fluorescence, tyrosine exposure, far-u.v. c.d. and gel-filtration retention time of the protein. At low denaturant concentration (less than 1 M for guanidinium chloride and less than 4.5 M for urea), a reversible dissociation step leading to inactivation of the enzyme was observed. At higher denaturant concentrations the monomer unfolds completely. The same unfolding behaviour was also observed with high hydrostatic pressure as denaturant. Our results indicate that the denaturation of GST Pi is a multistep process, i.e. dissociation of the active dimer into structured inactive monomers followed by unfolding.