Investigation of the active site of human placenta glutathione transferase π by means of a spin-labelled glutathione analogue

A spin-labelled analogue (sl-glutathione) has been used in order to characterize the active site of human placenta glutathione transferase π. The sl-glutathione shows a competitive inhibition towards glutathione (K_i = 14 μM). Binding of sl-glutathione to the enzyme, followed by electron paramagnetic resonance spectroscopy, gives a K_d of 3 μM and two identical binding sites for dimeric unit. Inhibition of the enzyme, by modification of the Cys-47 residue, completely prevents the binding of sl-glutathione. The same results are obtained by monitoring the binding of glutathione by means of fluorescence spectroscopy. It is concluded that integrity of the thiolate of Cys-47 is necessary to maintain an active conformation of the enzyme able to efficiently bind glutathione into the active site.