Fluorometric Titration Assay of Affinity of Tight-Binding Nonfluorescent Inhibitor of Glutathione S-transferase

To determine inhibition constant (K (i)) of tight-binding inhibitor, the putative method estimated an apparent K (i) from the response of initial rates to total concentrations of the inhibitor considering its depletion during binding for conversion into the true K (i), but was impractical with glutathione S-transferase of sophisticated kinetics. A fluorometric titration assay of dissociation constant (K (d)) was thus proposed. Schistosoma japonicum glutathione S-transferase (SjGST) action on a nonfluorescent divalent pro-inhibitor and glutathione yielded a divalent product in active site to act as a tight-binding inhibitor, whose binding quenched fluorescence of SjGST at 340 nm under the excitation at 280 nm. K (d) was estimated from the response of fluorescence of SjGST at 340 nm to total concentrations of the divalent product considering its depletion during binding. By fluorometric titration assay, K (d) of two tested nonfluorescent divalent products varied from subnanomolar to nanomolar, but both were resistant to change of SjGST levels and consistent with their apparent K (i) estimated via the putative method. Hence, fluorometric titration assay of K (d) of nonfluorescent tight-binding inhibitors/ligands was effective to GST and may be universally applicable to common enzymes/proteins; affinities of tight-binding inhibitors of GST can be approximated by their apparent K (i) estimated via the putative method.