文摘

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.