Enzymatic and Nonenzymatic Synthesis of Glutathione Conjugates:  Application to the Understanding of a Parasite's Defense System and Alternative to the Discovery of Potent Glutathione S-Transferase Inhibitors

A primary pathway for metabolism of electrophilic compounds in Schistosoma japonicum involves glutathione S-transferase (SjGST)-catalyzed formation of glutathione (GSH) conjugates. As part of a program aimed at gaining a better understanding of the defense system of parasites, a series of aromatic halides (1−8), aliphatic halides (9, 10), epoxides (11−20), α,β-unsaturated esters (21, 22), and α,β-unsaturated amides (23, 24) were prepared, and their participation in glutathione conjugate formation was evaluated. Products from enzymatic and nonenzymatic reactions of these substances with glutathione were characterized and quantified by using reverse-phase high-performance liquid chromatography (HPLC), NMR, and fast atom bombardment mass spectrometry (FAB-MS) analysis. Mechanisms for formation of specific mono(glutathionyl) or bis(glutathionyl) conjugates are proposed. Although the results of this effort indicate that SjGST does not catalyze addition or substitution reactions of 1, 3, 4, 7−9, 11−13, 15−17, 19−21, and 24, they demonstrate that 2, 5, 6, 14, 18, and 23 undergo efficient enzyme-catalyzed conjugation reactions. The k cat values for SjGST with 23 and 18 are about 886-fold and 14-fold, respectively, larger than that for 5. This observation suggests that 23 is a good substrate in comparison to other electrophiles. Furthermore, the initially formed conjugation product, 23a, is also a substrate for SjGST in a process that forms the bis(glutathionyl) conjugate 23b. Products arising by enzymatic and nonenzymatic pathways are generated under the conditions of SjGST-activated GSH conjugation. Interestingly, production of nonenzymatic GSH conjugates with electrophilic substrates often overwhelms the activity of the enzyme. The nonenzymatic GSH conjugates, 9a − 11a, 16a, 21a, and 22a, are inhibitors of SjGST with respective IC50 values of 1.95, 75.5, 0.96, 19.0, 152, and 0.36 μM, and they display moderate inhibitory activities against human GSTA2. Direct evidence has been gained for substrate inhibition by 10 toward SjGST and GSTA2 that is more potent than that of its GSH conjugate 10a. The significance of this work is found in the development of a convenient NMR-based technique that can be used to characterize glutathione conjugates derived from small molecule libraries as part of efforts aimed at uncovering specific potent SjGST and GSTA2 inhibitors. This method has potential in applications to the identification of novel inhibitors of other GST targets th