Schistosoma japonicum GSH S-transferase Sj26 is not the molecular target of praziquantel action.

It has been suggested that Sj26, a Schistosoma japonicum GSH S-transferase, is the molecular target of the antischistosomal drug praziquantel (McTigue et al., 1995, J. Mol. Biol. 246, 21-27). We tested this hypothesis by asking two questions: (1) does praziquantel inhibit Sj26 activity with a variety of model substrates; and (2) does praziquantel prevent the binding to Sj26 of physiologically relevant nonsubstrate ligands? High concentrations of praziquantel (up to 500 microM) did not inhibit Sj26 activity using the model substrates 1-chloro-2,4-dinitrobenzene, 3,4-dichloronitrobenzene, or ethacrynic acid. Sj26 had no measurable activity with two higher molecular weight GSH S-transferase substrates: 5-androsten-3,17-dione and sulfobromophthalein. We also assessed the ability of praziquantel to prevent the inhibition of Sj26 by a series of S-alkyl-GSH conjugates. The half-maximal inhibitory concentrations of S-hexyl-GSH, S-octyl-GSH, and S-decyl-GSH (10, 10, and 5 microM, respectively) for Sj26 were not affected by up to 500 microM praziquantel. This suggests that praziquantel does not compete with GSH for Sj26 binding. In order to determine if praziquantel disrupts binding of nonsubstrate ligands to Sj26, we tested praziquantel for its ability to prevent the inhibition of Sj26 by both bilirubin and hematin. Praziquantel (100 or 500 microM) did not alter inhibition of Sj26 by 3 microM bilirubin, but partially protected Sj26 against inhibition by hematin (0.1 to 2.0 microM). Interestingly, in a similar reaction, 100 microM S-methyl-GSH protected Sj26 from inhibition equally as well as praziquantel. Bovine serum albumin (5 microM) completely protected against inhibition by 1 microM hematin. These results indicate that although praziquantel partially protects Sj26 from hematin inhibition, this protection is neither specific to praziquantel nor physiologically relevant. Our results do not support the hypothesis that the mechanism of praziquantel action involves competitive inhibition of Sj26 catalytic activity or blocking binding of nonsubstrate ligands. We can, therefore, find no evidence that Sj26 is the molecular target of the antischistosomal activity of praziquantel.