THIOL ESTER HYDROLYSIS CATALYZED BY GLUTATHIONE-S-TRANSFERASE A1-1

rGSTA1-1 has been shown to catalyze the hydrolysis of the thiol ester glutathionyl ethacrynate (E-SC). In contrast, neither the retro-Michae l addition with the substrate EA-SG, to yield GSH and ethacrynic acid (EA), nor the conjugation reaction between GSH and EA to yield the thi ol ester E-SC was catalyzed to any measurable extent under similar con ditions. The steady state k(cat) and K-M for hydrolysis of E-SG by wil d type rGSTA1-1 were 0.11 +/- 0.009 min(-1) and 15.7 +/- 1.6 mM, respe ctively. The site directed mutant, Y9F, in which the catalytic Tyr-9 i s substituted with Phe, was completely inactive in this reaction. To u ncover a mechanistic signature that would distinguish between direct h ydrolysis and covalent catalysis involving acylation of Tyr-9, solvent isotope exchange and mass spectrometry experiments were performed. No O-18 incorporation into the starting thiol ester was detected with in itial velocity solvent isotope exchange experiments. However, covalent adducts corresponding to acylated protein also were not observed by e lectrospray ionization mass spectrometry, even with an assay that mini mized the experimental dead time and which allowed for detection of N- acetyltyrosine acylated with EA in a chemical model system. The k(on) and k(off) rate constants for association and dissociation of E-SG wer e determined, by stopped flow fluorescence, to be 5 x 10(5) s(-1) M-1 and 6.7 s(-1), respectively. Together with the isotope partitioning re sults, these rate constants were used to construct partial free energy profiles for the GST catalyzed hydrolysis of E-SG, assuming that Tyr- 9 acts as a general acid-base catalyst. The ''one-way flux'' of the th iol esterase reaction results directly from the thermodynamic stabilit y of the products after rate-limiting attack of the thiol ester by H2O or Tyr-9, and is sufficient to drive the hydrolysis to completion, in contrast to GST-catalyzed breakdown of other GSH conjugates.