NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE THIOLTRANSFERASE-CATALYZED GLUTATHIONE/GLUTATHIONE DISULFIDE INTERCHANGE REACTION

The kinetics of the thioltransferase-catalyzed symmetrical glutathione/glutathione disulfide (GSH/GSSG) interchange reaction have been studied by H-1-nuclear magnetic resonance spectroscopy. Kinetic parameters were determined by analysis of exchange-broadened multiplet patterns and by the inversion-magnetization transfer method using concentrations of GSH, GSSG and pig liver thioltransferase similar to intracellular concentrations. The rate constant for the reaction of GSSG with thioltransferase to form a thioltransferase-glutathlone mixed disulfide and GSH was estimated to be greater than or equal to 7.1 (+/- 0.4). 10(5) M(-1) s(-1). This reaction is proposed to be the first step in the mechanism by which the activity of some proteins is modulated by the thioltransferase-catalyzed formation of protein-glutathione mixed disulfides. The rate constant for the reaction of GSSG with thioltransferase is 4-5 orders of magnitude larger than rate constants for the analogous reaction of the thiolate groups of a variety of small molecules with GSSG. The symmetrical gamma-L-glutamyl-L-cysteine/gamma-L-glutamyl-L-cysteine disulfide (GCSH/GCSSCG), L-cysteinyl-glycine/L-cysteinyl-glycine disulfide (CGSH/CGSSGC) and cysteine/cystine (CSH/CSSC) thiol/disulfide interchange reactions were also studied as models for the GSH/GSSG interchange reaction. The GCSH/GCSSCG interchange reaction was found to be catalyzed by thioltransferase, and the rate constant for the reaction of GCSSCG with thioltransferase was estimated to be greater than or equal to 5.7(+/- 1.7). 10(4) M(-1) s(-1). In contrast, the CGSH/CGSSGC and CSH/CSSC interchange reactions were found to be slow on the NMR time-scale for the conditions used in this research, both in the absence and presence of thioltransferase. The results suggest that the gamma-L-glutamyl-L-cysteinyl moiety of GSSG and of GSH-containing mixed disulfides is essential for their recognition by thioltransferase.