P-164 - Reaction of glutathione and bio-related thiols with 2,2-Diphenyl-1-picrylhydrazyl radical solubilized by beta-cyclodextrin in water

Abstract

Glutathione (GSH) is known to act as a redox regulator. However, the radical-scavenging mechanism of GSH has yet to be fully clarified. Recently, we have succeeded in solubilizing water-insoluble 2,2-diphenyl -1-picrylhydrazyl (DPPH) radical in water using beta-cyclodextrin. In this study, the scavenging mechanism of the water-solubilized DPPH radical by GSH was investigated using the stopped-flow technique. Upon mixing of GSH with the DPPH radical in phosphate buffer (0.05 M, pH 7.4), the absorption band at 527 nm due to the DPPH radical decreased gradually. The pseudo-first order rated constants (kobs), determined from the decay of the absorbance at 527 nm, increased with increasing the GSH concentration and reached a constant value. Furthermore, the higher pH was, the larger the kobs values became. When H2O of the phosphate buffer was replaced with D2O, little change was observed for the kobs values. Thiol compounds with smaller pKa values than GSH, such as L-cysteine, showed higher scavenging activity than GSH under the same experimental conditions. These results suggest that the DPPH radical-scavenging reaction by GSH may proceed via deprotonation of GSH to produce the corresponding thiolate anion, GS-, followed by an electron transfer from GS- to the DPPH radical.