S-allylmercaptoglutathione Is a Substrate for Glutathione Reductase (E.C. 1.8.1.7) from Yeast (Saccharomyces cerevisiae)

Allicin (diallylthiosulfinate) is a potent thiol reagent and natural defense substance produced by garlic (Allium sativum) tissues when damaged. Allicin acts as a redox toxin and oxidizes the cellular glutathione (GSH) pool producing S-allylmercaptoglutathione (GSSA). The cellular enzyme glutathione reductase (GR) uses NADPH to reduce glutathione disulfide (GSSG) back to GSH and replenishes the GSH pool. It was not known whether GR could accept GSSA as a substrate. Here, we report that GR from yeast (Saccharomyces cerevisiae) shows Michaelis–Menten kinetics with GSSA as substrate in vitro (Km = 0.50 mM), but that GSSA is not as good a substrate as GSSG (Km = 0.07 mM). Furthermore, cells unable to synthesize GSH because the γ-glutamylcysteine synthetase (GSH1) gene is deleted, cannot grow without GSH supplementation and we show that the auxotrophic requirement for GSH in Δgsh1 mutants can be met by GSSA in the growth medium, suggesting that GSSA can be reduced to GSH in vivo