Intraellular Antioxidant Dietary Supplement Gluten Free Nutrition Glutathione is the most abundant intracellular thiol (i.e ., a compound containing a sulfhydryl [Sh] group) and low molecular weight tripeptide found in living cells."" The antioxidant functions of Glutathione Reduced include recycling vitamins E and C and serving as an antioxidant in all types of tissues.