A labile disulfide-containing compound under physiological conditions, comprising: the disulfide-containing compound having a labile disulfide bond that is either a disulfide bond that is cleaved more rapidly than oxidized glutathione or a disulfide bond constructed from thiols in which one of the constituent thiols has a lower pKa than glutathione or a disulfide bond that is activated by intramolecular attack from a free thiol.