Glutathione can occur in reduced (GSH), oxidized (GSSG), or in mixed disulfide forms and is ubiquitous in multiple biological systems serving as the major thiol-disulfide redox buffer of the cell. GSSG is the oxidized form of GSH . It can be reduced back to GSH through the NADPH-dependent enzyme glutathione reductase. GSSG functions as a hydrogen acceptor in the enzymatic determination of NADP+ and NADPH and can be a proximal donor in S-glutathionylation post translational modifications. The ratio of reduced glutathione to oxidized glutathione within cells is often used as an indicator of oxidative stress, with higher concentrations of GSSG predicting increased oxidative stress.