Neutralising free radicals

The sulfur group of cysteine in glutathione gives up an electron to neutralise hydrogen peroxide and lipid peroxides. The enzyme glutathione peroxidase runs this reaction, using selenium. Two used glutathione molecules join into GSSG, and glutathione reductase rebuilds them using NADPH. The ratio of GSH to GSSG is the cell's main read-out of oxidative stress.