FUNCTION: Involved in redox regulation of the cell. Can reduce H(2)O(2) and short chain organic, fatty acid, and phospholipid hydroperoxides. May play a role in the regulation of phospholipid turnover as well as in protection against oxidative injury. SUBUNIT: Homotetramer. SUBCELLULAR LOCATION: Cytoplasm. Lysosome. Also found in lung secretory organelles. MISCELLANEOUS: The active site is the redox-active Cys-47 oxidized to Cys-SOH. Cys-SOH may rapidly react with a Cys-SH of the other subunit to form an intermolecular disulfide with a concomitant homodimer formation. The enzyme may be subsequently regenerated by reduction of the disulfide by thioredoxin . MISCELLANEOUS: Irreversibly inactivated by overoxidation of Cys-47 (to Cys-SO(3)H) upon oxidative stress. SIMILARITY: Belongs to the ahpC/TSA family. Rehydrin subfamily.