Cyclooxygenase 1 (COX-1) is a bifunctional enzyme that exhibits both COX and peroxidase activities.{500,501} It is composed of an N-terminal signal peptide, an EGF-like domain, a membrane binding domain, a catalytic domain, and a C-terminal tail.{4376} COX-1 is constitutively expressed in the gastrointestinal tract, kidney, spleen, liver, and lung and localizes to the endoplasmic reticulum.{1299,1394} The COX component converts arachidonic acid (Item Nos. 90010 | 90010.1 | 10006607) to a hydroperoxyl endoperoxide prostaglandin G2 (PGG2; Item No. 17010) and the peroxidase component reduces the endoperoxide to the corresponding alcohol PGH2 (Item No. 17020), the precursor of PGs, thromboxanes, and prostacyclins.{500,501} COX-1 is the target of many non-steroidal anti-inflammatory drugs (NSAIDs) and is responsible for the undesirable gastrointestinal and renal side effects, such as ulcer formation and reductions in the glomerular filtration rate, respectively.{5282,4411} This product is intended to be used as a standard for Western blot (WB) and electrophoresis. This enzyme has been carefully purified to exclude all unrelated proteins and may not be catalytically active.