Glycolate oxidase is a member of the superfamily of the a-hydroxy acid oxidases (HAO), enzymes that are present in both plants and animals. It catalyzes the FMN-mediated oxidation of glycolate to glyoxylate and glyoxylate to oxalate with reduction of oxygen to hydrogen peroxide. The co-factor, FMN, is tightly bound but not covalently linked to the protein. In humans and other vertebrates, HAOs are found primarily in the peroxisomes of liver, kidney, and pancreas. Three HAOs have been identified in humans. HAO-1 is most highly expressed in liver and pancreas and is most active on two-carbon substrates such as glycolate. HAO-2 is expressed in liver and kidney and has greater activity against long-chain-a-hydroxy acid substrates such as 2-hydroxypalmitate. HAO-3 is expressed primarily in the pancreas. Recently, HAO has been identified as a major contributor to hyperoxaluria, a disorder in which large deposits of calcium oxalate form kidney stones.