Alkaline phosphatases are a group of isoenzymes, located on the outer layer of the cell membrane; they catalyze the hydrolysis of organic phosphate esters present in the extracellular space. Zinc and magnesium are important co-factors of this enzyme. Although alkaline phosphatases are present in different body tissues and have different physiochemical properties, they are true isoenzymes because they catalyze the same reaction. In the liver, alkaline phosphatase is cytosolic and present in the canalicular membrane of the hepatocyte. Alkaline phosphatase is present in decreasing concentrations in the placenta, ileal mucosa, kidney, bone, and liver. The majority of alkaline phosphatase in serum (more than 80%) is released from liver and bone, and in small amounts from the intestine. Even though alkaline phosphatases are present in many tissues throughout the body, their precise physiological function remains largely unknown.[1][2] Alkaline phosphatases are classified as tissue-specific and tissue nonspecific types. Alkaline phosphatases found in the intestine, placenta, and germinal tissue are tissue-specific. This means they are found only in the tissues where they are expressed in physiological conditions. They may also contribute to the circulating pool of serum alkaline phosphatase under specific situations when there is increased stimulation of their production. The tissue-nonspecific alkaline phosphatases form most of the fraction circulating in serum and, therefore, is of clinical interest. A single gene encodes it and is expressed in the liver, bone, and kidneys. Intestinal alkaline phosphatase is coded by a separate gene, which is different from the gene that codes for placental alkaline phosphatase and the Regan isoenzyme (produced in excess amounts in Hodgkin lymphoma). All tissue-nonspecific alkaline phosphatases have the same amino acid sequence but different carbohydrate and lipid side chains; post-translational modifications confer their unique physicochemical properties.[3][4]