Molybdenum is an essential trace element, and functions as a cofactor of various enzymes involved in the detoxification of pyrimidines, purines, pteridines. Xanthine oxidase and xanthine dehydrogenase are two important molybdoenzymes converting hypoxanthine to xanthine to uric acid. Molybdenum is also involved in sulfur metabolism as a cofactor of sulfite oxidase which converts potentially toxic sulfite to sulfate. This reaction is necessary for the normal metabolism of sulfur amino acids. Molybdenum is efficiently absorbed in the stomach and the small intestine. The organs that retain most of the absorbed molybdenum are the liver and kidney. Molybdenum metabolism is dynamic, and tissue turnover is high. Plasma levels are efficiently regulated by the kidneys which readily excrete any excess molybdenum. Significant amounts of molybdenum are also excreted via the bile.