Characterization of the Molybdenum Cofactor of Sulfite Oxidase, Xanthine Oxidase, and Nitrate Reductase

While the existence of a molybdenum cofactor common to nitrate reductase, xanthine oxidase, and sulfite oxidase has been finnly established by studies from several laboratories (1-6), the nature of the molecule has not been elucidated. Information available to date has shown that the molybdenum cofactor complex is noncovalently attached to the proteins (2, 4) and is probably a small, dialyzable molecule (2, 3). The extreme lability of the active cofactor released from various proteins in the presence of oxygen has thwarted previous attempts at characterization of the molecule. Using the rationale that structural studies on a stable oxygen-inactivated cofactor would provide clues to the structure of the active parent compound, we set out to develop a procedure for isolation of inactive molybdenum cofactor. The use of homogeneous molybdenum-containing enzymes as the starting material simplified the task of cofactor purification and allowed verification that the isolated, catalytically inactive material was indeed derived from the molybdenum cofactor. We report here the results of our initial characterization of the molybdenum cofactor and provide proof for the presence of a pteridine moiety as a structural component.