 Type I xanthinuria has been traced directly to mutations of the "XDH" gene which mediates xanthine oxidase activity.Type II xanthinuria may result from a failure of the mechanism which inserts sulfur into the active sites of xanthine oxidase and aldehyde oxidase, a related enzyme with some overlapping activities (such as conversion of allopurinol to oxypurinol).Inhibition of xanthine oxidase has been proposed as a mechanism for improving cardiovascular health.