XDH and chronic kidney disease: On the one hand, circulating XO activity may be dangerous because once in circulation, it has the ability to activated phagocytic cells and produce O2•− and H2O2, and it can be distributed to remote tissues [51,52], as well as internalized into vascular and other cells; therefore, it may further initiate oxidative damage exerting pathological effects in CKD patients [51,53].