ABSTRACT
Xanthine dehydrogenase (XDH; EC 1.1.1.204) reduces NADþ to NADH (7), as well as O2 to H2O2 (8) with xanthine as the second substrate [Eqs. (2) and (3)].
XanthineþNADþ ! uric acidþNADHþHþ ð2Þ XanthineþO2! uric acidþH2O2 ð3Þ
Steady-state kinetic rate constants of xanthine=O2 and xanthine=NADþ reactions catalyzed by XDH are 2:1 0:1 sec1 (7) and 6:3 sec1 (8) at 25C and pH 7.5, respectively. For comparison, the oxidation of NADH=O2 is 2:5 0:9 sec1 under the same conditions. Therefore, XDH has a significant and intrinsic xanthine oxidase activity on some substrates (7, 8).
