Oxidation--reduction potentials of turkey liver xanthine dehydrogenase and the origins of oxidase and dehydrogenase behaviour in molybdenum-containing hydroxylases

Abstract

Redox potentials for the various centers in the enzyme xanthine dehydrogenase (EC 1.2.1.37) from turkey liver were determined by potentiometric titration in the presence of mediator dyes, with low-temperature EPR spectroscopy. Values at 25.degree. C in pyrophosphate buffer, pH 8.2, are: Mo(VI)/Mo(V) (rapid), -350 .+-. 20 mV; Mo(V) (rapid)/Mo(IV), -362 .+-. 20 mV; Fe-S Iox./Fe-S Ired., -295 .+-. 15 mV; Fe-S IIox./Fe-S IIred., -292 .+-. 15 mV; FAD/FADH; -359 .+-. 20 mV; FADH.cntdot./FADH2, -366 .+-. 20 mV. This value of the FADH.cntdot./FADH2 potential, which is 130 mV lower than the corresponding one for milk xanthine oxidase, accounts for many of the differences between the 2 enzymes. When allowance is made for some interference by desulfo enzyme, then differences in the enzymes'' behavior in titration with xanthine are accounted for by the potentials. Increases in the Mo potentials of the enzymes caused by the binding of uric acid are discussed. Though the potential of uric acid/xanthine (-440 mV) is favorable for full reduction of the dehydrogenase, during turnover, for kinetic reasons, only FADH.cntdot. and very little FADH2 is produced from it. Since only FADH2 is expected to react with O2, lack of oxidase activity by the dehydrogenase is explained. Reactivity of the 2 enzymes with NAD+ as electron acceptor is discussed in relation to the potentials.