Multiple Phases in the Reduction of Xanthine Oxidase by Substrates

Abstract

1.A sytematic study has been made using electron paramagnetic resonance spectroscopy of the effects of enzyme concentration and relative proportions of active and inactivated enzyme on the anaerobic reduction of xanthine oxidase a t pH 8.2 by the substrates xanthine, purine and NADH. Changes in intensity of two molybdenum (V) signals (earlier designated Rapid and Slow) and iron-sulphur and flavin radical signals were monitored over the time-range 0.1 sec to 2 days. A few measurements with the inhibitory substrate allopurinol were also made.2. Reduction can occur in as many as four phases, all of which are clearly seen with xanthine as substrate. The Rapid type of signals from active enzyme first appear in a time comparable to the turnover time (phase I), then decrease in intensity over a somewhat longer time (phase 11). This is followed by a further increase in intensity over periods of up to a few hours (phase 111), to levels higher than those attained in phase I. Finally with a half-life of several hours (phase IV) the Slow signal, which is known to be derived from inactivated xanthine oxidase, appears also. Fewer phases can be clearly distinguished with the other substrates.3. I n accordance with other work, results are interpreted in terms of redox equilibria among different enzyme species with molybdenum signal intensity controlled by interconversions between the paramagnetic Mo(V) and the diamagnetic Mo(IV) and Mo(V1) oxidation states. The observed kinetics indicate that Mo(V) produced in the catalytic phase I is "over reduced" in phase I1 to Mo(1V) in a bimolecular reaction in which inactivated enzyme is not involved and €or which the reducing agent may possibly be a substrate radical. This Mo(1V) of active xanthine oxidase is then slowly reoxidised to Mo(V) by inactivated enzyme, which remains inert in the catalytic phase but undergoes reduction in the slower phases. The reducing equivalents appear in the inactivated form, first in the flavin and iron-sulphur and then in the molybdenum. However, with NADH, where catalytic reduction is known to occur via flavin instead of via molybdenum, both active and inactivated enzymes are reduced simultaneously, thus confirming that the difference between these enzyme forms is minimal.I n studies of enzyme catalysis in two-substrate reactions it is often possible to observe, by some spectroscopic method, changes in the enzyme when treated with the first substrate in the absence of the second. Only if the observed changes are shown to occur within the turnover time for catalysis, can it reasonably be assumed that they are relevant to the enzymatic mechanism. Slower reactions between enzyme and substrate may still provide useful information about the chemical nature of groups in the active site but the interpretation of such results must be approached with caution. I n particular, consideration must be given to the possibility of the substrate reaction with groups other than the active site, or of reaction involving catalytically inactive variants of the enzyme.Abbrevi...

NADH oxidase activity of rat and human liver xanthine oxidoreductase: potential role in superoxide production

“…The EPR signals from NADHreduced enzymes were simulated individually and the EPR parameters determined are shown in Tables 2 and 3. The simulation of the Mo(V) signal, as a mixture of slow and rapid type-1 signals, was consistent with the known simultaneous reduction by NADH of active and desulfo enzymes, via the flavin (with subsequent reduction of Mo(V) [47]. In addition, the Mo(V) signals were also found to split (D = 1.2 mT) in the presence of the Fe/S I centre (at temperatures below 60 K).…”

Flavocytochrome b2: Kinetic Studies by Absorbance and Electron‐Paramagnetic‐Resonance Spectroscopy of Electron Distribution among Prosthetic Groups

“…All radical intensities are thus multiplied per 1.14 according to Aasa and Vanngard [31]. The presence of isopentane in the frozen mixtures obtained with the rapid mixing-freezing apparatus, but not in the copper standard, have to be taken into account in this comparison; a dilution factor of 2 is used according to Swann and Bray [27] instead of 2.38 previously used by Palmer et a f .…”

“…However, it should be noticed that the uncertainty on the factor of dilution of the flavocytochrome b, in isopentance, after the freezing process, should be reflected in a systematic error in the radical absolute amount. The dilution factor considered was 2 [27] : an over-estimation should lead to a proportional over-estimation of the radical concentration. In any case, such an error must be small in view of the agreement (Fig.…”

“…In all the anaerobic experiments presented here, the substrate solution was freed from oxygen by bubbling argon (purified by passage through a catalytic purifier as previously described [27]) before its transfer into the syringe. Similarly the enzyme solution was rendered anaerobic with an aspirator pump and flushed with purified argon repeatedly, before the sample was transferred to the argon-filled anaerobic syringe.…”

“…The time is varied by changing the delay time between the injection and ejection pushes. The manual mixing was obtained in special tubes [27] maintained under anaerobic conditions as above ; these tubes have side arms and 2 min after mixing in a side arm, the solution was permitted to run in the EPR tube frozen.…”