Problems in studying the role of molybdenum in xanthine oxidase

The oxidation of azaheterocycles with mammalian liver aldehyde oxidase

1. Isoquinoline, cinnoline, quinoxaline, quinazoline and phthalazine were incubated with preparations of rabbit liver aldehyde oxidase. 2. The oxidation products, 1-hydroxyisoquinoline, 4-hydroxycinnoline, 2-hydroxy- and 2,3-dihydroxy-quinoxaline, 4-hydroxy- and 2,4-dihydroxy-quinazoline, and 1-hydroxyphthalazine were identified by comparison of their spectral and chromatographic characteristics with those of authentic compounds. 3. Michaelis-Menten constants are reported for the action of the parent heterocycles with aldehyde oxidase. The compounds reported in this study are among the most efficient substrates yet described for rabbit liver aldehyde oxidase. 4. The compounds in 1 above were incubated with bovine milk xanthine oxidase: only quinazoline and phthalazine yielded significant amounts of metabolites. Km values were calculated for these compounds. 5. Incubation of the heterocycles with rat liver preparations gave qualitatively the same results as those obtained using rabbit liver, but smaller amounts of the oxidation products were detected from rat liver incubations.

Tungsten vs. Molybdenum in models for biological systems

Biological systems show a marked preference for molybdenum over tungsten. Studies with methyliminodiacetic acid and L-cysteine have shown that the formation constants of the complexes with Mo(Vi) and W(VI) are very similar. These results imply that these elements would be bound with roughly equal strengths to an apoenzyme or a carrier whether or not these proteins contain a ligating sulfhydryl group. Similarly, transport across a membrane would not be expected to distinguish compounds of these metals providing they are carried in the same oxidation states. However, molybdenum could be distinguished from tungsten through the greater ease of reduction of the compounds of molybdenum.

Electron-electron double resonance measurements on xanthine oxidase

Electron-electron double resonance measurements were carried out on milk xanthine oxidase (xanthine:oxygen oxidoreductase EC 1.2.3.2) and the spectra obtained supported a previous model, based on EPR data, proposing a spin-spin interaction between unpaired electrons associated with Fe-S and Mo. The technique demonstrated that the additional apparently isotropic, splitting in the Mo EPR spectra observed at low temperature is produced by a single site giving two spectra interconverting at a rate consistent with the Fe-S spin lattice relaxation time. Other data concerning the model and the relaxation behaviour of the species are discussed.