The redox centers of xanthine oxidase are on independent structural domains of the enzyme

Microbial metabolism of quinoline and related compounds. XVIII. Purification and some properties of the molybdenum- and iron-containing quinaldic acid 4-oxidoreductase from Serratia marcescens 2CC-1

Serratia marcecens 2CC-1 utilizes quinaldic acid (quinoline 2-carboxylic acid) as sole source of carbon, nitrogen and energy. Growth of strain 2CC-1 on quinaldic acid as well as on nicotinic acid and hypoxanthine was inhibited completely by the molybdate antagonist tungstate, whereas growth on kynurenic acid and 6-hydroxynicotinic acid was not affected by tungstate. The synthesis of the molybdenum-containing hydroxylases quinaldic acid 4-oxidoreductase and nicotinic acid 6-oxidoreductase was found to be inducible. In addition, Serratia marcescens 2CC-1 produced a constitutively expressed xanthine oxidoreductase. Quinaldic acid 4-oxidoreductase was purified 1075-fold with a recovery of 5%. For catalytic activity, artificial electron acceptors were necessary. The 95-100-kDa enzyme was a heterodimer with subunit molecular masses of 75-80 kDa and 18-19 kDa. Quinaldic acid 4-oxidoreductase contained 2.3-3.7 g atom of iron and 0.5-0.6 g atom of molybdenum per mol of enzyme. The absorption spectrum exhibited maxima at 280 nm, 334 nm, 480 nm and a shoulder at 550 nm, with A280/A334 = 4.8, A280/A450 = 10.0, A280/A480 = 9.4, and A450/A550 = 1.6, suggesting the absence of a flavin cofactor. Acridine, quinacrine, ethylenediaminetetraacetate, 2,2'-dipyridyl, 1,10-phenanthroline and iodoacetate did not affect enzyme activity. p-Hydroxymercuribenzoate, m-arsenite, cyanide and methanol were effective inhibitors of quinaldic acid 4-oxidoreductase. Cyanide-inhibited enzyme was reactivated by treatment with S2-, indicating the presence of a pterin molybdenum cofactor with a monooxo-monosulfidotype molybdenum center. Quinaldic acid 4-oxidoreductase showed a very high substrate specificity, quinaldic acid being the only substrate found to be transformed significantly.

Purification and substrate inactivation of xanthine dehydrogenase from Chlamydomonas reinhardtii

Xanthine dehydrogenase (XDH) from the unicellular green alga Chlamydomonas reinhardtii has been purified to electrophoretic homogeneity by a procedure which includes several conventional steps (gel filtration, anion exchange chromatography and preparative gel electrophoresis). The purified protein exhibited a specific activity of 5.7 units/mg protein (turnover number = 1.9 .10(3) min-1) and a remarkable instability at room temperature. Spectral properties were identical to those reported for other xanthine-oxidizing enzymes with absorption maxima in the 420-450 nm region and a shoulder at 556 nm characteristic of molybdoflavoproteins containing iron-sulfur centers. Chlamydomonas XDH was irreversibly inactivated upon incubation of enzyme with its physiological electron donors xanthine and hypoxanthine, in the absence of NAD+, its physiological electron acceptor. As deduced from spectral changes in the 400-500 nm region, xanthine addition provoked enzyme reduction which was followed by inactivation. This irreversible inactivation also took place either under anaerobic conditions or whenever oxygen or any of its derivatives were excluded. Adenine, 8-azaxanthine and acetaldehyde which could act as reducing substrates of XDH were also able to inactivate it upon incubation. The same inactivating effect was observed with NADH and NADPH, electron donors for the diaphorase activity associated with xanthine dehydrogenase. In addition, partial activities of XDH were differently affected by xanthine incubation. We conclude that xanthine dehydrogenase inactivation by substrate is due to an irreversible process affecting mainly molybdenum center and that sequential and uninterrupted electron flow from xanthine to NAD+ is essential to maintain the enzyme in its active form.

Xanthine dehydrogenase and 2-furoyl-coenzyme A dehydrogenase from Pseudomonas putida Fu1: two molybdenum-containing dehydrogenases of novel structural composition

The constitutive xanthine dehydrogenase and the inducible 2-furoyl-coenzyme A (CoA) dehydrogenase could be labeled with [185W]tungstate. This labeling was used as a reporter to purify both labile proteins. The radioactivity cochromatographed predominantly with the residual enzymatic activity of both enzymes during the first purification steps. Both radioactive proteins were separated and purified to homogeneity. Antibodies raised against the larger protein also exhibited cross-reactivity toward the second smaller protein and removed xanthine dehydrogenase and 2-furoyl-CoA dehydrogenase activity up to 80 and 60% from the supernatant of cell extracts, respectively. With use of cell extract, Western immunoblots showed only two bands which correlated exactly with the activity stains for both enzymes after native polyacrylamide gel electrophoresis. Molybdate was absolutely required for incorporation of 185W, formation of cross-reacting material, and enzymatic activity. The latter parameters showed a perfect correlation. This evidence proves that the radioactive proteins were actually xanthine dehydrogenase and 2-furoyl-CoA dehydrogenase. The apparent molecular weight of the native xanthine dehydrogenase was about 300,000, and that of 2-furoyl-CoA dehydrogenase was 150,000. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of both enzymes revealed two protein bands corresponding to molecular weights of 55,000 and 25,000. The xanthine dehydrogenase contained at least 1.6 mol of molybdenum, 0.9 ml of cytochrome b, 5.8 mol of iron, and 2.4 mol of labile sulfur per mol of enzyme. The composition of the 2-furoyl-CoA dehydrogenase seemed to be similar, although the stoichiometry was not determined. The oxidation of furfuryl alcohol to furfural and further to 2-furoic acid by Pseudomonas putida Fu1 was catalyzed by two different dehydrogenases.