Effect of reduced pyridine nucleotides and tungstate on the in vitro insertion of molybdenum into demolybdo-nitrate reductase of Chlorella vulgaris

Microbial metabolism of quinoline and related compounds. XX. Quinaldic acid 4-oxidoreductase from Pseudomonas sp. AK-2 compared to other procaryotic molybdenum-containing hydroxylases

Quinaldic acid 4-oxidoreductase from Pseudomonas sp. AK-2 catalyses the hydroxylation of quinoline 2-carboxylic (quinaldic acid) to 4-hydroxyquinoline 2-carboxylic acid (kynurenic acid) with concomitant reduction of a suitable electron acceptor. An analogous hydroxylation in para-position relative to the N-heteroatom was only recently described for quinaldine 4-oxidoreductase (de Beyer & Lingens, 1993, Biol. Chem. Hoppe-Seyler 374, 101-110) and for quinaldic acid 4-oxidoreductase from Serratia marcescens 2CC-1 (Fetzner & Lingens, 1993, Biol. Chem. Hoppe-Seyler 374, 363-376). Quinaldic acid 4-oxidoreductase from Pseudomonas putida AK-2 was purified 78-fold to electrophoretic homogeneity with a recovery of 22%. The native enzyme (300 kDa) was composed of three subunits with molecular masses of 90, 34 and 20 kDa, indicating an alpha 2 beta 2 gamma 2 structure. Quinaldic acid 4-oxidoreductase contained FAD, molybdenum, iron and acid-labile sulfur in a ratio of 2:2:8:8. Molybdenum is probably associated with molybdopterin cytosine dinucleotide as organic part of the pterin molybdenum cofactor. The absorption spectrum of quinaldic acid 4-oxido-reductase exhibited the typical features of a molybdo-iron/sulfur-flavoprotein, namely, maxima at 274 nm, 340 nm and 450 nm, a shoulder at 550 nm, a ratio A280/A450 of 4.7 and a ratio A450/A550 of 3.5. The enzyme was susceptible to inactivation by methanol, sodium m-arsenite, p-hydroxymercuribenzoate, and potassium cyanide. Cyanide caused an alteration at 320 nm in the absorption spectrum, typical for the change in the coordination sphere of the molybdenum. Enzyme inactivated with cyanide was reactivated to 74% by incubation with sulfide. Thus, quinaldic acid 4-oxidoreductase possesses a monooxo-monosulfido-type molybdenum center.

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.

Inhibitory effects of sulphur compounds, copper and tungsten on nitrate reduction by mixed rumen micro-organisms

1. The inhibitory effects of inorganic and organic sulphur-containing compounds, copper and tungsten on nitrate reduction by mixed rumen micro-organisms were investigated in two in vitro studies. 2. Coarsely strained rumen fluid from nitrate-adapted (Expt 1) or non-adapted (Expt 2) Suffolk Down wethers maintained on lucerne (Medicago sativa) cubes was used as an inoculum. In Expt 1, anaerobic incubation was carried out for 24 h for each medium supplemented with 10 mM-sodium nitrate and the following chemicals: 0, 1, 2, 3, 5, 8 and 10 mM-sodium sulphide, 1 and 10 mM-sodium sulphite, 1 and 10 mM-sodium sulphate, 1 and 10 mM-L-cysteine, 1 and 10 mM-DL-methionine, 1 mM-sodium tungstate and 1 mM-copper sulphate. In Expt 2, 1 and 10 mM-Na2S, 1 and 10 mM-L-cysteine, 1 mM-Na2WO4, and 1 mM-CuSO4 were added to incubation media to test for chemical inhibition of microbial reduction of nitrate. 3. In Expt 1, the amount of nitrite formed decreased with increasing concentration of sulphide-S added. The additions of L-cysteine, W and Cu suppressed nitrite formation in media from both nitrate-adapted and non-adapted sheep. 4. In contrast to the effects of sulphide, L-cysteine and W counteracted, to some degree, nitrate-induced reduction of volatile fatty acid (VFA) production. Addition of Cu to the media resulted in a further depression of VFA production.

Repair in vitro of nitrate reductase-deficient tobacco mutants (cnxA) by molybdate and by molybdenum cofactor

Two nitrate reductase-deficient mutant cell lines (CnxA68/2, CnxA101) of Nicotiana tabacum are shown to be repairable under in-vitro conditions by (i) molybdate or (ii) by preparations of active molybdenum cofactor of homologous or heterologous origin, thereby yielding about 20% and 80%, respectively, of the corresponding wild-type NADH-nitrate reductase (EC 1.6.6.1) activity. In-vitro repair of nitrate reductase activity is dependent on sulphydryl-group protecting reagents and ethylenediaminetetraacetic acid (EDTA) in the extraction medium, the nitrogen source in the growth medium and the age of the cells. The results support the conclusion that the cnxA gene controls the insertion of molybdenum into the molybdenum cofactor. They are consistent with the idea of two interlinked pathways for the metabolic processing of molybdenum acquisition, one involving the synthesis of the structural moiety of the molybdenum cofactor and the other involving processing of the molybdate anion.