Quinoline oxidoreductase, an iron-sulfur molybdenum flavoprotein containing flavin adenine dinucleotide and molybdopterin cytosine dinucleotide, was purified from Pseudomonas putida 86 to homogeneity. The various electron-transfer centers of the purified enzyme were examined by electron paramagnetic resonance spectroscopy. Quinoline deuterated at position 2 was prepared by deuterodecarboxylation of 2-quinolinecarboxylic acid. Quinoline added to the enzyme elicited the Mo(V) "rapid" type Q signal arising from the complex of enzyme and substrate, whereas in oxidized quinoline oxidoreductase a Mo(V) "resting" signal was observed. EPR spectroscopy at helium temperatures below 70 K revealed the existence of two types of iron-sulfur centers, Fe-S I and Fe-S II. An organic free radical appeared upon reduction with sodium dithionite. Inactivation of the enzyme by cyanide led to the inactive desulfo quinoline oxidoreductase, which yielded another Mo(V) signal designated "slow" type Q upon reduction with dithionite. Desulfo quinoline oxidoreductase was partially reactivated by incubation with sulfide.

Quinaldine 4-oxidase from Arthrohacter sp. Rub1 a, an inducible molybdenum-containing hydroxylase, was purified to homogeneity by an optimized five-step procedure. Molecular oxygen is proposed as physiological electron acceptor. Electrons are also transferred to artificial electron acceptors with Eh > -8 mV. The molybdo-ironhlfur flavoprotein regiospecifically attacks its N-heterocyclic substrates :isoquinoline and phthalazine are hydroxylated adjacent to the N-heteroatom at C1, whereas quinaldine, quinoline, cinnoline and quinazoline are hydroxylated at C4. Additionally, the aromatic aldehydes benzaldehyde, salicylaldehyde, vanillin and cinnamaldehyde are oxidized to the corresponding carboxylic acids, whereas short-chain aliphatic aldehydes are not.Quinaldine 4-oxidase is compared to the two molybdenum-containing hydroxylases quinoline 2-oxidoreductase from Pseudomonas putida 86 [Tshisuaka, B., Kappl, R., Hiittermann, J. with respect to the substrates converted and the electron-acceptor specificities. These dehydrogenases hydroxylate their N-heterocyclic substrates exclusively adjacent to the heteroatom. Whereas the aldehydes tested are scarcely oxidized by quinoline 2-oxidoreductase, isoquinoline 1 -0xidoreductase catalyzes the oxidation of the aromatic aldehydes, although being progressively inhibited. Neither quinoline 2-oxidoreductase nor isoquinoline 1 -0xidoreductase transfer electrons to oxygen. Otherwise, the spectrum of electron acceptors used by quinoline 2-oxidoreductase and quinaldine 4-oxidase is identical. However, isoquinoline I-oxidoreductase differs in its electron-acceptor specificity.Quinaldine 4-oxidase is unusual in its substrate and electron-acceptor specificity. This enzyme is able to function as oxidase or dehydrogenase, it oxidizes aldehydes, and it catalyzes the nucleophilic attack of N-containing heterocyclic compounds at two varying positions depending on the substrate.Keywords: benzodiazines ; benzopyridine derivatives ; aldehyde oxidation ; molybdenum-containing hydroxylase ; quinaldine 4-oxidase.Quinaldine 4-oxidoreductase from the gram-positive bacterium Arthrobacter sp. Rii61 a, an inducible molybdenum-containing hydroxylase, catalyzes the initial step in quinaldine (2-methylquinoline) degradation (Hund et al., 1990) : quinaldine is converted to 1 H-4-oxoquinaldine with concomitant reduction of a suitable electron acceptor. The oxygen atom incorporated into the product derives from water. Quinaldine 4-oxidoreductase Abbreviations. INT, 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride; 11 OR, isoquinoline l-oxidoreductase; Q20R, quinoline 2-oxidoreductase; NBT, 2,2'-di-p-nitrophenyl-5,5'-diphenyl-3,3'-~3,3'-dimethoxy-4,4'-diphenylene]-dite~azolium chloride.Enzymes.

A series of interesting enzymes were discovered during investigations on the degradation of quinoline by microorganisms. These include the molybdenum-containing hydroxylases that catalyze the transformation 1→2 and the unusual 2,4-dioxygenases that catalyze the reaction 3→4. The application of the hydroxylases may even be interesting in industry, because several quinoline derivatives are used as pharmaceuticals or agrochemicals.

The iorA and iorB genes from the isoquinoline-degrading bacterium Pseudomonas diminuta 7, encoding the heterodimeric molybdo-iron-sulfur-protein isoquinoline 1-oxidoreductase, were cloned and sequenced. The deduced amino acid sequences IorA and IorB showed homologies (i) to the small (gamma) and large (alpha) subunits of complex molybdenum-containing hydroxylases (alpha beta gamma/alpha 2 beta 2 gamma 2) possessing a pterin molybdenum cofactor with a monooxo-monosulfido-type molybdenum center, (ii) to the N- and C-terminal regions of aldehyde oxidoreductase from Desulfovibrio gigas, and (iii) to the N- and C-terminal domains of eucaryotic xanthine dehydrogenases, respectively. The closest similarity to IorB was shown by aldehyde dehydrogenase (Adh) from the acetic acid bacterium Acetobacter polyoxogenes. Five conserved domains of IorB were identified by multiple sequence alignments. Whereas IorB and Adh showed an identical sequential arrangement of these conserved domains, in all other molybdenum-containing hydroxylases the relative position of "domain A" differed. IorA contained eight conserved cysteine residues. The amino acid pattern harboring the four cysteine residues proposed to ligate the Fe/S I cluster was homologous to the consensus binding site of bacterial and chloroplast-type [2Fe-2S] ferredoxins, whereas the pattern including the four cysteines assumed to ligate the Fe/S II center showed no similarities to any described [2Fe-2S] binding motif. The N-terminal region of IorB comprised a putative signal peptide similar to typical leader peptides, indicating that isoquinoline 1-oxidoreductase is associated with the cell membrane.

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