QUINALDINE 4-OXIDASE FROM ARTHROBACTER SP RU61A, A VERSATILE PROKARYOTIC MOLYBDENUM-CONTAINING HYDROXYLASE ACTIVE TOWARDS N-CONTAINING HETEROCYCLIC-COMPOUNDS AND AROMATIC-ALDEHYDES

Quinaldine 4-oxidase from Arthrobacter sp. Ru61a, an inducible molybde num-containing hydroxylase, was purified to homogeneity by an optimize d five step procedure. Molecular oxygen is proposed as physiological e lectron acceptor. Electrons are also transferred to artificial electro n accepters with E(0)' > -8 mV. The molybdo-iron/sulfur flavoprotein r egiospecifically attacks its N-heterocyclic substrates: isoquinoline a nd phthalazine are hydroxylated adjacent to the N-heteroatom at C1, wh ereas quinaldine, quinoline, cinnoline and quinazoline are hydroxylate d at C4. Additionally, the aromatic aldehydes benzaldehyde, salicylald ehyde, vanillin and cinnamaldehyde are oxidized to the corresponding c arboxylic acids, whereas short-chain aliphatic aldehydes are not. Quin aldine 4-oxidase is compared to the two molybdenum-containing hydroxyl ases quinoline 2-oxidoreductase from Pseudomonas putida 86 [Tshisuaka. B., Kappl, R., Huttermann, J. & Lingens, E (1993) Biochemistry 32, 12 928 -12934] and isoquinoline 1-oxidoreductase from Pseudomonas diminut a 7 [Lehmann, M., Tshisuaka, B., Fetzner, S., Roger, P. & Lingens, F. (1994) J. Biol. Chem. 269, 11254-11260] with respect to the substrates converted and the electron-acceptor specificities. These dehydrogenas es hydroxylate their N-heterocyclic substrates exclusively adjacent to the heteroatom Whereas the aldehydes tested are scarcely oxidized by quinoline 2-oxidoreductase, isoquinoline l-oxidoreductase catalyzes th e oxidation of the aromatic aldehydes, although being progressively in hibited. Neither quinoline 2-oxidoreductase nor isoquinoline l-oxidore ductase transfer electrons to oxygen. Otherwise, the spectrum of elect ron accepters used by quinoline 2-oxidoreductase and quinaldine 4-oxid ase is identical. However, isoquinoline l-oxidoreductase differs in it s electron-acceptor specificity. Quinaldine 4-oxidase is unusual in it s substrate and electron-acceptor specificity.Quinaldine 4-oxidase is unusual in its substrate and electron-acceptor specificity. This enzym e is able to function as oxidase or dehydrogenase, it oxidizes aldehyd es, and it catalyzes the nucleophilic attack of N-containing heterocyc lic compounds at two varying positions depending on the substrate.