title: Simultaneous formation of 2- and 4-quinolones from quinolinium cations catalysed by aldehyde oxidase.
abstract: Quinolinium salts were incubated with partially purified aldehyde oxidase, and the products were separated by high-pressure liquid chromatography and fully characterized by u.v. spectroscopy, i.r. spectroscopy and mass spectrometry. Oxidation of N-methylquinolinium salts with either rabbit or guinea-pig liver aldehyde oxidase in vitro gave two isomeric products, N-methyl-4-quinolone and N-methyl-2-quinolone. Incubation of N-phenylquinolinium perchlorate similarly yielded two oxidation products, N-phenyl-4-quinolone and N-phenyl-2-quinolone. The ratio of 2- to 4-quinolone production was species-dependent, the proportion of 4-quinolone with the guinea-pig enzyme being greater than that obtained with the rabbit liver enzyme. Kinetic constants were determined spectrophotometrically for both the quinolinium salts and a number of related quaternary compounds. In general, quaternization facilitated oxidation of a substrate, but a number of exceptions were noted, e.g. N-methylisoquinolinium and N-methylphen-anthridinium. Km values varied with the nature of electron acceptor employed, and this difference was more marked for quaternary substrates than the unquaternized counterparts. The product ratio obtained from N-methylquinolinium salts was found to be constant under various conditions, including purification of the enzyme and the use of either induced or inhibited aldehyde oxidase, but a change in the ratio was found at high pH values and in the presence of a competing substrate, N-methylphenanthridinium. This may indicate that a quaternary substrate binds to aldehyde oxidase in two alternative positions.
mesh_terms: D042931:Aldehyde Oxidase; D000445:Aldehyde Oxidoreductases / Q000302:isolation & purification / Q000378:metabolism*; D000818:Animals; D002851:Chromatography, High Pressure Liquid; D005260:Female; D006168:Guinea Pigs; D007700:Kinetics; D011804:Quinolines / Q000378:metabolism*; D011806:Quinolinium Compounds / Q000378:metabolism*; D011817:Rabbits; D013045:Species Specificity; D013056:Spectrophotometry, Ultraviolet; D013379:Substrate Specificity
chemical_list: D011804:Quinolines; D011806:Quinolinium Compounds; D000445:Aldehyde Oxidoreductases; D042931:Aldehyde Oxidase