HYDROXYLATION OF QUINALDIC ACID - QUINALDIC ACID 4-MONOOXYGENASE FROMALCALIGENES SP F2 VERSUS QUINALDIC ACID 4-OXIDOREDUCTASES

The N-heterocycles quinaldic acid (quinoline 2-carboxylic acid), kynur enic acid (4-hydroxyquinoline 2-carboxylic acid), 2-oxo-1,2-dihydroqui noline, and xanthine are utilized by Alcaligenes sp. F-2 as sole sourc e of carbon and energy. Although quinoline did not serve as growth sub strate, 8-hydroxy-2-oxo-1,2-dihydroquinoline and 8-hydroxycoumarin, me tabolites of the 'coumarin pathway' of quinoline catabolism, were isol ated from the culture fluid during growth on 2-oxo-1,2-dihydroquinolin e. Contrary to Serratia marcescens 2CC-1 and Pseudomonas sp. AK-2 (Sau ter et al. (1993) Biol. Chem. Hoppe-Seyler 374, 1037-1046), which poss ess different molybdenum-containing hydroxylases catalysing the 4-hydr oxylation of quinaldic acid to kynurenic acid with incorporation of ox ygen derived from water and concomitant reduction of an electron accep tor, Alcaligenes sp. F-2 contains an inducible quinaldic acid it-monoo xygenase that catalyses the very same conversion in the presence of O- 2 and NADH. The activity of the monooxygenase was enhanced 1.5-fold by Fe2+ ions. The extremely thermolabile enzyme (apparent molecular mass : 155 kDa) exclusively accepted quinaldic acid as substrate. The 'pseu dosubstrates' menadione, 8-hydroxyquinoline, and 8-hydroxy-2-oxo-1,2-d ihydroquinoline effected consumption of NADH and oxygen without being hydroxylated. Quinaldic acid 4-monooxygenase was inhibited by sulfhydr yl modifying and chelating agents, and by various divalent metal ions, whereas reducing agents did not affect enzymatic activity.