The utilization of quinaldine (2-methylquinoline) by Arthrobacter sp. Rü61a proceeds via 1H-4-oxoquinaldine, 1H-3-hydroxy-4-oxoquinaldine, and N-acetyl-anthranilic acid. By analogy, 1H-4-oxoquinoline is degraded by Pseudomonas putida 33/1 via 1H-3-hydroxy-4-oxoquinoline and N-formylanthranilic acid. Using the purified enzymes from both organisms, the mode of N-heterocyclic ring cleavage was investigated. The conversions of 1H-3-hydroxy-4-oxoquinaldine and 1H-3-hydroxy-4-oxoquinoline to N-acetyl- and N-formylanthranilic acid, respectively, were both accompanied by the release of carbon monoxide. The enzyme-catalysed transformations were performed in an [18O]O2 atmosphere and resulted in the incorporation of two oxygen atoms into the respective products, N-acetyl- and N-formylanthranilic acid, indicating an oxygenolytic attack at C-2 and C-4 of both 1H-3-hydroxy-4-oxoquinaldine and 1H-3-hydroxy-4-oxoquinolone.