2,4-dioxygenases catalyzing N-heterocyclic-ring cleavage and formation of carbon monoxide. Purification and some properties of 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase from Arthrobacter sp. Rü61a and comparison with 1H-3-hydroxy-4-oxoquinoline 2,4-dioxygenase from Pseudomonas putida 33/1.