Mechanism and Specificity of l‐ and d‐6‐Hydroxynicotine Oxidase

l ‐6‐Hydroxynicotine oxidase, a FAD‐protein, coverts in the presence of oxygen l ‐6‐hydroxynicotine to [6‐hydroxypyridyl(3)]‐(γ‐ N ‐methylaminopropyl)‐ketone; the overall process, whose stoichiometry has been established, consists of the enzyme‐catalyzed dehydrogenation of the substrate followed by spontaneous hydrolysis of the intermediate enamine. Titration of the enzyme anaerobically with substrate or dithionite spectrally does not indicate formation of a semiquinoid flavin, while upon irradiation in the presence of ethylenediaminetetraacetate, spectral changes take place which seem to relflect the existence of a transient radical stage. Molecular oxygen functions as electron acceptor in the oxidase reaction, other artificial or natural redox systems being inactive or even, as methylene blue, inhibitory to the l ‐6‐hydroxynicotine oxidation. Previous evidence for the almost absolute structural and stereochemical specificity of both l ‐and d ‐6‐hydroxynicotine oxidases was extended.