The spectral changes of the enzyme-bound FAD were investigated in the anaerobic and aerobic reactions of lysine monooxygenase, a flavoprotein. Under anaerobic conditions, the enzyme FAD was fully reduced with lysine via a transient intermediate having a broad absorption band in the long wavelength region. This anaerobic species was presumably involved in the dehydrogenation of lysine leading to the formation of an α-keto acid. During the steady state of the aerobic reaction, a separate spectral

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... species with a long wavelength absorption was observed. Both lysine and oxygen were required for its appearance. Kinetic evidence supports the intermediacy of this species in the oxygenation of lysine to an acid amide.