DEMONSTRATION of the actual binding of apo-protein, coenzyme and substrate of D-amino acid oxidase is necessary in order to resolve the processes involved in the dynamic enzyme complex formation resulting in the manifestation of chemical reaction catalyzed by this enzyme. Therefore, we have examined the change of absorption spectrum of flavin adenine dinucleotide by D-amino acid oxidase apo-protein and the substrate. D-Amino acid oxidase apo-protein and flavin adenine dinucleotide were prepared by the methods of Negelein, Bromel1 and Yagi et al. 2, respectively.