[The mechanism of action of d-amino acid oxidase. I. Evidence for a free radical mechanism of the reaction catalyzed b a dimeric form of the enzyme].

d-Amino acid oxidase can oxidize the substrate to a ketoacid in the absence of oxygen. The stoichiometry of this reaction is precisely 1 molecule of keto acid for 1 molecule of enzyme, containing two flavin groups. Hence, the flavin must be in the semi-reduced free radical state. But these free radicals cannot be visualized by ESR spectroscopy because of closeness and strong interaction. After the acid denaturation of the protein the coenzyme is released as a semi-reduced free radical. An alternative method of registration is the transfer of the free radical state to an added excess of free flavin molecules. By both methods it is quantitatively determined that each flavin of the enzyme is reduced to a free radical. Therefore, we believe to have evidenced unambiguously that this enzymatic reaction proceeds via a free radical transition state.