On the Reaction of Borohydride with d- and l-Amino Acid Oxidases

It has been confirmed that borohydride treatment of d-amino acid oxidase in the presence of d-alanine-1-14C leads to the strong attachment to the apoprotein of 1 mole of 14C per eq of FAD originally present. However this reaction, and a similar one with l-amino acid oxidase, does not result in any loss of catalytic activity. With both enzymes, borohydride reaction in the absence of substrate results in the production of a new flavin derivative, not previously reported. This modified flavin, characterized by an absorption maximum at 408 to 410 mµ, can be liberated by heat denaturation. It has intense blue fluorescence with an emission maximum at 475 mµ. On conversion to the flavin mononucleotide form there is an approximately 10-fold increase in fluorescence intensity.