The stabilization of d-amino-acid oxidase by flavin-adenine dinucleotide, substrates and competitive inhibitors

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Abstract
At pH 8.25 and at 37.5[degree], flavin-adenine dinucleotide (FAD), substrates and the competitive inhibitors L-leucine and Na benzoate protected D-amino acid oxidase apo-enzyme from thermal inactivation. Of several substrates which inhibited D-amino acid oxidase by antagonizing FAD, adenosine-5[image] -phosphate (AMP) was found to protect the oxidase. Quinine, mepacrine, adenosine, adenine and caffeine inhibited in this way, but did not protect the oxidase. The stabilization was investigated in relation to the kinetics of the enzyme at the same temp. and pH. Several substances, not being substrates or inhibitors, yet related to FAD or D-amino acids, did not protect the oxidase under the conditions investigated. The protection of the oxidase was discussed in terms of an equilibrium between the protein and protector. Evidence was presented that the site of protection by benzoate or by AMP was the same as the site of inhibition. The formation of non-fluorescent complexes between the inhibitor and FAD could account for the inhibition of the oxidase by quinine, adenosine and caffeine. This process did not account for the inhibition by AMP or adenosine diphosphate (ADP). It was concluded that AMP and ADP were able to combine with the enzyme protein in competition with the FAD. The affinity of the protein for ADP or AMP was at least 20 times the affinity for -adenosine-triphosphate, inosine-5[image] -phosphate, adenosine-3[image] -phosphate, guanosine-3[image] -phosphate or adenosine.

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