Spectroscopic demonstration of an initial stage of the complex of D-amino acid oxidase and its substrate D-alpha-aminobutyric acid

D-Amino acid oxidase [D-amino acid: O₂ oxidoreductase (deaminating), EC 1.4.3.3] was anaerobically mixed with its substrate D-α-aminobutyric acid at ?10°C and pa_H¹ 7 which were apart from their maxima for the enzymatic reaction. By an ordinary self-recording spectrophotometer, the absorption spectrum of an initial stage of the complex could be observed. The spectrum was in principle similar to that of the complex of this enzyme with benzoate, the enzyme-substrate complex model. The spectroscopic observation revealed that this species is in an equilibrium with the purple intermediate, a strong charge transfer complex between the enzyme and its substrate neutral D-amino acid.