A Study of the Equilibrium between the Monomer and the Dimer of D-Amino-Acid Oxidase

Abstract

By analyzing the reaction of D-amino-acid oxidase [EC 1.4.3.3] in equilibrium with p-aminobenzoate, we studied the functional difference between the affinities of the monomer and the dimer for p-vaminobenzoate, the interaction between the two active sites in the dimer (i.e.,cooperativity), and the shift in the equilibrium between monomer and dimer induced by p-aminobenzoate. We found that the affinity of the dimer for p-vaminobenzoate is about five times higher than that of the monomer, and there is no interaction between the two active sites in the dimer (i.e., no cooperativity). Based on our findings, we concluded that p-aminobenzoate induces dimerization of D-amino-acid oxidase. The apparent dime-rization constant in the presence of an infinite concentration of p-aminobenzoate is about 20 times greater than in its absence. The apparent dimerization constant was calculated as a function of the concentration of free p-aminobenzoate, using a Hitachi Hitac-10 digital computer. The relationship is not hyperbolic, but has a point of inflection.