Reactivity of d-Amino Acid Oxidase with 1,2-Cyclohexanedione: Evidence for One Arginine in the Substrate-Binding Site

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Abstract
D-Amino acid oxidase [pig kidney] is inactivated by reaction with 1,2-cyclohexanedione in borate buffer at pH 8.8. The reaction follows pseudo-1st-order kinetics. The presence of benzoate, a substrate-competitive inhibitor of the enzyme, protects substantially against inactivation. Partial reactivation could be obtained by removal of borate and its substitution with phosphate buffer. The reaction of 1,2-cyclohexanedione with the enzyme at different inhibitor concentrations appears to follow a saturation kinetics, indicating the formation of an intermediate complex between enzyme and inhibitor prior to the inactivation process. The partially inactivated enzyme shows the same apparent Km but a decreased Vmax as compared to the native D-amino acid oxidase. Similarly, the inhibited enzyme fails to bind benzoate. Amino acid analysis of the 1,2-cyclohexanedione-treated enzyme at various times of inactivation shows no loss of amino acid residues except for arginines. Analysis of the reaction data by statistical methods indicates that 3 arginine residues react with the inhibitor at slightly different rates and that one of them is essential for catalytic activity. The presence of benzoate, while it prevents the loss of activity, reduces by one the number of arginine residues hit by the reagent in the reaction of 1,2-cyclohexanedione with D-amino acid oxidase.