Enzymatic bromination of barbituric acid and some of its derivatives

Barbituric acid, 1-methyl- and 1,3-dimethylbarbituric acid, some of its 5-phenyl derivatives, and 5-chlorobarbituric acid are presented as new substrates for the bromoperoxidase isolated from the brown alga Ascophyllum nodosum. This enzyme is able to convert these substrates into the corresponding 5-bromo or 5,5-dibromo derivatives in good yields. Kinetic measurements show that the structure of the examined substrates has little or no effect on the enzymatic rate of bromination. However, at low substrate concentration the reaction rate depends on both the concentration of the organic substrate and the concentration of hydrogen peroxide. A mechanism is proposed for the reactions of bromoperoxidase with its substrates. These reactions involve the formation of free hypobromous acid which can either brominate the organic halogen acceptor or produce singlet oxygen by a competing reaction with hydrogen peroxide.