Biodegradation of bromoxynil using the cascade enzymatic system nitrile hydratase/amidase from Microbacterium imperiale CBS 498-74. Comparison between free enzymes and resting cells

Abstract

This work investigates the biodegradation of bromoxynil to the corresponding acid to reduce its acute toxicity. Sequential reactions catalysed by nitrile hydratase (NHase) and amidase (AMase), naturally present in Microbacterium imperiale CBS 498-74, have been used. The kinetic behaviour of the crude extract (CE) and of the resting cell (RC) confined enzymes (NHase and AMase) is compared. In both preparations the same NHase/AMase ratio has been measured. The study was performed using batch and continuous UF-membrane reactor configurations. This paper highlighted a different pH-optimum for each enzyme; a high acid to amide ratio at pH 5.5; and exponential temperature dependence for both enzymes. The halved activation energy indicated the presence of diffusional limitations for the RC-enzymes. However, the higher stability at pH 7.0 for RC-NHase (half-life = 1386 h) and the correct choice of operational conditions allowed the driving to completeness of the bromoxynil biotransformation into the corresponding acid in the batch reactor.