Interaction of reactive and inert chemicals in the presence of oxidoreductases: reaction of the herbicide bentazon and its metabolites with humic monomers.

Abstract

The herbicide bentazon (3-isopropyl-1H-2,1,3-benzothiadiazine-4(3H)-one-2,2-dioxide), a relatively inert chemical, and some of its metabolites were incubated with a laccase or a peroxidase in the presence or absence of humic monomers to evaluate the incorporation of the herbicide and its metabolites into humic material by oxidative enzymes. Guaiacol and ferulic acid were used as representative electron donor co-substrates in most of the oxidative coupling reactions. Bentazon and its metabolites, with the exception of hydroxy metabolites, underwent little or no transformation by the two enzymes in the absence of guaiacol and ferulic acid, but in the presence of these co-substrates transformation occurred. The reaction of bentazon with guaiacol in the presence of the laccase or a peroxidase was almost complete in 30 min. 6-Hydroxy- and 8-hydroxy-bentazon were completely transformed by each enzyme both with and without co-substrates. At pH 3.0 and in the presence of laccase and guaiacol, the concentrations of bentazon and its metabolites 2-amino-N-isopropyl-benzamide (AIBA), des-isopropyl-bentazon and 8-chloro-bentazon decreased by 27, 57, 20 and 4%, respectively. The corresponding levels of transformation with peroxidase at pH 3.0 were 9, 70, 30 and 5%, respectively. The extent of transformation decreased with increasing pH. At low pH, the hydroxy-bentazons were completely transformed, followed by (in order of percentage transformation) AIBA, des-isopropyl-bentazon, bentazon and 8-chloro-bentazon. Transformation of bentazon by the laccase increased with increasing guaiacol concentration. In the presence of the peroxidase, the most effective co-substrates for transformation of bentazon were (in decreasing order) catechol, vanillic acid, protocatechuic acid, syringaldehyde and caffeic acid, while in the presence of the laccase, catechol was most effective, followed by caffeic acid, protocatechuic acid and syringaldehyde.