The oxygenase reaction of acetolactate synthase detected by chemiluminescence

In addition to the synthesis of ketolacids the enzyme acetolactate synthase shows an oxygen-consuming side reaction. Partially purified acctolactate synthase from corn (Zea mays L.) and barley (Hordeum vulgare L.) exhibits chemiluminescence in the presence of oxygen, Mn²⁺ and low concentrations of pyruvate. Light emission is inhibited by azide, but not by catalase or superoxide dismutase. The data suggest the formation of singlet oxygen during the catalytic cycle, and provides a basis for a highly sensitive assay for the oxygenase reaction of acetolactate synthase. Both synthase activity and chemiluminescence are inhibited by sulfonylurea herbicides. The results add a new aspect to the irreversible inhibition of acetolactate synthase by these herbicides which may be enhanced by the presence of reactive oxygen species.