The oxygenase reaction of acetolactate synthase detected by chemiluminescence

Abstract

In addition to the synthesis of ketolacids the enzyme acetolactate synthase shows an oxygen-consuming side reaction. Partially purified acetolactate synthase from corn (Zea mays L.) and barley (Hordeum vulgare L.) exhibits chemiluminescence in the presence of oxygen, Mn" 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.

Expression, purification and characterization of Arabidopsis thaliana acetohydroxyacid synthase

“…With this possible exception, it is with our value of 7n8 units\mg that the published values should be compared. Southan and Copeland [18] obtained the wheat leaf enzyme with a specific activity of only 0n06 unit\mg, maize AHAS was obtained with a specific activity of 0n67 unit\mg [37], while Durner and Bo$ ger [17] obtained a preparation of the barley shoot enzyme with a specific activity of 1n6 units\mg and have mentioned preparations with activities as high as 3n1 units\mg [38]. Purification of plant AHAS is hampered by the very low abundance of the enzyme and its instability ; the difference in specific activity between the barley and wheat enzyme may be due to loss of activity in the latter.…”

Commercial Herbicides Can Trigger the Oxidative Inactivation of Acetohydroxyacid Synthase

“…However, in AHAS, this process has only been observed under turnover conditions, suggesting that there may be an alternative explanation for the observed time‐dependent and biphasic inhibition profiles. Adding to the conjecture, the inhibition of AHAS by herbicides has, in some cases, been reported to be irreversible, which is inconsistent with a mechanism of slow‐binding inhibition. In addition, it is also feasible that thiamin diphosphate (ThDP; a Breslow intermediate), a cofactor in the active site, can participate in alternative reactions with the herbicide (for example by protonation of the enamine or by formation of the keto form through tautomerization), thereby influencing inhibition.…”

An introduction to ALS-inhibiting herbicides

“…From a review of the literature, it is clear that ALS-inhibit-Ž ing herbicides affect many species of plants Ray, 1980Ray, , 1982Sweetser et al, 1981;Gressel and Segel, 1982;Brewster and Appleby, 1983;Hageman and Behrens, 1984;Turner, 1987;Blair and Martin, 1988;LaRossa and Van Dyk, 1988;Strek et al, 1989;Durner et al, 1990Durner et al, , 1994Moyer et al, 1990;Burnet and Hodgson, 1991;Mallory-Smith et al, 1991;Thill et al, 1991;Saari et al, 1992;Shaner, 1992;Fletcher et al, 1993Fletcher et al, , 1996Gerwick et al, . 1993;Forlani et al, 1995 . The ALS inhibitors have little, if any, effect on seed germination but affect both plant Ž .…”