Imidazolinones and Acetohydroxyacid Synthase from Higher Plants

Acetohydroxyacid synthase has been purified from maize (Zea mays, var Black Mexican Sweet) suspension culture cells 49-fold by a combination of ion exchange chromatography, gel filtration, and hydroxyapatite chromatography. Use of the nondenaturing, zwitterionic detergent 3-([3-cholamidopropyl]dimethyl-ammonio)-1-propanesulfonate was necessary to dissociate the enzyme from the heterogeneous, high molecular weight aggregates in which it appears to reside in vitro. The solubilized maize acetohydroxyacid synthase had a relative molecular mass of 440,000. The purified enzyme was highly unstable. Acetohydroxyacid synthase activities in crude extracts of excised maize leaves and suspension cultured cells were reduced 85 and 58%, respectively, by incubation of the tissue with 100 micromolar (excised leaves) and 5 micromolar (suspension cultures) of the imidazolinone imazapyr prior to enzyme extraction, suggesting that the inhibitor binds tightly to the enzyme in vivo. Binding of imazapyr to maize acetohydroxyacid synthase could also be demonstrated in vitro. Evidence is presented which suggests that the interaction between imazapyr and the enzyme is reversible. Imazapyr also exhibited slow-binding properties when incubated with maize cell acetohydroxyacid synthase in extended time course experiments. Initial and final K(i) values for the inhibition were 15 and 0.9 micromolar, respectively. The results suggest that imazapyr is a slow, tight-binding inhibitor of acetohydroxyacid synthase.