Kinetic Studies on the Inhibition of Acetolactate Synthase by Pyrimidinylsalicylic Acids

Abstract

The inhibition of acetolactate synthase (ALS) from etiolated pea seedlings by 2-(4, 6dimethoxypyrimidine-2-yloxy)benzoic acid (compound 2), one of the pyrimidinylsalicylic acid (PS) compounds, was desensitized by SR inhibitors but not at the alkaline pH. PS compounds including pyrithiobac (2-chloro-6-(4, 6-dimethoxypyrimidine-2-ylthio)benzoic acid) tested in this study inhibited ALS activity in the mixed-type manner with respect to pyruvate and in the non-competitive with respect to thiamine pyrophosphate (TPP). These compounds inhibited both of the two molecular species of ALS which were partially purified from pea seedlings and exhibited slow-binding properties to the crude preparation of ALS from pea seedlings in the extended-time-course experiments.The inhibition constants of the initial inhibition by PS compounds were 13-to 26-fold larger than those of the final steady state and the maximal first order rate constant (0. 69 min-1) for transition from the initial to the final steady state of the inhibition of pyrithiobac was nearly identical to those reported on other widely acknowledged ALS inhibitors.From these results, we conclude: PS compounds are slow-binding inhibitors to ALS activity, categorized in the sulfonylurea type.The binding site of PS compounds on the enzyme is considered to be located on the similar site to sulfonylureas. This binding site is judged not to be located on the regulatory center(s) but on the allosteric site in a wide sense near the catalytic center, which overlaps partially with the binding site of pyruvate and does not overlap with the binding site of TPP.Km= [E]

Thermal dependence of pyrithiobac efficacy inAmaranthus palmeri

“…slow-binding. mixed-type inhibition that combines initial and final steady-state inhibition as well as substrate-dependent and substrate-independent inhibition Both the accumulated dry weight and the Iso values increased rapidly above 34 C. (Shimizu et al 1994b). Therefore, while there was a clear correlation between the thermal dependencies of field activity and in vitro enzyme inhibition, the establishment of a quantitative relationship between the two cannot be derived from the results of these experiments.…”

The AHAS gene of Cichorium intybus is expressed in fast growing and inflorescential organs

“…Localized in the plastids of higher plants ( Bekkaoui et al 1993), the enzyme undergoes an allosteric inhibition by valine, leucine and at a lesser extent by isoleucine which are the end products of the biosynthetic pathway ( Miflin and Cave 1972; Shield et al 1996; for a review see Chipman et al 1998). It is also inhibited by several classes of herbicides ( Anderson and Hibberd 1985; Mazur and Falco 1989; Shimizu et al 1994; Dewaele et al 1997).…”