INHIBITION OF ACETYL-COENZYME-A CARBOXYLASE BY 2 CLASSES OF GRASS-SELECTIVE HERBICIDES

The selective grass herbicides diclofop, haloxyfop, and trifop ([(aryloxy)phenoxy] propionic acids) and alloxydim, sethoxydim, and clethodim (cyclohexanediones) are potent, reversible inhibitors of acetyl-coenzyme A carboxylase (ACC) partially purified from barley, corn, and wheat. Although inhibition of the wheat enzyme by clethodim and diclofop is noncompetitive versus each of the substrates adenosine triphosphate (ATP), HCO3', and acetyl-coenzyme A (acetyl-CoA), diclofop and clethodim are nearly competitive versus acetyl-CoA since the level of inhibition is most sensitive to the concentration of acetyl-CoA (Kis < Kii). To conclusively show whether the herbicides interact at the biotin carboxylation site or the carboxyl transfer site, the inhibition of isotope exchange and partial reactions catalyzed at each site was studied with the wheat enzyme. Only the [14C]acetyl-CoA-malonyl-CoA exchange and decarboxylation of [14C]malonyl-CoA reactions are strongly inhibited by clethodim and diclofop, suggesting that the herbicides interfere with the carboxyl transfer site rather than the biotin carboxylation site of the enzyme. Double-inhibition studies with diclofop and clethodim suggest that the [(aryloxy)phenoxy]propionic acid and cyclohexanedione herbicides may bind to the same region of the enzyme. © 1990, American Chemical Society. All rights reserved.