The selective grass herbicides diclofop, haloxyfop, and trifop were found to be potent reversible inhibitors of acetyl-CoA carboxylase from the susceptible species barley, corn, and wheat. Kisvalues with variable concentrations of acetyl-CoA ranged from 0.01 to 0.06 μm at pH 8.5 depending on the species of grass. Inhibition of the wheat enzyme by diclofop was noncompetitive versus acetyl-CoA with Kis< Kiiand noncompetitive versus MgATP and bicarbonate, but with Kis≈ Kii. Since the apparent inhibition constant was most sensitive to the level of acetyl-CoA, these compounds probably interact with the transcarboxylase site rather than the biotin carboxylation site. With the wheat enzyme the Kisvalue for the R-(+)-enantiomer of trifop was 1.98 ± 0.22 times lower than that of the racemic mixture. This confirms the Stereoselectivity observed in the whole plant. The enzyme from tolerant broadleaf species (spinach and mung bean) was much less sensitive to these herbicides (Kisvalues varied from 16 to 515 μm). These data confirm that acetyl-CoA carboxylase is the site of action for the aryloxyphenoxypropionic acid herbicides and may explain their selectivity for monocotyledenous Species. © 1988.
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