KINETICS OF INHIBITION OF ACETYL-COENZYME-A CARBOXYLASE BY SETHOXYDIM AND HALOXYFOP

The mechanism of inhibition of acetyl-CoA carboxylase by sethoxydim and haloxyfop was examined using a semipurified enzyme preparation extracted from Black Mexican Sweet Maize (Zea mays L.) suspension-culture cells. As determined by SDS-PAGE and Western blotting, the enzyme preparation contained a major biotin-containing polypeptide (Mr 222,000) and a minor biotincontaining polypeptide (Mr 73,400). The kinetics of enzyme inhibition by sethoxydim and haloxyfop were determined for the substrates MgATP, HCO3-, and acetyl-CoA. Sethoxydim and haloxyfop were linear, noncompetitive inhibitors for the three substrates, and the pattern of inhibition was similar for both herbicides. The Kis values for sethoxydim were 1.9, 5.6, and 13.3 μM for acetyl-CoA, HCO3-, and MgATP, respectively. The Kis values for haloxyfop were 0.36, 0.87, and 2.89 μM for acetyl-CoA, HCO3-, and MgATP, respectively. For both herbicides, Kis < Kii for acetyl-CoA, whereas Kii < Kis for MgATP and HCO3-. The kinetic data suggest that the transcarboxylation reaction catalyzed by acetyl-CoA carboxylase (acetyl-CoA → malonyl-CoA) is more sensitive to inhibition than is the biotin carboxylation reaction. Kinetic analysis also indicated that sethoxydim and haloxyfop are reversible, mutually exclusive inhibitors of acetyl-CoA carboxylase. © 1991.