Kinetics of inhibition of acetyl-coenzyme A carboxylase by sethoxydim and haloxyfop

Abstract

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 ( M r 222,000) and a minor biotincontaining polypeptide ( M r 73,400). The kinetics of enzyme inhibition by sethoxydim and haloxyfop were determined for the substrates MgATP, HCO 3 − , 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 K is values for sethoxydim were 1.9, 5.6, and 13.3 μ M for acetyl-CoA, HCO 3 − , and MgATP, respectively. The K is values for haloxyfop were 0.36, 0.87, and 2.89 μ M for acetyl-CoA, HCO 3 − , and MgATP, respectively. For both herbicides, K is < K ii for acetyl-CoA, whereas K ii < K is for MgATP and HCO 3 − . 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.