GRAMINICIDE-BINDING BY ACETYL-COA CARBOXYLASE FROM POA ANNUA LEAVES

A partially purified acetyl-CoA carboxylase (ACCase) preparation from leaves of a graminicide-resistant grass, Poa annua (annual meadow gras s), was weakly inhibited by an aryloxyphenoxypropionate graminicide, q uizalofop. The I-50 value of 10 mu M was ca 300-fold higher than that previously determined for an ACCase from a susceptible grass species, maize. Poa annua ACCase exhibited negative cooperativity in binding qu izalofop (Hill coefficient = 0.54 +/- 0.02). Inhibition was approximat ely equal in sensitivity to the concentrations of acetyl-CoA or ATP in the assay medium and appeared close to being non-competitive for thes e substrates. A double-reciprocal plot of initial velocity with the su bstrate pair acetyl-CoA plus ATP produced a parallel line pattern whic h was the expected pattern for a Ping-Pong reaction mechanism. The res ults from these kinetic measurements are discussed in relation to poss ible reasons for the insensitivity of P. annua ACCase to graminicides. It is concluded that cooperativity of herbicide-binding may be a poss ible indicator of such insensitivity. Copyright (C) 1997 Elsevier Scie nce Ltd