SPECIFIC BINDING OF A DICHLOROACETAMIDE HERBICIDE SAFENER IN MAIZE ATA SITE THAT ALSO BINDS THIOCARBAMATE AND CHLOROACETANILIDE HERBICIDES

Dichloroacetamide safeners such as N,N-diallyl-2,2-dichloroacetamide a nd (R,S)-3-dichloroacetyl-2,2,5-trimethyl-1 protect maize (Zea mays) a gainst injury from thiocarbamate and chloroacetanilide herbicides. Bin ding activity of tritium-labeled (R,S)-3-dichloroacetyl-2,2,5-trimethy l-1 (15 Ci/mmol; referred to as [H-3]Saf) was characterized in extract s of etiolated maize seedlings. The binding is saturable, involves a s ingle class of binding sites (K-d 0.12 mu M; maximal binding in coleop tiles 0.53 nmol/g fresh weight, equivalent to 55 pmol/mg protein), and is sensitive to boiling and protease treatment. Binding in etiolated maize seedlings is highest in the coleoptile and lowest in the leaves. Binding of [H-3]Saf also occurs in etiolated sorghum (Sorghom bicolor ) shoots but not several other cereals. There is a good correlation be tween known safener effectiveness and the concentration that inhibits [H-3]Saf binding half-maximally among 21 dichloroacetamides and relate d compounds. N,N-Diallyl-2,2-dichloroacetamide had the lowest inhibito r concentration that reduces specific binding by 50% (IC50), 0.01 mu M . [H-3]Saf binding is inhibited by 4 chloroacetanilide herbicides with IC50 values of 0.07 to 0.48 mu M and by 12 thiocarbamate herbicides a nd analogs with IC50 values of 0.06 to 2.3 mu M. The inhibition of [H- 3]Saf binding by alachlor and S-ethyl dipropylthiocarbamate is competi tive.