NATURE AND BIOAVAILABILITY OF NONEXTRACTABLE (BOUND) RESIDUES IN STORED WHEAT TREATED WITH CHLORPYRIFOS-METHYL

Wheat treated with chlorpyrifos-methyl [O,O-dimethyl O-[(3,5,6-trichlo ro-2-pyridyl) phosphorothioate] and stored for 28 months at 20 degrees C contained about 70% of the applied insecticide as nonextractable (b ound) residue mainly in the form of parent compound. The stored wheat containing bound chlorpyrifos-methyl was fed to rats. The bound residu e in wheat was released and metabolized predominantly by hydrolysis, a s indicated by the presence of a major metabolite, 3,5,6-trichloro-2-p yridinol, in urine and feces. The major route of elimination was via u rine, which contained 3,5,6-trichloro-2-pyridinol and O-methyl O-(3,5, 6-trichloro-2-pyridyl) phosphorothioate. The acetone-extractable porti on of feces contained both chlorpyrifos-methyl and 3,5,6-trichloro-2-p yridinol, whereas the nonextractable portion of feces contained only c hlorpyrifos-methyl. The results show that bound residues of chlorpyrif os-methyl in stored wheat, which cannot be extracted by methods common ly used in residue analysis of grain, are highly bioavailable.