THE CHEMICAL TRANSFORMATION OF ATRAZINE IN CORN SEEDLINGS

One of the possible detoxication pathways for atrazine in corn corresp onds to a chemical hydroxylation process. The aim of this work was to describe the mechanism of this reaction. Under in vitro experimental c onditions, a benzoxazinones mixture (10 mM) extracted from corn plantl ets was able to transform 91% of atrazine (6 mu M) into 2-hydroxyatraz ine during a 24-h period of incubation at 25 degrees C. This reaction was shown to be temperature-dependent; the half-life of atrazine was 6 7 h at 4 degrees C, 90 min at 25 degrees C, and only 30 min at 50 degr ees C. However, at this temperature a rapid degradation of the active benzoxazinones occurred. The pH value of the incubation medium was sho wn to influence greatly the hydroxylation rate of atrazine (no hydroxy lation process at pH 9, a relatively low rate at pH 7, and a maximum o ne at pH 5.5). The presence of an organic solvent (ethanol or acetone) in the reaction medium was responsible for a large decrease in hydrox ylation activity. In a water medium, an optimal rate was obtained when the benzoxazinones concentration was close to 10 mM (average rate of hydroxylation: 2 to 3.10(-4) nmol h(-1) nmol(-1) benzoxazinones). For concentration values lower than 1 mM, the rates remained very low. Ln the presence of 10 mM benzoxazinones, the hydroxylation rate appeared not to be saturable for concentrations in atrazine varying between 6 a nd 100 mu M. The comparison of the hydroxylation rates obtained with p urified benzoxazinones (DIMBOA, DIBOA, 2-monoglucosyl DIMBOA + 2-monog lucosyl DIBOA) suggested that the chemical reactivity of benzoxazinone s toward atrazine involved the 4-N-OH of the heterocycle and not the 2 -C-OH. This hypothesis was reinforced by the fact that the atrazine hy droxylating activity of DIMBOA or DIBOA remained unchanged even in the presence of AlCl3, (a chelator of the 2-OH group). The natural concen tration of benzoxazinones in the vacuolar sap of corn seedlings (great er than or equal to 10 mM) and the pH of this solution (close to 5.5) contribute to explain the high rate of atrazine chemical hydroxylation in vivo. (C) 1997 Academic Press.