The Chemical Transformation of Atrazine in Corn Seedlings

Abstract

One of the possible detoxication pathways for atrazine in corn corresponds to a chemical hydroxylation process. The aim of this work was to describe the mechanism of this reaction. Underin vitroexperimental conditions, a benzoxazinones mixture (10 mM) extracted from corn plantlets was able to transform 91% of atrazine (6 μM) into 2-hydroxyatrazine during a 24-h period of incubation at 25°C. This reaction was shown to be temperature-dependent; the half-life of atrazine was 67 h at 4°C, 90 min at 25°C, and only 30 min at 50°C. However, at this temperature a rapid degradation of the active benzoxazinones occurred. The pH value of the incubation medium was shown to influence greatly the hydroxylation rate of atrazine (no hydroxylation process at pH 9, a relatively low rate at pH 7, and a maximum one at pH 5.5). The presence of an organic solvent (ethanol or acetone) in the reaction medium was responsible for a large decrease in hydroxylation 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 3.10−4nmol h−1nmol−1benzoxazinones). For concentration values lower than 1 mM, the rates remained very low. In the presence of 10 mM benzoxazinones, the hydroxylation rate appeared not to be saturable for concentrations in atrazine varying between 6 and 100 μM. The comparison of the hydroxylation rates obtained with purified benzoxazinones (DIMBOA, DIBOA, 2-monoglucosyl DIMBOA + 2-monoglucosyl DIBOA) suggested that the chemical reactivity of benzoxazinones 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 hydroxylating activity of DIMBOA or DIBOA remained unchanged even in the presence of AlCl3, (a chelator of the 2-OH group). The natural concentration of benzoxazinones in the vacuolar sap of corn seedlings (≥10 mM) and the pH of this solution (close to 5.5) contribute to explain the high rate of atrazine chemical hydroxylationin vivo.