ABSTRACT:Endosulfan (6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,3,4-benzo(e)dioxathiepin-3-oxide) is a broad-spectrum chlorinated cyclodiene insecticide. This study was performed to elucidate the stereoselective metabolism of endosulfan in human liver microsomes and to characterize the cytochrome P450 (P450) enzymes that are involved in the metabolism of endosulfan. Human liver microsomal incubation of endosulfan in the presence of NADPH resulted in the formation of the toxic metabolite, endosulfan sulfate. The intrinsic clearances (CL int ) of endosulfan sulfate from ␤-endosulfan were 3.5-fold higher than those from ␣-endosulfan, suggesting that ␤-endosulfan would be cleared more rapidly than ␣-endosulfan. Correlation analysis between the known P450 enzyme activities and the rate of the formation of endosulfan sulfate in the 14 human liver microsomes showed that ␣-endosulfan metabolism is significantly correlated with CYP2B6-mediated bupropion hydroxylation and CYP3A-mediated midazolam hydroxylation, and that ␤-endosulfan metabolism is correlated with CYP3A activity. The P450 isoform-selective inhibition study in human liver microsomes and the incubation study of cDNA-expressed enzymes also demonstrated that the stereoselective endosulfan sulfate formation from ␣-endosulfan is mediated by CYP2B6, CYP3A4, and CYP3A5, and that from ␤-endosulfan is mediated by CYP3A4 and CYP3A5. The total CL int values of endosulfan sulfate formation catalyzed by CYP3A4 and CYP3A5 were consistently higher for ␤-endosulfan than for the ␣-form (CL int of 0.67 versus 10.46 l/min/pmol P450, respectively). CYP2B6 stereoselectively metabolizes ␣-endosulfan, but not ␤-endosulfan. These findings suggest that the CYP2B6 and CYP3A enzymes are major enzymes contributing to the stereoselective disposition of endosulfan.

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A significant improvement in river water quality cannot be expected unless nonpoint-source contaminants are treated in addition to the further treatment of point-source contaminants. If river water is sprayed over a floodplain, the consequent water filtration through the sediment profile can simultaneously remove organic matter and nitrogen in the water through aerobic and denitrifying reactions. This hypothesis was tested using lysimeters constructed from polyvinyl chloride (PVC) pipe (150 cm long, 15 cm in diameter) packed with loamy sand floodplain sediment. Water was applied to the top of the lysimeters at three different flow rates (48, 54, and 68 mm d(-1)). Concentrations of NO3 and dissolved oxygen (DO), chemical oxygen demand (COD), and redox potential (Eh) in the water were measured as functions of depth after the system reached steady states for both water flow and reactions. At the rate of 68.0 mm d(-1), a reducing condition for denitrification developed below the 5-cm depth due to the depletion of O2 by organic matter degradation in the surface oxidizing layer; Eh and DO were below 205 mV and 0.4 mg L(-1), respectively. At a depth of 70 cm, COD and NO3-N concentration decreased to 5.2 and 3.8 mg L(-1) from the respective influent concentrations of 17.1 and 6.2 mg L(-1). Most biodegradable organic matter was removed during flow and further removal of NO3 was limited by the lack of an electron donor (i.e., organic matter). These results indicate that the floodplain filtration technique has great promise for treatment of contaminated river water.

A field study was undertaken to investigate runoff and leaching loss of the herbicide pendimethalin in turfgrass land of loamy sand soil. A series of plots constructed in a golf course fairway were surface-applied with pendimethalin SC formulation at the rate of 2. 25 or 4.50 kg a.i./ha and subjected to simulated rainfall at 2.0 cm/day for 10 consecutive days. Runoff losses of pendimethalin were the highest at the first rainfall and then gradually decreased with time. The first runoff event contained pendimethalin in its highest concentration, and in subsequent runoff samples the concentration decreased exponentially. The ranges of pendimethalin concentration were 80.9-18.2 and 177.4-48.6 microgram/L in the standard and double doses, respectively. Total losses by 20 cm of rainfall for 10 days reached 0.81 and 1.22% of the initial deposits at 2.25 and 4.50 kg a. i./ha, respectively. Pendimethalin concentration in the leachate collected at 30-cm soil depth was quite lower than that in the runoff, and the concentration rapidly decreased from 4.3-4.7 to 0. 2-0.4 microgram/L during the 10 days of rainfall treatment. Soil residue analysis at 45 and 90 days after pendimethalin treatment showed that more than 90% of the residue remained at the top 10 cm of soil depth. Low runoff and leaching confirmed that lateral and downward movement of the herbicide should be limited in turf soil. The half-life of pendimethalin under field conditions was 23-30 days and was not affected by application dose and rainfall treatment, but longer persistence was observed under laboratory conditions. Considering low runoff and leaching, as well as relatively short persistence in soil, it is concluded that little environmental carryover of pendimethalin would be expected in turfgrass land.

The effects of drying on nine pesticides (chlorfenapyr, clothianidin, diethofencarb, folpet, imidacloprid, indoxacarb, methomyl, methoxyfenozide, and tetraconazole) in chili peppers were studied. The mean concentration factor calculated from weight reduction after drying by water loss was 5.28 for field 1 and 5.41 for field 2 (n = 10). Processing factors for pesticide residues in/on chili peppers after drying were 2.45-5.14 for field 1 and 1.71-4.53 for field 2, indicating that pesticide residues remaining in/on chili peppers were concentrated by usual drying process for pepper powder. The reduction factors reflecting net loss of pesticide residues during drying were 0.50-0.98 for field 1 and 0.44-0.98 for field 2. Drying caused a large reduction (37-49 %) in clothianidin, diethofencarb, imidacloprid, and tetraconazole; moderate reductions (16 and 22 %) in methomyl and methoxyfenozide; while did not affect the levels of chlorfenapyr, folpet, and indoxacarb (using conservative higher reduction factors from the two field trials).