Recently, much emphasis has been placed on understanding the toxic mode of action of the 1,2,4-triazole fungicides (i.e., conazoles) in an effort to improve and harmonize risk assessment. Relative to other conazoles, triadimefon is unique with respect to tumorigenesis in rodents, and it has been proposed that triadimefon does not share a common mechanism of toxicity with other conazoles. We postulate that one reason for this difference is that while many conazoles are metabolized via an oxidative P450-mediated pathway, triadimefon is not. In studies conducted with rat hepatic microsomes, triadimenol was identified as the major metabolite (approximately 80%) of triadimefon metabolism, and reduction of the carbonyl group in triadimefon occurred stereoselectively with preferential formation of the less toxic triadimenol B diastereomer. Using chemical inhibitors of P450s (i.e., clotrimazole and 1-aminobenzotriazole) and carbonyl reducing enzymes (i.e., glycyrrhetinic acid, quercitrin, and cortisone), both triadimefon depletion and triadimenol formation were found to be mediated by 11 beta-hydroxysteroid dehydrogenase type 1 (11 beta-HSD1). Studies examining NADPH production and inhibitor studies for glucose-6-phosphate translocation across the endoplasmic reticulum (ER) membrane implicated hexose-6-phosphate dehydrogenase (H6PDH) in the metabolism of triadimefon as well. These results ultimately associate triadimefon metabolism not only with steroidogenesis (i.e., 11 beta-HSD1) but carbohydrate metabolism (i.e., H6PDH) as well. Considering the impact of triadimefon on these biochemical pathways may help explain some of triadimefon's unique toxicological effects relative to other conazole fungicides.

Significant interspecies differences exist between human and rodent with respect to absorption, distribution, and excretion of bisphenol A (BPA) and its primary metabolite, BPA-glucuronide (BPA-G). ATP-Binding Cassette (ABC) transporter enzymes play important roles in these physiological processes, and their enzyme localization (apical vs. basolateral) in the plasma membrane allows for different cellular efflux pathways. In this study, we utilized an ATPase assay to evaluate BPA and BPA-G as potential substrates for the human and rat ABC transporters: P-glycoprotein (MDR1), multidrug resistance-associated proteins (MRPs), and breast cancer-resistant protein (BCRP). Based on high ATPase activity, BPA is likely a substrate for rat mdr1b but not for human MDR1 or rat mdr1a. Results indicate that BPA is a potential substrate for rat mrp2 and human MRP2, BCRP, and MRP3. The metabolite BPA-G demonstrated the highest apparent substrate binding affinity for rat mrp2 and human MRP3 but appeared to be a nonsubstrate or potential inhibitor for human MRP2, MDR1, and BCRP and for rat mdr1a, mdr1b, and bcrp. Analysis of ABC transporter amino acid sequences revealed key differences in putative binding site composition that may explain substrate specificity. Collectively, these results suggest that in both rat and human, apical transporters efflux BPA into the bile and/or intestinal lumen. BPA-G would follow a similar pathway in rat; however, in human, due to the basolateral location of MRP3, BPA-G would likely enter systemic and portal blood supplies. These differences between human and rodent ABC transporters may have significant implications for interspecies extrapolation used in risk assessment.

The uptake and phytotransformation of o,p'-DDT and p,p'-DDT were investigated in vitro using three axenically cultivated aquatic plants: parrot feather (Myriophyllum aquaticum), duckweed (Spirodela oligorrhiza), and elodea (Elodea canadensis). The decay profile of DDT from the aqueous culture medium followed first-order kinetics for all three plants. During the 6-day incubation period, almost all of the DDT was removed from the medium, and most of it accumulated in or was transformed by these plants. Duckweed demonstrated the greatest potential to transform both DDT isomers; 50-66% was degraded or bound in a nonextractable manner with the plant material after the 6-day incubation. Therefore, duckweed also incorporated less extractable DDT (32-49%) after 6 days than did the other plants. The capacity for phytotransformation/binding by elodea is between that of duckweed and parrot feather; approximately 31-48% of the spiked DDT was degraded or bound to the elodea plant material. o,p'-DDD and p,p'-DDD are the major metabolites in these plants; small amounts of p,p'-DDE were also found in duckweed (7.9%) and elodea (4.6%) after 6 days. Apparently, reduction of the aliphatic chlorine atoms of DDT is the major pathway for this transformation. This study, which provides new information on plant biochemistry as related to pollutant accumulation and phytotransformation, should advance the development of phytoremediation processes.

ABSTRACT:The extent to which membrane-disrupting agents, such as alamethicin, may alter cofactor transport and influence in vitro kinetic measurements of glucuronidation is a major concern regarding the characterization and extrapolation of inter-and intraspecies pharmacokinetics of bisphenol A (BPA). An additional concern is the omission of a BPA intestinal metabolism component in current pharmacokinetic models used to assess oral exposure. In this study, BPA glucuronidation in native hepatic microsomes from female rat and female human liver displayed higher V max values than that in males. In the presence of alamethicin, all hepatic V max values increased; however, this increase was disproportionately greater in males and gender differences were no longer observed. Female rats exhibited a much higher K m than all other species and genders; the addition of alamethicin had little influence on K m values for any of the test systems. The dissimilar K m measured for female rat suggests that different UDP-glucuronosyltransferase (UGT) enzyme(s) are involved in BPA glucuronidation. The presence of different UGTs in female rat was confirmed using Hill coefficients measured from diclofenac-mediated chemical inhibition assays within hepatic microsomes and purified human UGT2B7 and UGT2B15. Mixed-gender human intestinal microsomes showed little BPA glucuronidation reactivity compared with those from male rat intestine. Male rat intestinal microsomes in the presence of alamethicin exhibited a V max that was nearly 30-fold higher than that for mixed human microsomes. The species and gender metabolic differences we observed between rat and human liver and intestine provide key information for delineating BPA pharmacokinetics needed for human health risk assessment.