Pharmacokinetics of 2,4,5-T in mice as a function of dose and gestational status

A pharmacokinetic study was conducted in CD-1 mice with the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) as a function of dose (15, 30, 60, and 90 mg/kg, iv) and gestational status (nonpregnant, day 6, 10, 13, and 17 of gestation, and postpartum). Analysis for 2,4,5-T and its metabolites was based on an electron-capture gas-liquid chromatographic method. Pharmacokinetic stimulation of the blood, urine, and feces data from each mouse was performed on an analog-digital hybrid computer system based on a two-compartment model with parallel, first-order elimination kinetics. Data analysis demonstrated dose-independent kinetics for most pharmacokinetic parameters but a gestational status-dependence. There was a tendency, as indicated by an increase in the biologic half-life and AUC and decrease in clearance and total percent recovery, for pregnant animals to eliminate 2,4,5-T more slowly as gestation progressed, resulting in potentially increasing fetal exposure during the later stages of pregnancy.     

Developmental Toxicity of 2,4,5-Trichlorophenoxyacetic Acid (2,4,5-T): I. Multireplicated Dose-Response Studies in Four Inbred Strains and One Outbred Stock of Mice

A large-scaled multireplicated developmental toxicity studywas conducted in various strains/stocks of mice with the herbicide,2,4,5-trichiorophenoxyacetic acid (2,4,5-T), by gavage on GestationalDays 6 through 14. The most important attributes of the studydesign were replicated test groups, a minimum of four dose levelsper replicate, use of multiple stocks/strains of animals toobtain an estimate of the range in sensitivities due to genotype,complete pathological evaluation of maternal animals, and histopathologicalas well as teratological evaluation of the fetuses. Developmentaltoxicity was observed at doses below those producing discernibleor measurable maternal toxicity. Regression and/or probit analyseswere conducted to determine whether a dose-response relationshipexisted. Reduced fetal weight and increased incidence of cleftpalate and embryolethality were the most significant prenataleffects of 2,4,5-T exposure observed in this study. Each strain/stockexhibited a dose-related decrease in fetal weight with the CD-1mice having the steepest slope and the A/J mice having the shallowestslope. There was a striking similarity among the slopes of thedose-response curves for the various strains/stocks. The meanincidence of embryolethality in the A/J strain was significantlygreater than that of the other strains or stocks. There wassubstantial variation among replicates within strains. The useof the replicated study design was logistically necessary dueto the magnitude of the study and it also served to increasethe statistical power of the study.     

Sequential histopathologic, hematologic, and blood chemistry changes induced in mice by a technical and a purified preparation of 2,4,5-trichlorophenoxyacetic acid.

Maternal mice were given 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) on days 6 through 14 of pregnancy in a tetratologic study at the National Center for Toxicological Research. Sick or moribund mice sacrificed after 4-8 doses of 120 mg/kg 2,4,5-T often showed severe myocardial lesions, hypocellularity of the bone marrow, and depletion of lymphocytes in the thymus, spleen, or lymph nodes. Healthy mice sacrificed on day 17, 11 days after treatment began, showed few or no severe lesions. To determine if lesions earlier in gestation contributed significantly to an increase in fetal abnormalities in the healthy 17-day survivors, dihybrid croos F2 pregnant and nonpregnant mice received by gavage 0, 60, or 120 mg/kg 2,4,5-T on days 6 through 14 of pregnancy. One group received a technical preparation containing 97.9 +/- 0.4% 2,4,5-T; another received a purified preparation containing 99 +/- 0.3% 2,4,5-T. Mice were sacrificed when they became moribund and at 6, 24, and 30 hr, as well as at 4, 6, 8, and 11 days after beginning treatment. Almost all mice given 60 mg/kg and many given 120 mg/kg 2,4,5-T appeared normal at sacrifice either early or late in pregnancy and showed little or no pathologic changes. Mice that became ill or moribund often showed severe lesions; few survived 11 days. Severe myocardial lesions were seen in 26 of 70 moribund mice fiven the technical 2,4,5-T and 24 of 33 given the purified preparation of 2,4,5-T. The moribund mice, particularly those given the purified compound, also showed a high incidence of lesions in other organs and marked hematological and blood chemistry changes. These findings indicate that the lesions are primarily due to 2,4,5-T rather than to impurities in the technical preparation; also impaired maternal health is not the primary cause of the increase in fetal abnormalities.     

Cytoskeletal perturbation induced by herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)

To understand the mechanisms of toxicity of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), we have studied their effects on the cytoskeletal organization, particularly microtubules (MT) and microfilaments (MF), DNA synthesis, and the synthesis and composition of cytoskeletal proteins in mouse 3T3 cells. Exposure of cells to 2,4-D or 2,4,5-T resulted in a dose-dependent inhibition of DNA synthesis; 50% inhibition occurred at 2.21 mM and 0.90 mM for 2,4-D and 2,4,5-T, respectively. Furthermore, a strong synergistic inhibition of DNA synthesis was produced by mixtures (each having a total concentration of 1.25 mM) of 2,4-D with 2,4,5-T. Similarly, 2,4,5-T is more potent than 2,4-D in causing cytoskeletal perturbation as revealed by fluorescence microscopy. Treatment of cells with 2,4-D (2.5 mM) or 2,4,5-T (1.25 mM) for 20 h resulted in severe MT aggregation and the appearance of large bundles, which were organized in a rope-like structure in the former and a dramatic octopus-like pattern in the latter. Further, MT bundling is particularly severe in the cell center. Under these conditions, marked changes in MF organization also occurred as evidenced by clustering and crisscrossing of MF in the perinuclear region. A 1:1 mixture (final = 1.25 mM) of 2,4-D and 2,4,5-T, a formulation equivalent to Agent Orange composition, also induced a dramatic perturbation to the organization of MT and MF, resulting in the formation of ring-like structures. MT bundling is still apparent, especially around the outer edge of the "rings." MF are localized predominantly along the cell periphery, where they appear to be aggregated tightly forming patches. Surprisingly, the synthesis and composition of cytoskeletal proteins, which are resistant to detergent extraction but released by CaCl2, are essentially unaffected by 2,4-D or 2,4,5-T. These results suggest that the dramatic perturbation of the cytoskeletal morphology caused by these herbicides probably only results from a structural reorganization and redistribution of MT and MF.     

The fate of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) following oral administration to rats and dogs

Clearance of ¹⁴C activity from the plasma and its elimination from the body of rats and dogs were determined after single oral doses of [carboxy-¹⁴C]2,4,5-T. The half-life values for the clearance of ¹⁴C activity from the plasma of rats given doses of 5, 50, 100 or 200 mg/kg were 4.7, 4.2, 19.4 and 25.2 hr, respectively; half-lives for elimination from the body were 13.6, 13.1, 19.3 and 28.9 hr, respectively. The apparent volume of distribution also increased with dose. Urinary excretion of unchanged 2,4,5-T accounted for most of the ¹⁴C activity eliminated from the body of rats. A small amount of unidentified metabolite was detected in the urine when rats were given 100 or 200 mg/kg but not 5 or 50 mg/kg. These results show that the distribution, metabolism and excretion of 2,4,5-T are markedly altered when large doses are administered.In dogs given 5 mg/kg, the half-life values for clearance from plasma and elimination from the body were 77.0 and 86.6 hr, respectively, offering a plausible explanation of why 2,4,5-T is more toxic in dogs than in rats. Appreciable excretion in the feces was noted and three unidentified metabolites were detected in urine of dogs, indicating a considerable difference in metabolism of 2,4,5-T by dogs and rats given the same dose.     

The fate of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) following oral administration to man

Five human male volunteers ingested a single dose of 5 mg/kg without incurring detectable clinical effects. Concentrations of 2,4,5-T in plasma and its excretion were measured at intervals after ingestion. The clearances of 2,4,5-T from the plasma as well as its excretion from the body occurred via apparent first-order rate processes with half-lives of 23.10 and 23.06 hr, respectively. Essentially all of the 2,4,5-T was absorbed into the body and excreted unchanged in the urine. In the body, 65% of the 2,4,5-T resided in the plasma where 98.7% was bound reversibly to protein. The volume of distribution was 0.079 liters/kg. Utilizing the kinetic constants from the single dose experiment, the expected concentrations of 2,4,5-T in the plasma of individuals receiving repeated daily doses of 2,4,5-T were calculated. From these calculations, it was determined that the plasma concentrations would essentially reach a plateau after 3 days. If the daily dose ingested in mg/kg is A₀, the concentration in the plasma after attaining plateau would range from 12.7 A₀ to 22.5 A₀ μg/ml. This range would converge to approximately 17 A₀ μg/ml as the daily dose A₀ is distributed throughout the day.     

Specific teratogenic action of 2,4,5-trichlorophenoxyacetic acid on mouse and rat whole-embryo cultures

The effects of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were studied at concentrations ranging from 0.17 to 3.52 m on post-implanted mouse and rat embryos cultured for 48 hr, either in the absence of any metabolic activation system or in the presence of mouse and rat S-9 mix. 2,4,5-T proved to be a potential teratogen, at 0.88 and 1.76 m , on mouse embryos in the absence of any metabolic activation system. In the presence of mouse S-9 mix, 2,4,5-T showed a high teratogenic potential at 0.17, 0.88 and 1.76 m . In contrast, in the presence of rat S-9 mix, 2,4,5-T induced structural defects only at 1.76 m . At 3.52 m , 2,4,5-T was 100% embryolethal with or without S-9 mix. On rat embryos, 2,4,5-T was potentially teratogenic only in the presence of mouse S-9 mix, causing a significant increase in dysmorphogenic effects at 0.17, 0.88 and 1.76 m . With or without rat S-9 mix, 2,4,5,-T was only embryolethal to rat embryos at 1.76 and 3.52 m . The abnormalities mainly involved the forebrain, the midbrain and the branchial arches. These results are consistent with the known in vivo embryotoxic action of this compound.     

Distribution of 2,4,5-trichlorophenoxyacetic acid in the mouse fetus

The herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) readily crosses the placenta in the mouse on gestational day 13. Concentrations of 2,4,5-T in maternal tissues and fetuses were obvious at 30 min, highest at 8 h, diminished by 24 h and almost entirely eliminated by 48 h. Autoradiographs of the whole fetus showed that initially 2,4,5-T was mainly in the highly vascularized tissues such as liver followed by the skin, eyes and ventricles of the brain. At 48 h, there was a general distribution of 2,4,5-T in the skeletal muscles. None was detected in the palate. The excretion rate in the mouse was approx. 60 gm/h; about 52% of the dose was recovered unchanged in the urine in 24 h.     

Fate and Distribution of the Herbicides 2,4-Dichloro-phenoxyacetic Acid (2,4-D) and 2,4,5-Trichlorophenoxy-acetic Acid (2,4,5-T) in the Dogfish Shark

1. The urinary and biliary excretion, tissue distribution and metabolism of ¹⁴C-labelled 2,4-dichloro- or 2,4,5-trichloro-phenoxyacetic acids (2,4-D or 2,4,5-T) were measured in dogfish sharks, Squalus acanthias.

2. Both herbicides are extensively metabolized (> 90%) to the corresponding taurine conjugates, and are excreted predominantly via the urine, where ca. 70% of the administered dose appears within 4–6 days after treatment.

3. The highest tissue levels of 2,4-D or 2,4,5-T were found in liver and kidney. Penetration of both herbicides into the CNS was restricted.

4. Plasma elimination was rapid and the t_0.5 for either phenoxyacetic acid was < 45?min. Similarly, rapid clearance as seen from renal tissue. Final t_0.5 values for muscle were about 2–3 days while the major organ showing 2,4-D or 2,4,5-T retention was the liver, where t_0.5 values were about 5 days for both the herbicides.

5. The overall pharmacokinetics in the dogfish shark for these herbicides resembled those seen in some mammals.     

Specificity of 2,4,5-trichlorophenoxyacetate on tetraethylammonium transport.

Renal cortical slices from rats pretreated acutely with 2,4,5-trichlorophenoxyacetate (2,4,5-T) show a reduced ability to transport 2,4-dichlorophenoxyacetate (2,4-D) and tetraethylammonium (TEA). The depression in renal transport is dependent on the 2,4-D or TEA concentration in the incubation medium and is surmountable at high concentrations of 2,4-D or TEA. The steady-state transport of TEA by normal renal slices is inhibited by the organic anion 2,4,5-T, in a dose-related manner. Kinetic analysis of the effect of 2,4,5-T on TEA transport appears to demonstrate that 2,4,5-T is a competitive inhibitor of the steady-state accumulation of TEA. However, the initial influx of TEA was found to be unaffected by 2,4,5-T. The transport of another organic cation, N¹-methylnicotinamide, was depressed only modestly by 2,4,5-T.     

Inhibition of Intercellular Communication in Cultures of Chinese Hamster V79 Cells by 2,4-Dichlorophenoxyacetic Acid and 2,4,5-Trichlorophenoxyacetic Acid

Inhibition of Intercellular Communication in Cultures of ChineseHamster V79 Cells by 2,4-Dicfalorophenoxyacctic Acid and 2,4,5-TrichlorophenoxyaceticAcid. RUBINSTEIN, C, JONE, C, TROSKO, J. E., AND CHANG, C. C.(1984). Fundam. Appt. Toxicol. 4, 731–739. Using the Chinesehamster V79 in vitro cell system designed to measure intercellularcommunication, 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyaceticacid (2,4,5-T), and several mixtures of these compounds weretested for their ability to inhibit this biological process.The ability of these chemicals to inhibit colony-forming abilityof these cells was tested prior to the studies to measure intercellularcommunication. 2,4-D was less cytotoxic than 2,4,5-T. Both 2,4,5-Tand 2,4-D were able to inhibit intercellular communication attheir respective noncytotoxic dose ranges. Various mixturesof both chemicals were also able to inhibit intercellular communication,snowing some kind of addititvity. No-effect levels were alsonoted in the intercellular communication assay. These resultswere interpreted as being consistent with the hypothesis thatthese compounds might be teratogenic by their ability to inhibitintercellular communication during development     

2,4,5-Trichlorophenoxyacetic Acid Influence on 2,6-Dinitrotoluene-Induced Urine Genotoxicity in Fischer 344 Rats: Effect on Gastrointestinal Microflora and Enzyme Activity

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) and 2,6-dinitrotoluene(2,6-DNT) are hazardous chemicals that have potential harmfuleffects. 2,6-DNT is recognized as a hepatotoxicant while 2,4,5-T,a component of Agent Orange, is also suspect. 2,6-DNT requiresboth oxidative and reductive metabolism to elicit genotoxiceffects. To determine what effect 2,4,5-T had on 2,6-DNT metabolism,intestinal enzymes, microbial populations, and urine mutagenicitywere examined during 2,4,5-T treatment. Weanling Fischer 344male rats were treated daily with 54.4 mg/kg 2,4,5-T by gavagefor 4 weeks. One, two, and four weeks after the initial 2,4,5-Tdose, rats were administered (po) 2,6-DNT (75 mg/kg) and urinewas collected for 24 hr in metabolism cages. Azo reductase,nitroreductase, ß-glucuronidase, dechlorinase, anddehydrochlorinase activities were examined concurrently. Treatmentof rats for 1 week reduced the transformation of 2,6-DNT tomutagenic urinary metabolites. This was accompanied by a decreasein the fecal anaerobic microorganisms. The elimination ofLactobacillusfermentum from the small intestine and cecum of treated animalsaccompanied a significant increase in oxygen-tolerant lactobacilliand other unidentified aerobic microorganisms. However, therewere no significant alterations in the intestinal enzyme activitiesexamined. By 2 weeks of 2,4,5-T treatment, microbiota and urinegenotoxicity returned to the levels observed in control animals.This trend continued for the duration of the experiment After2 weeks, while cecal nitroreductase and azo reductase activitiesincreased, small intestinal ß-glucuronidase activitydecreased. By 4 weeks, treated and untreated animal intestinalenzyme activities were indistinguishable. The transient increasein azo reductase and nitroreductase after treatment with 2,4,5-Tfor 2 weeks may have been counteracted by the reduced ß-glucuronidaseactivity, thus resulting in no change in 2,6-DNT-derived urinemutagenicity. However, other environmental chemicals, unaffectedby ß-glucuronidase, potentially could be activatedby 2,4,5-T exposure.     

Developmental Toxicity Evaluation of Inhaled Methyl Isobutyl Ketone in Fischer 344 Rats and CD-1 Mice

Developmental Toxicity Evaluation of Inhaled Methyl IsobutylKetone in Fischer 344 Rats and CD-1 Mice. Tyl, R. W., FRANCE,K. A., FISHER, L. C., PRITTS, I. M., TYLER, T. R., PHILLIPS,R. D., and MORAN, E. J. (1987). Fundam. Appl. Toxicol. 8, 310–327.Pregnant Fischer 344 rats and CD-1 mice were exposed to methylisobutyl ketone vapor (CAS No. 108-10-1) by inhalation on GestationalDays 6 through 15 at concentrations of 0, 300, 1000, or 3000ppm (mean analytical values of 0, 305, 1012, and 2997 ppm, respectively).The animals were sacrificed on Gestational Day 21 (rats) or18 (mice), and live fetuses were examined for external, visceral,and skeletal alterations. In rats, exposure to 3000 ppm resultedin maternal toxicity expressed as clinical signs, decreasedbody weight and body weight gain, increased relative kidneyweight, and decreased food consumption, and in fetotoxicityexpressed as reduced fetal body weight per litter and reductionsin skeletal ossification. In mice, exposure to 3000 ppm resultedin maternal toxicity expressed as exposure-related increasesin deaths (12.0%, 3/25 dams), clinical signs, and increasedabsolute and relative liver weight, and in fetotoxicity expressedas increased incidence of dead fetuses, reduced fetal body weightper litter, and reductions in skeletal ossification. No treatment-relatedincreases in embryotoxicity or fetal malformations were seenin either species at any exposure concentration tested. Therewas no evidence of treatment-related maternal, embryo, or fetaltoxicity (including malformations) at 1000 or 300 ppm in eitherSpecies.     

Relationship between 2,4,5-trichlorophenoxyacetate and the renal organic base transport system.

At concentrations which did not alter tissue oxygen consumption, 2,4,5-trichlorophenoxyacetate (2,4,5-T) increased the rate of tetraethylammonium (TEA) efflux from renal slices by 35–130 per cent. The efflux of N¹-methylnicotinamide (NMN) was unaffected by 2,4,5-T. The steady-state transport of 2,4,5-T by renal slices was not altered by competitive or irreversible inhibitors of the base transport system. These data and others indicate that, despite 2,4,5-T-TEA interaction, 2,4,5-T is not transported totally or even to a great extent by the renal organic base secretory mechanism. A model is presented that is consistent with all the observations.     

The metabolism and distribution of 2,4,5-trichlorophenoxyacetic acid in female rats

[1-¹⁴C]-2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) was fed to pregnant and non-pregnant female rats at various dosages, and expired air, urine, feces, internal organs and tissues were analyzed for radioactivity. During the first 24 hr, 75 ± 7% of the radioactivity was excreted in the urine and 8.2 ± 4.6% in the feces. No ¹⁴C was found in the expired air. There was no significant difference in the rate of elimination between the pregnant and nonpregnant rats, or among the dosages used. Radioactivity was detected in all tissues, with the highest concentration being found in the kidney. The maximum concentration of radioactivity in all tissues was generally reached between 6 to 12 hr after po dosing and then started to decline rapidly. Radioactivity was also detected in the fetuses and in the milk. The average biological half-life of 2,4,5-T in the organs was 3.4 hr for the adult rats and 97 hr for the newborn.     

Effects of the Herbicide 2,4,5-T on the Incorporation of 14C-Thymidine, 3H-Uridine and 3H-L-Leucine into L 929 Cells

Abstract: A rapid inhibition of the incorporation of ³H-L-leucine, ³H-uridine and ¹⁴C-thymidine into the macromolecular fraction of L 929 cells was found when 2.25 mM 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) was added to the incubation medium. The reduced incorporation of ³H-uridine and ¹⁴C-thymidine could be explained by an inhibitory effect of 2,4,5-T on the uptake of these compounds into the acid-soluble precursor pool, whereas a direct effect on protein synthesis was indicated. Kinetic analysis of the uptake of uridine into the acid-soluble pool indicated that 2,4,5-T inhibited the facilitated transport of this nucleoside into the cell. The overall results are interpreted as indicating that 2,4,5-T induced alterations in the permeability of the cellular plasma membrane.     

Inhibition of human glutathione S-transferases by 2,4-dichlorophenoxyacetate (2,4-D) and 2,4,5-trichlorophenoxyacetate (2,4,5-T)

The phenoxyacid herbicides, 2,4-dichlorophenoxyacetate (2,4-D) and 2,4,5-trichlorophenoxyacetate (2,4,5-T), inhibit all known isoenzymes of human liver and erythrocyte glutathione (GSH) S-transferase. However, the maximal inhibition and the I50 values vary significantly for different isoenzymes. GSH peroxidase II activity of GSH S-transferases is also inhibited by both these compounds. These studies suggest that the effect of these compounds on human GSH S-transferases is significantly different from that reported for rat liver enzymes.     

In vitro uptake of organic ions by renal cortical tissue of rats treated acutely with 2,4,5-trichlorophenoxyacetic acid.

The effect of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) on renal cortical function has been studied in male adult rats. Significant decreases in organic acid and organic base transport were measured when rats were pretreated with 2,4,5-T 24 hr before in vitro analysis of renal function. Experiments in which injections of p-aminohippurate were given showed no effect on organic acid or organic base transport by renal cortical slices. The data are interpreted to mean that pretreatment with 2,4,5-T has a depressive influence on the transport of some organic ions. Adult animals which were treated daily with 2,4,5-T accumulated a large body store of this herbicide during the first 6 days of administration. However, by 9 days the animals excreted essentially the dose of 2,4,5-T administered, and by 16 days the herbicide accumulated during the first 6 days had been almost totally eliminated. This chronic excretion pattern may explain why only moderate toxicity has been reported for this herbicide.     

Teratogenic and toxicological examination of 2,4,5-trichlorphenoxyacetic acid in developing chick embryos

Practical grade 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), dissolved in dimethylsufoxide (DMSO), was injected into the air space of fertilized chicken eggs prior to incubation. Doses of 2,4,5-T administered were 12.5, 25, 50, 75, 100, and 125 mg/kg to determine herbicide toxicity on the first day of incubation. A similar group was studied on day 5 of incubation with doses of 2,4,5-T at 50, 75, 100 and 250 mg/kg. LD₅₀ was estimated to be 62 mg/kg on day zero and 68 mg/kg on day 5. Additional, embryos were exposed to 2,4,5-T at 50 mg/kg on day zero of gestation and sacrificed after 48 h of incubation. Serial sections were examined for teratological and developmental anomalies. None were found.     

Differential activation and inhibition of different forms of rat liver glutathione S-transferase by the herbicides 2,4-dichlorophenoxyacetate (2,4-D) and 2,4,5-trichlorophenoxyacetate (2,4,5-T)

The predominant forms of the dimeric enzyme glutathione S-transferase were purified from rat liver. Forms Y_bY′_b and Y_bY_b (also known as forms C and A, respectively) could be almost completely inhibited by 2,4-dichlorophenoxyacetate (2,4-D). Half-maximal inhibition was obtained at 0.5 mm 2,4-D. Inhibition was seen even at extrapolated infinite concentrations of both substrates for Y_bY_b but not Y_bY′_b. These same forms could also be inhibited 70 to 80% by 2,4,5-trichlorophenoxyacetate (2,4,5-T) with half maximal inhibition occurring at 0.2 mm. Glutathione S-transferase from Y_aY_a was maximally inhibited by 72 and 30%, respectively, by 2,4-D and 2,4,5-T. The 30% inhibition of Y_aY_a caused by 2,4,5-T was shown to reduce the nearly complete inhibition caused by a previously characterized inhibitor, namely bile acids. This suggests competition for a common binding site on the enzyme. In contrast to the above results, it was found that form Y_cY_c (also termed AA) was activated 2.7-fold by 2,4,5-T and 1.4-fold by 2,4-D. This activation could be blocked by chenodeoxycholate which, by itself, did not affect the activity of the enzyme. The effects of 2,4,5-T and 2,4-D on the heterodimer Y_aY_c (also termed form B) were intermediate between their effects on Y_aY_a and Y_cY_c, suggesting that each subunit contributes its unique property to the heterodimer. The microsomal membrane-bound form of glutathione S-transferase was also examined and found to be inhibited by both 2,4-D and 2,4,5-T. However, unlike the inhibitions of soluble forms, 2,4,5-T caused more extensive inhibition than 2,4-D. It is concluded that exposure to 2,4-D and 2,4,5-T can limit the ability of glutathione S-transferase forms Y_bY_b and Y_bY′_b to metabolize electrophilic toxins. This capacity to potentiate the toxicity of certain electrophiles indicates a need to study the effect of herbicides on glutathione S-transferases from human tissues.