Triorthotolyl phosphate inhibition of carboxylesterases and potentiation of procaine toxicity

Groups of mice were given single doses of triorthotolyl phosphate (TOTP) (1–125 mg/kg, i.p.) 18hr prior to sacrifice or procaine challenge. A dose-dependent decrease was recorded in liver carboxylesterase hydrolysis of diethyl succinate, triacetin and procaine. TOTP pretreatment shortened the time to onset and prolonged the duration of procaine (175 mg/kg, i.p.)-induced loss of righting ability. The correlation coefficient for inhibition of liver procaine hydrolysis and prolongation of procaine action was 0.97 (P < 0.0001). No deaths were observed in procaine-injected controls yet mortality after procaine injection in TOTP-pretreated mice increased as a function of the TOTP pretreatment dose. In addition, plasma procaine concentrations after procaine injection (150 mg/kg, i.p.) reached peak levels sooner, remained elevated longer and were 3-fold higher in the TOTP-pretreated mice as compared to corn oil-pretreated procaine-injected controls. Results demonstrate potentiation of procaine toxicity by TOTP and suggest that this results from inhibition of procaine metabolism in vivo.     

Iso-OMPA-induced potentiation of soman toxicity in rat correlates with the inhibition of plasma carboxylesterases

Recently, the question was raised as to why iso-OMPA, generally known as a selective irreversible inhibitor of butyrylcholinesterase (BuChE), potentiates soman toxicity in rats but not in mice. Mice are known to have higher carboxylesterase (CarbE) and lower BuChE activity in plasma than rat. It could be hypothesized that it is the iso-OMPA inhibition of plasma CarbE, and not of BuChE, which is responsible for potentiation of soman toxicity in iso-OMPA-pretreated rats. In order to test this hypothesis two doses of iso-OMPA were administered to rats prior to soman. The two doses were selected in such a way that both were high enough to inhibit more than 90% of plasma BuChE activity; plasma CarbE activity, however, was only slightly inhibited by the lower and substantially by the higher dose of iso-OMPA. Our results demonstrate that iso-OMPA-induced potentiation of soman toxicity correlates with the inhibition of CarbE and not with the inhibition of BuChE activity in rat plasma. Relative resistance of mice to iso-OMPA-induced potentiation of soman toxicity could therefore be explained by a higher proportion of CarbE activity remaining uninhibited after iso-OMPA pretreatment. By having their active centers unoccupied, CarbE molecules can bind soman and reduce its concentration in neuronal tissue and motor end-plates.This is a summarized report. The full material is available from the authors on request     

iso-OMPA-induced potentiation of soman toxicity in rat

Male Sprague-Dawley rats, injected with the irreversible acetylcholinesterase (AChE) inhibitor soman (pinacolyl methylphosphonofluoridate) 25 g/kg sc, showed no signs of toxicity. Pretreatment with iso-OMPA (tetra-isopropylpyrophosphoramide) 1 mg/kg sc 1 h before the soman administration, caused severe signs of hypercholinergic activity, similar to those seen with an acute signs-producing nonlethal dosage (100 g soman/kg sc). Within 1 h iso-OMPA alone significantly reduced the activity of carboxylesterases (CarbE) in all tissues studied and butyrylcholinesterase (BuChE) activity was significantly reduced in plasma (22%) and liver (27%). Soman (25 g/kg) alone significantly reduced the plasma activity of CarbE (15%), BuChE (53%) and AChE (18%), but had no effect on these enzymes of liver. The combined treatment of iso-OMPA and soman, however, reduced CarbE activity in liver (0%) and produced significantly greater effects than iso-OMPA or soman alone on AChE and BuChE in all the brain areas and skeletal muscles tested. The number of necrotic lesions found in skeletal muscles was many times higher with the combined treatment than seen with soman (25 g/kg) alone, and was equal to those seen with an acute toxicity signs-producing dose of soman. It is concluded that the observed iso-OMPA-induced potentiation of soman toxicity is probably caused via reduced nonspecific binding sites (BuChE and CarbE) for soman leading to greater inhibition of AChE.     

Cytogenetic biomarkers of carbofuran toxicity utilizing human lymphocyte cultures in vitro

Short-term lymphocyte cultures from human peripheral blood samples were incubated with various aliquots of the carbofuran pesticide. After 48 h of initiation and 24 h of exposure to the carbofuran pesticide aliquots, it was seen that carbofuran caused an increase in the frequency of chromosomal aberrations, and the increase was significant (p < 0.05) in treated samples compared to controls. Karyotype analysis revealed more satellite associations, gaps, and breaks in treated samples. Single-strand breaks in the DNA assessed by comet assay revealed that the pesticide caused increase in the comet tail length implicating genotoxicity in somatic cells. The LD(50) of carbofuran was found to be 18 microM as calculated by probit analysis and determined by trypan blue dye exclusion method. The results presented here indicate that in vitro assays could be used as indicators of cyto- and genotoxicity of the pesticide, and their end points could be used as biomarkers.     

Acetone potentiation of acute acetonitrile toxicity in rats

The purpose of these studies was to investigate the nature and mechanism of a toxicologic interaction between acetonitrile and acetone. Results of oral dose-response studies utilizing a 1:1 (w/w) mixture of acetonitrile and acetone, or varying doses of acetonitrile administered together with a constant dose of acetone, indicated that acetone potentiated acute acetonitrile toxicity three- to fourfold in rats. The onset of severe toxicity (manifested by tremors and convulsions) was delayed in the groups dosed with both solvents compared to the groups that received acetonitrile or acetone alone. Blood cyanide (a metabolite of acetonitrile) and serum acetonitrile and acetone concentrations were measured after oral administration of 25% aqueous solutions of acetonitrile, acetone, or acetonitrile plus acetone. Concentrations of cyanide in the blood of rats given acetonitrile plus acetone remained near baseline, in contrast to the high concentrations found in rats dosed with acetonitrile alone. At 34-36 h, high blood cyanide concentrations were found in rats dosed with both of the solvents. This delayed onset of elevation of blood cyanide coincided with the occurrence of clinical signs and with the disappearance of serum acetone. In further pharmacokinetic studies, blood cyanide concentrations were measured after similar dosage regimens of acetone and acetonitrile. Peak cyanide concentrations were found to be significantly greater in rats dosed with both solvents than in rats given only acetonitrile. Administration of either sodium thiosulfate or a second dose of acetone prevented the toxicity associated with exposure to both solvents. These results suggest that the effects of acetone on acetonitrile toxicity are due to a biphasic effect on the metabolism of acetonitrile to cyanide, that is, an initial inhibition followed by a stimulation of this metabolism upon acetone elimination.     

Polybrominated biphenyl-induced potentiation of chloroform toxicity.

ICR male mice were fed diets containing 0, 1, 25, or 100 ppm of polybrominated biphenyls (PBB) for 14 days prior to challenge with a single injection of chloroform (CHCl₃). Dietary PBB potentiated the CHCl₃-induced depression of p-aminohippurate accumulation by slices of renal cortex and the CHCl₃-induced rise in blood urea nitrogen concentration in a PBB-intake-related manner. The kidney weight-to-body weight ratio was elevated in PBB mice but not in mice consuming control (0 ppm) diet following administration of 50 μl/kg of CHCl₃. Mice ingesting food containing 100 ppm of PBB also exhibited a greater increase in serum glutamic-oxaloacetic transaminase activity than did control mice following CHCl₃ administration. The 24-hr LD50 of CHCl₃ in control mice was 1.28 ml/kg, while that in mice consuming 100 ppm of PBB was 0.62 ml/kg.     

Comparative toxicity of the pesticides carbofuran and malathion to the freshwater flagellate Euglena gracilis

Pesticides are toxic chemicals used for agricultural as well as non-agricultural purposes. The toxicity of pesticides does not remain limited to the site of application but they also cause toxicity to non-target organisms in terrestrial as well as in aquatic environments. This study discusses the comparative toxicity of a carbamate (carbofuran) and an organophosphorus (malathion) pesticide to the freshwater flagellate Euglena gracilis during short- and long-term exposures. To evaluate the toxicity of the pesticides, different parameters of the flagellate, like cell density, motility, swimming velocity, cell shape, gravitactic orientation, photosynthetic efficiency, and concentration of light harvesting pigments, were used as end points. Carbofuran was found to be more toxic to E. gracilis than malathion and adversely affected almost all the tested parameters in short- and long-term experiments. The only significant adverse effect by malathion could be demonstrated on the swimming velocity of cells in short-term experiments. The adverse effects of the pesticides were more pronounced during short-term than during long-term exposure.     

The potentiation of the antitumor activity but not toxicity of bleomycin by 3-aminobenzamide

Our previous studies have demonstrated that 3-aminobenzamide (3AB), an inhibitor of adenosine diphosphate-ribosyl transferase (ADPRT) could enhance the cytotoxicity (in vitro) and antitumor activity (in vivo) of bleomycin (BLM) A5 and peplomycin (PEP) against S-180, hepatoma and Ehrlich ascites carcinoma (EAC). In this study, it was shown that the inhibition rates (INR's) of S-180 in two experiments were increased from 42.5 and 46.1% to 66.2 and 75.9% when BLM 2.5 mg/kg/day x 8 was combined with 3AB 385.4 mg/kg/day x 8, while the decrease of body weight could not be enhanced. BLM at a dose of 5 mg/kg/day x 8 gave INR's of 64.8 and 75%, similar to the combined group but decreased the body weight more significantly. However, the addition of 3AB 385.4 mg/kg/day to BLM did not increase the acute toxicity of BLM alone. There was no significant difference of change of the body weight and subacute toxicity between the BLM and BLM + 3AB group. There was no difference of peripheral blood white cell count and the pathomorphological and ultrastructural change, wet weight and hydroxyproline content (to reflect the collagen content) of the lung of the mice between BLM alone and BLM + 3AB group. Therefore, the study provided experimental evidences for the reasonable use of nontoxic ADPRT inhibitors in adjunct to the chemotherapy of BLM in cancers.     

The potentiation of 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity by tamoxifen in female CD1 mice.

Tamoxifen, an antiestrogen commonly used in breast cancer therapy, potentiated the lethality of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) when coadministered to female CD1 mice, despite the virtual lack of toxicity associated with the administration of tamoxifen alone. The 58-day ip LD50 of TCDD was reduced from 330 to 185 micrograms/kg by sc administration of 1 mg/kg/day tamoxifen. A significant dose-response relationship was observed for the potentiating effect of tamoxifen on TCDD lethality. All mice receiving TCDD developed a centrilobular pattern of hepatocellular degeneration and necrosis with perivascular infiltration of inflammatory cells. Clinical chemistry parameters were indicative of liver disease. Abnormalities in mice receiving tamoxifen plus TCDD were similar to, but more severe than, those in mice receiving TCDD only. Seven days after administration of [14C]TCDD, liver retention of radioactivity was increased 80-100% by coadministration of tamoxifen. This elevated retention was associated with a 50 and 37% decrease in excretion of radioactivity by the urinary and fecal routes, respectively. Our results suggest that the potentiation of TCDD toxicity by tamoxifen is associated with decreased excretion of TCDD, leading to elevated liver retention and enhanced severity of liver pathology.     

Dose-response relationships in 1,3-butanediol-induced potentiation of carbon tetrachloride toxicity

Pretreatment of rats with 1,3-butanediol (BD) (1.0, 5.0, or 10.0% in drinking water) for 7 days enhanced the hepatotoxic, but not the nephrotoxic, effect of a single dose of CCl₄ (0.1 ml/kg, ip) in a dose-related manner. Biochemical tests and light microscopy were employed to assess toxicity. The smallest dosage of BD needed to potentiate CCl₄ hepatotoxicity was estimated to be between 0.1 and 1.0 g/kg/day. The potentiated response appeared to be related to the severity of the metabolic ketosis produced by BD. Pretreatment with BD (10.0%) resulted in increased hepatic microsomal activity and cytochrome P-450 content; the in vitro binding of ¹⁴CCl₄-derived radioactivity to microsomal protein was increased both aerobically and anaerobically, but was greater under aerobic conditions. BD appeared to potentiate CCl₄ liver injury, at least in part, by producing an increase and/or alteration in mixed function activation of CCl₄ to toxic metabolites. Pretreatment with BD (1.0, 10.0%) also resulted in a reduction of hepatic glutathione content. Thus, BD potentiation may arise through several, interrelated pathways.     

Effect of chloramphenicol pretreatment on malathion-induced acute toxicity in the rat

Pretreatment of rats with chloramphenicol (CAP) (100 mg/kg, ip) 30 min prior to a single oral LD50 dose of malathion (MTH) at 340 mg/kg completely protected against MTH-induced signs of cholinergic toxicity. Pretreatment with CAP also decreased the extent and duration of MTH-induced inhibition of cholinesterase (ChE). It was previously established that CAP inhibits (1) the cytochrome-P-450-catalyzed oxidative desulfuration of methylparathion to the much more toxic oxygen analog methylparaoxon (MOX) and (2) the carboxyesterase in rat liver. Since carboxyesterases account for 60% or more of the catabolism of MTH in the rat, the present results were surprising. Thus it appears that the inhibition of MTH toxicity by CAP pretreatment is attributable to inhibition by CAP of the metabolic activation of MTH to MOX.     

Cholinesterase inhibition and behavioral toxicity of carbofuran on Oreochromis niloticus early life stages

Nile tilapia Oreochromis niloticus at 9 days post-hatch were exposed in semi-static experiments to the carbamate insecticide carbofuran, which is applied in agricultural systems in Brazil. Although the molecular mechanism of carbofuran toxicity is well known, a detailed understanding of the ecological mechanisms through which carbofuran effects can propagate towards higher levels of biological organization in fish is incomplete. Mortality rates were quantified for larvae exposed for 96 h to 8.3, 40.6, 69.9, 140, 297 and 397 μg/L carbofuran, and the LC(50) 96 h was 214.7 μg/L. In addition, the biochemical biomarker cholinesterase inhibition and behavioral biomarkers related to vision, swimming, prey capture and predator avoidance were quantified in individual larvae, as well as their growth in weight. The behavioral parameters were quantified by analysis of digitally recorded videos of individual larvae within appropriate experimental setups. The activity of the enzyme cholinesterase decreased after exposure to carbofuran with a lowest observed effects concentration (LOEC) of 69.9 μg/L. Visual acuity deficits were detected after carbofuran exposure with a LOEC of 40.6 μg/L. Swimming speed decreased with carbofuran exposure, with a LOEC of 397.6 μg/L. The number of attacks to prey (Daphnia magna nauplii) decreased in larvae exposed to carbofuran, with a LOEC of 397.6 μg/L. Growth in weight was significantly reduced in a dose dependent manner, and all carbofuran groups exhibited a statistically significant decrease in growth when compared to controls (p<0.05). The number of predator attacks necessary to capture larvae decreased after exposure to carbofuran, and the LOEC was 69.9 μg/L. These results show that exposure of sensitive early life stages of tilapia O. niloticus to sublethal concentrations of carbofuran can affect fundamental aspects of fish larval ecology that are relevant to recruitment of fish populations, and that can be better understood by the application of behavioral biomarkers.     

Influence of 3-methylcholanthrene pretreatment on sanguinarine toxicity in mice

We examined the effect of 3-methylcholanthrene (3-MC) on the liver toxicity of sanguinarine in mice. Administration of 10 mg sanguinarine/kg bw ip to male mice resulted in significant decreases in liver glutathione and P450 enzymes activities, and increased in sorbitol dehydrogenase and alanine aminotransferase levels in serum suggestive of liver damage. However, pretreatment with 20 mg 3-MC/kg/d ip, an inducer of P450 enzymes, for 3 d mitigated the sanguinarine toxic effects suggesting 3-MC induced cytochrome P450 enzymes that promote detoxification of sanguinarine.     

Cocaethylene metabolism and interaction with cocaine and ethanol: role of carboxylesterases.

Carboxylesterases are important in the metabolism of cocaine, catalyzing the hydrolysis of cocaine to its two major metabolites, benzoylecgonine and ecgonine methyl ester. In the presence of ethanol, some cocaine undergoes transesterification with ethanol instead of hydrolysis with water producing the active metabolite, cocaethylene. The metabolic fate of cocaethylene is unknown, but given its structural similarity to cocaine, it was hypothesized that cocaethylene would also be metabolized by carboxylesterases and its elimination decreased in the presence of ethanol, as is cocaine's. Dogs were given cocaine alone, cocaethylene alone, cocaine and ethanol, cocaethylene and ethanol, and cocaine and cocaethylene on separate study days and sequential blood samples drawn. Plasma concentrations of cocaine, benzoylecgonine, and cocaethylene were determined by high-performance liquid chromatography. The pharmacokinetic dispositions of cocaine and cocaethylene were similar with clearance values of 0.91 +/- 0.22 and 0.79 +/- 0.16 l/min, and volumes of distribution of 2.6 +/- 0.82 and 2.7 +/- 0.47 l/kg, respectively. Both cocaine and cocaethylene clearances were decreased about 20% when given with ethanol. Following administration of cocaethylene alone, benzoylecgonine achieved similar plasma concentrations as those attained following cocaine alone, which indicates that benzoylecgonine is a major metabolite of cocaethylene. Carboxylesterases play an important role in the elimination of both cocaine and cocaethylene.     

Ginkgolide A contributes to the potentiation of acetaminophen toxicity by Ginkgo biloba extract in primary cultures of rat hepatocytes

The present cell culture study investigated the effect of Ginkgo biloba extract pretreatment on acetaminophen toxicity and assessed the role of ginkgolide A and cytochrome P450 3A (CYP3A) in hepatocytes isolated from adult male Long-Evans rats provided ad libitum with a standard diet. Acetaminophen (7.5-25 mM for 24 h) conferred hepatocyte toxicity, as determined by the lactate dehydrogenase (LDH) assay. G. biloba extract alone increased LDH leakage in hepatocytes at concentrations > or =75 mug/ml and > or =750 mug/ml after a 72 h and 24 h treatment period, respectively. G. biloba extract (25 or 50 mug/ml once every 24 h for 72 h) potentiated LDH leakage by acetaminophen (10 mM for 24 h; added at 48 h after initiation of extract pretreatment). The effect was confirmed by a decrease in [(14)C]-leucine incorporation. At the level present in a modulating concentration (50 mug/ml) of the extract, ginkgolide A (0.55 mug/ml), which increased CYP3A23 mRNA levels and CYP3A-mediated enzyme activity, accounted for part but not all of the potentiating effect of the extract on acetaminophen toxicity. This occurred as a result of CYP3A induction by ginkgolide A because triacetyloleandomycin (TAO), a specific inhibitor of CYP3A catalytic activity, completely blocked the effect of ginkgolide A. Ginkgolide B, ginkgolide C, ginkgolide J, quercetin, kaempferol, isorhamnetin, and isorhamnetin-3-O-rutinoside did not alter the extent of LDH leakage by acetaminophen. In summary, G. biloba pretreatment potentiated acetaminophen toxicity in cultured rat hepatocytes and ginkgolide A contributed to this novel effect of the extract by inducing CYP3A.