Metabolism of toxic substances in isolated hepatocytes

Treating chronic arsenic toxicity with high selenium lentil diets

Arsenic (As) toxicity causes serious health problems in humans, especially in the Indo-Gangetic plains and mountainous areas of China. Selenium (Se), an essential micronutrient is a potential mitigator of As toxicity due to its antioxidant and antagonistic properties. Selenium is seriously deficient in soils world-wide but is present at high, yet non-toxic levels in the great plains of North America. We evaluate the potential of dietary Se in counteracting chronic As toxicity in rats through serum biochemistry, blood glutathione levels, immunotoxicity (antibody response), liver peroxidative stress, thyroid response and As levels in tissues and excreta. To achieve this, we compare diets based on high-Se Saskatchewan (SK) lentils versus low-Se lentils from United States. Rats drank control (0ppm As) or As (40ppm As) water while consuming SK lentils (0.3ppm Se) or northwestern USA lentils (<0.01ppm Se) diets for 14weeks. Rats on high Se diets had higher glutathione levels regardless of As exposure, recovered antibody responses in As-exposed group, higher fecal and urinary As excretion and lower renal As residues. Selenium deficiency caused greater hepatic peroxidative damage in the As exposed animals. Thyroid hormones, triiodothyronine (T3) and thyroxine (T4), were not different. After 14weeks of As exposure, health indicators in rats improved in response to the high Se lentil diets. Our results indicate that high Se lentils have a potential to mitigate As toxicity in laboratory mammals, which we hope will translate into benefits for As exposed humans.

Mechanisms of chloroform-induced hepatotoxicity: oxidative stress and mitochondrial permeability transition in freshly isolated mouse hepatocytes

The role of mitochondrial permeability transition (MPT) and oxidative stress in chloroform toxicity was determined in freshly isolated female B6C3F1 mouse hepatocytes. Incubation of chloroform (12 mM) with hepatocytes resulted in cell death (alanine aminotransferase release and propidium iodide fluorescence). Chloroform had volatilized from the incubation and glutathione was depleted by 1 h; however, toxicity was not significantly different between control and chloroform-incubated cells. Hepatocytes were washed and reincubated in fresh media at 1 h. Subsequent reincubation of chloroform-treated hepatocytes resulted in significant toxicity at 3-5 h. Inclusion of the MPT inhibitor cyclosporine A or the antioxidant N-acetylcysteine (NAC) in the reincubation media at 1 h prevented toxicity. Confocal microscopy studies with the dye calcein AM indicated MPT that was blocked by cyclosporine A or NAC. Fluorescence microscopy studies utilizing JC-1 indicated loss of mitochondrial membrane potential, which was also blocked by cyclosporine A or NAC. Dichlorofluorescein fluorescence increased during the reincubation phase, indicating increased oxidative stress, and the increase was blocked by cyclosporine A. Since oxidative stress may occur by peroxynitrite, its role in toxicity was examined. Either of the nitric oxide synthase inhibitors N(G)-methyl-L-arginine (L-NMMA) and 7-nitroindazole (7-NI) at 1 h blocked toxicity. Western blot analysis of hepatocytes for 3-nitrotyrosine in proteins, a biomarker of peroxynitrite, indicated one major nitrated protein at 81 kD. Nitration of this protein was inhibited by cyclosporine A, L-NMMA, 7-NI, or NAC. The data indicate that chloroform-induced cell death occurs in two phases: a metabolic phase characterized by glutathione depletion, and an oxidative phase characterized by MPT and protein nitration.

Acute toxicity and urinary excretion of diphenyldiselenide

The acute toxicity of diphenyldiselenide (DPDS) in the male Swiss mouse was found to be enhanced by pretreatment with phenobarbital of SKF-525A. DPDS decreased hepatic glutathione content by 50% at 1 h after administration. Following administration of 14C-DPDS, labelled metabolites were found in urine but not in bile or feces. Analysis of the urinary metabolites of 14C-DPDS showed that selenium-containing metabolites elute from a DEAE-Sephadex column in two fractions: the first has not been chemically characterized, while the second peak contained the glucuronide conjugates of C6H4(OH)SeH and C6H5SeH. Virtually all of the administered selenium is excreted within 5 days, while only about 36% of the 14C is excreted in the same time period. This discrepancy indicates metabolic scission of the carbon-selenium bond.

A rapid method of preparing hepatic parenchymal cells for studying drug metabolism

Parenchymal cells were prepared from the livers of male Sprague-Dawley rats by collagenase perfusion and purified by a self-generating Percoll gradient. The method consisted of mixing 31% Percoll and 5 x 10(6) cells/ml, followed by centrifugation at 10,000 x g for 10 min. A self-generated gradient provided a rapid and efficient recovery of highly viable parenchymal cells. The parenchymal cells were determined to be very stable during incubation at 37 degrees C for at least 2 h. Cell integrity was evaluated by trypan blue dye exclusion, lactate dehydrogenase leakage, and membrane peroxidation. In addition, drug metabolism and conjugation were evaluated as markers of intracellular integrity. With increasing p-nitroanisole (pNA) concentration, the formation of p-nitrophenol (pNP) increased. The rate of sulfation was maximal at a pNA concentration of 0.25 mM and decreased greatly above 1.0 mM. Glucuronidation increased from 0.25 mM to a maximum rate of 2.0 mM pNA. Above 1.0 mM pNA, nonconjugated pNP increased proportionately to the decrease in sulfation. These results indicate that the cell integrity was maintained, and that these cells can be used as a model for studying drug metabolism.

Isolation of hepatocytes from postnatal mice

A method for the isolation of hepatocytes from postnatal (1- to 3-week-old) mice has been developed. Cell isolation was carried out by retrograde perfusion of the liver with a collagenase-containing bicarbonate buffer. Viable cells were separated by selective adsorption onto collagen membranes. Cell viability was assessed by measuring ATP and glutathione content, lactate:pyruvate ratio, and the rate of protein synthesis. Comparisons of these parameters were made with those in cells isolated from adult mice and with values in whole liver. These hepatocytes were capable of metabolizing acetaminophen to its known hepatic conjugates and were susceptible to acetaminophen toxicity. This procedure for isolating mouse hepatocytes should be useful for the study of hepatic drug metabolism and its relationship to toxicity in the postnatal mouse.

Involvement of glutathione reductase in selenite metabolism and toxicity, studied in isolated rat hepatocytes

Cellular lysis in freshly isolated hepatocytes incubated with varying concentrations of selenite could be related to the reductive metabolism of selenite. A decrease in intracellular GSH levels was observed concomitant with an increased rate of accumulation of oxidized glutathione in the incubation medium. Pretreatment of hepatocytes with an inhibitor of GSSG-reductase (1,3-bis(2-chloroethyl)-1-nitrosourea), prior to the addition of 50 microM selenite, resulted in substantially lower GSH-levels. The rate of GSSG reductase-catalyzed metabolism of selenite (50 microM) could be estimated to approximately 7 nmoles reduced/h per 10(6) cells. The results indicate that this was the major metabolic pathway for toxic concentrations of selenite in isolated hepatocytes. Furthermore, selenite considerably decreased cellular NADPH levels. In hepatocytes isolated from starved rats, the presence of alanine and glucose in the incubation medium protected against selenite-mediated cellular lysis. These observations suggest that an insufficient NADPH generation could be critical for selenite reduction and toxicity in isolated hepatocytes.