Inhibition of paranitroanisole and antipyrine monooxygenation in isolated rat hepatocytes by compounds interacting with mitochondrially related carbohydrate metabolism

Isolation of intact mitochondria and hepatocytes using vibration

A method of isolation of rat liver cells and mitochondria suggesting tissue disaggregation by vibration has been proposed. Mitochondria obtained using vibration have better parameters of oxidative phosphorylation as compared to homogenization. Hepatocytes isolated by vibration had viability about 90%, as determined by trypan blue exclusion, and rates of respiration and xenobiotic metabolism comparable to that observed by the enzymatic method.

Cytotoxic effects of N-acetyl-p-benzoquinone imine, a common arylating intermediate of paracetamol and N-hydroxyparacetamol

The cytotoxic effects of N-acetyl-p-benzoquinone imine (NAPQI), a postulated ultimate reactive metabolite of paracetamol (pHAA), was studied in suspensions of isolated rat hepatocytes. Incubation of cells for 10-300 min with 0.1-0.5 mM NAPQI led to concentration dependent cell damage, as determined by increased trypan blue exclusion, lactate dehydrogenase release and glutathione (GSH) depletion. NAPQI and N-hydroxyparacetamol (N-OH-pHAA), a postulated proximate metabolite of pHAA, caused cytotoxic effects in the same concentration range. In contrast, no toxic effects of pHAA (less than or equal to 20 mM) could be demonstrated. With the short half-life of NAPQI, less than 0.5% of the NAPQI added is expected to be left in the incubation medium after a 2 min incubated period. Nevertheless, 10-120 min (depending on the concentration of NAPQI) elapsed before the cells responded with increased membrane permeability. Clearly, the initial damage caused by NAPQI must be followed by subsequent cellular steps before toxicity becomes apparent. The addition of N-acetylcysteine, GSH or ascorbate during the NAPQI exposure period fully protected the hepatocytes from NAPQI damage. Lesser effects were demonstrated when these agents were added after the 5 min NAPQI exposure period. The results presented in this study further support the hypothesis that NAPQI is the ultimate reactive formed from pHAA.

Effects of xylene exposure on the metabolism of antipyrine in vitro and in vivo in the rat

Exposure of male rats to different concentrations of xylene for 3 days induced, in a dose-dependent way, the in vitro liver microsomal metabolism of antipyrine. The degree of induction was statistically significant at an exposure level of 250 ppm and maximal (2.5-fold increase) at 2000 ppm. This increase was of the same magnitude as after phenobarbital treatment. Female rats had a lower basal antipyrine metabolism than males, but exhibited a greater relative increase in antipyrine metabolism following xylene exposure. Cytochrome P-450 isozymes, purified from xylene- and phenobarbital-treated animals, were efficient catalysts of antipyrine metabolism, with turnover numbers of 33.3 and 21.1, respectively. A reduction in the half-life of antipyrine to 39% of preexposure values occurred after exposure of male rats to 1000 ppm of xylene for 3 days. Exposure to lower xylene levels did not produce significant alterations in antipyrine elimination half-life. In vitro, xylene was shown to be a non-competitive metabolic inhibitor of antipyrine. Experiments in vivo indicated that inhibition is not important at relatively low xylene exposure levels. It is concluded that induction of hepatic monooxygenases by xylene can be demonstrated, with antipyrine as a test drug, both in vitro and in vivo.

Drug metabolism activities of isolated rat hepatocytes in monolayer culture

The levels of cytochrome P-450 in hepatocytes cultured as monolayers for 22 hrs in Dulbecco's modified Eagle medium supplemented with serum and insulin was reduced to approximately 40% of initial values of freshly isolated hepatocytes. In correspondence with this the activities of the cytochrome P-450 monooxygenases aryl hydrocarbon (benzo(a)pyrene) hydroxylase (AHH) and ethylmorphine (EM) N-demethylase were reduced to 40 and 22% of their initial activities, respectively. Modifying the culture medium through omission of cysteine and cystine, and adding dexamethazone and delta-amino levulinic acid, increased the content of cytochrome P-450 to 59% and EM N-demethylase to 46% of initial values, but was without effect on AHH activity. However, further modifications by adding high concentrations of asparagine and leucine increased AHH activity to 62% of initial values, but did not further enhance the total content of cytochrome P-450 or the EM N-demethylase activity. The activities of cytochrome P-450 reductase, flavin containing monooxygenase, epoxide hydrolase and glutathione S-transferase decreased less (to about 70-80% of initial values) than cytochrome P-450 associated monooxygenase activities, whereas UDP-glucuronyl transferase decreased to about 50% of initial values. In contrast to what was observed regarding cytochrome P-450 and associated monooxygenase activities, modification of the incubation conditions did not affect the non-cytochrome P-450 enzymatic activities.