Age- and sex-related differences in acetaminophen metabolism in the rat

The effect of age and gender on glucuronidation and sulphation in rat liver: a study using paracetamol as a model substrate

The effect of age and sex on glucuronidation and sulphation was studied in vitro in rat liver fractions, using paracetamol (acetaminophen) as substrate. Glucuronidation was more rapid in females; sulphation in males. Age was not a determinant of either metabolic pathway in either sex.

Carcinogen activation by sulfate conjugate formation

The foregoing pages presented a substantial body of data that established that sulfotransferase conjugation can transform many xenobiotics into agents that can modify cellular macromolecules. However, activation by sulfation is rarely the only metabolic pathway that is open to these compounds; other pathways can become more important in response to a variety of factors. This metabolic switching can be produced by substrate concentration, cofactor availability, kinetic factors that dictate the velocity of the various possible conjugation reactions, and, in some cases, competition between Phase-I and Phase-II metabolism. Also, it is important to realize that demonstration of activation by sulfate ester formation in vitro does not necessarily mean that a similar activation process will occur in vivo. Experience also teaches that argument by analogy can be very misleading in the case of sulfate activation. Small structural differences can upset the delicate balance between sulfate activation and the various other competing pathways. Nevertheless, sulfation is an important mechanism by which a number of chemicals are transformed to their activated forms.

Sulfation and glucuronidation of acetaminophen by cultured hepatocytes reproducing in vivo sex-differences in conjugation on Matrigel and type 1 collagen

The sulfate and glucuronide conjugation of acetaminophen (APAP) by hepatocytes cultured on Matrigel or type 1 collagen was compared to APAP metabolism in vivo. The metabolic fate of low (15 mg/kg), medium (125 mg/kg), and high (300 mg/kg) doses of APAP injected intraperitoneally were determined in male and female rats. Males excreted more APAP as the sulfate conjugate than females, which correlated with the twofold greater APAP sulfotransferase activity in the male vs. females (301 +/- 24 vs. 156 +/- 18 pmol.mg-1 protein.min-1). Also, as sulfate conjugation became saturated, there was a dose-related shift in APAP metabolism from sulfate to glucuronide conjugation in both sexes. After death, the livers of the same animals were perfused with collagenase and the hepatocytes cultured in modified Waymouth's medium on either Matrigel or rat-tail collagen, with various doses of APAP (0, 0.125, 0.25, 0.5, and 1.0 mM). Sex differences in APAP sulfation and glucuronidation persisted in culture for up to 4 days, with sulfation predominating in the male similar to in vivo. With increasing APAP concentration (dose), there was a saturation of sulfate conjugation and a shift to glucuronidation as observed in vivo. Sex differences in APAP sulfation and glucuronidation were no longer significant by Day 4 in culture. Sulfation, and to a lesser extent, glucuronidation, were more stable on Matrigel than collagen. We concluded that APAP metabolism of freshly isolated hepatocytes could replicate in vivo sex differences in conjugation, and that Matrigel was superior to collagen as substrate.

Zonal distribution of paracetamol glucuronidation in the isolated perfused rat liver

1. The lobular distribution of paracetamol conjugation was studied using antegrade and retrograde perfusion of isolated rat liver. After addition of 25 mg of paracetamol, recovery of sulphate from antegrade perfusions was greater than from retrograde perfusions indicating periportal predominance of this metabolic pathway. 2. Recovery of sulphate was greater than recovery of glucuronide after addition of 25 mg of paracetamol during both antegrade and retrograde perfusions. Recovery of sulphate was similar after addition of 25 or 100 mg of paracetamol, whereas recovery of the glucuronide was increased at the higher dose, indicating a higher capacity for glucuronidation than for sulphation, but a higher affinity for sulphation. 3. Recovery of glucuronide from antegrade and retrograde perfusions was similar after addition of 25 mg of paracetamol, but was greater from antegrade perfusions after addition of 100 mg. This indicates that drug concentration may be an important determinant of the zonal distribution of drug metabolism. Factors other than localization of drug-metabolizing enzymes, such as competition with other metabolic pathways or availability of cofactors, may also influence the apparent zonal distribution of drug metabolism.

Impaired mitochondrial oxidative energy metabolism following paracetamol-induced hepatotoxicity in the rat

1. Effects of paracetamol treatment in vivo at subtoxic (375 mg kg-1 body weight) and toxic (750 mg kg-1 body weight) doses on energy metabolism in rat liver mitochondria were examined. 2. Paracetamol treatment resulted in a significant loss in body weights without affecting the liver protein contents. Toxic doses, however, resulted in 21% decrease in the yield of mitochondrial proteins. 3. Subtoxic doses of paracetamol did not, in general, affect the respiratory parameters in the liver mitochondria except in the case of succinate where both the state 3 respiration and the ADP-phosphorylation rates increased by 28%. 4. Toxic doses of paracetamol caused 25 to 47% decrease in the state 3 respiration rates depending on the substrate used. ADP/O ratios also decreased significantly with pyruvate + malate and succinate as the substrates. Consequently, ADP-phosphorylation was impaired significantly from 20 to 63%. 5. Subtoxic doses of paracetamol resulted in increased contents of cytochrome c + c1 while the toxic doses caused lowering of the cytochromes aa3 and b contents. 6. Glutamate and succinate dehydrogenase activities decreased in both the experimental groups while Mg2+-ATPase activity was impaired only after toxic dose-treatment. 7. The results show that toxic doses of paracetamol result in impaired energy coupling in the liver mitochondria. Effects of subtoxic doses were also demonstrable in terms of impaired dehydrogenases activities.

Age-related susceptibility to the insulin-depleting action of 4-diphenylmethylpiperidine in young rats

Studies were undertaken to investigate age-related changes in the ability of 4-diphenylmethylpiperidine (4-DPMP) to reduce levels of pancreatic insulin in young rats. Oral doses of 4-DPMP (5.0 or 10.0 mg/kg) were given once daily for two days to 9-, 15- and 21-day-old rats. Twenty-four hours after the last dose, pancreatic insulin content, non-fasting serum glucose, and the amount of unchanged 4-DPMP present in the whole body were estimated. 4-DPMP treatment produced a decline in pancreatic insulin and the extent of this action was greater in younger animals. The observed changes in pancreatic insulin were not reflected in altered serum glucose levels, showing this parameter is a relatively poor indicator of pancreatic insulin loss. Younger animals had a larger fraction of the total dose of 4-DPMP in the body at the end of the experimental period when compared to the fraction retained by older rats. The age-related susceptibility of young rats to the diabetogenic action of 4-DPMP may be related to the differences in the rate of elimination of the chemical at different ages.

Hepatotoxicity of acetaminophen in neonatal and young rats. I. Age-related changes in susceptibility

The susceptibility of neonatal (11 days) and young rats (19 and 33 days) to acetaminophen-induced hepatic necrosis was examined. Acetaminophen-induced lethality (LD50) was slightly lower in 19-day-old animals (840 mg/kg) compared to 11- and 33-day-old animals (1220 and 1580 mg/kg, respectively). A toxic dose of the drug ( LD20 ) produced elevated serum glutamate-pyruvate transaminase and lactate dehydrogenase activities 20-24 hr after drug administration only in 19- and 33-day-old animals. Serum enzyme elevation was not observed after a toxic dose of acetaminophen ( LD20 or LD50) in 11-day-old rats. Histological evaluation showed that both 19- and 33-day-old rats developed extensive hepatic centrilobular damage, whereas morphological parameters in 11-day-old animals given acetaminophen were not different from controls. It appears that high doses of acetaminophen are lethal to young rats, but that 11-day-old animals are different from 19-day-old and older rats in that the neonatal animals lack susceptibility to the hepatotoxic effects of the drug. Lower susceptibility of the neonatal rat liver to the hepatic effects of two other hepatotoxicants (bromobenzene and tannic acid) was also observed.

Hepatotoxicity of acetaminophen in neonatal and young rats. II. Metabolic aspects

The effects of a toxic dose of acetaminophen on levels of hepatic glutathione and [14C]acetaminophen-derived metabolites covalently bound to tissue macromolecules were investigated in 11- (1000 mg/kg), 19- (750 mg/kg), and 33-day-old (1250 mg/kg) rats. Although hepatic glutathione levels are lower in untreated 11- and 19-day-old rats compared to 33-day-old animals, a toxic dose of acetaminophen produced similar maximal depletion in all ages (to 0.5 mg/g). Covalent binding of [14C]acetaminophen to hepatic and renal macromolecules was higher in the younger animals. Glutathione-derived conjugates of APAP were present in the liver and kidney of neonatal and young rats given a toxic dose of the drug. In addition, age-related differences in the levels of the sulfate and glucuronide conjugates of the drug in these tissues were observed. The data presented suggest that neonatal rats, while less susceptible than older rats to the hepatic necrosis produced by the drug, are more able to metabolically activate a toxic dose of acetaminophen.

Sex related differences in acetaminophen toxicity in the mouse

Acetaminophen LD50 was two fold lower in female than male mice, the greater sensitivity of female mice for acetaminophen was also reflected in the serum enzyme levels of glutamic oxaloacetic and pyruvic transaminases (SGOT/SGPT), where the enhancement of both enzymes was higher in female than male mice. We could also observe the L-cysteine protection against the toxic effect of acetaminophen. Our results demonstrate that the lethal action of acetaminophen administrated either P.O. or S.C., is sex related.

Evidence for the active renal secretion of S-pentachlorophenyl-N-acetyl-L-cysteine by female rats

Male and female rats received 50 mumoles of pentachloronitrobenzene/kg by oral intubation daily for seven days. The final excreta were hydrolysed and analysed by electron capture GLC for the presence of pentachlorobenzenethiol and tetrachloro-1,4-benzenedithiol (derived from the equivalent N-acetylcysteine conjugates). No differences were found between the sexes for faeces and bile but the urinary excretion of both thiols by females was more than 10-fold greater than males. A similar result for urine was obtained following i.p. administration of a single 20 mumoles/kg dose of S-pentachlorophenyl-N-acetyl-L-cysteine (pentachlorophenyl mercapturate); in addition co-treatment with probenecid did not greatly change excretion by the males but considerably reduced excretion by females. The sex difference in the urinary levels of pentachlorophenyl N-acetylcysteine after 40 and 100 mumoles/kg doses of pentachloronitrobenzene was confirmed by h.p.l.c. of the mercapturate and again probenecid inhibited the excretion. Analysis of urine by TLC following a dose of [14C]hexachlorobenzene (8 muCi/kg; 0.67 mumoles/kg) showed that more radioactivity was associated with the mercapturate from female rats than males. The results suggest that S-pentachlorophenyl-N-acetyl-L-cysteine, a metabolite of hexachlorobenzene and pentachloronitrobenzene, may be excreted by an active renal secretion which is particularly developed in female F344 rats.

Hepatotoxicity and metabolism of acetaminophen in male and female rats

This present study was designed to assess the role of metabolic and pharmacokinetic factors in the lower susceptibility of female rats compared to male rats to xenobiotics metabolized by the cytochrome P-450-dependent mixed-function oxidase (MFO) system. Adult intact male and female Sprague-Dawley rats were administered labeled acetaminophen (1 g/kg body weight + 5 microCi [3H]acetaminophen) after an overnight fast. They were bled and killed at 0.5, 1, 2, 3, 6, 12, 24, and 36 h after drug administration. The percentage of [3H]acetaminophen radioactivity remaining in blood, liver, GI tract, and excreted in the urine was determined at all time intervals. Plasma prothrombin time and serum transaminases were determined as indices of hepatotoxicity. Hepatic GSH and glycogen were assayed. Total urinary acetaminophen and its metabolites and the molar percent of various metabolites excreted during the first 6 h were determined. Castrated male and ovariectomized female rats and their respective controls were also given acetaminophen (APAP) and were killed 24 h later to determine hepatotoxicity. The extent of hepatic damage in the intact male rats was greater and appeared sooner than in the female rats. Hepatic GSH and glycogen were depleted earlier in female rats. The percent of the administered dose excreted in the urine during the first 6 h was 17.5 for the male rat versus 24.5 for the female rat. While the APAP glucuronide conjugate concentration was significantly higher, the APAP sulfate conjugate concentration was lower in the female than it was in the male rat. Although peak radioactivity in serum was reached by 30 min in both male and female rats, suggesting quick intestinal absorption, it was significantly higher in female rats and was associated with decreased intestinal and hepatic levels and increased urinary excretion when compared to male rats. While castration of male rats decreased susceptibility to hepatotoxicity, ovariectomy of female rats tended to increase susceptibility to hepatotoxicity in comparison to their respective controls. Our data suggest that aside from the reported sex differences in the cytochrome P-450-dependent MFO enzymes, there are significant differences in GSH utilization. There are also significant changes in glucuronidation and sulfation pathways, as well as in the pharmacokinetics of acetaminophen, which tend to protect female rats against acetaminophen hepatotoxicity.

Sex differences in the metabolism of hexachlorobenzene by rats and the development of porphyria in females

Male and female F 344 rats were dosed every other day for 103 days with 50 mumole of hexachlorobenzene (HCB)/kg. Females developed a hepatic porphyria, the urine and liver levels of porphyrins being 40- and 310-fold higher respectively than those of males. Urine was periodically hydrolysed and analysed for the three metabolites pentachlorophenol, 2,3,5,6-tetrachlorobenzene-1,4-diol and pentachlorothiophenol (derived from the mercapturate). The combined urinary excretion of these was greater in females than males, especially during the first 10 weeks. Pentachlorothiophenol was particularly high in female urine. After 103 days this metabolite was slightly less in female faeces than in male's but free hepatic pentachlorothiophenol was 3.6-fold greater. Although total 24 hr excretions of metabolites were higher by females than males and after 7 daily doses of HCB, a difference in this respect was not conclusively proven. However, total pentachlorothiophenol excretion was always significantly greater by females. The male/female ratios for pentachlorophenol and pentachlorothiophenol in bile were identical to those for faeces. Excretion of metabolites by both adult males and females was stimulated by pretreatment with diethylstilboestrol (DES). No sex differences in metabolism were observed with immature rats.