Effects of glucose and fructose on conjugation of p-nitrophenol in hepatocytes of normal and streptozotocin-diabetic rats

Effect of experimental diabetes and insulin replacement on intestinal metabolism and excretion of 4-nitrophenol in rats

Luminal appearance of 4-nitrophenol (PNP) metabolites (4-nitrophenol-β-glucuronide (PNP-G) and 4-nitrophenol-sulfate (PNP-S)) and activity of the related metabolic enzymes have been investigated in control and experimental diabetic rats. Experimental diabetes was induced by administration of streptozotocin (65 mg/kg i.v.). PNP (500 μmol/L) was luminally perfused in the small intestine and the metabolites were determined in the perfusion solution. Effect of insulin replacement was also investigated in the diabetic rats. It was found that experimental diabetes increased the luminal appearance of PNP-G, which could be completely compensated by rapid-acting insulin administration (1 U/kg i.v.). Activities of the enzymes involved in PNP-G production (UDP-glucuronyltransferase and β-glucuronidase) were also elevated; however, these changes were only partially compensated by insulin. Luminal appearance of PNP-S was not significantly changed by administration of streptozotocin and insulin. Activities of the enzymes of PNP-S production (sulfotransferases and arylsulfatases) did not change in the diabetic rats. The results indicate that experimental diabetes can provoke changes in intestinal drug metabolism. It increased intestinal glucuronidation of PNP but did not influence sulfate conjugation. No direct correlation was found between the changes of metabolic enzyme activities and the luminal appearance of the metabolites.

Measurement of glucuronidation by isolated rat liver cells using [14C]fructose

We have developed a simple and sensitive method for the study of the relative rates of glucuronidation of compounds, in isolated liver cells, based on the incorporation of 14C from fructose into glucuronide conjugates. Liver cells from fasted rats are used to minimize any reduction of the specific activity by glycogenolysis. Although rates of glucuronidation are lower in isolated liver cells from fasted rats than in those from fed rats, because of a reduction in the concentration of UDP-glucuronic acid, it is possible to compare the rates of glucuronidation of different compounds. Radiolabelled glucuronides are separated from [14C]fructose and [14C]glucose, produced by the liver cells, by normal-phase HPLC on a polar amino-cyano column. The specific activity of the glucuronide was found to be approximately 50% of that of the [14C]fructose. Absolute amounts of glucuronide can be determined by measuring the specific activity of the [14C]glucose, also produced by liver cells from fructose, which reflects that of the glucose-6-phosphate and hence the UDP-glucuronic acid used for glucuronidation, although for the measurement of relative rates this would not be necessary. We have used this method to examine the kinetics of the glucuronidation of N-acetyl-p-aminophenol (acetaminophen), 4-nitrophenol and 1-naphthol in isolated rat liver cells. The method should be applicable to the study of the rates of glucuronidation of a range of aglycones and, unlike other methods, does not require glucuronide standards or radiolabelled aglycone.

Effect of glucose and gluconeogenic substrates on fasting-induced suppression of acetaminophen glucuronidation in the rat

Previous studies in rats have shown that an acute fast decreases the apparent rate constant for glucuronidation of hepatotoxic doses of acetaminophen which results in a prolongation of the mean residence time of the drug in the animals and, hence, increased acetaminophen reactive metabolite formation and liver injury. Since acetaminophen glucuronidation under these conditions is limited by UDPGA formation, we have attempted to reverse the potentiating effects of fasting by administering glucose or gluconeogenic substrates. Histological and pharmacokinetic studies revealed that glucose (2 g/kg, i.p.) given 0.25 and 1.5 hr after acetaminophen (700 mg/kg, i.p.) did not protect the rats from liver injury or enhance acetaminophen glucuronidation. The administered glucose did not increase hepatic levels of UDP-glucose or UDPGA either basally or following administration of a hepatotoxic dose of acetaminophen. Administration of the gluconeogenic substrates, lactate, alanine, fructose and galactose, raised blood glucose levels, but did not protect the rats from liver injury or enhance glucuronidation, suggesting that the glucose-6-phosphate formed from these compounds was not available for UDPGA production for acetaminophen glucuronidation. Collectively, these studies indicate that administration of glucose and these gluconeogenic substrates does not reverse the fasting-induced potentiation of acetaminophen hepatotoxicity, and that the rate-determining step for UDPGA synthesis for glucuronidation of hepatotoxic doses of acetaminophen is prior to UDP-glucose formation.

Mechanism of decreased acetaminophen glucuronidation in the fasted rat

The mechanism by which an acute fast decreases the glucuronidation of hepatotoxic doses of acetaminophen in the rat was examined. Fasting did not depress the level of the enzyme, glucuronyl transferase, or the basal level of the co-substrate, UDP-glucuronic acid (UDPGA). Administration of a hepatotoxic dose of acetaminophen rapidly depleted UDPGA levels in both fed and fasted rats to the same nadir. Fed and fasted rats differed in that the rate of repletion of UDPGA levels was markedly slower in fasted rats. The total hepatic levels of UDP-glucose dehydrogenase and its cofactor, NAD+, were not decreased by fasting. In fasted rats, hepatic levels of the UDPGA precursor, UDP-glucose, were approximately 60% those of fed rats both before and after a hepatotoxic dose of acetaminophen. In fed rats, acetaminophen induced a marked depletion of hepatic glycogen levels and a dramatic increase in blood glucose levels. Acetaminophen induced a similar marked increase in blood glucose levels in fasted rats in spite of the fact that they lacked hepatic glycogen. It is concluded that the fasting-induced decrease in the glucuronidation of hepatotoxic doses of acetaminophen results from decreased production of UDPGA. The decreased synthetic capacity for UDPGA does not appear to be due to the inability of the liver to produce glucose units per se, but rather to the fasting-induced altered activities of the enzymes of carbohydrate metabolism which, in turn, alter the fate of glucose-6-phosphate derived from gluconeogenesis.

Conjugation reactions in hepatocytes isolated from streptozotocin-induced diabetic rats

The activities of three drug conjugation reactions, glutathione, glucuronic acid and sulphate conjugation and the synthesis of glutathione, have been measured in hepatocytes isolated from streptozotocin-induced male diabetic rats. The intracellular content of reduced glutathione (GSH) was decreased in diabetic rat hepatocytes compared with controls. Following depletion of the intracellular GSH stores with diethylmaleate, the resynthesis of GSH in the presence of 0.5 mM L-methionine, occurred faster in diabetic rat hepatocytes than in those from control rats indicating that the cystathione pathway may be more efficient in the diabetic animals. In contrast, there was no significant difference in the resynthesis of GSH between control and diabetic rat hepatocytes in the presence of L-cysteine. The GSH conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) and 3,4-dichloronitrobenzene (DCNB) was deficient in diabetic rat hepatocytes, although only the effect on the former reaction was statistically significant (P less than 0.05). The Vmax for CDNB conjugation was significantly lower (P less than 0.05) in cytosolic fractions prepared from diabetic rat liver than in control rat liver fractions. This was accompanied by an increase in the affinity of the enzyme for CDNB. In contrast, the Vmax and Km for the conjugation of DCNB in cytosolic fractions were unaffected by the induced-diabetes. Glucuronic acid conjugation of both 1-naphthol and phenolphthalein was markedly deficient in diabetic rat hepatocytes. The intracellular concentrations of the cofactor for glucuronidation, UDP-glucuronic acid, were decreased in diabetic rat liver and this was thought to contribute to the defect in glucuronidation. The sulphation of 1-naphthol was not significantly altered by the induced diabetes. Deficiencies in glutathione and glucuronic acid conjugation in streptozotocin-induced diabetic rats may result in an increased susceptibility to xenobiotic induced cytotoxicity.

The kinetics of 4-nitrophenol conjugation by perfused livers and hepatic microsomes from streptozocin-induced diabetic rats

The formation of both glucuronide and sulphate conjugates of 4-nitrophenol is deficient in perfused livers from male diabetic rats. Experiments with 'native' hepatic microsomes demonstrated that the defect in glucuronidation is due to a decrease in the maximal velocity of the reaction. There is no alteration in the affinity of the glucuronyltransferase for 4-nitrophenol. Non-linear regression analysis of the 4-nitrophenol liver perfusate concentrations showed that the elimination follows saturable Michaelis-Menten kinetics. Clearance values in 'native' microsomal preparations and in perfused livers were calculated and found to be similar in both systems. This provides evidence that glucuronyltransferase is 'native' in the intact liver.

The metabolism and disposition of [2-14C]diazepam in the streptozotocin-diabetic rat

The oral administration of [2-14 C]diazepam to streptozotocin (STZ)-diabetic rats resulted in decreased faecal and biliary levels of metabolites with a concomitant rise in urinary radioactivity when compared to control values. This situation was reversible upon insulin treatment. The increased urinary metabolite excretion could not be ascribed to the diuresis observed in STZ-diabetic rats. No alteration in the phase I or II routes of [14C]diazepam metabolism in diabetic animals was observed either in vivo or in vitro. Following i.v. administration of [2-14C]diazepam, blood 14C levels in diabetic rats were elevated above those observed in normal animals.

Glucuronidation in rat intestinal epithelial cells. Dependence of glucose supply and resistance to inhibition by ethanol and fasting

Glucuronidation of 1-naphthol was studied in mucosal cells isolated from the rat intestine. Glucuronidation was directly dependent on the supply of extracellular carbohydrates. Basal glucuronidation (ca. 0.3 nmoles/min X mg cell protein) was increased 2- to 3-fold by adding glucose or fructose to the incubation medium. Saturation of glucuronidation was achieved by adding 0.3 mM glucose, while saturation by fructose was not reached at concentrations below 2 mM. No carbohydrate reserves able to support glucuronidation appear to be present in intestinal cells, since no difference in glucuronidation was observed between cells prepared from fasted (18 or 42 h) and control rats. Glucuronidation was decreased by adding D-galactosamine to the incubation medium, but only when extracellular glucose was present. Various chemicals which are known to inhibit glucuronidation in hepatocytes (ethanol, diethyl ether, sorbitol) did not influence the glucuronidation of 1-naphthol in isolated intestinal cells. Only when ethanol was added to mucosal cells in the absence of extracellular glucose was a small decrease in glucuronidation observed.