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

HPLC quantification of 4-nitrophenol and its conjugated metabolites from bile

An isocratic ion pair RP-HPLC method with UV-Vis detection has been developed and validated for simultaneous analysis of 4-nitrophenol (PNP), 4-nitrophenyl β-glucuronide (PNP-G), and 4-nitrophenyl sulfate (PNP-S) in rat bile samples using 4-ethylphenol (ETP) as internal standard. Chromatographic separation was achieved on a C(18) column by isocratic elution with a mobile phase consisted of methanol-0.01 M citrate buffer pH 6.2 (47:53 v/v) containing 0.03 M TBAB. The flow rate was 1.0 ml min(-1), the detection was affected at 290 nm. Calibration plots were generated over the concentration range 1-100 μM PNP, PNP-G, PNP-S with a common lower limit of quantification of 2.5 μM. Intra- and inter-day precision and repeatability were determined at six different concentrations. Results obtained by application of the method for determination of PNP, PNP-G and PNP-S in bile fractions collected during intestinal perfusion of PNP in hyperglycemic rats are presented.

A simple and rapid ion-pair HPLC method for simultaneous quantitation of 4-nitrophenol and its glucuronide and sulfate conjugates

Because of its simple and well characterized metabolic profile, 4-nitrophenol is widely used as a model substrate to investigate the influence of drug therapy, disease, nutrient deficiencies and other physiologically altered conditions on conjugative drug metabolism in animal studies. For simultaneous determination of 4-nitrophenol (PNP), 4-nitrophenyl-beta-D-glucuronide (PNP-G) and 4-nitrophenyl-sulfate (PNP-S) in samples generated in rat small intestine luminal perfusion experiments, an ion-pair HPLC assay coupled with UV detection was set up. The RP-HPLC separation was achieved with a methanol-water mixture (50:50, v/v) containing 0.01 M tetrabutyl-ammonium-bromide with UV detection of the analytes at 290 nm. The isocratic system was operated at ambient temperature and required less than 7 min of chromatographic time. The method provided good enough within-day precision, between-day precision and linearity in the target concentration ranges of 6-1200 microM (PNP) and 2.5-100 microM (PNP-G and PNP-S). The instrumental limit of quantification for PNP-G and PNP-S was found to be 2.7 microM and 2.1 microM, respectively. The assay was applied for determination of PNP, PNP-G and PNP-S in rat small intestine perfusates.

A brief report on some physiological parameters of streptozocin-diabetic rat

Several biological changes occur when streptozocin is given to experimental animals. The rat streptozocin (STZ) model is extensively used in diabetic experiments. In this brief report, the main physiological characteristics of rats injected with streptozocin are presented. These characteristics are manifested by weight loss, organ weight reduction, serum glucose elevation, decrease in serum insulin level, and other enzyme and hormonal changes. A collection of these parameters may be helpful in establishing a database to describe this model.

Development of a small-scale bioreactor for drug metabolism studies maintaining hepatospecific functions

1. The aim of the study was the development of a small-scale liver cell bioreactor maintaining tissue monoxygenase activity and hepatospecific activities over at least 2 weeks. 2. For characterization the antihypertensive drug urapidil was used as a model compound to study maintenance of metabolic activity. Tissue-specific parameters assessed included urea and albumin secretion as well as cellular integrity. The problem of the use of serum in bioreactor cultures is addressed. 3. Bioreactor runs could be performed in serum- and lactate-free cultures with a joint recovery of oxidative biotransformation capacity for urapidil as well as tissue-specific markers. LDH release was reduced with older cultures. Fibronectin was shown as a contributing factor for cell attachment. 4. In the present study the design and function of a modular, small-scale-type bioartificial liver cell culture model is thus described lending itself for drug metabolism studies but maintaining also typical hepatospecific properties.

High-performance liquid chromatographic analysis of p-nitrophenol and its conjugates in biological samples

p-Nitrophenol (pNP) and its conjugated metabolites, generated in a perfused rat liver preparation, are readily separated and quantitated in serum perfusate and bile samples using a reverse-phase high-performance liquid chromatographic method. Serum perfusate samples can be analyzed following protein precipitation with acetonitrile: following protein precipitation with 1.5 M perchloric acid (1 part to 2 parts serum) there was degradation of pNP sulfate to pNP when samples were stored at room temperature. pNP can also be analyzed in blood perfusate samples following extraction with a number of organic solvents including ethyl acetate or isobutanol-methylene chloride (4:1, v/v). Rat liver perfusions at a constant input concentration of 40 microM demonstrated a high hepatic extraction ratio of pNP (mean of 0.90) due to the formation of the sulfate and glucuronide conjugates; no pNP glucoside was detected in perfusate or bile samples.

Impaired hepatic elimination of paranitrophenol and its metabolites in the rat following chronic ethanol pretreatment

Chronic ethanol feeding has been shown to enhance hepatic microsomal drug oxidation in humans and in laboratory animals. However, the effects of chronic ethanol administration on drug conjugation are less conclusive. We have studied the effects of chronic ethanol feeding on (a) the conjugation and elimination of p-nitrophenol (PNP) by the isolated perfused rat liver, (b) the formation of PNP glucuronide by hepatic microsomal PNP-glucuronyltransferase in vitro and (c) the hepatic content of UDP-glucuronic acid (UDPGA). PNP elimination from the isolated perfused rat liver was best described as a combination of parallel saturable and first-order processes. Ethanol pretreatment did not influence the former but resulted in a 48% reduction in the rate of elimination by the latter. This was associated with a significant reduction in recovery of PNP-glucuronide from bile, but no change in concentrations of PNP glucuronide or sulfate in perfusate. Michaelis constants and Vmax for PNP-glucuronyltransferase in native and solubilized microsomes and UDPGA concentrations in liver were not influenced by ethanol pretreatment. These results suggest that chronic ethanol treatment reduces PNP elimination in the intact liver primarily via a reduction in the biliary excretion of PNP glucuronide without altering glucuronidation per se.

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.