Glucuronic acid conjugation by hepatic microsomal fractions isolated from streptozotocin-induced diabetic rats

Vitamin E (α-Tocopherol) Does Not Ameliorate the Toxic Effect of Bisphenol S on the Metabolic Analytes and Pancreas Histoarchitecture of Diabetic Rats

This study investigated whether the coadministration of vitamin E (VitE) diminishes the harmful effects provoked by plasticizer bisphenol S (BPS) in the serum metabolites related to hepatic and renal metabolism, as well as the endocrine pancreatic function in diabetic male Wistar rats. Rats were divided into five groups (n = 5-6); the first group was healthy rats (Ctrl group). The other four groups were diabetic rats induced with 45 mg/kg bw of streptozotocin: Ctrl-D (diabetic control); VitE-D (100 mg/kg bw/d of VitE); BPS-D (100 mg/kg bw/d of BPS); The animals from the VitE + BPS-D group were administered 100 mg/kg bw/d of VitE + 100 mg/kg bw/d of BPS. All compounds were administered orally for 30 days. Body weight, biochemical assays, urinalysis, glucose tolerance test, pancreas histopathology, proximate chemical analysis in feces, and the activity of antioxidants in rat serum were assessed. The coadministration of VitE + BPS produced weight losses, increases in 14 serum analytes, and degeneration in the pancreas. Therefore, the VitE + BPS coadministration did not have a protective effect versus the harmful impact of BPS or the diabetic metabolic state; on the contrary, it partially aggravated the damage produced by the BPS. VitE is likely to have an additive effect on the toxicity of BPS.

Influence of gestational diabetes mellitus on the stereoselective kinetic disposition and metabolism of labetalol in hypertensive patients

PURPOSE

This study investigated the influence of gestational diabetes mellitus on the kinetic disposition and stereoselective metabolism of labetalol administered intravenously or orally.

METHODS

Thirty hypertensive women during the last trimester of pregnancy were divided into four groups: non-diabetic and diabetic women treated with intravenous or oral labetalol.

RESULTS

The pharmacokinetics of labetalol was not stereoselective in diabetic or non-diabetic pregnant women receiving the drug intravenously. However, oral administration of labetalol resulted in lower values of the area under the plasma concentration versus time curve (AUC) for the β-blocker (RR) than for the other enantiomers in both diabetic and non-diabetic women. Gestational diabetes mellitus caused changes in the kinetic disposition of the labetalol stereoisomers when administered orally. The AUC values for the less potent adrenoceptor antagonist (SS) and for the α-blocking (SR) isomers were higher in diabetic than in non-diabetic pregnant women.

CONCLUSIONS

The approximately 100% higher AUC values obtained for the (SR) isomer in diabetic pregnant women treated with oral labetalol may be of clinical relevance in terms of the α-blocking activity of this isomer.

The role of intracellular signaling in insulin-mediated regulation of drug metabolizing enzyme gene and protein expression

Endogenous factors, including hormones, growth factors and cytokines, play an important role in the regulation of hepatic drug metabolizing enzyme expression in both physiological and pathophysiological conditions. Diabetes, fasting, obesity, protein-calorie malnutrition and long-term alcohol consumption produce changes in hepatic drug metabolizing enzyme gene and protein expression. This difference in expression alters the metabolism of xenobiotics, including procarcinogens, carcinogens, toxicants and therapeutic agents, potentially impacting the efficacy and safety of therapeutic agents, and/or resulting in drug-drug interactions. Although the mechanisms by which xenobiotics regulate drug metabolizing enzymes have been studied intensively, less is known regarding the cellular signaling pathways and components which regulate drug metabolizing enzyme gene and protein expression in response to hormones and cytokines. Recent findings, however, have revealed that several cellular signaling pathways are involved in hormone- and growth factor-mediated regulation of drug metabolizing enzymes. Our laboratory has reported that insulin and growth factors regulate drug metabolizing enzyme gene and protein expression, including cytochromes P450 (CYP), glutathione S-transferases (GST) and microsomal epoxide hydrolase (mEH), through receptors which are members of the large receptor tyrosine kinase (RTK) family, and by downstream effectors such as phosphatidylinositol 3-kinase, mitogen activated protein kinase (MAPK), Akt/protein kinase B (PKB), mammalian target of rapamycin (mTOR), and the p70 ribosomal protein S6 kinase (p70S6 kinase). Here, we review current knowledge of the signaling pathways implicated in regulation of drug metabolizing enzyme gene and protein expression in response to insulin and growth factors, with the goal of increasing our understanding of how disease affects these signaling pathways, components, and ultimately gene expression and translational control.

Molecular basis of bilirubin UDP-glucuronosyltransferase induction in spontaneously diabetic rats, acetone-treated rats and starved rats

The co-ordinated induction of several hepatic drug-metabolizing enzymes is a common feature in the regulation of drug biotransformation under normal and pathological conditions. In the present study the activity and expression of bilirubin UDP-glucuronosyltransferase (UGT1A1) were investigated in livers of BioBreeding/Worcester diabetic, fasted and acetone-treated rats. Bilirubin glucuronidation was stimulated by all three treatments; this was correlated with an increase in the UGT1A1 protein concentration in hepatic microsomes. Transcriptional induction of UGT1A1 was also observed in diabetes and starvation but not with acetone treatment, which apparently caused translational stabilization of the enzyme protein. The hormonal/metabolic alterations in diabetes and starvation might be a model for postnatal development. The sudden interruption of maternal glucose supply signals the enhanced expression of UGT1A1, giving a novel explanation for the physiological induction of bilirubin glucuronidation in newborn infants.

Sex-dependent metabolism of xenobiotics

Sex-dependent differences in xenobiotic metabolism have been most extensively studied in the rat. Because sex-dependent differences are most pronounced in rats, this species quickly became the most popular animal model to study sexual dimorphisms in xenobiotic metabolism. Exaggerated sex-dependent variations in metabolism by rats may be the result of extensive inbreeding and/or differential evolution of isoforms of cytochromes P450 in mammals. For example, species-specific gene duplications and gene conversion events in the CYP2 and CYP3 families have produced different isoforms in rats and humans since the species division over 80 million years ago. This observation can help to explain the fact that CYP2C is not found in humans but is a major subfamily in rats (Table 11). Animal studies are used to help determine the metabolism and toxicity of many chemical agents in an attempt to extrapolate the risk of human exposure to these agents. One of the most important concepts in attempting to use rodent studies to identify sensitive individuals in the human population is that human cytochromes P450 differ from rodent cytochromes P450 in both isoform composition and catalytic activities. Xenobiotic metabolism by male rats can reflect human metabolism when the compound of interest is metabolized by CYP1A or CYP2E because there is strong regulatory conservation of these isoforms between rodents and humans. However, problems can arise when rats are used as animal models to predict the potential for sex-dependent differences in xenobiotic handling in humans. Information from countless studies has shown that the identification of sex-dependent differences in metabolism by rats does not translate across other animal species or humans. The major factor contributing to this observation is that CYP2C, a major subfamily in rats, which is expressed in a sex-specific manner, is not found in humans. To date, sex-specific isoforms of cytochromes P450 have not been identified in humans. The lack of expression of sex-dependent isoforms in humans indicates that the male rat is not an accurate model for the prediction of sex-dependent differences in humans. Differences in xenobiotic metabolism among humans are more likely the consequence of intraindividual variations as a result of genetics or environmental exposures rather than from sex-dependent differences in enzyme composition. A major component of the drug discovery and development process is to identify, at as early a stage as possible, the potential for toxicity in humans. Earlier identification of individual differences in xenobiotic metabolism and the potential for toxicity will be facilitated by improving techniques to make better use of human tissue to prepare accurate in vitro systems such as isolated hepatocytes and liver slices to study xenobiotic metabolism and drug-induced toxicities. Accurate systems should possess an array of bioactivation enzymes similar to the in vivo expression of human liver. In addition, the compound concentrations and exposure times used in these in vitro test systems should mimic those achieved in the target tissues of humans. Consideration of such factors will allow the development of compounds with improved efficacy and low toxicity at a more efficient rate. The development of accurate in vitro systems utilizing human tissue will also aid in the investigation of the molecular mechanisms by which the CYP genes are regulated in humans. Such studies will facilitate the study of the basis for differences in expression of isoforms of CYP450 in humans.

Biochemical basis for deficient paracetamol glucuronidation in cats: an interspecies comparison of enzyme constraint in liver microsomes

Unlike most other mammalian species, domestic cats glucuronidate phenolic compounds poorly and are therefore highly susceptible to the toxic side effects of many drugs, including paracetamol. In this study, we evaluated the role of enzyme constraint, a characteristic that limits the activity of all uridine 5'-diphosphoglucuronosyltransferase (UGT) enzymes, in the aetiology of this species-dependent defect of drug metabolism. Detergent activation experiments were performed using hepatic microsomes from cats (4), dogs (4), man (4), and 6 other mammalian species (1 liver each). In addition, we used microsomes from Gunn rats which are sensitive to paracetamol toxicity because of a genetic defect affecting all family 1 UGTs. Increase in paracetamol-UGT activity at optimum concentrations of detergent was used as an index of enzyme constraint. Native activity (measured in the absence of detergent) was less than one-sixth in cats compared with other species. Optimum detergent treatment tended to enhance rather than abolish this difference, however, indicating relatively lower levels of constraint of paracetamol-UGT in cats compared with other species. Similarly, detergent treatment failed to reduce the native activity difference between homozygous mutant and normal Gunn rats. Initially CHAPS (3-(3-cholamidopropyl)-dimethylammonio-1-propanesulphonic acid) was used as the detergent activator; in 3 of 4 microsomal preparations from man, however, inhibition rather than activation was observed at all detergent concentrations used. Studies were repeated using the non-ionic detergent, Brij 58 (polyoxyethylene 20-cetyl ether), which resulted in similar although more profound activation and no inhibition. We conclude that deficient paracetamol glucuronidation in cats does not result from increased paracetamol-UGT constraint in this species compared with other mammalian species. Other causes, such as differences in enzyme protein concentration or substrate affinity might be responsible.

Applicability of cultured hepatocytes derived from goat, sheep and cattle in comparative drug metabolism studies

1. Using trimethoprim (TMP), scoparone (SCOP), ethylmorphine (EtM), 1-naphthol (1-N) and phenol red (PhR) as test substrates, biotransformation activities were investigated in cultured hepatocytes from male and female rat, male and female goat, and female sheep and cattle. 2. As compared with rat hepatocytes, the total culture cytochrome P450 content was relatively well maintained in ruminant hepatocytes. In 72 h, it decreased to approximately half the initial content, whereas in rat hepatocytes only 30% was maintained. In ruminant hepatocytes, sulphation of 1-N remained fairly stable, glucuronidation of PhR decreased gradually, and glucuronidation of 1-N increased during the 72-h culture period. 3. Oxidative metabolism of TMP was rapid in goat and sheep hepatocytes, as compared with rat hepatocytes, reflecting species differences in TMP pharmacokinetics in vivo. In contrast with rat hepatocytes, 6-O-demethylation was by far the major pathway of scoparone metabolism in ruminant hepatocytes. The glucuronidation and sulphation activities were similar among the species. 4. In goat liver cells, sex differences in some oxidative biotransformations were observed, females being more active than males. In rat hepatocytes, a reverse sex difference was observed. 5. In conclusion, cultured hepatocytes from agricultural target species appear a useful in vitro model to study comparative metabolism of veterinary drugs and other xenobiotics. Comparing rat and ruminant, sex and species differences and similarities in drug metabolism can be observed that reflect the in vivo situation.

Paracetamol elimination in patients with non-insulin dependent diabetes mellitus

The pharmacokinetics and metabolism of an intravenous dose (500 mg) of paracetamol were studied in a group of non-insulin dependent diabetic patients (n = 10) and in a group of healthy control subjects (n = 9). Paracetamol clearance, half-life and the partial clearance to paracetamol glucuronide were not significantly different, but the partial clearance to paracetamol sulphate was significantly reduced (62 +/- 18 vs 86 +/- 17 ml h-1 kg-1 (mean +/- s.d.)) and the renal clearance of paracetamol was significantly increased (56 +/- 20 vs 22 +/- 6 ml h-1 kg-1 (mean +/- s.d.)) in the non-insulin dependent diabetic patients, compared with the control group.

Long-term kinetic vitreous fluorophotometry in normal and diabetic subjects

Nine normal and 24 diabetic subjects were examined by long-term vitreous and plasma fluorescein fluorophotometry and the observed concentration profiles were described by biexponential time courses. The rate constant of elimination of fluorescein from the body (K10) was significantly decreased in diabetics with background and proliferative retinopathy, presumably caused by affection of the liver and possibly representing alterations in membranes of liver cells. Increased kidney albumin excretion was observed with increasing degree of retinopathy. The apparent rate constant of fluorescein penetration into the eye (Kin) was found significantly decreased in background as well as in proliferative retinopathy; while the permeability index, calculated as areas under vitreous and plasma fluorescein curves, was significantly increased. In the normal subjects Kin was significantly higher than the rate constant of fluorescein transfer (K12) from the apparent central to the peripheral tissue compartment, whereas in the diabetics this difference was only found in the group with background retinopathy. The findings seem compatible with the concept that the breakdown of the blood-ocular barrier could be caused at least partly by affection of an active transport system for fluorescein, but thickening and compositional changes of the basement membranes in the eye might also be of importance.

Optimal detergent activation of rat liver microsomal UDP-glucuronosyl transferases toward morphine and 1-naphthol: contribution to induction and latency studies

The detergent-activation profiles of UDP-glucuronosyl transferases (UGTs, EC 2.4.1.17) toward 1-naphthol and toward morphine have been determined: three non-ionic detergents, Triton X-100, Brij 58 and Lubrol Px and one zwitterion detergent, 3-(3-cholamidopropyl)-dimethylammonio-1-propanesulfonic acid (CHAPS) were studied. The results showed that marked inhibition of 1-naphthol-UGT and morphine UGT activities occurred with high concentrations of Triton X-100. Lubrol Px, at high concentrations, inhibited 1-naphthol-UGT but not morphine-UGT. It appeared that the detergent/protein ratio suitable for optimal activation of both isoenzymes was limited to 0.2 for these detergents. In contrast, Brij 58 and CHAPS displayed optimal activation of the two enzymes for a large range of detergent/microsomal protein ratios (respectively from 0.2 to 1 and from 0.4 to 1), making them the most suitable for induction and/or latency studies of both isoenzymes. The influence of maximal activation status on the effect of 3-methylcholanthrene and phenobarbital treatment on morphine-UGT and 1-naphthol-UGT activity has also been evaluated. The findings provided evidence that detergent-activation profiles and optimal detergent-activated versus "native" UGT activity determination give crucial informations about the characteristics of a given isoenzymic form of UGT, i.e. its sensitivity to specific alterations of the phospholipid environment, its latency and its inducibility.

Male rat hepatic UDP-glucuronosyltransferase activity toward thyroxine. Activation and induction properties--relation with thyroxine plasma disappearance rate

Detergent-activation of UDP-glucuronosyltransferase (UGT) isoenzyme(s) involved in thyroxine (T4) glucuronidation in control, phenobarbital (PB)- and 3-methylcholanthrene (3-MC)-treated rats showed that between the four tested detergents, i.e. Triton X-100, Brij 58, Lubrol Px and 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonic acid (CHAPS), optimal activation of T4 UGT was displayed by the zwitterion CHAPS. "Native" versus optimal detergent-activated T4 UGT activity determination revealed that the latency of T4 UGT in microsomes from 3-MC-treated rats was decreased while the latency of T4 UGT in microsomes from PB-treated rats was increased compared to control, and suggest that the UGT isoenzyme(s) involved in the hepatic glucuronidation of T4 is (are) different in PB-treated rats than in 3-MC-treated rats. After a 7-day treatment with 20 mg/kg 3-MC, the activity of T4 UGT was increased 5-fold when determined in "native" and 4-fold when determined in optimal detergent-activated microsomes compared to controls. After a 7-day treatment with 75 mg/kg PB, T4 UGT was equivalent to the control when determined in "native", and increased 1.3-fold when determined in optimal detergent-activated microsomes. The results thus extend evidence that both 3-MC and PB induce the synthesis of UGT protein(s) involved in the glucuronidation of T4, 3-MC being a strong and PB a weak inducer. Hyperthyroid and hypothyroid status, achieved respectively by a 7-day treatment with 100 microns/kg T4 or a 7-day treatment with 10 mg/kg of one of the antithyroid drugs propylthiouracile or methymazole, did not modify T4 UGT activity, suggesting that the isoenzyme(s) conjugating T4 in microsomes from control rats is (are) unlikely to be either 4-nitrophenol or bilirubin UGT isoenzymes. After 14 days of treatment with 75 mg/kg PB, the hepatic glucuronidation rate of T4 was not different from the control when enzyme activity was expressed per mg microsomal protein but was significantly increased 1.4-fold when expressed per whole liver. A significant (1.5-fold) increase in the 125I-T4 plasma elimination rate was also observed in PB-treated rats compared to controls. A strong (3.6-fold) increase in the T4 glucuronidation rate was observed in rats treated with 5 mg/kg 3-MC for 14 days while the 125I-T4 plasma elimination rate was equivalent to the controls. These results demonstrate that there is no direct relation between T4 UGT activity (and subsequent biliary secretion of T4-glucuronides) and T4 plasma clearance and suggest an important contribution of the intestinal exchangeable thyroid hormone pool to the maintenance of blood thyroid hormone levels.

Hepatic transport of bilirubin in rats with streptozotocin-induced diabetes

This study was undertaken to evaluate the effects of streptozotocin-induced diabetes on the hepatic transport of bilirubin in male Wistar rats. Rats were pretreated with streptozotocin (60 mg/kg i.p.) to induce uncontrolled diabetes. Six days later endogenous biliary excretion and plasma bilirubin concentration were significantly enhanced compared to control animals (+36% and +46%, respectively), while the blood levels of free hemoglobin remained unchanged. Following a bilirubin load, the maximal biliary excretion of the pigment (Tm) in diabetic animals was significantly enhanced compared to control animals (+49%). Liver and plasma bilirubin concentrations at the end of bilirubin administration were significantly reduced (-28% and -30%, respectively). Bilirubin UDP-glucuronosyltransferase activity and UDP-glucose concentration in liver were significantly enhanced (+31% and +81%, respectively), as was the biliary excretion of unconjugated bilirubin (+37%) and bilirubin mono- (+38%) and diconjugates (+53%). When streptozotocin-diabetic rats were treated with insulin, the parameters of bilirubin transport and metabolism were significantly reduced compared to diabetic animals receiving no hormone replacement. In summary, our data indicate that in short-term streptozotocin-diabetic rats there is increased bilirubin production as well as enhanced hepatic conjugation and subsequent biliary excretion of the pigment. These effects appear to be a direct consequence of diabetes.

Determination of 125I-labelled thyroxine glucuronide by reversed-phase high-performance liquid chromatography using on-line radiochemical detection to determine UDPglucuronosyltransferase activity

A convenient, fast and highly sensitive high-performance liquid chromatographic method, using on-line radiochemical detection, is described for the determination of [125I]thyroxine glucuronide. The method involves direct injection of the supernatant, a total analysis time of 30 min and a detection limit of 1 pmol. The results demonstrate that the method is suitable for the determination of UDPglucuronosyltransferase activity with thyroxine as substrate in native hepatic microsomes. The rate of thyroxine glucuronidation in microsomes from rats treated with Arodor 1254 was ten times higher than in control microsomes, indicating that with this method, increases of UDPglucuronosyltransferase thyroxine activities, often associated with hepatic induction process involved in thyroid hypertrophy, can be easily detected. This method could also be applied to all experimental biological systems that involve the separation and quantification of [125I]thyroxine and [125I]thyroxine glucuronide.

Diabetes in female rats; changes in liver microsomal aminopyrine N-demethylase and UDP-glucuronyl transferase activities

Short or long term diabetes in female rats produced remarkable activation of aminopyrine N-demethylation, inhibition of oestrone and p-nitrophenol glucuronidation and no changes in morphine UDP-glucuronyltransferase activity in vitro. Km and Vmax for these reactions were determined. Insulin treatment partially antagonized diabetes activation of aminopyrine N-demethylation: it restored decreased UDP-glucuronyltransferase activities for oestrone and p-nitrophenol only in long term and short term diabetes, respectively. Insulin also markedly inhibited morphine glucuronidation. Triton X-100 also displayed a differential pattern of activation for the glucuronidation reactions in liver microsomes of diabetic rats. Results suggest that diabetes in female rats may increase the actual amount of enzyme protein for aminopyrine metabolism and to decrease that for oestrone and p-nitrophenol.

The toxicity of menadione and mitozantrone in human liver-derived Hep G2 hepatoma cells

The cytotoxic properties of quinone drugs such as menadione and adriamycin are thought to be mediated through one-electron reduction to semiquinone free radicals. Redox cycling of the semiquinones results in the generation of reactive oxygen species and in oxidative damage. In this study the toxicity of mitozantrone, a novel quinone anticancer drug, was compared with that of menadione in human Hep G2 hepatoma cells. Mitozantrone toxicity in these cells was not mediated by the one-electron reduction pathway. In support of this, inhibition of the enzymes glutathione reductase and catalase, responsible for protecting the cells from oxidative damage, did not affect the response of the Hep G2 cells to mitozantrone, whereas it exacerbated menadione toxicity. In addition, the toxicity of menadione was preceded by depletion of reduced glutathione which was probably due to oxidation of the glutathione. Mitozantrone did not cause glutathione depletion prior to cell death. DT-diaphorase activity and intracellular glutathione were found to protect the cells from the toxicity of both quinones. Inhibition of epoxide hydrolase potentiated mitozantrone toxicity but did not affect that of menadione. Our experiments indicate that mitozantrone toxicity may involve activation to an epoxide intermediate. Both quinone drugs inhibited cytochrome P-450-dependent mixed-function oxidase activity, although menadione was more potent in this respect.

Mixed function oxidase and UDP-glucuronyltransferase activities in the human Hep G2 hepatoma cell line

In cultured human hepatoma cells phenolphthalein glucuronidation was increased 3-fold by 2 mM phenobarbitone (PB) in the culture medium but not by 25 microM benz(a)anthracene (BA), while 1-naphthol glucuronidation was not increased by either PB or BA. Ethoxyresorufin O-deethylation (EROD) was increased 15-fold by BA but not by PB, while the O-dealkylations of pentoxyresorufin (PROD) and benzyloxyresorufin (BROD) were increased by either PB or BA. The BROD activity increased by BA was sensitive to inhibition by alpha-naphthoflavone whereas that induced by PB was not. This suggests induction of different cytochrome P-450 isoenzymes. Control Hep G2 cells had similar glucuronide conjugation and cytochrome reductase activities to freshly isolated human adult hepatocytes, but had lower O-dealkylation and elevated microsomal epoxide hydrolase activities.

The effect of long-term streptozotocin-induced diabetes on the hepatotoxicity of bromobenzene and carbon tetrachloride and hepatic biotransformation in rats

To exclude the possibility that changes in hepatotoxicity and biotransformation were induced by diabetogen administration, the influence of long-lasting experimental insulin-dependent diabetes on the activities of benzphetamine demethylase, styrene oxide hydrolase, and UDP-glucuronosyl-transferases toward 1-naphthol, diethylstilbestrol, estrone and testosterone, and glutathione S-transferases toward 1-chloro-2,4-dinitrobenzene, ethacrynic acid, and sulfobromophthalein was studied. Adult male Sprague-Dawley rats injected with 45 mg streptozotocin/kg rapidly developed the classical symptoms of diabetes which persisted throughout the 90-day test period. Ketonemia was detectable at 6 but not at either 35 or 90 days after streptozotocin administration. After acute challenge with bromobenzene or carbon tetrachloride (CCl4), aspartate and alanine aminotransferase activities in rats diabetic for 35 and 90 days were markedly higher than those in normal rats, suggesting that diabetes potentiated the hepatotoxicity of these chemicals. Administration of 25 microliters CCl4/kg, ip, to diabetic rats decreased enzyme activities toward benzphetamine, sulfobromophthalein, 1-chloro-2,4-dinitrobenzene, and 1-naphthol. In normal rats, a dose of 400 microliters CCl4/kg, ip, was required to cause similar changes in enzyme activities. Bromobenzene (500 microliters/kg, ip) elicited opposing responses in diabetic and normal rats in N-demethylase activity, in UDP-glucuronosyltransferase activity toward 1-naphthol, estrone, and testosterone, and in glutathione S-transferase activity toward 1-chloro-2,4-dinitrobenzene. Total cytochrome P450 concentrations were reduced by both induction of diabetes and hepatotoxicant challenge. Thus, chronic uncontrolled diabetes alters the response of hepatic xenobiotic biotransformation enzymes in a non-uniform, substrate-dependent manner, independent of initial diabetogen effects. The role of cytochrome P450j in potentiating CCl4 toxicity is discussed.

Hepatic phase II biotransformation in C57Bl/KsJ db/db mice: comparison to that in Swiss Webster and 129 REJ mice

1. Cytochrome P-450 concentrations were similar in male and female carrier (db/+) and diabetic (db/db) mice. Benzphetamine N-demethylase and styrene oxide hydrolase activities were 47 and 65% lower in db/+ than in db/db mice. 2. UDP-Glucuronosyltransferase activity toward 1-naphthol, estrone and diethylstilbestrol was not different between db/db and db/+, but was 40% higher in db/db mice toward testosterone. 3. Glutathione S-transferase activity toward 1-chloro-2,4-dinitrobenzene and ethacrynic acid was 47 and 59% lower in db/db mice than in male db/+ mice. Female db/+ mice had similar activities to those found in diabetic animals. 4. The differences in enzyme activity between hyperinsulinemic and normal animals suggest that insulin can influence both phase I and phase II biotransformations. 5. Enzyme activities in db/+ and db/db mice were compared to those in 129 REJ and Swiss Webster mice.

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.

Human adult hepatocytes in primary monolayer culture. Maintenance of mixed function oxidase and conjugation pathways of drug metabolism

The stabilities of several drug oxidation and conjugation pathways in human adult hepatocytes have been investigated during 72 hr culture. Cytochrome P-450-dependent mixed function oxidase was measured by the O-dealkylations of ethoxyresorufin (EROD), pentoxyresorufin (PROD) and benzyloxyresorufin (BROD), which are probes for different isozymes of cytochrome P-450 in the rat. EROD declined to 64% of initial fresh cell values after 72 hr in culture, whereas PROD increased to 162% and BROD remained relatively constant. Addition of phenobarbitone to the culture medium selectively increased PROD to a greater extent than EROD and did not affect BROD. NADPH-cytochrome c reductase and NADH-cytochrome b5 reductase were markedly labile during culture, declining to 32% and 22% of fresh cell values respectively. Epoxide hydrolase (EH) showed a large transient increase (2-5-fold) in enzyme activity 24 hr after culture, declining to fresh cell values by 48 hr. UDP-glucuronyltransferase (GT) activity towards phenolphthalein and 1-naphthol also increased (2-3-fold) during the 72 hr of culture, the greater and more rapid increase being observed with phenolphthalein glucuronidation. Sulphotransferase activity declined rapidly within 24 hr of culture, whereas reduced glutathione (GSH) levels and GSH conjugation were maintained at fresh cell values for 72 hr.

Hepatic biotransformation in lean and obese Wistar Kyoto rats: comparison to that in streptozotocin-pretreated Sprague-Dawley rats

1. Phase I and phase II biotransformation was compared in streptozotocin-induced hypoinsulinemic (STZ) and genetic hyperinsulinemic (WKY-fatty) rats. 2. Total cytochrome P-450 concentrations were reduced in both STZ and WKY, whereas styrene oxide hydrolase and benzphetamine N-demethylase activities were normal in STZ and reduced in WKY. 3. UDP-glucuronosyltransferase activity was decreased toward testosterone and 1-naphthol in STZ and WKY, and was increased toward estrone in the obese female WKY. 4. Glutathione S-transferase activity was decreased in STZ toward 1-chloro-2,4-dinitrobenzene, ethacrynic acid and sulfobromophthalein, but was similar to that in normal rats for WKY.

Inhibition and activation of UDP-glucuronyltransferase in alloxanic-diabetic rats

Short or long term alloxan diabetes produced activation of oestrone and morphine glucuronidation and inhibition of p-nitrophenol glucuronidation in rat liver microsomes. Insulin treatment restored decreased glucuronyltransferase (GT) activity for p-nitrophenol and it did not abolish diabetes activation on oestrone glucuronidation. Triton X-100 detergent activation reduced differences between normal, diabetic and insulin treated rats in the glucuronidation rates of the substrates assayed. 1,4-Benzodiazepines inhibited morphine GT activity and stimulated oestrone GT activity in normal, diabetic and insulin treated diabetic rats. Activation and inhibition of GT activities for oestrone and xenobiotics in diabetes mellitus appears to be related with membrane perturbations of liver microsomes.

Conjugation of organic compounds in isolated hepatocytes from a marine fish, the plaice, Pleuronectes platessa

A method is described for the preparation of viable hepatocytes from a marine fish, the plaice. Their ability to detoxify organic compounds was measured by the formation of glucuronic acid and sulphate conjugates with the model substrates 1-naphthol and phenolphthalein. 1-Naphthol was conjugated three- to four-fold faster than phenolphthalein and glucuronidation predominated with both substrates. Strong substrate inhibition of glucuronidation was observed with 200 microM 1-naphthol or phenolphthalein. No measurable sulphate conjugation was detected with phenolphthalein. Treatment of fish with 3-methylcholanthrene induced formation of both glucuronide and sulphate conjugates by two- to three-fold. Compared with rat hepatocytes, the extent of sulphation was 100-fold lower in plaice hepatocytes whereas glucuronide formation was only 10-fold lower. The observations indicate that isolated plaice hepatocytes provide a suitable system for studies of the detoxication of xenobiotic pollutants in fish liver.