Influence of morphine concentration on detergent activation of rat liver morphine-UDP-glucuronosyltransferase

Lack of interaction between metoclopramide and morphine in vitro and in mice

1. This study examined interactions via common metabolism or via common pharmacodynamic pathways between frequently co-prescribed metoclopramide (a prokinetic) and morphine (an opioid analgesic). 2. In human liver microsomes, morphine 3-glucuronide and morphine 6-glucuronide formation had V(max) estimates of 6.2 +/- 0.07 and 0.75 +/- 0.01 (nmole min(-1) mg(-1) protein) and K(m) estimates of 1080 +/- 37 and 665 +/- 55 (microM), respectively. The in vitro K(i) for morphine 3-glucuronide formation in the presence of metoclopramide in human liver microsomes or recombinant uridine diphosphoglucuronosyltransferase 2B7 predicted a lack of in vivo interaction. 3. Morphine (2 mg kg(-1) subcutaneously) delayed gastrointestinal meal transit in mice, metoclopramide (10 mg kg(-1) subcutaneously) had no effect on meal transit, and metoclopramide did not alter this effect of morphine. 4. Morphine (2 or 5 mg kg(-1) subcutaneously) was antinociceptive in mice (hot plate test) and metoclopramide (10 mg kg(-1) subcutaneously) did not alter the antinociceptive effects of morphine. 5. Together, the data suggest a lack of interaction between morphine and metoclopramide.

Copper modifies liver microsomal UDP-glucuronyltransferase activity through different and opposite mechanisms

Treatment of hepatic microsomes with Fe(3+)/ascorbate activates UDP-glucuronyltransferase (UGT), a phenomenon totally prevented and reversed by reducing agents. At microM concentrations, iron and copper ions catalyze the formation of ROS through Fenton and/or Haber-Weiss reactions. Unlike iron ions, indiscriminate binding of copper ions to thiol groups of proteins different from the specialized copper-binding proteins may occur. Thus, we hypothesize that incubation of hepatic microsomes with the Cu(2+)/ascorbate system will lead to both UGT oxidative activation and Cu(2+)-binding induced inhibition, simultaneously. We studied the effects of Cu(2+) alone and in the presence of ascorbate on rat liver microsomal UGT activity. Our results show that the effects of both copper alone and in the presence of ascorbate were copper ion concentration- and incubation time-dependent. At very low Cu(2+) (25nM), this ion did not modify UGT activity. In the presence of ascorbate, however, UGT activity was increased. At higher copper concentrations (10 and 50microM), this ion led to UGT activity inhibition. In the presence of ascorbate, 10microM Cu(2+) activated UGT at short incubation periods but inhibited this enzyme at longer incubation times; 50microM Cu(2+) only inhibited UGT activity. Thiol reducing agent 2,4-dithiothreitol prevented and reversed UGT activation while EDTA prevented both, UGT activation and inhibition. Our results are consistent with a model in which Cu(2+)-induced oxidation of UGT leads to the activation of the enzyme, while Cu(2+)-binding leads to its inhibition. We discuss physiological and pathological implications of these findings.

METABOLISM OF OPIOIDS IS ALTERED IN LIVER MICROSOMES OF SICKLE CELL TRANSGENIC MICE

Pain in sickle cell anemia (SCA) is clinically managed with opioid analgesics. There are reports that SCA patients tolerate high doses of these drugs without adequate pain relief. The current study investigated the in vitro hepatic metabolism of opioids in mouse models of sickle cell anemia, with the hypothesis that higher dose requirements in SCA could be explained by an increased metabolism rate of opioids. Various rodent cytochrome P450 substrates, i.e., buprenorphine and codeine, and rodent uridine glucuronosyltransferase substrates, i.e., morphine, buprenorphine, and estradiol, were studied. The three groups used were: 1) control C57BL mice, 2) mice with the human alpha-globin and sickle beta-globin transgenes (SC), and 3) mice with the human alpha-globin and sickle beta-globin transgenes, and homozygous for the murine alpha-globin and heterozygous for the beta(major)-gene knockout (SCKO). In vitro hepatic microsomal incubations were carried out for each substrate, and data were fit to the Michaelis-Menten equation. Morphine formation had a higher V(max) in SCKO microsomes (0.4 +/- 0.009 nmol/min. mg; estimate +/- S.E.) than controls (0.25 +/- 0.007). Morphine-3-glucuronide formation had V(max) estimates of 18.9 +/- 0.6, 25.1 +/- 0.4, and 27.06 +/- 1.1 nmol/min. mg in control, SC, and SCKO microsomes, respectively. The control V(max) for estradiol-3-glucuronide formation was 2-fold greater than in SCKO microsomes. The control V(max) for estradiol 17-glucuronide formation was 3.4- and 2.2-fold greater than in SC and SCKO microsomes. Thus, in vitro metabolism of opioids is altered in SCA mouse models, which may lead to altered clearances of these drugs.

Comparison of radical scavenging effect, inhibition of microsomal oxygen free radical generation, and serum lipoprotein oxidation of several natural antioxidants

Typical components of the Mediterranean diet, such as olive oil and red wine, contain high concentrations of complex phenols, which have been suggested to have an important antioxidant role. The aim of the present work was to determine the inhibitory potency of compounds such as oleuropein, hydroxytyrosol, and other structurally related compounds, such as gallic acid, toward reactive oxygen species generation and free radical scavenging ability. The potency of these compounds was also examined with respect to protecting in vitro low-density lipoprotein oxidation. These studies indicate that complex phenols, such as hydroxytyrosol, and gallic acid both inhibit free radical generation and act as free radical scavengers. The use of three different approaches to determine antioxidant potency demonstrates that activity in one test does not necessarily correlate with activity in another. It was also demonstrated that the presence of two phenolic groups is not always associated with antioxidant activity.

No effect of phenobarbital pretreatment of rats on methotrexate pharmacokinetics in the isolated liver perfused in a single-pass way

Pretreatment of the rat with phenobarbital (PB) is known to increase gene expression of the canalicular multispecific organic anion transporter (cMOAT) and hepatobiliary transport of its substrates (glutathione, sulfobromophthalein). To determine the effect of PB on the hepatobiliary transport of methotrexate (MTX, another substrate of cMOAT) and its metabolism to 7-hydroxymethotrexate (7-OHMTX) in the rat, we compared the steady-state pharmacokinetics of MTX in the isolated liver of either PB-pretreated (80 mg/day/kg bw for 4 days, i.p.) or nonpretreated rats. The livers were perfused in a single-pass way at a flow rate of 15 ml/min using a perfusate which consisted of Krebs-Henseleit buffer containing glucose, taurocholate, bovine albumin and erythrocytes. During the perfusion with 50 micromol/l MTX, the steady-state biliary clearance (1.26 +/- 0.24 ml/min) in 7 nonpretreated rats accounted for a major proportion of the hepatic clearance (1.30 +/- 0.33 ml/min), metabolism of MTX to 7- OHMTX was minor (partial metabolic clearance = 0.041 +/- 0.023 ml/min). MTX concentrations in bile surpassed those in the input perfusate by approximately 100-fold. Pretreatment of rats (n = 7) with PB did not change significantly the steady-state hepatic, biliary and partial metabolic clearances of 50 micromol/l MTX. An interesting result is a 38% increase in the hepatic vascular resistance of non-pretreated livers caused by MTX. The results suggest that in rats, pretreatment with PB has no effect on the hepatobiliary transport and hydroxylation of MTX.

Glucuronidation of dihydrocodeine by human liver microsomes and the effect of inhibitors

1. Glucuronidation is the major route of metabolism of dihydrocodeine (DHC) and accounts for 25-30% of an oral dose in urine. The kinetics of DHC-6-glucuronide formation in liver microsomes from five human donors and the effect of a number of potential inhibitor drugs were examined using a newly developed and validated HPLC assay. 2. The formation of DHC-6-glucuronide exhibited atypical kinetics that conformed to the Hill equation. The mean intrinsic dissociation constant (Ks) and maximum velocity (Vmax) values were 1566 micromol/L and 0.043 micromol/min per g, respectively. The Ks and Vmax values varied 1.5- and 3.5-fold, respectively. 3. Seven drugs were tested for inhibitory effects on DHC glucuronidation at low (50 micromol/L) and high (500 micromol/L) concentrations. At 50 micromol/L, only diclofenac produced greater than 50% inhibition, while at concentrations of 500 micromol/L inhibition was greater than 35% for diclofenac, amitriptyline, oxazepam, naproxen, chloramphenicol and probenecid, but not paracetamol. 4. The present study found little interindividual variation in the activity of human liver microsomes for glucuronidation of DHC. Comparison of the results from the inhibition studies with those reported previously for codeine and morphine suggest that the UDP-glucuronosyltransferase isoform UGT2B7 is involved in the glucuronidation of DHC.

Glucuronidation of diflunisal in liver and kidney microsomes of rat and man

1. The glucuronidation of diflunisal to its phenolic (DPG) and acyl glucuronide (DAG) was measured in vitro using microsomes prepared from rat (n = 4) and human (n = 6) liver and kidney tissue. UGT activities towards bilirubin, 4-nitrophenol and (-)-morphine were also determined. 2. beta-Glucuronidase activity towards phenolphthalein glucuronide was much lower in microsomes prepared from human liver (45.2 +/- 3.1 Fishman Units/mg protein), human kidney (22.0 +/- 3.3 FU/mg), and rat kidney (25.1 +/- 2.5 FU/mg) as compared with rat liver (118.7 +/- 8.8 FU/mg). 3. The formation rate of DAG significantly increased when saccharo-1,4-lactone, a beta-glucuronidase inhibitor, was added to the rat liver microsomal incubation medium. beta-Glucuronidase inhibition, however, had little effect on the formation rate of DAG in human liver microsomes, and no effect in rat and human kidney microsomes. The formation of DPG was not affected by the microsomal beta-glucuronidase activity. 4. Unlike rat kidney microsomes, which only formed DAG, human kidney microsomes formed both diflunisal glucuronides. Formation of both diflunisal glucuronides in human kidney microsomes (Vmax = 0.97 +/- 0.21 and 0.27 +/- 0.07 nmol/min/mg for formation of DAG and DPG respectively) represented 60-70% of the activity found in liver microsomes (Vmax = 1.58 +/- 0.32 and 0.40 +/- 0.08 nmol/min/mg for formation of DAG and DPG respectively). 5. These results demonstrate that the in vitro glucuronidation rate of diflunisal may be affected by the microsomal beta-glucuronidase activity particularly when using rat liver microsomes. Our results also demonstrate that the human kidney has an important UGT-activity towards diflunisal.

Effect of cyclophosphamide administration on the activity and relative content of hepatic P4502D1 in rat

1. The effects of the administration of the anticancer and immunosuppressive drug, cyclophosphamide, to the rat on hepatic P4502D1 activity and content in the microsomal fraction have been examined. 2. Liver microsomes were obtained from male Hooded Wistar rats administered a single dose (i.p.) of saline or cyclophosphamide (200 mg/kg). Rats receiving cyclophosphamide were killed 1, 4, 7, 10 or 14 days after cyclophosphamide administration. The O-demethylation of dextromethorphan to dextrorphan was used to monitor 2D1 activity. 3. The mean Vmax for dextrorphan formation was reduced significantly (p < 0.0001) 7, 10 and 14 days after cyclophosphamide administration compared with the control group (control, 0.32 +/- 0.07; 7-day, 0.20 +/- 0.08; 10-day, 0.11 +/- 0.02; and 14-day group, 0.15 +/- 0.02 nmol/mg/min). 4. Western blotting revealed that there was a significant reduction (p < 0.0005) in the microsomal relative 2D1 content 10 days after cyclophosphamide administration compared with the control group (control, 1.25 +/- 0.44; and 10-day group, 0.65 +/- 0.14). 5. The activity of reduced nicotinamide adenine dinucleotide phosphate P450 reductase was significantly reduced (p < 0.0001) 7, 10 and 14 days following cyclophosphamide administration (control, 215 +/- 24; 7-day, 102 +/- 20; 10-day, 59 +/- 4 and 14-day group, 76 +/- 8 nmol/mg/min). Cytochrome b5 content was significantly reduced (p < 0.0001) 7 and 10 days following cyclophosphamide administration (control, 0.46 +/- 0.13; 7-day, 0.28 +/- 0.07 and 10-day group, 0.20 +/- 0.03 nmol/mg). 6. The significant reductions in the activity of rat hepatic microsomal 2D1 following cyclophosphamide administration, as seen by the alterations in mean Vmax for dextrorphan formation, do not appear to be due to a single factor, but may result from a combination of several events, including reductions in relative 2D1 content, reduced nicotinamide adenine dinucleotide phosphate P450-reductase activity and cytochrome b5 content.

Cyclophosphamide administered repeatedly to the male rat and as a single dose to the female rat. Its effects on hepatic and pulmonary P450 and associated enzymes

1. Two different aspects of the effects of the cytotoxic agent cyclophosphamide (CP) on rat P450 and associated enzymes have been examined. 2. First, the effects of CP, administered as a single 200 mg/kg dose, on hepatic and pulmonary P450 and some associated enzymes in the female rat have been investigated. Second, the effects of repeat doses of CP (40 mg/kg on days 0-4 with killing on days 5, 8 and 11) to the male rat have been examined. 3. CP decreased the activity of the female rat hepatic enzymes 2A1, 2C6 and/or 2C12 and 2E1, NADPH-P450 oxidoreductase and 17 beta-oxidoreductase and the pulmonary enzyme 2B, 7 days after its administration. The decreases in the activity of the enzymes 2E1 and NADPH-P450 oxidoreductase were accompanied by a corresponding change in the amount of enzyme protein indicating that the alteration in expression of these enzymes occurred via changes in transcription and/or translation or protein degradation. 4. CP also impaired its own activation 7 days after its administration to the female rat. 5. The change in female enzyme profile was accompanied by a reduction in the hormones oestradiol, T4 and T3 7 days after CP administration. 6. Despite an apparent trend for an increase in activity on day 5, a decrease on day 8 and a subsequent increase on day 11, repeat doses of CP to the male rat generally did not alter the P450 isoforms 2A2, 2B1, 2C11, 2E1 and 3A2 or 17 beta-oxidoreductase, NADPH-P450 oxidoreductase and steroid 5 alpha-reductase. 7. Chronic administration of CP to the male rat significantly reduced erythromycin demethylase and NADPH-P450 oxidoreductase 8 days following commencement of dosing and significantly increased 2A2 11 days following commencement of dosing. There was also a statistically significant increase in pulmonary 2B 5 days following commencement of dosing. 8. Plasma testosterone and TSH were unchanged following repeated dosing with CP while T3 was significantly decreased on days 5, 8 and 11 and T4 was significantly decreased on day 8.

Hormonal perturbation as a possible mechanism for the alteration of cytochrome P450 by cyclophosphamide

The mechanism by which the cytotoxic agent cyclophosphamide (CP) alters cytochrome P450 and some associated enzymes in the male rat has been investigated. CP, administered as a high single dose, decreases the activity of the enzymes CYP2C6, CYP2C11, CYP3A2 and CYP2E1 and NADPH P450 reductase and increases the activity of steroid 5 alpha-reductase. CP appears to exert its effect via an indirect mechanism that reaches its maximal effect 7 days after administration. The decreases in the activity of the enzymes CYP2C11, CYP2E1 and CYP3A2 are accompanied via a corresponding change in the amount of enzyme protein indicating that the alteration in expression of these enzymes occurs via changes in transcription and/or translation. Michaelis-Menten analysis confirmed this conclusion for the enzymes CYP2C11 and CYP3A2. The change in enzyme profile is accompanied by a reduction in the hormones, testosterone, TSH, T4 and T3. The reduction in hormone levels is also maximal 7 days after CP administration. To determine whether CP alters enzyme expression in the male rat via perturbation of hormonal regulation, daily replacement doses of hCG and/or T3 were administered for 7 days after a single dose of CP and hepatic CYP and associated enzyme activities assessed. Results indicated that daily hormone replacement with either hCG and/or T3 prevented the changes in expression of the hormone dependent enzymes NADPH P450 reductase, steroid 5 alpha-reductase and CYP3A2 following a single dose of CP. In contrast for other enzymes including the male sex-specific enzyme CYP2C11, the female predominant enzyme CYP2E1 and the sex-independent enzyme CYP2C6, daily replacement of hCG and/or T3 did not prevent the changes that occur 7 days following CP administration. As rats appeared anorexic and dehydrated and significant weight losses were recorded following CP treatment, blood was collected at the time of killing and subjected to biochemical analysis and a complete blood picture to identify any changes in such parameters that may have contributed to the changes in hormones and/or enzyme expression that occurred. However, significant variation in the treatment groups compared with controls for all parameters was not observed to occur except for an anticipated leukopoenia. We have concluded that CP alters the enzymes NADPH P450 reductase, steroid 5 alpha-reductase and CYP3A2 via perturbation of the regulation of these enzymes by testosterone and/or thyroid hormones. However, while interference with regulation by testosterone and/or thyroid hormones may be part of the mechanism by which CP alters CYP2C11 and CYP2E1, other factors are contributing.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of 5-fluorouracil treatment on rat liver microsomal enzymes

1. The effects of the administration of the anticancer drug 5-fluorouracil on P450-mediated metabolizing capacity, and the activity of several related enzymes were examined by comparing two different dosage regimes, i.e. a single ip dose of 5-fluorouracil (120 mg/kg) at 1, 4 and 7 days prior to killing, and a repeated dose regimen of 5-fluorouracil (24 mg/kg/day for 5 consecutive days) with animals being killed at 1, 4 and 7 days after the last 5-fluorouracil dose. 2. Rat liver microsomes were assayed for total P450 content, reduced nicotinamide adenine dinucleotide phosphate-P450-reductase, aminopyrine demethylase, cyclophosphamide 4-hydroxylase, aniline hydroxylase, androstenedione 6 beta- and 16 alpha-hydroxylase and delta 4-3-oxo-steroid 5 alpha-oxidoreductase activity. 3. The animals that received 5-fluorouracil as a single, high dose did not show any alterations in any of the measured parameters at any of the times at which they were killed. 4. The animals that received chronic low doses of 5-fluorouracil, however, showed a statistically significant decrease in androstenedione 6 beta- and 16 alpha-hydroxylase and cyclophosphamide 4-hydroxylase activity and a statistically significant increase in delta 4-3-oxo-steroid 5 alpha-oxidoreductase activity 4 and 7 days after completion of dosing. 5. Immunoquantified P4502C11 and 3A content was also decreased at these time points. These alterations occurred concurrently with depleted plasma testosterone levels in the groups that received repeated low doses of 5-fluorouracil.

Effects of cyclophosphamide and adriamycin on rat hepatic microsomal glucuronidation and lipid peroxidation

The effect of cytotoxic drug administration, as a single dose i.p. to rats (six rats/treatment group), on hepatic microsomal UDP-glucuronosyltransferase (UGT) activity was investigated. Glucuronidation of morphine in microsomes from control rats apparently involved at least two enzymes. Administration of cyclophosphamide (CP; 200 mg/kg 7 days prior to killing) significantly increased the rate of morphine glucuronidation over the range 0.05-10 mM, and significantly increased the apparent Vmax for the high capacity isoenzyme from 1.25 +/- 0.12 to 1.95 +/- 0.39 nmol/mg/min. In contrast, the activity of 1-naphthol UGT was not significantly altered by administration of CP. Rats treated with the same dose of CP 1 day prior to killing showed a significant decrease in microsomal morphine-UGT activity at 0.05 and 2.5 mM morphine, but a significant increase in activity was observed following administration of CP or Adriamycin (AD; 10 mg/kg) 4 days prior to killing. The extent of microsomal lipid peroxidation was significantly increased in microsomes obtained from rats treated with CP or AD 4 days prior to killing, and was positively correlated (P less than 0.001) with the rate of glucuronidation of 0.05 and 2.5 mM morphine. Preincubation of microsomes in the presence of CP (5 mM) and AD (100 microM) significantly decreased the rate of glucuronidation of 2.5 mM morphine. In vitro NADPH-mediated lipid peroxidation significantly increased the activity of both the high and low affinity morphine-UGT isoenzymes. Administration of the cytotoxic drugs CP and AD may alter microsomal morphine-UGT activity via the process of lipid peroxidation, although other mechanisms cannot be excluded.

Investigation of the mechanism by which cyclophosphamide alters cytochrome P450 in male rats

The effects of administration of the cytotoxic agent cyclophosphamide on cytochrome P450 have been examined in the liver microsomes of male rats. Microsomes were prepared after cyclophosphamide administration 1, 4 or 7 days prior to killing. The coadministration of cyclophosphamide with N-acetylcysteine has also been investigated. The microsomes were assayed for NADPH cytochrome P450 reductase, aminopyrine demethylase, erythromycin demethylase and androstenedione hydroxylase activities. Activities were generally unchanged 1 and 4 days after cyclophosphamide administration and were significantly decreased at 7 days. N-Acetylcysteine did not alter the effects of cyclophosphamide at 7 days. The effect of cyclophosphamide in vitro has also been examined. Microsomes from untreated animals were subjected to the above assays following in vitro metabolic activation of cyclophosphamide in a reconstituted system in the presence and absence of N-acetylcysteine. All enzyme activities were significantly reduced by the cyclophosphamide metabolites. The presence of N-acetylcysteine prevented this inactivation. The results of these investigations suggest that cyclophosphamide inactivates hepatic cytochrome P450 in vitro and in vivo via different mechanisms.