P-450 Mixed Function Oxidase System Research Articles

MODULATION OF CARCINOGEN-METABOLIZING ENZYME BY MADINAH MINT (Mentha spp) IN RAT LIVER.

Background:The present study was undertaken to assess whether boiling water mint extract (BWME) modulates the cytochrome P450 mixed function oxidase system.Materials and methods:Male albino rats were randomly divided into two groups, comprising 12 animals each. The first group served as control, whereas the second was maintained on BWME (10 % w/v) as its sole drinking liquid for six weeks. Liver microsomal were separated and subjected for phase I and II enzymes (cytochrome P450 mixed function oxidase) analysisResults:The results obtained showed that, BWME caused a significant elevation in the activity of epoxide hydrolase (p<0.001) when compared with the control. However, glutathione S-transferase and glucuronosyl transferase activities were significantly decreased (p<0.001 and p<0.01) respectively compared with control. The mutagenic activity of N-nitrosopiperidine was lower in the mint-treated hepatic microsomal compared with the controls.Conclusion:It can be concluded that BWME has the potential to suppress the activity of cytochrome enzymes involved in the bio-activation of chemical carcinogen; hence may display chemo preventive activity.

Measuring cytochrome P450 activity in aquatic invertebrates: a critical evaluation of in vitro and in vivo methods.

The first step in xenobiotic detoxification in aquatic invertebrates is mainly governed by the cytochrome P450 mixed function oxidase system. The ability to measure cytochrome P450 activity provides an important tool to understand macroinvertebrates' responses to chemical stressors. However, measurements of P450 activity in small aquatic invertebrates have had variable success and a well characterized assay is not yet available. The general lack of success has been scarcely investigated and it is therefore the focus of the present work. In particular, the suitability of the substrate selected for the assay, the sensitivity of the assay and the possible inhibition/attenuation of enzymatic activity caused by endogenous substances were investigated. 7-ethoxycoumarin-O-dealkylation activity of Daphnia magna, Chironomus riparius larvae and Hyalella azteca was assessed in vivo and in vitro and possible inhibition of enzymatic activity by macroinvertebrates homogenate was investigated. Activities of D. magna and C. riparius larvae measured in vivo were 1.37 ± 0.08 and 2.2 ± 0.2 pmol h(-1) organism(-1), respectively, while activity of H. azteca could not be detected. In vitro activity could be measured in C. riparius larvae only (500-1000 pmol h(-1) mg microsomal protein(-1)). The optimization of the in vitro assay has been especially long and resource consuming and particularly for D. magna, substances that inhibited cytochrome P450 activity seemed to be released during tissue homogenization preventing activity measurements in vitro. We therefore recommend testing the P450 inhibition potential of homogenate preparations prior to any investigation of P450 activity in vitro in macroinvertebrates.

Pharmacology Review

After completing this article, readers should be able to: 1. Review the actions of the diuretic agents commonly used in neonates. 2. Delineate the adverse effects of diuretic therapy. 3. Determine the roles of diuretics in therapy. Diuretics represent one of the most common classes of drugs administered to sick neonates and infants. Unwanted fluid retention arising from myriad neonatal disorders can and does complicate the optimal care of sick infants. To appreciate the many factors that influence a patient’s response to diuretic therapy and clinical and laboratory monitoring, we direct readers to the article “The Ontogeny of Human Kidney Development: Influence on Neonatal Diuretic Therapy” in this issue. All the clinically relevant diuretics, with the exception of aldosterone receptor antagonists (eg, spironolactone), must achieve sufficient free drug concentrations within the renal tubular lumen to reach their sites of action. Thus, diuretic effectiveness ultimately is guided by the functional capacity of the kidney, which clearly is influenced by infants’ gestational and postnatal ages. This article addresses the most common diuretics used in the neonate and young infant, focusing on the pharmacokinetic and pharmacodynamic characteristics of each drug relative to the infant’s postconceptional age and other factors that influence the infant’s overall response to therapy. ### “Loop” or “High Ceiling” Diuretics All diuretics, with the exception of spironolactone, must reach the renal tubular lumen to exert their pharmacologic effect. A lower renal blood flow (RBF) in preterm infants reduces the amount of drug delivered to the kidney, potentially limiting diuretic effectiveness. Highly protein-bound, loop diuretics (Table 1) are filtered minimally at the glomerulus. Instead, these agents are carried to the proximal tubule where they are actively secreted into the lumen via the organic acid transport pump. Luminal flow then carries the loop diuretics to their sites of action. | Loop Diuretics | || | •Bumetanide | | •Furosemide | | •Torsemide | | Thiazide Diuretics | | •Chlorothiazide | | •Chlorthalidone | | •Hydrochlorothiazide | | •Indapamide | | •Metolazone | | Aldosterone Antagonists | | •Eplerenone | | •Spironolactone | Table 1. Commonly Used Diuretic Drugs in Preterm and Newborn Infants Loop diuretics bind …

Important role of prodromal viral infections responsible for inhibition of xenobiotic metabolizing enzymes in the pathomechanism of idiopathic Reye's syndrome, Stevens-Johnson syndrome, autoimmune hepatitis, and hepatotoxicity of the therapeutic doses of acetaminophen used in genetically predisposed persons.

Upper respiratory tract febrile illnesses caused by various viruses, mycoplasma, chlamydia infections, and/or inflammatory diseases are usually observed a few days to a few (several) weeks before the onset of Reye's syndrome, Stevens-Johnson syndrome, autoimmune hepatitis (hepatotropic virus infections), or hepatotoxicity associated with therapeutic administration of acetaminophen in persons with varying degrees of deficits of important enzymatic activity. Activation of systemic host defense mechanisms by inflammatory component(s) results in depression of various induced and constitutive isoforms of cytochrome P-450 mixed-function oxidase system superfamily enzymes in the liver and most other tissues of the body. Because several cytochrome P-450 enzymes activities important for biotransformation of many endogenous and egzogenous substances show considerable variability between individuals, in some genetically predisposed persons, even the administration of therapeutic doses of a drug may result in serious clinical mishaps, if an important concomitant risk factor (eg, acute viral infection) is involved. Several inflammatory cytokines, such as interleukins, transforming growth factor beta1, human hepatocyte growth factor, and lymphotoxin, downregulate gene expression of major cytochrome P-450 enzymes with the specific effects on mRNA levels, protein expression, and enzyme activity observed with a given cytokine varying for each P-450 studied, thus eventually leading to metabolite-mediated adverse drug reactions and immunometallic diseases which sometimes result in tissue injury beyond the site(s) where metabolic bioactivation takes place. On the other hand, it must be emphasized that inhibition of metabolism of several drugs, as well as influence on the concentration and/or ratio of various cytokines in inflamed tissues, may exert beneficial effects in patients with different diseases, thus opening new therapeutic possibilities. Clinically relevant interactions may be exemplified by the effects of some fluoroquinolone antibiotics, such as pefloxacin and ciprofloxacin, which probably have a steroid-sparing effect in some patients with frequently relapsing nephrotic syndrome, and an increased bioavailability of several drugs following concomitant intake with freshly pressed grapefruit juice, eventually caused by inhibition of their metabolism, mediated mainly by CYP3A and specifically inhibited by naturally occurring flavonoids.

Association of Multiple Developmental Defects and Embryonic Lethality with Loss of Microsomal NADPH-Cytochrome P450 Oxidoreductase

The microsomal flavoprotein NADPH-cytochrome P450 oxidoreductase (CYPOR) is believed to function as the primary, if not sole, electron donor for the microsomal cytochrome P450 mixed-function oxidase system. Development of the mammalian embryo is dependent upon temporally and spatially regulated expression of signaling factors, many of which are synthesized and/or degraded via the cytochromes P450 and other pathways involving NADPH-cytochrome P450 oxidoreductase as the electron donor. Expression of CYPOR as early as the two-cell stage of embryonic development (The Institute for Genomic Research Mouse Gene Index, version 5.0, www.tigr.org/tdb/mgi) suggests that CYPOR is essential for normal cellular functions and/or early embryogenesis. Targeted deletion of the translation start site and membrane-binding domain of CYPOR abolished microsomal CYPOR expression and led to production of a truncated, 66-kDa protein localized to the cytoplasm. Although early embryogenesis was not affected, a variety of embryonic defects was observable by day 10.5 of gestation, leading to lethality by day 13.5. Furthermore, a deficiency of heterozygotes was observed in 2-week-old mice as well as late gestational age embryos, suggesting that loss of one CYPOR allele produced some embryonic lethality. CYPOR -/- embryos displayed a marked friability, consistent with defects in cell adhesion. Ninety percent of CYPOR -/- embryos isolated at days 10.5 or 11.5 of gestation could be classified as either Type I, characterized by grossly normal somite formation but having neural tube, cardiac, eye, and limb abnormalities, or Type II, characterized by a generalized retardation of development after approximately day 8.5 of gestation. No CYPOR -/- embryos were observed after day 13.5 of gestation. These studies demonstrate that loss of microsomal CYPOR does not block early embryonic development but is essential for progression past mid-gestation.

Caffeine Potentiation of Allyl Alcohol-Induced Hepatotoxicity. II. In Vitro Study*

We examined the effects of caffeine (C) on allyl alcohol (AA)- and acrolein (A)-induced hepatotoxicity on freshly-isolated, rat hepatocytes obtained from livers of adult, male, Sprague-Dawley rats. Isolated rat hepatocytes in suspension were incubated in each test with one of the following: 0, 1.0, or 2.5 mM of AA alone; or with 0, 2.5, or 5 mM of C alone; or a combination of AA and C at the same range of concentrations as used alone, for 15, 30, 60, 90, and 120 minutes at 37 degrees C. A dose- and time-dependent potentiation of cytotoxicity as measured by cellular viability (using trypan blue exclusion) were observed. The AA (2.5 mM)-induced lactate dehydrogenase (LDH) leakage observed after 60 minutes incubation was completely prevented when pretreated for 15 minutes with 4-methylpyrazole (MP) (0.5 mM). Such pretreatment, even with a double dose of 4-MP, only partially, and not significantly, prevented LDH leakage when the hepatocytes were incubated with a mixture of 2.5 mM AA and 5 mM C. The depletion of hepatocyte nonprotein sulfhydryl (NPSH) content caused by AA was further enhanced in the presence of C, as early as 15 minutes after their exposure. The AA-induced increase in lipid peroxidation was also potentiated by C; however, potentiation started later, and only after sufficient depletion of NPSH (mostly glutathione) occurred resulting from the presence of AA plus C. A significant loss of protein sulfhydryls in rat hepatocytes could be noted following a 60-minute incubation period with either AA (1 mM) or AA (1 mM) plus C (5 mM). Similarly, C produced a dose-and time-dependent potentiation of A-induced liver cytotoxicity, which was preceded by severe loss of NPSH content within 15 minutes of exposure, whereas the potentiation of lipid peroxidation (LPO) resulting from A plus C was found to be a relatively late event, as with AA plus C. Furthermore, combined treatment with AA and C produced a significantly higher cytotoxicity (as measured by cellular viability) than that due to the combined treatment with A plus C based on equimolar concentration. These results suggest that two increased bioactivation pathways of AA involving the P-450 mixed-function oxidase system resulting from C may be involved in the potentiation of AA hepatotoxicity.

Influence of Caffeine on Allyl Alcohol-Induced Hepatotoxicity in Rats* I. In Vivo Study

Cotreatment of rats with a low hepatotoxic dose (30.7 mg/kg, i.p.) of allyl alcohol (AA) and a higher, but nontoxic, dose (150 mg/kg, oral) of caffeine (CF) potentiated the hepatotoxicity of AA. This was verified by significantly higher levels of plasma alanine aminotransferase (ALT) activity and histopathologically greater severity of lesions in the periportal hepatocytes than those due to AA alone. Treatment of rats with 4-methylpyrazole (4-MP) (0.5 mmol/kg, i.p.) (an inhibitor liver alcohol dehydrogenase) for 30 minutes, followed by similar cotreatment with AA and CF, completely prevented the elevation of plasma levels of ALT and histological damage induced by cotreatment with CF and AA 24 hours following their administration. Severe liver damage induced by cotreatment with CF and AA was further, markedly enhanced by phenobarbital pretreatment (80 mg/kg, i.p., 3 days). Thus, extensive necrosis of periportal hepatocytes was noted, as well as edema and accumulation of inflammatory cells in the necrotic foci caused by such pretreatment. The depression of hepatic nonprotein sulfhydryls resulting from CF plus AA was much more severe than that caused by AA or CF alone and appeared as early as 30 minutes after administration. However, much less marked depletion of protein thiols was observed following similar treatments. Significant increase in lipid peroxidation (as measured by melondialdehyde [MDA] formation) was also observed in rat liver but only 24 hours after administration. The production ofMDA in the rat liver was significantly higher after administration of AA plus CF than after administration of AA alone. Pretreatment of rats with phenobarbital further significantly enhanced the formation of 2,4-dinitrophenylhydrazine (DNP)-reactive metabolite(s) (measured as DNP-acrolein adduct equivalents) in rat liver induced by AA (30.7 mg/kg) plus CF (150 mg/kg) within 1 hour following such treatment. Cotreatment with AA and a higher dose of CF resulted in significantly higher excretion of urinary thioethers or mercapturic acids than in rats treated with AA alone. Thus, these data suggest that an increased bioactivation pathway of acrolein involving a P450 mixed-function oxidase system caused by CF may be involved in such potentiating effects of CF on AA-induced hepatotoxicity in rats.

Cytochrome P-450 3A: Interactions with dermatologic therapies

Recent case reports and studies suggest that interactions involving the cytochrome P-450 mixed function oxidase system are important causes of medication toxicity and de-creased efficacy during combination drug therapy. The cytochrome P-450 3A3/4 isoenzyme is involved in many significant drug interactions. New and familiar drugs continue to be implicated as having potentially serious interactions with this group of enzymes. An understanding of the basic principles of these interactions may have a major impact on patient outcome. (J Am Acad Dermatol 1997;37:765-71.)

Barbiturate-induced expression of neuronal nitric oxide synthase in the rat cerebellum

Neuronal nitric oxide synthase (nNOS) is known to share some structural and functional similarities with the cytochrome P450 mixed function oxidase system. Unlike P450, it does not require a second enzyme, reductase, to transfer electrons. This characteristic is similar to P450 BM-3 of Bacillus megaterium. P450 BM-3 and certain mammalian subfamilies of P450, such as P4502B, are known to be induced by phenobarbital (PB), and these P450s share a consensus sequence called the Barbie box. Because of the similarities nNOS shares with P450 BM-3 and other mammalian P450s, we have examined whether nNOS also responds to PB treatment. We have used semi-quantitative PCR, Western blot analysis, a functional assay, and immunohistochemistry in order to answer this question. These data show a threefold increase in nNOS mRNA expression, more modest nNOS protein and activity induction, and no discernible changes in localization of nNOS within the cerebellum following PB treatment.

Treatment of Severe Acetaminophen–Induced Hepatocellular Injury with Prostaglandin E: Two Case Reports

Drugs have long been recognized as culprits in hepatocellular injury. Acetaminophen is one example of a commonly used over-the-counter medication that can cause severe centrolobular hepatic necrosis when ingested in large quantities in suicide attempts or unintentional overdoses. Acetaminophen hepatotoxicity is mediated by a toxic reactive metabolite formed from the parent compound by the cytochrome P450 mixed-function oxidase system of the hepatocyte. Conventional treatment of acetaminophen-induced liver injury consists of supportive measures and prevention of further drug absorption. In addition, in patients with high serum acetaminophen concentrations, the severity of hepatic necrosis appears to diminish with the timely use of sulfhydryl compounds such as N-acetyl cysteine. Two patients in whom acetaminophen hepatotoxicity was successfully treated using prostaglandin E1are described.

Nitric oxide is a mediator of the decrease in cytochrome P450-dependent metabolism caused by immunostimulants.

Bacterial lipopolysaccharide (LPS) and a diverse array of other immunostimulants and cytokines suppress the metabolism of endogenous and exogenous substances by reducing activity of the hepatic cytochrome P450 mixed-function oxidase system. Although this effect of immunostimulants was first described almost 40 yr ago, the mechanism is obscure. Immunostimulants are now known to cause NO overproduction by cells via induction of nitric oxide synthase. We have investigated whether NO overproduction is involved in suppressing hepatic metabolism by LPS. In vitro treatment of hepatic microsomes with NO, produced by chemical decomposition of 3-morpholinosydnonimine or by nitric oxide synthase, substantially suppressed cytochrome P450-dependent oxygenation reactions. This effect of NO was seen with hepatic microsomes prepared from two species (rat and chicken) and after exposure to chemicals that induce distinct molecular isoforms of cytochromes P450 (beta-naphthoflavone, 3-methylcholanthrene, and phenobarbital). Spectral studies indicate that NO reacts in vitro with both Fe(2+)- and Fe(3+)-hemes in microsomal cytochromes P450. In vivo, LPS diminished the phenobarbital-induced dealkylation of 7-pentoxyresorufin by rat liver microsomes and reduced the apparent P450 content as measured by CO binding. These LPS effects were associated with induction of NO synthesis; LPS-induced NO synthesis showed a strong positive correlation with the severity of cytochrome P450 inhibition. The decrease in both hepatic microsomal P450 activity and CO binding caused by LPS was largely prevented by the selective NO synthase inhibitor N omega-nitro-L-arginine methyl ester. Our findings implicate NO over-production as a major factor mediating the suppression of hepatic metabolism by immunostimulants such as LPS.

Effect of protein and calorie restriction on the cytochrome P-450 isozyme (P-450 IID6) activity in rats

Dextromethorphan (DXM) is commonly used as probe for the metabolic activity of the debrisoquine isozyme (P-450 IID6) of the cytochrome P-450 mixed-function oxidase system. Several reports have described changes in P-450 activity with dietary alteration. This study was conducted to determine the effect of protein and calorie restriction on DXM metabolism in Sprague Dawley (SD) rats. Four- to five-week-old male SD rats were fed one of four dietary regimens: a regular diet containing 22.5% protein ad libitum (REGLIB); a low protein (8%) diet fed ad libitum (PROLIB); a regular protein diet, but only 50% by weight of that consumed by the REGLIB group (REG50); a low protein (8%) diet, but only 50% by weight of that consumed by the PROLIB group (PRO50). The DXM log m molar metabolic ratio (LMR) calculated as the ratio of the urinary DXM to its metabolite dextrorphan concentration was measured at the beginning of the study and after 5 weeks of feeding. Groups REG50, PROLIB, and PRO50 received 49%, 57%, and 29% of the calorie intake and 49%, 24%, and 12% of the protein intake of group REGLIB, respectively. Their weight gain was 24%, 40%, and 3% of that gained by REGLIB. LMR significantly decreased for REGLIB and PROLIB indicating an increase in DXM metabolism. The LMR of REG50 and PRO50 did not change. DXM metabolism continues to develop in these rats during the first 7 to 8 weeks of life. Energy restriction inhibited the maturation of DXM metabolism, while protein restriction did not significantly affect DXM metabolism.

Critical Ketoconazole Dosage Range for Ciclosporin Clearance Inhibition in the Dog

Ciclosporin (CsA) is metabolized exclusively by the hepatic cytochrome P-450 mixed function oxidase system. Ketoconazole (KC) is a potent inhibitor of this enzyme system. CsA was administered alone and in combination with five different doses of KC (1.25, 2.5, 5.0, 10.0, 20.0 mg/kg/day) under steady-state conditions to 7 adult mongrel dogs. KC produced a highly significant (p = 0.0001), dose-dependent decrease in CsA total body clearance [Cl(T)]. The critical KC dosage range for this to occur was found to be between 2.5 and 10 mg/kg/day. The reduction of CsA CL(T) was insignificant (p greater than 0.05) at a KC dose of less than 2.5 mg/kg/day, and the 92% reduction observed using 20 mg/kg/day KC was not significantly greater than the 85% reduction occurring after only 10 mg/kg/day KC (p greater than 0.05). The dose of concomitant KC was also highly correlated with a reduction in the whole blood CsA parent/parent + metabolite ratio as determined using high-performance liquid chromatography and polyclonal fluorescent polarization immunoassay for CsA measurement (r = 0.998, p less than 0.0001). The absolute oral bioavailability of CsA as well as the time required to reach its maximum concentration in the blood following oral administration did not change significantly over the course of the study (p greater than 0.05). We conclude from these new observations that the KC-induced decrease in CsA Cl(T) in the dog in vivo is dose-dependent and maximized within the KC dosage range of 2.5-10 mg/kg/day. The effect does not appear to involve a decrease in the rate of CsA oral absorption, and may be compensated for by an appropriate reduction in the concomitantly administered dose of CsA.

Comparison of the Parenteral Histamine2-Receptor Antagonists

The chemical structure, pharmacokinetic properties, and drug-drug interaction profiles of the parenterally available histamine2 (H2)-receptor antagonists were compared. Famotidine is a guanidinothiazole derivative, ranitidine contains an aminomethylfuran ring, and cimetidine has an imidazole ring. Data from the literature indicate that because of its chemical structure famotidine has a much greater potency and affinity for the H2-receptor and a notable lack of drug-drug interactions when compared with ranitidine and cimetidine. As a result, famotidine should be considered the H2-receptor antagonist of choice for critically ill patients who require gastric-acid suppression and at the same time are being treated with other drugs that depend on the cytochrome P-450 mixed-function oxidase system for their metabolism and/or on renal tubular mechanisms for their excretion.

Chlorophenoxyacid herbicides induce microsomal cytochrome P-450 IVA1 ( P-452) in rat liver

Induction of the cytochrome P-450 mixed-function oxidase and specifically the cytochrome P-450 IVA1 isoenzyme by seven phenoxyacid herbicides in rat liver have been studied. Liver microsomes from rats orally treated with the herbicides at 3 dose levels showed a significant increase in total cytochrome P-450 content with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) being the most potent inducer. Benzphetamine N-demethylation, as a marker of cytochrome P-450 b ( P-450 IIB1) activity, was not affected by any of the herbicides studied, whereas cytochrome P-450 c ( P-450 IA1), as assayed by ethoxyresorufin- O-deethylation activity, was significantly increased (up to 2.2-fold) by some of the herbicides. The 12-hydroxylation of lauric acid was significantly induced (3–8-fold) by all seven herbicides, 4-chlorophenoxyacetic acid (CPA) and 2,4,5-trichlorophenoxypropionic acid (2,4,5-TP) being the weakest and most potent inducers respectively. This increase in lauric acid 12-hydroxylase activity was accompanied by an increase in the hepatic content of cytochrome P-450 IVA1 as assessed by both a qualitative Western blot procedure and a quantitative ELISA method. By comparison, lauric acid 11-hydroxylase activity was not or only marginally increased by phenoxyacid herbicide pretreatment. Our results suggest that the phenoxyacid herbicides act as mixed inducers of the cytochrome P-450 mixed function oxidase system, preferentially inducing the cytochrome P-450 IVA1 isoenzyme.

EXACERBATION OF CYCLOSPORINE TOXICITY BY CONCOMITANT ADMINISTRATION OF ERYTHROMYCIN

Cyclosporine (CsA), an immunosuppressive drug widely used in clinical organ transplantation, causes a variety of side effects, including parenchymal complications of nephrotoxicity and hepatotoxicity. Erythromycin ethinylsuccinate (EES), a macrolide antibiotic frequently administered to transplant patients afflicted with pneumonias caused by Mycoplasma pneumoniae and Legionella pneumophila, markedly potentiated parenchymal drug toxicity in nine (three renal and six cardiac) CsA-treated allograft recipients. The mean and median blood urea nitrogen (BUN), creatinine, and total bilirubin increased upon initiation of EES treatment: in the renal recipients from 27, 1.7, and 0.5 mg/dl, respectively, before, to a mean and median of 81/101, 8.3/3.9, and 2.1/1.2 mg/dl during, and to 72/22, 1.9/1.7, and 0.6/0.5 mg/dl after cessation of EES treatment. The median serum radioimmunoassay (RIA)-determined CsA trough value of 147 ng/ml prior, rose to a zenith of 1125 ng/ml during, EES therapy. In the six cardiac recipients, the mean and median BUN, creatinine, and total bilirubin of 51/45, 1.5/1.3, 1.2/1.3 mg/dl, respectively, before, rose to 100/91, 3.7/3.6, and 2.3/2.1 mg/dl during, and fell to 49/44, 1.8/2.1, and 1.0/0.8 mg/dl after, cessation of EES. The mean serum CsA trough value of 185 ng/ml rose to 815 ng/ml during EES administration. Since EES and CsA are both metabolized by the hepatic cytochrome P450 mixed-function oxidase system, simultaneous use of these two drugs may decrease CsA metabolism, with consequent elevation of blood levels and induction of CsA toxicity. Therefore, blood level monitoring and careful regulation of CsA dose are necessary, in order to achieve the safe use of EES in transplant recipients.

Effects of acetaminophen on cadmium metabolism in mice

Acetaminophen (ACM) administration to mice of the ( C57BL 6 × DBA 2 )F 1 strain produced a typical hepatic centrilobular necrosis similar to that observed in rodents and humans. To determine the effects of this drug-induced necrosis on cadmium (Cd) metabolism, mice were given a sublethal dose of CdCl 2 · 2.5 H 2O containing 109CdCl 2 and maintained for a period of time sufficient for Cd-metallothionein (Cd-MT) to be synthesized and distributed. Subsequent administration of ACM ip or po evoked a marked redistribution of Cd from livers to kidneys of mice, and increased the amount of Cd excreted in urine and feces. There were only minimal or no effects on Cd concentrations in other organs assessed. The effect of ACM on Cd redistribution was antagonized by administration of cysteine, a glutathione precursor, and was enhanced by pretreatment with phenobarbital, a potent inducer of the cytochrome P-450 mixed-function oxidase system. Pretreatment of mice with ACM 6 or 24 hr prior to Cd administration caused aberrations of the normal Cd distribution pattern, but no effect was noted when Cd administration was delayed for 48 hr after ACM injection, indicating recovery of the mechanisms of Cd-MT synthesis and sequestration. Sephadex G-75 gel filtration chromatography of serum from ACM-treated mice showed that most of the Cd was associated with high-molecular-weight proteins, and only a minor portion was present as Cd-MT. Cd excreted in urine was predominantly in a lowmolecular-weight form, but there was evidence of two minor components of higher molecular weight, neither of which eluted as Cd-MT. Cd excreted in feces was insoluble following homogenization in 0.25 m sucrose solution. Cd in livers and kidneys of ACM-treated mice eluted as Cd-MT. It was concluded that persons who have a moderately high Cd burden may be at risk of Cd nephrotoxicity if they incur hepatic necrosis subsequent to ACM abuse.

Potentiation of the toxic effects of acetaminophen in mice by concurrent infection with influenza B virus: a possible mechanism for human Reye's syndrome?

Using weanling mice of two different genetic strains we demonstrated a potentiation of the toxic effects of acetaminophen by prior infection with influenza B virus. The C57BL/6N (B6) strain of mice is genetically predisposed to increased toxicity from acetaminophen when the hepatic cytochrome P-450 mixed function oxidase system is preinduced. When B6 animals are pretreated with influenza B virus and an mixed function oxidase system inducing agent before administering acetaminophen, we observed a significant incidence of atypical "fatty" liver pathology on light microscopy similar to the microvesicular steatosis seen in human Reye's syndrome. Electron microscopic changes in the liver of these animals resemble those published to date in human Reye's syndrome.

The generation of oxidation products of benzo[a]pyrene by lipid peroxidation: a study using gamma-irradiation.

The role which active oxygen and radicals generated by the peroxidation of unsaturated fatty acids could play in the oxidation of benzo[a]pyrene has been studied using gamma-irradiation. Irradiation of benzo[a]pyrene resulted in the formation of benzo[a]pyrene 1,6-, 3,6- and 6,12-quinones and other more polar products which were analysed by h.p.l.c. OH. radicals are believed to be involved in this oxidation. The presence of polyunsaturated fatty acids and polyunsaturated lipids stimulated the formation of benzo[a]pyrene products following gamma-irradiation. Oxidation of benzo[a]pyrene also occurred over a period of days in the presence of autoxidising mackerel oil. The rate of benzo[a]pyrene oxidation was related to the extent of lipid peroxidation as determined by malonaldehyde formation. Malonaldehyde production as a result of peroxidising lipids was inhibited by benzo[a]pyrene which suggested that benzo[a]pyrene reacted directly with lipid peroxy radicals or hydroperoxides generated in the process of lipid peroxidation. These results demonstrate that oxidation products of the peroxidation of lipids and fatty acids are able to react directly with benzo[a]pyrene to form products including benzo[a]pyrene quinones without the presence of enzymes such as the cytochrome P-450 mixed function oxidase system and prostaglandin synthetase. It is possible that benzo[a]pyrene may be activated by these types of reactions in vivo or in vitro when benzo[a]pyrene is in contact with polyunsaturated lipids in foodstuffs or the intestinal lumen and peroxidation of unsaturated fats may play an important role in human carcinogenesis.

Cimetidine and acetylcysteine as antidote for acetaminophen overdose.

The incidence of acetaminophen overdose, often resulting in hepatotoxicity or even death, steadily increases as its use (instead of aspirin) as an antipyretic and analgesic increases. Acetaminophen-induced hepatotoxicity is though to be dependent on the dose ingested and the activity of the cytochrome P450 enzyme system. The actual hepatotoxic mechanism is believed to be the formation of a toxic tissue arylating metabolite(s). Current therapy attempts to inactivate this toxic metabolite(s) with acetylcysteine, as opposed to inhibiting its production. Because cimetidine can inhibit the cytochrome P450 mixed-function oxidase system, we believe it could decrease the production of the hepatotoxic acetaminophen metabolite(s) providing an effective antidote to be used in conjunction with acetylcysteine.