Vitamin E and drug metabolism

Tocopherols and tocotrienols are metabolized by side chain degradation initiated by cytochrome P450 (CYP)-catalyzed omega-hydroxylation followed by beta-oxidation. Whereas alpha-tocopherol is only poorly metabolized, high amounts of the final products, carboxyethyl hydroxychroman (CEHC), are found from other tocols in HepG2 cells and in human urine. CYP3A4 and CYP4F2 were suggested to be involved in tocopherol degradation. CYP3A4 metabolizes most of the drugs and is induced by many of its substrates via the activation of the pregnane X receptor (PXR). Also tocopherols and in particular tocotrienols induce the expression of a PXR-driven reporter gene and the expression of endogenous CYP3A4 and CYP3A5 which is supported by sporadic publications spread over the last 30 years. The potential interference of vitamin E with drug metabolism is discussed in the light of related complications evoked by herbal remedies.     

Curcumin modulates drug metabolizing enzymes in the female Swiss Webster mouse

Curcumin, the yellow pigment found in turmeric, exhibits potent chemopreventative properties in both in vivo and in vitro cancer models. We hypothesized that this effect may occur via curcumin-mediated changes in enzymes involved in both carcinogen bioactivation and estrogen metabolism. Female Swiss Webster mice were treated with either curcumin (200 mg/kg or 400 mg/kg, p.o.) or vehicle control for 1 or 2 weeks. The results demonstrated that curcumin had no effect on the catalytic activities of ovarian aromatase, hepatic catechol-O-methyltransferase or hepatic UDP-glucuronosyltransferase. However, both doses of curcumin caused a 25% decrease in CYP1A catalytic activity, but not polypeptide levels, following 2 weeks of treatment. Additionally, following 2 weeks of curcumin at 400 mg/kg, there was a 20% decrease in the catalytic activity and a 28% decrease in polypeptide levels of CYP3A. While 2 weeks of curcumin treatment (400 mg/kg) caused a 20% increase in glutathione S-transferase activity, there was no parallel increase in hepatic stores of the co-factor glutathione. In conclusion small changes in CYP1A, CYP3A and GST following long term treatment (2 weeks) suggest that the combination of all three metabolic pathways may play a small role in curcumin's chemopreventative action.     

Biological functions and metabolic fate of vitamin E revisited

Information accumulated lately has confirmed the essentiality of vitamin E for humans and provided a better understanding of its biological function and metabolic fate. The discovery of -tocopherol transfer protein, which preferentially binds to RRR--tocopherol, not only provides conclusive evidence of the essentiality of vitamin E for humans, but also sheds light on the superiority of RRR--tocopherol biologically over other isomers. The presence of tocopherol regeneration systems and multiple interdependent antioxidant systems is largely responsible for the lack of a widespread deficiency in humans and the difficulty to deplete vitamin E in the adult. The bulk of excess tocopherols consumed is excreted to feces unchanged or to urine with the side chain shortened but the chroman ring intact. The ability of dietary vitamin E to mediate mitochondrial superoxide generation affords a possible mode of action of vitamin E at the tissue levels. By decreasing the generation and/or the levels of reactive oxygen/nitrogen species, dietary vitamin E not only protects against oxidative damage, but also modulates the expression and/or activation of redox-sensitive biological response modifiers that regulate important cellular events.     

Membrane permeability changes with vitamin A/vitamin E mixed bilayers

Vitamin A (all trans-retinol) enhances the permeability of egg phosphatidylcholine liposomes to glucose, urea, and erythritol while vitamin E (α-tocopherol) decreases permeability to the same solutes. Egg phosphatidylcholine bilayers containing both vitamin A and vitamin E are shown to have an altered permeability more similar to that affected by vitamin E alone. The membrane stabilizing effect of vitamin E appears dominant over the membrane destabilizing effect of vitamin A.     

Induction of rat hepatic drug metabolizing enzymes by dimethylcyclosiloxanes

Low molecular weight dimethylcyclosiloxanes (DMCS) are important precursors in the synthesis of polydimethysiloxane polymers widely used in industry, and in medical and personal care products. The objective of this study was to characterize the ability of two DMCS, octamethylcyclosiloxane (D4) and decamethylcyclopentasiloxane (D5) to induce drug metabolizing enzymes in rats. Male and female Sprague-Dawley rats were administered 1, 5, 20, or 100 mg/kg D4 or D5 in corn oil daily by gavage for 4 days. Changes in the levels of activity and/or immunoreactivity of CYP1A1/2, CYP2B1/2, CYP3A1/2 and NADPH cytochrome P450 reductase in liver microsomes were examined. Significant increases were observed in the liver to body weight ratio in female rats administered either D4 or D5 at doses > or = 20 mg/kg. Increases in the liver to body weight ratio were observed in male rats treated with > or = 100 mg/kg D5 but not with D4. Relatively large increases in CYP2B1/2 enzymatic activity and immunoreactive protein were observed with increasing concentrations of both D4 and D5. Significant increases in 7-pentoxyresorufin O-depentylase (PROD) activity were also detected in male and female rats given D4 at doses > or = 5 mg/kg. D5 increased PROD activity in male rats at doses > or = 20 mg/kg and in female rats at doses > or = 5 mg/kg. 7-Ethoxyresorufin O-deethylase (EROD) activity was increased in both male and female rats receiving > or = 20 mg/kg D4 or > or = 5 mg/kg D5; however, no changes were detected in CYP1A1/2 immunoreactive protein in rats of either sex. D4 and D5 caused significant increases in CYP3A1/2 immunoreactive protein in only male rats treated with 100 mg/kg of either compound. However, significant increases were detected in CYP3A1/2 immunoreactive protein in female rats at D4 doses > or = 20 mg/kg and D5 doses > or = 5 mg/kg. Induction of NADPH cytochrome P-450 reductase immunoreactive protein was observed with D4 in female rats and in both male and female rats with D5. Induction of CYP2B/1/2, CYP3A1/2 and NADPH cytochrome P450 reductase was observed in rats treated with 50 mg/kg phenobarbital by intraperitoneal injection. Maximal CYP2B induction detected with D4 was approximately 50% of the increase observed with phenobarbital. In summary, D4 and D5 induced CYP2B1/2 in adult rat liver in a manner similar to that observed with phenobarbital; however, differences were observed between D4 and D5 in their ability to induce CYP3A1/2 and NADPH cytochrome P450 reductase. Female rats were more sensitive to the inductive properties of low doses of both DMCS than male rats whereas male rats were more responsive to phenobarbital induction.     

Induction of hepatic drug metabolizing enzymes by DL-methionine in rats

A single intraperitoneal injection of DL-methionine (500 mg/kg body wt.) to adult male Wistar rats was shown to significantly induce all the components of the hepatic microsomal mixed function oxidase system such as NADPH cytochrome C reductase activity, cytochromes P-450 and b₅, as well as activities of drug metabolizing enzymes such as aminopyrine demethylase and uridine 5′ -diphosphate-glucuronosyltransferase. Combined administration of nicotinamide (250 mg/kg body wt.) and DL-methionine (500 mg/kg body wt.) was shown to bring about an additional increase (25-30%) in the activities of these enzymes as compared to their induction on independent administration of the two endobiotics. In rats bearing Yoshida sarcoma (ascites) tumour as well as in normal rats injected with serum from tumour bearing animals, the decreased activities of hepatic mixed function oxidases could be restored to their normal levels by administration of DL-methionine (500 mg/kg body wt.) to these rats. Whereas actinomycin D (1 mg/kg body wt.) had no effect on the increased incorporation of [¹⁴C] labelled leucine into microsomal proteins following administration of nicotinamide, the enhanced incorporation of the label following DL-methionine administration was completely inhibited by the same dose of actinomycin D. Administration of cycloheximide (0·5 mg/kg body wt.) to rats could completely inhibit the increased incorporation of [¹⁴C] leucine into hepatic microsomal proteins following independent administration of nicotinamide and DL-methionine. Similar inhibitory pattern with actinomycin D and cycloheximide was also demonstrated in case of induction of NADPH cytochromeC reductase activity by both these endobiotics.     

Bioengineering anabolic vitamin D-25-hydroxylase activity into the human vitamin D catabolic enzyme, cytochrome P450 CYP24A1, by a V391L mutation

CYP24A1 is a mitochondrial cytochrome P450 (CYP) that catabolizes 1α,25-dihydroxyvitamin D(3) (1α,25-(OH)(2)D(3)) to different products: calcitroic acid or 1α,25-(OH)(2)D(3)-26,23-lactone via multistep pathways commencing with C24 and C23 hydroxylation, respectively. Despite the ability of CYP24A1 to catabolize a wide range of 25-hydroxylated analogs including 25-hydroxyvitamin D(3), the enzyme is unable to metabolize the synthetic prodrug, 1α-hydroxyvitamin D(3) (1α-OH-D(3)), presumably because it lacks a C25-hydroxyl. In the current study we show that a single V391L amino acid substitution in the β3a-strand of human CYP24A1 converts this enzyme from a catabolic 1α,25-(OH)(2)D(3)-24-hydroxylase into an anabolic 1α-OH-D(3)-25-hydroxylase, thereby forming the hormone, 1α,25-(OH)(2)D(3). Furthermore, because the mutant enzyme retains its basal ability to catabolize 1α,25-(OH)(2)D(3) via C24 hydroxylation, it can also make calcitroic acid. Previous work has shown that an A326G mutation is responsible for the regioselectivity differences observed between human (primarily C24-hydroxylating) and opossum (C23-hydroxylating) CYP24A1. When the V391L and A326G mutations were combined (V391L/A326G), the mutant enzyme continued to form 1α,25-(OH)(2)D(3) from 1α-OH-D(3), but this initial product was diverted via the C23 hydroxylation pathway into the 26,23-lactone. The relative position of Val-391 in the β3a-strand of a homology model and the crystal structure of rat CYP24A1 is consistent with hydrophobic contact of Val-391 and the substrate side chain near C21. We interpret that the substrate specificity of V391L-modified human CYP24A1 toward 1α-OH-D(3) is enabled by an altered contact with the substrate side chain that optimally positions C25 of the 1α-OH-D(3) above the heme for hydroxylation.     

Adverse effects of vitamin E by induction of drug metabolism

Observational studies with healthy persons demonstrated an inverse association of vitamin E with the risk of coronary heart disease or cancer, the outcome of large-scale clinical trials conducted to prove a benefit of vitamin E in the recurrence and/or progression of such disease, however, was disappointing. Vitamin E did not provide benefits to patients with cardiovascular diseases, cancer, diabetes or hypertension. Even harmful events and worsening of pre-existing diseases were reported, which are hard to explain. Since vitamin E is metabolized along the same routes as xenobiotics and induces drug-metabolizing enzymes in rodents, it is hypothesized that a supplementation with high dosages of vitamin E may also lead to an induction of the drug-metabolizing system in patients that depend on drug therapy. Compromising essential therapy might therefore outweigh any benefit of vitamin E in patients. It is recommended to work out at which threshold the drug-metabolizing system can be induced in humans before new trials with high dosages of vitamin E are started.     

Kinetics of the reaction by which natural vitamin E is regenerated by vitamin C

The rate constant and activation energy of the regeneration reaction of natural vitamin E by vitamin C were determined with a double-mixing stopped-flow spectrophotometer. The formation of vitamin C radical was observed in the absorption spectrum. The kinetic effect of methyl substitution on the aromatic ring of vitamin E radical indicates that partial charge-transfer plays a role in the reaction. Since a substantial deuterium kinetic isotope effect was not found, the tunneling effect may not play an important role under the present experimental conditions.     

Expression and induction of CYP3As in human fetal hepatocytes

CYP3A4 and CYP3A7 mRNA expression levels were markedly up-regulated by dexamethasone (DEX), but not by rifampicin (RIF). CYP3A5 mRNA level was not increased significantly by DEX, RIF, or phenobarbital. Testosterone 6beta-hydroxylase activity was induced to about 2-fold of control by DEX. However, concomitant treatment with RIF did not alter DEX-mediated induction of CYP3A mRNA expression and testosterone 6beta-hydroxylase activity. DEX-mediated induction of CYP3A mRNA was suppressed in a dose-dependent manner by RU486, a glucocorticoid receptor (GR) antagonist. At 5microM RU486, DEX-mediated induction of CYP3A4, CYP3A5, and CYP3A7 mRNA expression was inhibited almost completely. These results suggest that, in human fetal hepatocytes, PXR is not involved in DEX-mediated induction of CYP3A4 and CYP3A7, and that the induction is mediated directly by GR.     

Curcumin and its derivatives inhibit 2,3,7,8,-tetrachloro-dibenzo-p-dioxin-induced expression of drug metabolizing enzymes through aryl hydrocarbon receptor-mediated pathway

Abstract

Certain dioxins, including 2,3,7,8,-tetrachloro-dibenzo-p-dioxin (TCDD), are exogenous ligands for an aryl hydrocarbon receptor (AhR) and induces various drug-metabolizing enzymes. In this study, we examined the effect of curcumin on expression of drug-metabolizing enzymes through the AhR and NF-E2 related factor 2 (Nrf2) pathways. Curcumin dose-dependently inhibited TCDD-induced expression of phase I enzyme cytochrome P450 1A1 (CYP1A1) and phase II enzymes NAD(P)H:quinone oxidoreductase-1 (NQO1) and heme oxygenase 1 (HO-1) but not tert-butyl hydroquinone-induced NQO1 and HO-1, suggesting that curcumin inhibited only AhR pathway, but not Nrf2 one directly. Furthermore, we used 14 curcumin derivatives and obtained the correlation between hydrophobicity of the compounds and suppressive effect against AhR transformation. Results from the quantitative structure active correlative analysis indicated that methoxy groups and β-diketone structure possessing keto-enol tautomerism in curcumin were necessary to inhibit AhR transformation, and the addition of methyl and methoxy group(s) to the curcumin increased the inhibition effect.     

Influence of high vitamin E dosages on retinol and carotinoid concentration in body tissues and eggs of laying hens

The aim of the study was to contribute to the discussion of overdosing vitamin E in laying hens. A total of 45 laying hens, divided into 5 groups were fed diets supplemented with either 0; 100; 1000; 10 000 or 20 000 mg dl‐α‐tocopheryl acetate/kg diet over a period of 10 weeks. Concentrations of vitamins A and E were measured in plasma, various tissues and egg yolk. Furthermore egg yolk colour and some carotinoids were measured in egg yolks. None of the vitamin E doses significantly influenced performance of the hens. As expected, vitamin E concentration in plasma, all tissue samples and egg yolk was significantly increased with increasing tocopherol content in the diet. The egg yolk showed the highest vitamin E concentration, followed by liver and muscles. Feeding 1000 mg α‐tocopheryl acetate per kg diet resulted in an increase of vitamin A concentration in the liver. Very high doses (10 000 and 20 000 mg/kg diet) significantly decreased retinol concentration in the liver and egg yolk, as well as carotinoid concentration in the egg yolk. The lower carotinoid concentration in egg yolk resulted in a decreased intensity of egg yolk colour. A prooxidative and/or competitive effect of very high doses of vitamin E with other fat soluble substances has been discussed.     

Induction of drug metabolizing enzymes in the liver of diabetic mice

The effects of two classical inducers, phenobarbital and 3-methylcholanthrene, have been tested on some liver microsomal drug-metabolizing enzymes (monooxygenases and phase II enzymes) and on benzo(a)pyrene metabolism in genetically (ob/ob) and chemically (streptozotocin) diabetic mice. 1) In ob/ob mice, the basal activities and the inducibility of phase I and phase II enzymes, as well as the electrophoretic pattern of microsomal proteins, were not notably different from those of similarly treated lean mice. 2) A possibly common form of cytochrome P 450 present both in microsomes from steptozotocin-diabetic non-induced mice and in those from phenobarbital-treated non-diabetic mice could explain the increased "phenobarbital-like" enzyme activities in chemically diabetic animals. 3) The increase of monooxygenase activities produced by streptozotocin treatment is partially depressed by 3-methylcholanthrene, probably as a result of the dilution of "phenobarbital-like" cytochrome P 450 forms by 3-methylcholanthrene-induced cytochrome P 448. 4) The increased formation of the most carcinogenic metabolites of benzo(a)pyrene, and the slight decrease of phase II conjugation enzyme activities, may add their deleterious effects in 3-methylcholanthrene-induced streptozotocin-diabetic animals.     

Progress of vitamin E metabolic engineering in plants

Vitamin E is important for human and animal health. Many human diseases, such as certain cancers and neurodegenerative and cardiovascular disease, are associated with the insufficient intake of vitamin E. The daily requirements for vitamin E in men and women have been increased to 15–30 mg. Because the primary source of dietary vitamin E comes from plants, there is a need to increase vitamin E production through plant engineering in order to meet the demand in human consumption. Numerous studies have been carried out in this field, leading to many successful examples. In this review, we summarized the recent progress in vitamin E metabolic engineering in plants aimed at improving the vitamin E content and regulating composition of vitamin E.     

Modulation of signal transduction by vitamin E

The ability of vitamin E to modulate signal transduction and gene expression has been observed in numerous studies; however, the detailed molecular mechanisms involved are often not clear. The eight natural vitamin E analogues and synthetic derivatives affect signal transduction with different potency, possibly reflecting their different ability to interact with specific proteins. Vitamin E modulates the activity of several enzymes involved in signal transduction, such as protein kinase C, protein kinase B, protein tyrosine kinases, 5-, 12-, and 15-lipoxygenases, cyclooxygenase-2, phospholipase A2, protein phosphatase 2A, protein tyrosine phosphatase, and diacylglycerol kinase. Activation of some these enzymes after stimulation of cell surface receptors with growth factors or cytokines can be normalized by vitamin E. At the molecular level, the translocation of several of these enzymes to the plasma membrane is affected by vitamin E, suggesting that the modulation of protein-membrane interactions may be a common theme for vitamin E action. In this review the main effects of vitamin E on enzymes involved in signal transduction are summarized and the possible mechanisms leading to enzyme modulation evaluated. The elucidation of the molecular and cellular events affected by vitamin E could reveal novel strategies and molecular targets for developing similarly acting compounds.     

维生素E拮抗急性二(噁)英对小鼠肝脏细胞色素P450酶和抗氧化物酶基因转录的影响

为了探讨维生素E对急性2,3,7,8-四氯代二苯并二噁英（TCDD）染毒小鼠肝脏细胞色素P450酶（CYP450）基因和抗氧化物酶基因转录的影响,我们设计了对照组、TCDD（30μg／kg）染毒组和染毒同时给予100mg／kg维生素E组（实验组）3组实验。用Real—Time PCR法检测小鼠细胞色素P450酶1A1、1A2、181、超氧化物歧化酶（SOD）、过氧化氢酶（CAT）、谷胱甘肽过氧化物酶（GSH-Px）基因表达的变化。结果显示：1）急性TCDD染毒小鼠肝脏细胞色素P450酶1A1、1A2、181mRNA水平与对照组相比明显升高,100mg／kg的维生素E具有部分拮抗肝脏细胞色素P450酶1A1、1A2、181 mRNA水平升高的作用;2）急性TCDD染毒小鼠肝脏SOD和GSH-PX mRNA水平呈现降低的趋势,CAT的mRNA水平明显降低,染毒并给予100mg／kg维生素E,SOD和CATmRNA水平与染毒组相比明显升高,GSH．PxmRNA水平也有升高的趋势。上述结果表明：TCDD能影响或部分影响小鼠肝脏细胞色素P450酶1A1、1A2、181、SOD、CAT、GSH-Px的基因转录,而维生素E能在一定程度上缓解TCDD的作用[动物学报54（6）：1038—1043,2008]。     

Exploring the topology of the Gid complex, the E3 ubiquitin ligase involved in catabolite-induced degradation of gluconeogenic enzymes

In the yeast Saccharomyces cerevisiae, key regulatory enzymes of gluconeogenesis such as fructose-1,6-bisphosphatase are degraded via the ubiquitin proteasome system when cells are replenished with glucose. Polyubiquitination is carried out by the Gid complex, a multisubunit ubiquitin ligase that consists of seven different Gid (glucose-induced degradation-deficient) proteins. Under gluconeogenic conditions the E3 ligase is composed of six subunits (Gid1/Vid30, Gid2/Rmd5, Gid5/Vid28, Gid7, Gid8, and Gid9/Fyv10). Upon the addition of glucose the regulatory subunit Gid4/Vid24 appears, binds to the Gid complex, and triggers ubiquitination of fructose-1,6-bisphosphatase. All seven proteins are essential for this process; however, nothing is known about the arrangement of the subunits in the complex. Interestingly, each Gid protein possesses several remarkable motifs (e.g. SPRY, LisH, CTLH domains) that may play a role in protein-protein interaction. We, therefore, generated altered versions of individual Gid proteins by deleting or mutating these domains and performed co-immunoprecipitation experiments to analyze the interaction between distinct subunits. Thus, we were able to create an initial model of the topology of this unusual E3 ubiquitin ligase.     

Inhibition of human drug metabolizing cytochrome P450 enzymes by plant isoquinoline alkaloids

The human cytochrome P450 (CYP) enzymes play a major role in the metabolism of endobiotics and numerous xenobiotics including drugs. Therefore it is the standard procedure to test new drug candidates for interactions with CYP enzymes during the preclinical development phase. The purpose of this study was to determine in vitro CYP inhibition potencies of a set of isoquinoline alkaloids to gain insight into interactions of novel chemical structures with CYP enzymes. These alkaloids (n = 36) consist of compounds isolated from the Papaveraceae family (n = 20), synthetic analogs (n = 15), and one commercial compound. Their inhibitory activity was determined towards all principal human drug metabolizing CYP enzymes: 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6 and 3A4. All alkaloids were assayed in vitro in a 96-well plate format using pro-fluorescent probe substrates and recombinant human CYP enzymes. Many of these alkaloids inhibited the CYP3A4 form, with 30/36 alkaloids inhibiting CYP3A4 with at least moderate potency (IC₅₀ < 10 μM) and 15/36 inhibiting CYP3A4 potently (IC₅₀ < 1 μM). Among them corydine, parfumine and 8-methyl-2,3,10,11-tetraethoxyberbine were potent and selective inhibitors for CYP3A4. CYP2D6 was inhibited with at least moderate potency by 26/34 alkaloids. CYP2C19 was inhibited by 15/36 alkaloids at least moderate potently, whereas CYP1A2, CYP2B6, CYP2C8, and CYP2C9 were inhibited to a lesser degree. CYP2A6 was not significantly inhibited by any of the alkaloids. The results provide initial structure-activity information about the interaction of isoquinoline alkaloids with major human xenobiotic-metabolizing CYP enzymes, and illustrate potential novel structures as CYP form-selective inhibitors.     

Recent developments in vitamin E nutrition of turkeys

Research and field observations have shown that vitamin E status of poults is often inadequate during the 5 to 21-day posthatching period. Although overt consequences of this inadequacy may not be evident and clear-cut, subtle adverse effects on health and metabolic efficiency are probable. These latter effects can result in uneconomical performance of turkeys. It therefore seems advisable to supplement the first diet (starter or prestarter) of poults with 100 to 150 IU of vitamin E. The cost of doing so will contribute little to the total cost of production and may facilitate more efficient turkey production. In the meantime, other alternatives that may be effective and economical need to be evaluated.