Chromanol Ring Research Articles

Role of plastoglobules in metabolite repair in the tocopherol redox cycle.

Plants are exposed to ever changing light environments and continuously forced to adapt. Excessive light intensity leads to the production of reactive oxygen species that can have deleterious effects on photosystems and thylakoid membranes. To limit damage, plants increase the production of membrane soluble antioxidants such as tocopherols. Here, untargeted lipidomics after high light treatment showed that among hundreds of lipid compounds alpha-tocopherol is the most strongly induced, underscoring its importance as an antioxidant. As part of the antioxidant mechanism, α-tocopherol undergoes a redox cycle involving oxidative opening of the chromanol ring. The only enzyme currently known to participate in the cycle is tocopherol cyclase (VTE1, At4g32770), that re-introduces the chromanol ring of α-tocopherol. By mutant analysis, we identified the NAD(P)H-dependent quinone oxidoreductase (NDC1, At5g08740) as a second enzyme implicated in this cycle. NDC1 presumably acts through the reduction of quinone intermediates preceding cyclization by VTE1. Exposure to high light also triggered far-ranging changes in prenylquinone composition that we dissect herein using null mutants and lines overexpressing the VTE1 and NDC1 enzymes.

Vitamin E and antioxidant activity; its role in slow coronary flow

SummaryAimOxidative stress, which is widely recognised as an important feature of many diseases, can be defined as an increased formation of reactive oxygen species or decreased antioxidant defense. In this study we measured plasma vitamin E levels and total antioxidant activity (AOA) in patients with slow coronary flow (SCF).MethodsThe plasma vitamin E levels and AOA were measured in 40 patients with angiographically diagnosed SCF. Forty subjects with normal coronary flow (NCF) served as the control group. SCF and NCF were analysed, and blood samples were taken for plasma vitamin E levels and AOA. Plasma vitamin E levels and AOA in patients with SCF were evaluated and compared to those of patients with NCF.ResultsThere was no significant difference between the two groups in terms of plasma AOA, lipid profile and C-reactive protein (CRP) levels but there was a significant difference in vitamin E levels between the two groups (p = 0.001).ConclusionVitamin E levels were found to be lowered in patients with SCF compared to the NCF group. The association between smoking and vitamin E levels is worth further investigating in larger samples.

Abstract A46: Vitamin E intake and the lung cancer risk among female nonsmokers: A report from the Shanghai Women's Health Study

Abstract Background: Tocopherols, as the major source of vitamin E in diet in China and US, include four isoforms (α, β, γ, and δ) differentiated by the number and position of methyl groups on the chromanol ring. Tocopherols may inhibit carcinogenesis through inhibition of cell proliferation, induction of apoptosis, inhibition of angiogenesis, and enhancement of immune function. Results of past prospective studies and randomized controlled trials that evaluated the association between tocopherols intake and female lung cancer risk have been inconsistent and did not take into consideration lung cancer histology. Methods: We prospectively evaluated the association between tocopherol isoforms from diet, vitamin E supplement use, and lung cancer risk in 72,829 Chinese female nonsmokers aged 40-70 years and participating in the Shanghai Women's Health Study (SWHS). Tocopherol exposure from dietary and supplements sources were assessed through a validated food-frequency questionnaire at baseline and reassessed during follow-up. Cox proportional hazards models with time-dependent analysis were used to calculate multivariate-adjusted Hazard ratios (HRs) and 95% confidence interval (CIs) for lung cancer. All statistical tests were two sided. Results: Between January 1997 and December 2010, 481 study participants were diagnosed with lung cancer after study enrollment. Total dietary tocopherol was inversely associated with lung cancer risk among women complying with suggested dietary guidelines for adequate intake (AI) of tocopherol (14 mg/day or more: HR: 0.73; 95% CI = 0.54-0.99; compared to the lowest category) with a significant dose-response trend (P trend=0.045). Furthermore, borderline significant inverse associations were observed for combined dietary combined β and γ tocopherol (HR: 0.79; 95% CI = 0.59-1.06), and δ-tocopherols intake (HR: 0.78; 95% CI = 0.59-1.04). Additionally, there was a significant interaction effect between vitamin E or multivitamin supplements use and total dietary tocopherol intake in analysis using AI of tocopherol (P interaction = 0.018). In contrast, vitamin E supplement use was associated with a significant increase in lung cancer risk (HR: 1.39; 95% CI = 1.07-1.82). Analyses of lung cancer histology found that vitamin E supplement use was associated with increased risk of adenocarcinoma (HR: 1.79; 95% CI = 1.23-2.60). Exclusion of 45 cases diagnosed within two years of follow-up did not change the HR estimations appreciably. Conclusion: Total dietary tocopherol may be associated with lower lung cancer risk among female non-smokers, while tocopherol from vitamin supplements use was significantly associated with increased lung adenocarcinoma risk. Our results require confirmation in other prospective studies. Citation Format: Qi-Jun Wu, Yong-Bing Xiang, Gong Yang, Hong-Lan Li, Qing Lan, Yu-Tang Gao, Wei Zheng, Xiao-Ou Shu, Jay Fowke. Vitamin E intake and the lung cancer risk among female nonsmokers: A report from the Shanghai Women's Health Study. [abstract]. In: Proceedings of the Twelfth Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2013 Oct 27-30; National Harbor, MD. Philadelphia (PA): AACR; Can Prev Res 2013;6(11 Suppl): Abstract nr A46.

Tocopherol biosynthesis: chemistry, regulation and effects of environmental factors

Tocopherols are lipophilic antioxidants and together with tocotrienols belong to the vitamin-E family. The four forms of tocopherols (α-, β-, γ- and δ-tocopherols) consist of a polar chromanol ring and lipophilic prenyl chain with differences in the position and number of methyl groups. The biosynthesis of tocopherols takes place mainly in plastids of higher plants from precursors derived from two metabolic pathways: homogentisic acid, an intermediate of degradation of aromatic amino acids, and phytyldiphosphate, which arises from methylerythritol phosphate pathway. The regulation of tocopherol biosynthesis in photosynthetic organisms occurs, at least partially, at the level of key enzymes as such including p-hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27), homogentisate phytyltransferase (HPT, EC 2.5.1.-), tocopherol cyclase (TC, EC 5.4.99.-), and two methyltransferases. Tocopherol biosynthesis changes during plant development and in response toward different stresses induced by high-intensity light, drought, high salinity, heavy metals, and chilling. It is supposed that scavenging of lipid peroxy radicals and quenching of singlet oxygen are the main functions of tocopherols in photosynthetic organisms. The antioxidant action of tocopherols is related to the formation of tocopherol quinone and its following recycling or degradation. However, until now, the mechanisms of tocopherol degradation in plants have not been established in detail. This review focuses on mechanisms of tocopherols biosynthesis and its regulation in photosynthetic organisms. In addition, available information on tocopherol degradation is summarized.

Vitamin E tocotrienols: life beyond tocopherols

An extensive amount of work has been carried out on vitamin E, which was first identified as a “fertility factor” in 1938 by two researchers from the University of California, Berkeley. This vitamin E, referred to as tocopherol, consists of a chromanol ring with a 15-carbon isoprenoid chain. Based on the number and position of methyl groups on the chromanol ring, there are four saturated analogues. Almost 16,000 pubmed citations indicate that a lot is known about this vitamin. However, there is another vitamin E, called tocotrienol, that has three trans double bonds in the isoprenoid tail and was first described in Nature in 1965. There is very little known about this vitamin E, as indicated by less than 1,000 (about 700) pubmed citations. Therefore, the first International Conference on Tocotrienols and its Role in Chronic Diseases was organized in Las Vegas, USA, from July 29–30th 2010 to discuss the chemistry, biology, and clinical implication of tocotrienols and was sponsored by the Malaysian Palm Oil Board. This was also timely as tocotrienol received GRAS status from USFDA in April 2010. It was a two-day conference, with 27 speakers. From this conference, it became clear that tocotrienols exhibit health benefits quite different from that of tocopherols, and in most cases, these activities are superior for human use. There were also indications that different isomers of tocotrienols exhibit distinct activities. For instance, while α-tocotrienol was highly effective in the brain for cerebral ischemia, γ- and δ-tocotrienol exhibited strong anticancer activities. The current special issue of Genes and Nutrition is devoted to various aspects of tocotrienols as discussed at this conference. We sought to build upon the various unique and novel benefits of tocotrienols, for example, the role of tocotrienols in cardioprotection based on structure–function relationships, the radioprotective properties of γ-tocotrienol as well as the protective effects of vitamin E against hypercholesterolemia-induced age-related disease (Das; Kartal Ozer). The molecular mechanism of tocotrienols was equally appreciated and addressed using comparative gene expression profiling in tocol-treated endothelial cells (Hauer Jensen). While information continues to emerge on the anticancer effects of tocotrienols, a review on tocotrienols and breast cancer aims to update on the present understanding of tocotrienols in clinical trials (Nesaretnam), likewise for the anti-cancer mechanism of tocotrienols where multiple cell signaling pathways were addressed and compared against the pathways involved in tocopherol (Aggarwal; Galli). The concept of combination approach with γ-tocotrienol and statin or receptor tyrosine kinase inhibitor was also reviewed (Sylvester). In addition to that, the role of tocotrienols in attenuating tumor angiogenesis was re-examined using pro-angiogenic markers in human umbilical vein endothelial cells and murine mammary cancer cells (Selvaduray). Although not every presentation made at the conference is represented in this issue, it gives the readers an idea about the diversity of applications for tocotrienols. Finally, our sincere thanks are extended to all participants and working committee in making this event a success. The meeting stimulated fruitful discussions on tocotrienols among international researchers, and we look forward to meeting again in July 2011 at the second conference.

Abstract 190: Targeting ceramide and endoplasmic reticulum-stress in vitamin E induced apoptosis in human breast cancer cells

Abstract Vitamin E consists of eight plant-derived forms classified as tocopherols (α, β, Δ, γ) and tocotrienols (α, β, Δ, γ) which have a chromanol ring and a saturated and unsaturated aliphatic side chain, respectively. The anti-cancer actions of tocopherols and tocotrienols are limited to the gamma and delta forms. Here, we report that both γ-tocopherol (γT) and γ-tocotrienol (γT3) display potent anti-cancer actions with γT3 being more potent. Both forms target similar pro-apoptotic and anti-apoptotic pathways. γT and γT3 induced apoptosis in human breast cancer cells by enhancing death-receptor DR5 mediated activation of caspase-8 and mitochondria dependent caspase-9, as well as by activating ER-stress markers: GADD153/ C/EBP homologous protein (CHOP), GRP-78, ATF-4 and eIF2α. JNK and CHOP are involved in DR5 mediated apoptosis triggered by both vitamin E forms. γT and γT3 decreased protein levels of anti-apoptotic factors c-FLIP, BCl-2 and survivin. Since ceramide is a known upstream inducer of ER-stress and downstream mediator of death receptor apoptotic pathway mediators, treatments with γT and γT3 were conducted in the presence of specific inhibitors of sphingomyelin hydrolysis, de novo ceramide pathways and ER-stress in order to distinguish between the pathways modulated by the vitamin E forms. Data show that both γT and γT3 induced apoptosis via activation of sphingomyelin hydrolysis and the de novo ceramide pathways; however, activation of the ER-stress pathway contributed to apoptosis induced by γT3 only. Taken together, our data suggest that γT3s more potent apoptotic effects in comparison to γT are due to the ability of γT3 to activate ER stress pathways. This better understanding of the anti-cancer properties and mechanistic actions by these two natural occurring agents in vitro provides the necessary impetus to test them in combination with other known anti-cancer agents in pre-clinical xenograft studies to identify novel and safer therapeutic strategies. Research funded by The Clayton Foundation for Research. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 190. doi:10.1158/1538-7445.AM2011-190

Vitamin E Responds to its Lipid Environment

Vitamin E (α-tocopherol) is recognized as the premier lipid soluble antioxidant in biological membranes, but many questions persist of how the vitamin actually functions. We have hypothesized that α-tocopherol preferentially locates in domains that are enriched in polyunsaturated phospholipids, increasing the concentration of the vitamin in the vicinity of the lipid species most vulnerable to oxidation. These highly disordered domains depleted in cholesterol are analogous, although organizationally the antithesis, of the well-studied lipid rafts that are highly ordered and rich in cholesterol. Here we investigate the molecular organization of α-tocopherol in model membranes as a function of unsaturation. Using solid state 2H NMR and ESR, we see that there is less restriction to the motion of α-tocopherol and that it has less impact on chain order in more unsaturated membranes. We also have now measured the location of the chromanol ring of α-tocopherol using neutron diffraction. The results show that the vitamin sits lower in the membrane and is more intimately tied to the lipid chains when they are more unsaturated (where it's antioxidant role is needed), and generally sits higher in the membrane and is excluded vertically from the chains when they are more saturated (where it's antioxidant role is not needed). Together these findings support α-tocopherol's putative structure/function role as a free-radical scavenger by locating itself in the right place at the right time.

Antiproliferative and Proapoptotic Effects of Tocopherol and Tocol on Activated Hepatic Stellate Cells

Activated hepatic stellate cells (HSCs) play crucial roles in liver fibrosis. In the course of liver injury, HSCs, which reside in perisinusoidal spaces and lose lipid droplets, morphologically change into a myofibroblastic phenotype and acquire an increased proliferation activity in what is known as the activated state. We have investigated therapeutic strategies for liver fibrosis by promoting spontaneous reversion or inducing apoptosis in activated HSCs. Vitamin E consists of four tocopherols and four tocotrienols, all of which are well-known antioxidants. In this study, the antiproliferative and proapoptotic effects of a tocol, which lacks methyl groups attached to the chromanol ring, and four tocopherols were investigated using activated HSCs. δ-Tocopherol and tocol exhibited relatively high proliferation inhibitory and proapoptotic abilities. However, they did not show proliferation inhibition ability on primary hepatocytes or HepG2 cells. Significant cell detachment was also observed in δ-tocopherol- and tocol-treated HSCs. Decreased protein expressions of α-smooth muscle actin and β1 integrin were observed in a dose-dependent manner. These results indicate that δ-tocopherol and tocol induce anoikis in activated HSCs.

Vitamin E Prevents Lipid Raft Modifications Induced by an Anti-cancer Lysophospholipid and Abolishes a Yap1-mediated Stress Response in Yeast

We have previously established that the anti-cancer lysophospholipid edelfosine (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine, Et-18-OCH(3)) induces cell death in yeast by selective modification of lipid raft composition at the plasma membrane. In this study we determined that alpha-tocopherol protects cells from the edelfosine cytotoxic effect, preventing the internalization of sterols and the plasma membrane proton pump ATPase, Pma1p. Two non-mutually exclusive hypotheses were considered to explain the protective effect of alpha-tocopherol: (i) its classical antioxidant activity is necessary to break progression of lipid peroxidation, despite the fact Saccharomyces cerevisiae does not possess polyunsaturated fatty acids and (ii) due to its complementary cone shape, insertion of alpha-tocopherol could correct membrane curvature stress imposed by edelfosine (inverted cone shape). We then developed tools to distinguish between these two hypotheses and dissect the structural requirements that confer alpha-tocopherol its protective effect. Our results indicated its lipophilic nature and the H donating hydroxyl group from the chromanol ring are both required to counteract the cytotoxic effect of edelfosine, suggesting edelfosine induces oxidation of membrane components. To further support this finding and learn more about the early cellular response to edelfosine we investigated the role that known oxidative stress signaling pathways play in modulating sensitivity to the lipid drug. Our results indicate the transcription factors Yap1 and Skn7 as well as the major peroxiredoxin, Tsa1, mediate a response to edelfosine. Interestingly, the pathway differed from the one triggered by hydrogen peroxide and its activation (measured as Yap1 translocation to the nucleus) was abolished by co-treatment of the cells with alpha-tocopherol.

Vitamin E supplementation and mammalian lifespan

Vitamin E refers to a family of several compounds that possess a similar chemical structure comprising a chromanol ring with a 16-carbon side chain. The degree of saturation of the side chain, and positions and nature of methyl groups designate the compounds as tocopherols or tocotrienols. Vitamin E compounds have antioxidant properties due to a hydroxyl group on the chromanol ring. Recently, it has been suggested that vitamin E may also regulate signal transduction and gene expression. We previously reported that lifelong dietary vitamin E (alpha-tocopherol) supplementation significantly increased median lifespan in C57BL/6 mice by 15%. This lifespan extension appeared to be independent of any antioxidant effect. Employing a transcriptional approach, we suggest that this increase in lifespan may reflect an anti-cancer effect via induction of the P21 signalling pathway, since cancer is the major cause of death in small rodents. We suggest that the role of this pathway in life span extension following supplementation of vitamin E now requires further investigation.

Abstract 961: Higher activity of δ-tocopherol (δ-T) than γ-tocopherol and α-tocopherol in the inhibition of lung cancer H1299 xenograft tumors in mice

Abstract Our previous study showed that γ-TmT (a tocopherol mixture containing 57% γ-T, 24% δ-T, 13% α-T and 1.5% β-T) effectively inhibited colon and lung carcinogenesis as well as the growth of transplanted human lung cancer in mice. This study aims to determine the more active constituents in this mixture in a xenograft model. After injection of human lung cancer H1299 cells into NCr nu/nu nude mice, the mice were maintained on a standard AIN93M diet or diet enriched with α-T, γ-T, δ-T or γ-TmT (each at 0.3% and 0.17%) for 50 days. Dietary δ-T showed the strongest inhibition based on final tumor weight (35.6% and 57.6% reduction from the control by 0.17% and 0.3% δ-T, respectively). Dietary 0.3% γ-T and γ-TmT also showed significant inhibition (37.4% and 46% reduction from control, respectively). However, an inhibitory effect was not observed with dietary α-T group. Consistent with the animal study, the growth of H1299 cells was inhibited by tocopherols with their effectiveness following the order of δ-T > γ-TmT > γ-T, but α-T was not effective. We also found that dietary α-T, γ-T and δ-T treatment respectively increased serum α-T, γ-T and δ-T levels (up to 45, 9.7 and 1.2μM, respectively). However, the serum α-T level was decreased by dietary γ-T and δ-T; for example, at 0.3% they decreased α-T levels by 50% and 42% from the control group, respectively. The results suggest a competition among different forms of tocopherols. Dietary treatment with each form of tocopherol also increased the corresponding side-chain degradation metabolites: carboxyethyl hydroxychromans (CEHC) and carboxymethylbutyl hydroxychromans (CMBHC). With 0.3% α-, γ- and δ-T in the diet, the mean serum levels of α-, γ- and δ-CEHC were 0.08, 0.7 and 3.0μM, respectively; and α-, γ- and δ-CMBHC were 0.14, 0.83 and 4.0μM, respectively. These metabolites retain the intact chromanol ring and, similar to their parent tocopherols, could trap reactive oxygen and nitrogen species. δ-CEHC and δ-CMBHC, which are present at rather high concentrations, may play an important role in inhibiting xenograph and tumor growth. The present results show that δ-T and γ-T effectively inhibited human lung cancer xenograft tumors and the inhibition may be due to the apoptosis-inducing, antioxidative, and reactive nitrogen species-trapping activities of these tocopherols and their metabolites (Supported by NIH grants CA120915, CA122474, and CA133021). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 961.

Redox agents modulate neuronal activity and reproduce physiological aspects of neuronal aging

The high oxygen consumption and post-mitotic nature of the central nervous system (CNS) makes it particularly susceptible to oxidative stress, the impact of which is widely regarded as a root cause of functional impairment of the aging brain in vertebrates and invertebrates alike. Using an invertebrate model system we demonstrate that the lipid soluble antioxidant α-tocopherol can both reverse 2,2-azobis(2-methylpropion-amidine) dihydrochloride (AAPH) induced decline in excitability in young neurons as well as restore the electrical activity and excitability of aged neurons not unlike the level of their younger equivalents. Furthermore, using two analogs of α-tocopherol where either the acyl chain has been removed (Trolox) or the hydroxyl group of the chromanol ring has been methylated we were able to assert that the restorative effect of α-tocopherol requires both insertion into the plasma membrane as well as an active OH group. Thus, our results indicate peroxidation is an important modulator of neuronal excitability as well as support the growing body of evidence suggesting α-tocopherol's actions may extend well beyond its established role as a lipid domain preventative antioxidant.

Structural and Kinetic Properties of α-Tocopherol in Phospholipid Bilayers, a Molecular Dynamics Simulation Study

Structural and kinetic properties of vitamin E in biomembranes provide the key to understanding the biological functions of this lipophilic vitamin. We report a series of molecular dynamics simulations of two alpha-tocopherol/phosphatidylcholine systems and two alpha-tocopherol/phosphatidylethanolamine systems in water at 280, 310, and 350 K. The preferential position, hydrogen bonding, orientation, and dynamic properties of the alpha-tocopherol molecule in the bilayers have been examined. In all the four systems simulated, the vitamin remains in one leaflet of lipid bilayer at 280 and 310 K but flips over from one side to the other at 350 K within 200 ns of the simulation. The hydroxyl oxygen in the headgroup of alpha-tocopherol preferred a location between the third and the fifth carbon atom in the sn-2 acyl chains of the lipids. Hydrogen bonding analysis shows that the hydrogen bonds are mainly with the oxygens of the fatty acid esters rather than with the phosphate oxygens of the lipid molecule, and those with the amino groups are trivial in the case of phosphatidylethanolamines, at all three temperatures. The hydrogen bonds with phosphatidylethanolamines are more stable than those with phosphatidylcholines at low temperatures. The orientation of alpha-tocopherol in the bilayers is relatively flexible: the chromanol ring takes various tilt angles with respect to the bilayer normal, and the isoprenyl chain is mobile and able to adopt many different conformers. Calculation of lateral diffusion coefficients of alpha-tocopherol and phospholipid molecules shows that alpha-tocopherol has a comparable diffusion rate with phospholipid molecules at the gel phase but diffuses more rapidly than lipid molecules at the liquid-crystal phase.

Biomimetic Chromanol Cyclisation: A Common Route to α‐Tocotrienol and α‐Tocopherol

AbstractA common synthetic route to α‐tocotrienol and α‐tocopherol has been accomplished by a biomimetic cyclisation that yields the chromanol ring. The chirality at C2 of the chromanol was induced by a covalently attached chiral dipeptide. Its terminal Asp participates in the enantioface‐selective protonation of the double bond of the α‐tocotrienol precursor.α‐Tocotrienol was diastereoselectively hydrogenated toα‐tocopherol. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Diastereoselective Synthesis of α-Tocopherol: A New Concept for the Formation of Chromanols

A diastereoselective synthesis of alpha-tocopherol 1 (93% de) was achieved via two key steps, (i) a highly diastereoselective Shi epoxidation of a trisubstituted alkene and (ii) an acid supported, "anti-Baldwin" epoxide ring opening under inversion of configuration leading to the 6-membered chromanol ring.

Tocochromanols, plastoquinol, and other biological prenyllipids as singlet oxygen quenchers—determination of singlet oxygen quenching rate constants and oxidation products

Singlet oxygen quenching rate constants for tocopherol and tocotrienol homologues have been determined in organic solvents of different polarities, as well as for other biological prenyllipids such as plastoquinol, ubiquinol, and α-tocopherolquinol. The obtained results showed that the quenching activity of tocochromanols was mainly due to the chromanol ring of the molecule and the activity increased with the number of the methyl groups in the ring and solvent polarity. Among prenylquinols, α-tocopherolquinol was the most active scavenger of singlet oxygen followed by ubiquinol and plastoquinol. The oxidation products of tocopherols were identified as 8a-hydroperoxy-tocopherones which are converted to the corresponding tocopherolquinones under acidic conditions. The primary oxidation products of prenylquinols, containing unsaturated side chains, were the corresponding prenylquinones that were further oxidized to hydroxyl side-chain derivatives. In the case of plastochromanol, the γ-tocotrienol homologue found in some seed oils, mainly the hydroxyl derivatives were formed, although 8a-hydroperoxy-γ-tocopherones were also formed to a minor extent, both from plastochromanol and from its hydroxyl, side-chain derivatives. The obtained results were discussed in terms of the activity of different prenyllipids as singlet oxygen scavengers in vivo.

Influence of major structural features of tocopherols and tocotrienols on their ω-oxidation by tocopherol-ω-hydroxylase

Human cytochrome P450 4F2 (CYP4F2) catalyzes the initial omega-hydroxylation reaction in the metabolism of tocopherols and tocotrienols to carboxychromanols and is, to date, the only enzyme shown to metabolize vitamin E. The objective of this study was to characterize this activity, particularly the influence of key features of tocochromanol substrate structure. The influence of the number and positions of methyl groups on the chromanol ring, and of stereochemistry and saturation of the side chain, were explored using HepG2 cultures and microsomal reaction systems. Human liver microsomes and microsomes selectively expressing recombinant human CYP4F2 exhibited substrate activity patterns similar to those of HepG2 cells. Although activity was strongly associated with substrate accumulation by cells or microsomes, substantial differences in specific activities between substrates remained under conditions of similar microsomal membrane substrate concentration. Methylation at C5 of the chromanol ring was associated with markedly low activity. Tocotrienols exhibited much higher Vmax values than their tocopherol counterparts. Side chain stereochemistry had no effect on omega-hydroxylation of alpha-tocopherol (alpha-TOH) by any system. Kinetic analysis of microsomal CYP4F2 activity revealed Michaelis-Menten kinetics for alpha-TOH but allosteric cooperativity for other vitamers, especially tocotrienols. Additionally, alpha-TOH was a positive effector of omega-hydroxylation of other vitamers. These results indicate that CYP4F2-mediated tocopherol-omega-hydroxylation is a central feature underlying the different biological half-lives, and therefore biopotencies, of the tocopherols and tocotrienols.

Inhibition of Monosodium Urate Monohydrate-mediated Hemolysis by Vitamin E

Microcrystals of monosodium urate monohydrate (MSUM) induce cytolysis and hemolysis in erythrocytes. In this report, we studied the effect of vitamin E on MSUM-mediated hemolysis in human erythrocytes. Vitamin E significantly inhibited hemolysis induced by MSUM. The hydroxyl group in the chromanol ring of vitamin E is dispensable for protecting erythrocytes against hemolysis induced by MSUM, indicating that the inhibitory effect of vitamin E is not due to its antioxidant properties. However, both the chromanol ring and the isoprenoid side chain are important for vitamin E to suppress MSUM-induced hemolysis. Our current study suggests that vitamin E inhibits hemolysis induced by MSUM as a membrane stabilizer.

Singlet Oxygen Oxidation Rates of ∝‐, γ‐, and δ‐Tocopherols

ABSTRACT: The reaction rate constants of 5 × 10−4 M, 10 × 10−4 M, and 20 × 10−4 M α‐, γ‐, and δ‐tocopherols with singlet oxygen in methylene chloride containing 1 × 10−5 M chlorophyll under light at 25 °C for 60 min were studied. The oxidation of tocopherols determined by a spectrophotometric method showed that the losses of 20 × 10−4 M α‐, γ‐, and δ‐tocopherols after 60 min under light were 21%, 16%, and 9%, respectively. The degradation of α‐, γ‐, and δ‐tocopherols was undetectable in the absence of chlorophyll under light or in the presence of chlorophyll in dark. The losses of tocopherols under light were mainly due to singlet oxygen oxidation. The degradation rates of 20 × 10−4 M α‐, γ‐, and δ‐tocopherols were 6.6 ×10−6 M/min, 5.0 × 10−6 M/min, and 2.9 × 10−6 M/min, respectively. The reaction rates between α‐, γ‐, or δ‐tocopherol and singlet oxygen were 4.1 ×106/M s, 3.3 × 106/M s, and 1.4 × 106/M s, respectively. The singlet oxygen oxidation rate of δ‐tocopherol was significantly lower than α‐ or γ‐tocopherol at α= 0.05. As the electron density in the chromanol ring of tocopherol increased, the singlet oxygen oxidation was increased.

Vitamin E inhibits hemolysis induced by hemin as a membrane stabilizer

Hemin is a potential cytolytic agent. To test the effect of vitamin E on hemin-mediated permeability in cell membranes, sheep erythrocytes were chosen as an appropriate model to study hemolysis induced by hemin. Hemin-induced hemolysis but did not elicit lipid peroxidation in sheep erythrocytes. Vitamin E was effective in inhibiting hemin-mediated hemolysis. Both chromanol ring and the isoprenoid side chain of tocopherols were essential for inhibition of hemin-induced hemolysis. There was a strong correlation between the inhibitory effects of tocopherols on hemin-induced erythrocyte hemolysis and their effects on fluorescence anisotropy of cell membranes. Our results suggested that, in contrast to its antioxidant activity, vitamin E inhibits hemolysis induced by hemin as a membrane stabilizing agent.