alpha-tocopherol-binding proteins: purification and characterization

Oral vitamin E absorption in English Cocker Spaniels with familial vitamin E deficiency and retinal pigment epithelial dystrophy

BACKGROUND

Retinal Pigment Epithelial Dystrophy (RPED) with neuroaxonal degeneration in English Cocker Spaniels (ECS) is associated with systemic vitamin E deficiency in the absence of dietary insufficiency.

OBJECTIVE

To evaluate the ability of ECS with RPED to absorb orally administered vitamin E and establish a basis for vitamin E supplementation in affected dogs.

ANIMALS STUDIED

8 RPED-affected ECS and five clinically normal dogs.

PROCEDURES

An oral vitamin E tolerance test (OVETT) was conducted in each dog. Blood samples were obtained prior to and at 3, 6, 9, 12, 24, 120, and 240 h following oral administration of 90 iu/kg of RRR-α-tocopherol. Plasma alpha tocopherol (αTOC) content was measured by normal phase, high-performance liquid chromatography, and indices of vitamin E absorption calculated.

RESULTS

There was marked variation in OVETT results between individuals. In RPED-affected ECS, mean peak plasma αTOC concentration (17.87 ± 13.21 μg/mL), attained after administration of a large oral dose of the vitamin, was significantly lower than the mean peak plasma αTOC concentration attained in normal dogs (47.61 ± 17.17 μg/mL; P < 0.005). However, the plasma concentrations achieved in 7/8 RPED-affected dogs remained within the normal reference range for plasma αTOC in vitamin E-replete dogs, for at least 12 h postdose.

CONCLUSIONS

Vitamin E-deficient ECS with RPED are capable of absorbing orally administered vitamin E. Twice daily administration of 600-900 iu tocopherol is likely to restore plasma vitamin E concentrations to the normal range in most affected dogs.

The immunoregulatory role of vitamins A, D and E in patients with primary Sjogren's syndrome

OBJECTIVE

The aim of the present study was to investigate the immunomodulating role of fat-soluble vitamins in 25 patients with primary SS (pSS) and 15 healthy individuals.

METHODS

Plasma levels of vitamins A, D and E were determined by HPLC. Peripheral NK, NK T cells, T-cell subsets, B cells, IL-10 producing Tr1 cells, CD4(+)CD25(+) Treg cells and Th17 were determined by flow cytometry. Various Th1- and Th2-soluble cytokines were assessed by ELISA, whereas intracytoplasmic cytokines (IFN-gamma, IL-4, -10 and -17) were measured by flow cytometry. Correlation was assessed between vitamin levels and immunological and clinical parameters.

RESULTS

Vitamin A levels did not differ between patients and controls, yet in patients with extraglandular manifestations (EGMs) a significant decrease in vitamin A levels was apparent compared with pSS patients without EGMs (P = 0.005). Vitamin E levels were increased in patients compared with controls (P = 0.004), whereas vitamin D levels were similar in pSS and control subjects. In patients, vitamin A showed a positive correlation with both NK cell (P = 0.038) and Th17 cell (P = 0.025), and a negative correlation with Schirmer's test values (P = 0.035). Positive correlation was found between vitamin E and NK cells (P = 0.043), Th1 cells (P = 0.049) and the Th1/Th2 ratio (P = 0.043). In the control group, we found correlation between vitamin E and serum IL-10 levels (P = 0.003).

CONCLUSIONS

Our data suggest that fat-soluble vitamins may be important in immunoregulatory processes in patients with pSS.

Vitamin E: An overview of major research directions

During the last 90 years since the discovery of vitamin E, research has focused on different properties of this molecule, the focus often depending on the specific techniques and scientific knowledge present at each time. Originally discovered as a dietary factor essential for reproduction in rats, vitamin E has revealed in the meantime many more important molecular properties, such as the scavenging of reactive oxygen and nitrogen species with consequent prevention of oxidative damage associated with many diseases, or the modulation of signal transduction and gene expression in antioxidant and non-antioxidant manners. Research over the last 30 years has also resolved the biosynthesis and occurrence of vitamin E in plants, the proteins involved in the cellular uptake, tissue distribution and metabolism, and defined a congenital recessive neurological disease, ataxia with vitamin E deficiency (AVED), characterized by impaired enrichment of alpha-tocopherol in plasma as a result of mutations in the liver alpha-tocopherol transfer gene. This review is giving a brief introduction about vitamin E by following the major research directions since its discovery with a historical perspective.

Vitamin E and neurodegenerative disorders associated with oxidative stress

Several neurodegenerative disorders are associated with oxidative stress that is manifested by lipid peroxidation, protein oxidation and other markers. Included in these disorders in which oxidative stress is thought to play an important role in their pathogenesis are Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), tardive dyskinesia, Huntington's disease (HD), and multiple sclerosis. This review presents some of the chemistry of vitamin E as an antioxidant and summarizes studies in which vitamin E has been employed in these disorders and models thereof.

Expression and refolding of recombinant human alpha-tocopherol transfer protein capable of specific alpha-tocopherol binding

alpha-Tocopherol transfer protein (alpha-TTP) is a cytosolic protein found predominantly in mammalian liver that is proposed to be responsible for the stereoselective uptake of alpha-tocopherol from the diet. Although recombinant alpha-TTP has been reported previously, little detail has been provided about the yields and competency of the recovered protein at binding tocopherols and other ligands. In this work, we report the successful expression and refolding of a recombinant human alpha-TTP. Ligation-independent cloning generated a construct in pET-30 encoding an alpha-TTP fusion protein (pET-30/ttp) containing a six-histidine tag and an S-tag, each cleavable by a separate protease upon expression in Escherichia coli. Overexpression of the protein led to the formation of inclusion bodies that were solubilized in 8 M urea and purified by metal chelate affinity chromatography. Another construct in pET-28b (pET-28b/ttp) provided a soluble protein product after expression that contained a 40-amino-acid N-terminal extension, which can be reduced to 21 amino acids by cleavage with thrombin. The success of different refolding experiments was assessed using a Lipidex gel-based tocopherol binding assay. The best recovery of refolded recombinant alpha-TTP fusion capable of binding alpha-tocopherol was provided by matrix-assisted refolding in the presence of 0.5 M arginine. Cleavage of the fusion protein with Factor Xa successfully generated the full-length wild-type protein with no additional N-terminal amino acids. The resulting purification scheme provides recombinant alpha-TTP in good yield and purity for investigation of both its structure and its binding affinities for different ligands including natural and synthetic tocols.

A novel human tocopherol-associated protein: cloning, in vitro expression, and characterization

Vitamin E (alpha-tocopherol) is an essential dietary nutrient for humans and animals. The mechanisms involved in cellular regulation as well as in the preferential cellular and tissue accumulation of alpha-tocopherol are not yet well established. We previously reported (Stocker, A., Zimmer, S., Spycher, S. E., and Azzi, A. (1999) IUBMB Life 48, 49-55) the identification of a novel 46-kDa tocopherol-associated protein (TAP) in the cytosol of bovine liver. Here, we describe the identification, the molecular cloning into Escherichia coli, and the in vitro expression of the human homologue of bovine TAP, hTAP. This protein appears to belong to a family of hydrophobic ligand binding proteins, which have the CRAL (cis-retinal binding motif) sequence in common. By using a biotinylated alpha-tocopherol derivative and the IASys resonant mirror biosensor, the purified recombinant protein was shown to bind tocopherol at a specific binding site with K(d) 4.6 x 10(-7) m. Northern analyses showed that hTAP mRNA has a size of approximately 2800 base pairs and is ubiquitously expressed. The highest amounts of hTAP message are found in liver, brain, and prostate. In conclusion, hTAP has sequence homology to proteins containing the CRAL_TRIO structural motif. TAP binds to alpha-tocopherol and biotinylated tocopherol, suggesting the existence of a hydrophobic pocket, possibly analogous to that of SEC14.

Synthesis of phytyl- and chroman-derivatized photoaffinity labels based on alpha-tocopherol

Photoaffinity analogues of alpha-tocopherol have been prepared by substituting photosensitive functional groups at either the terminus of an alkyl chain of varying length mimicking the phytyl tail or on C-3 of the chroman portion of tocopherol. The alkyl chain-modified compounds 2a-d contain a hexyl to nonyl alkyl chain extending from C-2 of the chroman, terminating in a tetrafluoroazidobenzyloxy group. These compounds were prepared starting from the commercially available Trolox acid 4, followed by esterification, protection, and reduction to the silyl-protected Trolox aldehyde 7, which was coupled using Wittig chemistry to different omega-hydroxyphosphonium bromides. Reduction of the alkene product, coupling with p-azidotetrafluorobenzyl bromide, and deprotection of the phenolic silyl group gave compounds 2a-d in excellent yields. Chroman-functionalized photoaffinity labels were synthesized starting from the protected tocopherol chromene 16b which was a key intermediate for preparation of a 3-hydroxy derivative, either by reduction of epoxides produced directly with Jacobsen's catalysts or by treatment with NBS in wet DME to give two stereoisomeric bromohydrins which were cyclized and reduced to give the phenol-protected C-3 alcohols 19a,b. These alcohols were then converted to diazoacetate esters, and the protecting group was removed to give 3-diazoacetoxy alpha-tocopherols 3a,b.

Molecular mechanism of cellular uptake and intracellular translocation of alpha-tocopherol: role of tocopherol-binding proteins

Vitamin E (alpha-tocopherol) is a lipid-soluble antioxidant which is present in cellular membranes where it plays an important role in the suppression of free radical-induced lipid peroxidation. There are eight naturally occurring homologues of vitamin E which differ in their structure and in biological activity in vivo and in vitro. Various studies have suggested that the tocopherol distribution system favours the accumulation of alpha-tocopherol both in the plasma and different tissues. Mechanisms involved in the preferential accumulation of alpha-tocopherol are not yet well established; however, recent data indicate that both intracellular and membrane alpha-tocopherol-binding proteins may be involved in these processes. A 30 kDa alpha-tocopherol-binding protein (TBP) in the liver cytoplasm is now known to regulate plasma vitamin E concentrations by preferentially incorporating alpha-tocopherol into nascent very low density (VLDL) whereas the 15 kDa TBP may be responsible for intracellular distribution of alpha-tocopherol. The 30 kDa TBP is unique to the hepatocyte whereas the 15 kDa TBP is present in all major tissues. The 15 kDa TBP specifically binds alpha-tocopherol in preference to the delta- and gamma-tocopherol and may exclusively transport alpha-tocopherol to these intracellular sites. In addition, the presence of a membrane TBP (TBPpm) in tissues may regulate their alpha-tocopherol levels. Activity of erythrocyte TBPpm appears to be reduced in smokers, which may lead to reduced levels of alpha-tocopherol in these cells despite smokers have similar plasma levels of vitamin E as in non-smokers. The current status of the evidence for this directed flow of alpha-tocopherol through interactions with these proteins (TBP and TBPpm) is discussed.