Developmental changes in the expression of alpha-tocopherol transfer protein during the neonatal period of rat

Identification and expression analysis of alpha tocopherol transfer protein in chickens fed diets containing different concentrations of alpha-tocopherol

Among the eight forms of vitamin E, the liver preferentially releases α-tocopherol into the circulation and it is distributed to the non-liver tissues. In the hepatocytes, alpha tocopherol transfer protein (TTPA) specifically recognizes α-tocopherol with 2R-configuration and facilitates its intracellular transfer. The identification and characterization of TTPA expression have not been demonstrated in avian species. Therefore, the objectives of this study were to identify avian TTPAs, to compare the sequence conservation, phylogenetic relationship, protein interactions, and disease associations of chicken TTPA with those of human and vertebrate TTPA, and to characterize the tissue expression of the TTPA gene in chickens fed diets supplemented with different amounts of α-tocopherol. Our results suggest that the chicken TTPA was highly conserved with the human and vertebrate TTPA, and consisted of a cellular retinaldehyde binding protein and TRIO guanine exchange factor (CRAL_TRIO) domain. Feeding diets supplemented with increasing amounts of α-tocopherol (25 IU/Kg, 50 IU/Kg, or 100 IU/Kg) to broiler chickens had no effects on growth performance compared with feeding basal diets containing no supplemental α-tocopherol. The expression of TTPA gene was detected high in the liver of chickens in response to dietary α-tocopherol concentrations, whereas its expression was very low or undetectable in the non-liver tissues. In conclusion, the chicken TTPA protein sequence is highly conserved with other avian and vertebrate TTPA protein sequences. The higher expression of TTPA gene in the chicken liver in response to dietary α-tocopherol concentrations may suggest its crucial role in transporting α-tocopherol in the chicken liver.

Studies in Vitamin E: Biochemistry and Molecular Biology of Tocopherol Quinones

Tocopherols and tocotrienols, parent congeners in the vitamin E family, function as phenolic antioxidants. However, there has been little interest in their quinone electrophiles formed as a consequence of oxidation reactions, even though unique biological properties were suggested by early studies conducted immediately after the discovery of vitamin E. Oxidation of tocopherols and tocotrienols produces para- and ortho-quinones, and quinone methides, while oxidation of their carboxyethyl hydroxychroman derivatives produces quinone lactones. These quinone electrophiles are grouped in two subclasses, the nonarylating fully methylated alpha-family and the arylating desmethyl beta-, gamma-, and delta-family. Arylating quinone electrophiles form Michael adducts with thiol nucleophiles, provided by cysteinyl proteins or peptides, which can be identified and quantified by tetramethylammonium hydroxide thermochemolysis. They have striking biological properties which differ significantly from their nonarylating congeners. They are highly cytotoxic, inducing characteristic apoptotic changes in cultured cells. Cytotoxicity is intimately associated with the induction of endoplasmic reticulum stress and a consequent unfolded protein response involving the pancreatic ER kinase (PERK) signaling pathway that commits overstressed cells to apoptosis. The step-function difference between arylating and nonarylating tocopherol quinones is conceivably the basis for distinct biological properties of parent tocopherols, including the epigenetic modification of a histone thiol, the ceramide pathway, natriuresis, and the activity of COX-2, NF-kappaB, PPARgamma, and cyclin. The role of alpha-tocopherol in the origin and evolution of the western hominin diet, the so-called "Mediterranean" diet, and the prominence of alpha-tocopherol in colostrum, mother's milk, and infant nutrition are considered. Finally, the discordance introduced into the diet by arylating tocopherol quinone precursors through the wide use of vegetable oils in deep-frying is recognized.

Biochemical consequences of heritable mutations in the alpha-tocopherol transfer protein

Tocopherol transfer protein (TTP) regulates vitamin E status by facilitating the secretion of tocopherol from liver to circulating lipoproteins. Heritable mutations in the ttpA gene, encoding for TTP, result in ataxia with vitamin E deficiency (AVED) syndrome, typified by low vitamin E levels and a plethora of neurological disorders. The molecular mechanisms by which TTP facilitates tocopherol secretion are presently unknown. We recently showed that vitamin E is taken up by hepatocytes through an endocytic process and that, shortly following uptake, the vitamin is found primarily in lysosomes. We showed further that TTP is localized to late endocytic vesicles and that it facilitates the intracellular trafficking of tocopherol from lysosomes to the plasma membrane. To gain insight into the molecular mechanisms that underlie TTP actions, we studied the physiological impact of three naturally occurring heritable mutations in the ttpA gene (the R59W, R221W, and A120T substitutions). We found that these mutations impair the ability of TTP to facilitate the secretion of vitamin E from cells. Furthermore, the degree of impairment corresponded to the severity of the AVED pathology associated with each mutation. In cells that express mutated TTP proteins, vitamin E did not traffic to the plasma membrane and remained "trapped" in lysosomes. In addition, we observed that substitution mutations that cause the AVED syndrome impart a marked instability on the TTP protein. These observations suggest that the physiological role of TTP is anchored in its ability to direct vitamin E trafficking from the endocytic compartment to transport vesicles that deliver the vitamin to the site of secretion at the plasma membrane.

Mutagenicity of tocopheryl quinones: evolutionary advantage of selective accumulation of dietary alpha-tocopherol

We have shown that phenolic antioxidant tocopherols are oxidized to nonarylating alpha-tocopheryl quinone (alpha-TQ) and arylating gamma- and delta-TQ electrophiles. The arylating quinones stimulate apoptosis and are highly cytotoxic in mammalian cells. Some xenobiotic phenolic antioxidants are mutagens, and it has been suggested that their arylating quinone metabolites are the active agents in mutagenesis related to carcinogenesis. We found that neither alpha- nor gamma-TQ was directly genotoxic in supercoiled-to-nicked circular DNA conversions, but these agents interacted with the cytomegalovirus reporter-driven plasmid and enhanced luciferase transfection, with gamma-TQ > alpha-TQ. The Ames test, using gamma-TQ and a number of Salmonella strains, showed no evidence of bacterial mutagenesis. gamma-TQ was highly cytotoxic and alpha-TQ slightly cytotoxic in eukaryocyte AS52 cells. A guanosine phosphoribosyltransferase gene assay showed that gamma-TQ was highly mutagenic and alpha-TQ slightly mutagenic in AS52 cells. A review of the literature identified associations where a decrease in dietary gamma-tocopherol (gamma-T) diminishes and an increase in dietary gamma-T and its quinone enhances carcinogenicity. Humans and other omnivores selectively accumulate alpha-tocopherol, even though gamma-T is their principal dietary tocopherol. We suggest that this selectivity confers an evolutionary advantage by limiting tissue gamma-T, a putative precursor of the mutagen gamma-TQ.

Dietary vitamin A modulates the concentrations of RRR-alpha-tocopherol in plasma lipoproteins from calves fed milk replacer

The practice of supplementing milk replacers fed to neonatal calves with high concentrations of vitamin A has raised concerns regarding the effect of excess vitamin A on the bioavailability of vitamin E. A 4 x 2 factorial experiment evaluated the effects of four dietary amounts of vitamin A [0, 1.78 [National Research Council (NRC)(6) requirement, control], 35.6 and 71.2 micromol daily as retinyl acetate] and two forms of vitamin E (RRR-alpha-tocopherol and RRR-alpha-tocopheryl acetate, 155 micromol daily) on plasma RRR-alpha-tocopherol and RRR-gamma-tocopherol and RRR-alpha-tocopherol associated with plasma lipoproteins (Lp) from milk replacer-fed Holstein calves from birth to 28 d of age. The VLDL, LDL, HDL and very high-density lipoprotein (VHDL) fractions were separated by ultracentrifugal flotation, and the amount of vitamin E associated with each fraction was determined by normal-phase HPLC. The amount and distribution of RRR-alpha-tocopherol in Lp fractions were unaffected by the form of dietary vitamin E. Plasma and Lp RRR-alpha-tocopherol concentrations increased with age (P < 0.0001) and were maximal at 28 d of age. Concentrations of RRR-alpha-tocopherol associated with Lp were 25% (P < 0.01) to 39% (P < 0.0001) lower in calves fed 35.6 and 71.2 micromol of vitamin A daily than in control calves at 28 d of age. The RRR-gamma-tocopherol concentrations were unaffected by dietary vitamin A (P >/= 0.05). In conclusion, dietary vitamin A modulated the amount and distribution of RRR-alpha-tocopherol in the circulation of milk replacer-fed neonatal calves. Because of the essential antioxidant role of vitamin E, the health-related consequences associated with the depression of the LP RRR-alpha-tocopherol concentrations in calves fed vitamin A at 35.6 and 71.2 micromol need to be investigated.