Occurrence of -tocopherol binding protein in rat liver cell sap

Intracellular trafficking of vitamin E in hepatocytes: the role of tocopherol transfer protein

The term vitamin E denotes a family of tocopherols and tocotrienols, plant lipids that are essential for vertebrate fertility and health. The principal form of vitamin E found in humans, RRR-alpha-tocopherol (TOH), is thought to protect cells by virtue of its ability to quench free radicals, and functions as the main lipid-soluble antioxidant. Regulation of vitamin E homeostasis occurs in the liver, where TOH is selectively retained while other forms of vitamin E are degraded. Through the action of tocopherol transfer protein (TTP), TOH is then secreted from the liver into circulating lipoproteins that deliver the vitamin to target tissues. Presently, very little is known regarding the intracellular transport of vitamin E. We utilized biochemical, pharmacological, and microscopic approaches to study this process in cultured hepatocytes. We observe that tocopherol-HDL complexes are efficiently internalized through scavenger receptor class B type I. Once internalized, tocopherol arrives within approximately 30 min at intracellular vesicular organelles, where it co-localizes with TTP, and with a marker of the lysosomal compartment (LAMP1), before being transported to the plasma membrane in a TTP-dependent manner. We further show that intracellular processing of tocopherol involves a functional interaction between TTP and an ABC-type transporter.

Purification and partial characterisation of an alpha-tocopherol-binding protein from rabbit heart cytosol

An alpha-tocopherol-binding protein has been isolated and purified from rabbit heart cytosol. The purified protein had an apparent molecular mass of 14,200, as derived from SDS-PAGE. The content of the protein in rabbit heart was around 11.8 micrograms per g of tissue. The binding of alpha-tocopherol to the purified protein was rapid, reversible, and saturable. Neither gamma nor delta tocopherol could displace the bound alpha-tocopherol from the protein, suggesting a high specificity for alpha-tocopherol. alpha-Tocopherol-binding protein did not bind oleate. Transfer of alpha-tocopherol from liposomes to mitochondria was stimulated 8-fold in the presence of the binding protein, suggesting that this protein may be involved in the intracellular transport of alpha-tocopherol in the heart.

Purification and characterization of the alpha-tocopherol transfer protein from rat liver

alpha-Tocopherol transfer protein was purified from the 10,000 x g supernatant of rat liver. Two isoforms of the transfer protein exist, of which the isoelectric points are 5.0 and 5.1 as determined by chromatofocusing. These two isoforms have the same molecular weight; both showed molecular weight of approx. 30,500 on SDS-polyacrylamide gel electrophoresis. They cannot be distinguished from each other by amino acid composition or substrate specificity.

Studies on the hyperplasia ('regeneration') of the rat liver following partial hepatectomy. Changes in lipid peroxidation and general biochemical aspects

Using the experimental model of partial hepatectomy in the rat, we have examined the relationship between cell division and lipid peroxidation activity. In rats entrained to a regime of 12 h light/12 h dark and with a fixed 8 h feeding period in the dark phase, partial hepatectomy is followed by a rapid regeneration of liver mass with cycles of synchronized cell division at 24 h intervals. The latter phenomenon is indicated in this study by pulses of thymidine kinase activity having maxima at 24 h, 48 h and 72 h after partial hepatectomy. Microsomes prepared from regenerating livers show changes in lipid peroxidation activity (induced by NADPH/ADP/iron or by ascorbate/iron), which is significantly decreased relative to that in microsomes from sham-operated controls, again at 24 h, 48 h and 72 h after the operation. This phenomenon has been investigated with regard to possible underlying changes in the content of microsomal fatty acids, the microsomal enzymes NADPH:cytochrome c reductase and cytochrome P-450, and the physiological microsomal antioxidant alpha-tocopherol. The cycles of decreased lipid peroxidation activity are apparently due, at least in part, to changes in microsomal alpha-tocopherol content that are closely associated in time with thymidine kinase activity.

Modeling cortical cataractogenesis: IX. Activity of vitamin E and esters in preventing cataracts and gamma-crystallin leakage from lenses in diabetic rats

Normal and streptozotocin diabetic female Wistar rats were given vitamin E in the diet as the tocopherol, acetate, or succinate form (2,850 IU/kg food). At the end of 6 weeks, the rats were examined for weight gain or loss, general body condition, and cataracts. At sacrifice, blood was collected for measurement of serum glucose, and gamma-crystallin levels were measured in aqueous and vitreous humors using a radioimmunoassay. One lens was homogenized in 8 M guanidinium chloride for ATP analysis. In normal rats, gamma-crystallin was detected in both aqueous and vitreous humors, with the higher concentration in the vitreous humor. Diabetes caused a sixfold increase in gamma-crystallin in both the aqueous and vitreous humors. Diabetes also led to a significant worsening in general body condition, loss of body weight, formation of cataracts, and decrease in lens ATP levels. Addition of vitamin E and vitamin E succinate, but not vitamin E acetate, to the diet resulted in reduction of gamma-crystallin leakage into the vitreous humors and an increase in body weight. There was no improvement noted for the lens ATP levels, the general body condition, or visual cataract score. Neither streptozotocin-induced diabetes nor vitamin E in the diet appeared to affect the weight of the lenses.

Anomalous antioxidant effects in selenium- and vitamin E-deficient liver mitochondria

In this study, we investigate the mechanisms of two anomalous protective effects of exogenous vitamin E that had previously been postulated to involve either a specific antioxidant effect or a non-antioxidant function of the vitamin. These atypical vitamin E effects were observed during the prevention of NAD-induced respiratory decline occurring in homogenates and mitochondria prepared from vitamin E- and selenium-deficient rat liver. The study showed neither hypothesis to be true; rather, the two effects, one in homogenates and the other in isolated mitochondria, were explained by other mechanisms. The protective effect against respiratory decline in homogenates was found to result from interference in the thiobarbituric acid assay for lipid peroxidation by ethanol (the conventional solvent for vitamin E addition). With other non-interfering solvents, inhibition of lipid peroxidation by vitamin E, in contrast to previous studies, correlated perfectly with prevention of respiratory decline. The atypical vitamin E effect occurring in isolated mitochondria-and consisting of a requirement for cytosol proteins for the prevention of respiratory decline by exogenous vitamin E-was found to be caused by the prevention of adverse glass effects and not by the action of vitamin E-specific binding proteins. Frequent failures in the combined protective effect of vitamin E and cytosol, which had been a major complication of respiratory decline studies, were found to be caused by phospholipase activity generated during isolation procedures. Irreversible deactivation of respiratory enzymes by lipid peroxidation was found not to be involved in the respiratory decline mechanism.

Enhancement of the transfer of alpha-tocopherol between liposomes and mitochondria by rat-liver protein(s)

alpha-Tocopherol transfer from liposomes to mitochondria was enhanced by a cytosolic protein in rat liver. This protein was separated from the exchange protein specific for phosphatidylcholine. The stimulation was specific for alpha-tocopherol and the "affinity" order of tocopherols for the cytosolic protein was found to be as follows: alpha-tocopherol greater than gamma-tocopherol much greater than alpha-tocopheryl acetate, alpha-tocopherol quinone. The transfer-stimulating protein may act as a carrier, since an intermediate (alpha-tocopherol)-protein complex could be detected when liposomes containing alpha-tocopherol were incubated with the cytosol. This transfer-stimulating activity was present not only in liver, but also in heart, spleen and lung.

A possible role of rabbit heart cytosol tocopherol binding in the transfer of tocopherol into nuclei

An alpha-tocopherol-binding macromolecule was isolated from the heart cytosol of rabbits fed for 1 month with an alpha-tocopherol-deficient diet. The amount of [3H]-tocopherol bound to nuclear chromatin was increased when the alpha-tocopherol-deficient heart nuclei were incubated in the presence of [3H]tocopherol-cytosol complex. In this condition, large amounts of [3H]tocopherol were associated with a subnuclear fraction that contained non-histone acidic proteins.

Anomalous vitamin E effects in mitochondrial oxidative metabolism

Three different vitamin E effects, suggestive of specific antioxidant effects, were discovered in the protective action of vitamin E against respiratory decline (a decrease in mitochondrial respiration attributed to a "leakage" of electron transport radicals). No correlation was found between respiraotry decline and random lipid peroxidation. The mechanisms behind two of the three atypical vitamin E effects were defined. Both involve an artifact in the TBA assay for lipid peroxidation. This artifact occurs when TBA assays are carried out in the presence of sucrose and acetaldehyde; the latter is produced from ethanol, the solvent used to add vitamin E to preparations. The artifact in the TBA assay for peroxidations appears also to be responsible for differing interpretations of the hepatotoxic effect of ethanol.

Phospholipase A2 of rat liver mitochondria in vitamin E deficiency

1. There is a more than 2-fold increase in phospholipase A(2) activity (EC 3.1.1.4) of liver mitochondria isolated from vitamin E-deficient rats compared with that in normal rats. 2. alpha-Tocopherol in lipoprotein-bound form is more effective than free alpha-tocopherol in restoring the enzyme activity to normal.

Effect of protein deficiency on absorption, transport and distribution of alpha-tocopherol in the rat

The absorption, transport and distribution of alpha-[3H]tocopherol were greatly decreased in protein deficiency. This was reflected in the subcellular distribution of alpha-[3H]tocopherol in livers of protein-deficient rats. The ratio, bound:free for alpha-[3H]tocopherol, also decreased in both serum and liver cytosol. After protein refeeding, absorption, transport and distribution patterns of alpha-[3H]tocopherol for the protein-deficient rats were restored to patterns similar to those of control animals.

Binding of D-alpha-tocopherol to rat liver nuclear components

When D-alpha-tocopherol is administered i.v. to vitamin E deficient rats, significant amounts of this vitamin are bound to a nucleoprotein complex in hepatic nuclei, and this complex can be solubilized by high concentrations of sodium chloride (0.6 M). The bound vitamin in this complex, extractable by ethanol, was found to be identical with authentic alpha-tocopherol by thin layer chromatography.

Involvement of binding lipoproteins in the absorption and transport of alpha-tocopherol in the rat

1. Specific lipoproteins binding alpha-tocopherol but not its known metabolites have been isolated and identified from cytosol of rat intestinal mucosa and from serum. 2. A timestudy of the appearance of the orally administered alpha-[(3)H]tocopherol with these lipoproteins indicates that very-low-density lipoprotein of serum acts as a carrier of the vitamin. 3. The involvement of the mucosal lipoprotein in the absorption of the vitamin from the intestine has been inferred from observations on the amounts of alpha-tocopherol in serum of orotic acid-fed rats where release of lipoproteins from the liver to serum is completely inhibited. A considerable decrease in the association of alpha-tocopherol with serum very-low-density lipoprotein under this condition is interpreted to mean that serum lipoproteins are limiting factors for the transport of the vitamin across the intestine and that this is possibly effected by exchange of alpha-tocopherol between serum very-low-density lipoprotein and mucosal lipoprotein.