Tocopherol content of adipose tissue from vitamin E-deficient humans

The high performance liquid chromatographic isolation of tocopherol with fluorometric quantitation, which we have previously described, has been extended to the analysis of needle aspiration biopsies of adipose tissue. Results are expressed relative to triglyceride content (ng tocopherol per mg triglyceride). In normal subjects adipose tissue content was 262 +/- 33; this value was increased two- to three-fold in normal persons ingesting additional vitamin E. Abetalipoproteinaemic patients have very low adipose tissue tocopherol values--about 10-20% of normal subjects; with massive supplementation of vitamin E (grams per day) a number of such patients have achieved normal tissue tocopherol concentrations. Patients with cholestatic liver disease and low plasma content of tocopherol also had low adipose tissue values; these could be increased by parenteral administration of vitamin E. Neuromuscular improvement noted in response to treatment with supplementary vitamin E has not as yet been correlated to the adipose tissue increment of tocopherol. While extensive observations on the intestinal absorption of tocopherol in humans have been reported, the mode of transfer from plasma lipoproteins to tissues has been less studied. Our results from patients with lipoprotein lipase deficiency and other abnormalities of lipid metabolism suggest that considerable transfer occurs during the initial catabolism of the chylomicrons.

Localization of alpha-tocopherol transfer protein in the brains of patients with ataxia with vitamin E deficiency and other oxidative stress related neurodegenerative disorders

Vitamin E (alpha-tocopherol) is an essential nutrient and an important antioxidant. Its plasma levels are dependent upon oral intake, absorption and transfer of the vitamin to a circulating lipoprotein. The latter step is controlled by alpha-tocopherol transfer protein (alpha-TTP), which is a 278 amino acid protein encoded on chromosome 8, known to be synthesized in the liver. Mutations in alpha-TTP are associated with a neurological syndrome of spinocerebellar ataxia, called ataxia with vitamin E deficiency (AVED). Earlier studies suggested that alpha-TTP is found only in the liver. In order to establish whether alpha-TTP is expressed in the human brain, and what relationship this has to AVED, we studied immunohistochemically the presence of alpha-TTP in the brains of a patient with AVED, normal subjects, and patients with Alzheimer's disease (AD), Down's syndrome (DS), cholestatic liver disease (CLD) and abetalipoproteinemia (ABL). The neuropathology of both AD and DS is thought to be related in part to oxidative stress. The diseases of AVED, of cholestatic liver disease, and of abetalipoproteinemia are thought to be due to lack of circulating tocopherol, leading to inadequate protection against oxidative damage. We demonstrate the presence of alpha-TTP in cerebellar Purkinje cells in patients having vitamin E deficiency states or diseases associated with oxidative stress.

Vitamin E dose-response studies in humans with use of deuterated RRR-alpha-tocopherol

BACKGROUND

Supplemental vitamin E does not raise plasma alpha-tocopherol concentrations more than approximately 3-fold.

OBJECTIVE

To elucidate the mechanism for the limitation in plasma alpha-tocopherol, we undertook human supplementation trials using incrementally increased doses of deuterated vitamin E.

DESIGN

Plasma was obtained from 6 healthy, young adults (4 men and 2 women) during 3 sequential supplementation trials with doses of 15, 75, and 150 mg RRR-alpha-tocopheryl acetate labeled with deuterium (d3-RRR-alpha-tocopheryl acetate). A defined diet was provided on the day of deuterated vitamin E administration, but otherwise subjects ate ad libitum.

RESULTS

The areas under the curves calculated from the plasma d3-RRR-alpha-tocopherol concentrations increased linearly with dose--a 10-fold increase in dose resulted in a 10-fold increase in area under the curve. d3-RRR-alpha-Tocopherol absorption and incorporation into plasma did not decrease with increasing dose. At 11 h, the 15-, 75-, and 150-mg doses resulted in 8+/-4%, 21+/-10%, and 37+/-20% labeling, respectively, of plasma vitamin E. Plasma total (labeled plus unlabeled) alpha-tocopherol concentrations before supplementation were 12+/-3 micromol/L and over the 96 h after the dose averaged 13.3+/-2.6, 15.4+/-3.0, and 16.7+/-4.9 micromol/L for the 15-, 75-, and 150-mg doses, respectively.

CONCLUSIONS

d3-RRR-alpha-Tocopherol was incorporated into the plasma in preference to circulating plasma RRR-alpha-tocopherol. This could occur if the newly absorbed d3-RRR-alpha-tocopherol was preferentially used to replenish circulating vitamin E.

Ataxia with isolated vitamin E deficiency: heterogeneity of mutations and phenotypic variability in a large number of families

Ataxia with vitamin E deficiency (AVED), or familial isolated vitamin E deficiency, is a rare autosomal recessive neurodegenerative disease characterized clinically by symptoms with often striking resemblance to those of Friedreich ataxia. We recently have demonstrated that AVED is caused by mutations in the gene for alpha-tocopherol transfer protein (alpha-TTP). We now have identified a total of 13 mutations in 27 families. Four mutations were found in >=2 independent families: 744delA, which is the major mutation in North Africa, and 513insTT, 486delT, and R134X, in families of European origin. Compilation of the clinical records of 43 patients with documented mutation in the alpha-TTP gene revealed differences from Friedreich ataxia: cardiomyopathy was found in only 19% of cases, whereas head titubation was found in 28% of cases and dystonia in an additional 13%. This study represents the largest group of patients and mutations reported for this often misdiagnosed disease and points to the need for an early differential diagnosis with Friedreich ataxia, in order to initiate therapeutic and prophylactic vitamin E supplementation before irreversible damage develops.

Dissociation of LPL and LDL: effects of lipoproteins and anti-apoB antibodies

We have shown previously that the activity of lipoprotein lipase (LPL), the major enzyme responsible for hydrolysis of triglyceride contained in circulating lipoproteins, is associated with lipoproteins in postheparin plasma. In other studies, microtiter plate assays showed that LPL interaction with low density lipoprotein (LDL) and very low density lipoprotein (VLDL) was decreased by antibodies to apolipoprotein (apo)B. To test whether antibodies to apoB affected LPL-LDL association in solution, two types of assays were performed, gel filtration and coprecipitation. First we showed that LPL activity and immunoreactive mass co-eluted during gel filtration of normal postheparin plasma, approximately with the peak of low density lipoproteins. Then LPL was used for gel filtration studies in the presence and absence of LDL and anti-apoB monoclonal antibodies. LPL association with LDL was diminished by antibodies to the amino-terminal region of apoB; antibodies to the carboxyl-terminal LDL receptor binding region of apoB were less effective. LDL binding to LPL containing heparin-agarose was also disrupted by the amino-terminal antibodies to apoB. To determine the LPL-lipoprotein association in situations in which the distribution of plasma lipoproteins was altered, we studied plasma from two types of subjects with dyslipidemias. The addition of 125I-labeled LPL to type 1 postheparin plasma produced two peaks of radioactivity, one peak eluted in the void volume of the column (with the chylomicrons) and a second peak eluted just prior to the normal elution of low density lipoproteins. In postheparin plasma from an abetalipoproteinemic subject, LPL eluted with HDL. We conclude that LPL associates primarily with apoB-containing lipoproteins. The reason for this appears to be that LPL interacts with the apoB.

Translocation of apolipoprotein B across the endoplasmic reticulum is blocked in abetalipoproteinemia

Abetalipoproteinemia (ABL) is an autosomal recessive disease characterized by the inability of the liver and intestine to secrete apolipoprotein B (apoB). Mutations in the microsomal triglyceride transfer protein (MTP) gene, but not the apoB gene, are responsible for the ABL phenotype. It is not clear how loss of MTP in ABL patients leads to a complete, but specific, block in the secretion of apoB. It is to this question that our work is directed. In cultured cells lacking MTP, translocation of apoB is completely arrested, leading to the hypothesis that apoB requires MTP in order to completely enter the lumen of the endoplasmic reticulum, the site of lipoprotein assembly. We examined this hypothesis by determining the presence in plasma of distinct N-terminal apoB peptides, produced exclusively from translocation arrested apoB, in the plasma of six ABL patients and six normal subjects. The data show that N-terminal apoB peptides are present in the plasma of six ABL patients, whereas intact apoB-100 was barely detectable. Moreover, the plasma of all six ABL patients displayed a 2000-fold increase in the amount of an 85 kDa N-terminal apoB peptide relative to apoB-100. These data provide the first in vivo data supporting the essential role that MTP plays in apoB translocation. In normal humans, varied expression of MTP may be responsible for the post-transcriptional regulation of apoB secretion.

A novel function for apolipoprotein B: lipoprotein synthesis in the yolk sac is critical for maternal-fetal lipid transport in mice

Apolipoprotein (apo) B, the principal structural component necessary for the synthesis and secretion of triglyceride-rich lipoproteins by the intestine and liver, is highly expressed in the yolk sac visceral endoderm of mammals, although its function in this tissue has been hitherto unclear. Disruption of the apoB gene in mice results in embryonic lethality (approximately 9.5 - 10.5 d). Here we demonstrate that apoB is normally expressed at early time points in embryonic development in yolk sac visceral endodermal cells, and that this expression is associated with the synthesis and secretion of apoB-containing lipoproteins. The lack of apoB in the visceral endoderm resulted in an accumulation of intracellular lipid droplets, an absence of lipoproteins from the secretory pathway, and reduced concentrations of cholesterol and alpha-tocopherol in tissues of apoB-/- embryos. Visceral endoderm of apoB+/- embryos exhibited an intermediate phenotype. Our results suggest that apoB plays an essential role in the transport of lipid nutrients to the developing mouse embryo via the yolk sac-mediated synthesis and secretion of apoB-containing lipoproteins.

Lipoproteins containing apolipoprotein B isolated from patients with abetalipoproteinemia and homozygous hypobetalipoproteinemia: identification and characterization

Abetalipoproteinemia (ABL) and homozygous hypobetalipoproteinemia (HBL) are inherited disorders which are classically characterized by progressive retinal and spinocerebellar disease, fat-soluble vitamin deficiency, and absence of apolipoprotein (apo) B from the plasma. Using immunoaffinity chromatography with an anti-apo B antiserum, we isolated apo B-containing lipoprotein (LpB) particles from the plasma of 4 ABL and 2 HBL patients. The LpB particles were characterized and compared with low density lipoprotein (LDL) and LpB isolated from normal plasma. The ABL/HBL LpB particles were similar in size and charge to normal LpB particles but were relatively enriched in several other apolipoproteins. They contained alpha-tocopherol in a ratio to cholesterol that was proportionately much higher than the very low ratio of alpha-tocopherol to cholesterol in plasma. They bound saturably to fibroblasts and were internalized and degraded similarly to LDL. Hence, the molecular defects in ABL and HBL permit the secretion of a very small number of apo B-containing lipoproteins which may be important for transport of alpha-tocopherol to peripheral tissues.

Human alpha-tocopherol transfer protein: cDNA cloning, expression and chromosomal localization

alpha-Tocopherol transfer protein (alpha TTP), which specifically binds this vitamin and enhances its transfer between separate membranes, was previously isolated from rat liver cytosol. In the current study we demonstrated the presence of alpha TTP in human liver by isolating its cDNA from a human liver cDNA library. The cDNA for human alpha TTP predicts 278 amino acids with a calculated molecular mass of 31,749, and the sequence exhibits 94% similarity with rat alpha TTP at the amino acid level. The recombinant human alpha TTP expressed in Escherichia coli exhibits both alpha-tocopherol transfer activity in an in vitro assay and cross-reactivity to the anti-(rat alpha TTP) monoclonal antibody. Northern blot analysis revealed that human alpha TTP is expressed in the liver like rat alpha TTP. The human and rat alpha TTPs show structural similarity with other apparently unrelated lipid-binding/transfer proteins, i.e. retinaldehyde-binding protein present in retina, and yeast SEC14 protein, which possesses phosphatidylinositol/phosphatidylcholine transfer activity. Both Southern-blot hybridization of human-hamster somatic cell hybrid lines and fluorescence in situ hybridization revealed a single alpha TTP gene corresponding to the 8q13.1-13.3 region of chromosome 8, which is identical to the locus of a recently described clinical disorder, ataxia with selective vitamin E deficiency (AVED). The relationship between alpha TTP and AVED will be discussed.

Exencephaly and hydrocephaly in mice with targeted modification of the apolipoprotein B (Apob) gene

Apolipoprotein B (apoB) is a key structural component of several lipoproteins. These lipoproteins transport cholesterol, lipids, and vitamin E in the circulation. Humans that produce truncated forms of apoB have low plasma concentrations of apoB, beta-lipoproteins, cholesterol, and often vitamin E. This condition has been modeled in mice by targeted modification of the apoB gene. Homozygous transgenic mice display all of the hallmarks of the human disorder. Unexpectedly, approximately 30% of the perinatal homozygotes are exencephalic and of those that have closed neural tubes, approximately 30% are hydrocephalic. The latter condition has also been noted in a relatively small proportion of the heterozygous mice. Vital staining of gestational day 9 (GD9) homozygous offspring has illustrated a striking pattern of excessive cell death involving the alar plate of the hindbrain. Histological and scanning electron microscopic analyses have confirmed this finding. We speculate that varying degrees of affect, as noted among GD 9 and 10 embryos, lead to the spectrum of malformations, including hydrocephaly, present in term fetuses. Analysis of vitamin E deficiency as a possible causative factor has illustrated that homozygous fetuses, indeed, show this deficiency. Amelioration of the defects through alpha-tocopherol supplementation of the maternal diet has been explored. Further analyses of this transgenic mutant promise to provide significant information relative to the role of deficiency of vitamin E and other apoB dependent compounds in dysmorphogenesis.

Human plasma vitamin E kinetics demonstrate rapid recycling of plasma RRR-alpha-tocopherol

A kinetic model of vitamin E transport in humans is described using data from our studies with deuterium-labeled stereoisomers of alpha-tocopherol (RRR- and SRR-). In normal subjects, both alpha-tocopherols are present at similar concentrations in chylomicrons, but by 24 hr, RRR-alpha-tocopherol is at higher plasma concentrations because RRR-alpha-tocopherol is preferentially incorporated into very low density lipoproteins, which are then secreted into plasma. In three nondiscriminator patients with familial isolated vitamin E deficiency, the fractional disappearance rates (mean +/- SD) of deuterium-labeled RRR- and SRR-alpha-tocopherols in plasma were 1.4 +/- 0.6 and 1.3 +/- 0.3 pools per day, respectively (difference, 0.1 +/- 0.3). In these patients, plasma concentrations of both RRR- and SRR-alpha-tocopherols decreased similarly to SRR-alpha-tocopherol in controls. In six controls, fractional disappearance rates of deuterium-labeled RRR-alpha-tocopherol (0.4 +/- 0.1 pool per day) were significantly (P < 0.01) slower than for SRR- (1.2 +/- 0.6). The differences (0.8 +/- 0.6 pool per day) between these two rates in controls estimate the rate at which RRR-alpha-tocopherol, which had left the plasma, was returned to the plasma. Although plasma labeled RRR-alpha-tocopherol concentrations in controls appear to change slowly, these data show that both RRR- and SRR-alpha-tocopherols leave the plasma rapidly, but only RRR-alpha-tocopherol is returned to the plasma, likely in nascent very low density lipoproteins. This recycling of RRR-alpha-tocopherol accounts for nearly 1 pool of alpha-tocopherol per day.

beta-Carotene transport in human lipoproteins. Comparisons with a-tocopherol

The purpose of this study was to investigate the temporal relationships of the transport of beta-carotene in human lipoproteins. We administered 60 mg beta-carotene with breakfast to nine fasting subjects, then blood samples were collected at intervals of up to 75 h, lipoproteins were isolated, and beta-carotene was quantitated. beta-Carotene concentrations in chylomicrons and very low density lipoproteins (VLDL) peaked at 6 and 9 h, respectively. Nonetheless, at all time points the majority of plasma beta-carotene was contained in low density lipoproteins (LDL), while high density lipoproteins (HDL) carried a smaller portion (at 24 h, 73 +/- 8% in LDL as compared with 23 +/- 5% in HDL). In three subjects, transport of beta-carotene was compared with the results of earlier studies on the transport of stereoisomers of alpha-tocopherol. Unlike plasma RRR-alpha-tocopherol concentrations, which are maintained by the preferential incorporation of RRR-alpha-tocopherol into VLDL by the liver, beta-carotene increased and decreased in VLDL similarly to SRR-alpha-tocopherol, a stereoisomer whose concentrations are not maintained in plasma. In conclusion, beta-carotene is primarily transported in the plasma in LDL, but its incorporation by the liver into lipoproteins does not appear to be enhanced.

Discrimination between RRR- and all-racemic-alpha-tocopherols labeled with deuterium by patients with abetalipoproteinemia

The ability to discriminate between stereoisomers of alpha-tocopherol was studied in five patients with abetalipoproteinemia (ABL) because an impairment in secretion of apolipoprotein B-containing lipoproteins might impede the normally enhanced plasma transport of RRR-alpha-tocopherol. An oral dose containing 3.7 g of each 2R, 4'R,8'R-alpha-[5-C2H3]tocopheryl acetate (d3RRR-alpha-tocopheryl acetate) and 2RS,4'RS,8'RS-alpha-[5,7-(C2H3)2]tocopheryl acetate (d6 all rac-alpha-tocopheryl acetate) was administered, then the labeled and unlabeled alpha-tocopherol contents of plasma and red blood cells from multiple blood samples obtained at selected times up to 72 h following the dose were quantitated. ABL plasma contained about 1%-10% of the d3-RRR-alpha-tocopherol concentrations of normal subjects given only 150 mg of each isotope. Three of the patients discriminated between forms of alpha-tocopherol with ratios of RRR-/allrac-alpha-tocopherol > or = 1.8, similar to normals. These data suggest that the hepatic tocopherol binding protein is present and functional in ABL patients. Although two of the patients did not discriminate between stereoisomers of alpha-tocopherol, it is likely that this resulted from nearly a complete block in very low density lipoprotein (VLDL) secretion. Thus, the ability of ABL patients to absorb and transport orally administered vitamin E is markedly impaired and variable among patients.

Efficacy of water-soluble vitamin E in the treatment of vitamin E malabsorption in short-bowel syndrome

A water-soluble form of vitamin E, tocopheryl succinate polyethylene glycol 1000 (TPGS), was used as an oral vitamin E supplement in a 71-y-old patient with severe fat malabsorption and vitamin E deficiency secondary to short-bowel syndrome. An absorption test with deuterium-labeled TPGS demonstrated that TPGS was absorbed and the released alpha-tocopherol was transported normally in lipoproteins. The disappearance portion of the deuterated alpha-tocopherol curves were parallel to those in control subjects, suggesting normal metabolic turnover of alpha-tocopherol. Long-term (3 y) supplementation with orally administered TPGS (10,360 mg or 4000 IU/d) maintained normal plasma alpha-tocopherol concentrations, raised adipose tissue alpha-tocopherol concentrations, and prevented further progression of the neurological abnormalities resulting from vitamin E deficiency. Thus, TPGS can be an effective vitamin E supplement in short-bowel syndrome despite severe fat malabsorption.

Concomitant brainstem axonal dystrophy and necrotizing myopathy in vitamin E-deficient rats

The purpose of this study was to simultaneously evaluate in rats the effects of vitamin E depletion on tissue alpha-tocopherol (alpha-T) concentrations, electrophysiologic measurements and histopathology. Rats (21-day-old male Wistar) were fed either vitamin E-deficient or supplemented (control) diets (n = 6/group) for 10, 16, and 61 weeks. At these times, electrophysiologic tests (electromyography, spinal and somatosensory evoked potentials, and motor nerve conduction velocity) were performed, the rats were killed and alpha-T concentrations of adipose tissue, sciatic nerve, and cervical and lumbar spinal cord were measured along with histopathologic evaluation of skeletal muscles and the nervous system. By 61 weeks, depletion of alpha-T from adipose tissue and peripheral nerve was more severe (< 1% of controls) than from cervical and lumbar spinal cord (15 and 8% of controls, respectively). Electrophysiologic tests were normal at all times. Histopathologic evaluation at 61 weeks revealed normal peripheral nerve structure, but necrosis of type 1 muscle fibers and increased numbers of spheroids in the gracile and cuneate nuclei. Our results confirm that low alpha-T concentrations in tissues precede histologic changes in peripheral nerves and skeletal muscle. Furthermore, pathologic changes associated with vitamin E deficiency occur independently in muscle and nervous tissue of rats.

Intestinal expression of human apolipoprotein A-IV in transgenic mice fails to influence dietary lipid absorption or feeding behavior

Two transgenic mouse lines, expressing low or high amounts of human apo A-IV were created. In low and high expressor HuAIVTg mice on a chow diet, serum human apo A-IV levels were 6 and 25 times the normal human level and on a high fat diet, they were 12 and 77 times higher. Human apo A-IV was equally distributed between lipoprotein (mainly HDL) and lipid-free fractions. Intestinal absorption of radiolabeled cholesterol and triglycerides was unaffected in HuAIVTg mice. Vitamin A, carried exclusively in chylomicrons and their remnants, was catabolized normally. When an intragastric vitamin E bolus is given to the HuAIVTg mice, the initial absorption and appearance in triglyceride-rich lipoproteins was similar to that observed in normal mice. However, elevated amounts of vitamin E were subsequently observed in the VLDL of the HuAIVTg mice. Furthermore, in the fed state, serum VLDL triglycerides were markedly elevated in HuAIVTg mice. This effect was greater in high expressor mice. Serum total cholesterol was not elevated, but the distribution was altered in the HuAIVTg mice; VLDL-C was increased at the expense of VLDL-C. Kinetic studies suggested a delayed clearance of VLDL in HuAIVTg mice. Apo A-IV has been suggested to be a satiety factor, but no effect on feeding behavior or weight gain was observed in these HuAIVTg mice. In summary, our studies with HuAIVTg mice show that additional apo A-IV does not effect intestinal absorption of fat and fat-soluble vitamins, and at least chronic elevation of plasma apo A-IV does not effect feeding behavior in this model system.

Vitamin E deficiency in dogs does not alter preferential incorporation of RRR-alpha-tocopherol compared with all rac-alpha-tocopherol into plasma

The plasma and lipoprotein transport of RRR and all rac-alpha-tocopherols, labeled with different amounts of deuterium [2R,4'R,8'R-alpha-[5-C2H3]tocopheryl acetate (d3RRR-alpha-tocopheryl acetate] and 2RS, 4'RS, 8'RS-alpha-[5,7-(C2H3)2]tocopheryl acetate (d6all rac-alpha-tocopheryl acetate), was studied in adult beagle dogs that had been fed a vitamin E-deficient (-E; two dogs) or supplemented (+E; two dogs) diet for two years. We set out to test the hypothesis that the activity of the hepatic tocopherol binding protein (which is thought to preferentially incorporate RRR-alpha-tocopherol into the plasma) is up-regulated by vitamin E deficiency. Labeled alpha-tocopherols increased and decreased similarly in plasma of both -E and +E dogs. Irrespective of diet, d3RRR-alpha-tocopherol was preferentially secreted in plasma. Thus, vitamin E deficiency in dogs does not markedly increase the apparent function of the hepatic tocopherol binding protein. We also studied vitamin E transport in a German Shepherd dog with degenerative myelopathy (DM). Based on the coincident appearance of d3RRR-alpha-tocopherol in plasma and chylomicrons, we suggest that the abnormality in DM may be associated with abnormal vitamin E transport resulting from an impaired function of the hepatic tocopherol binding protein.

Depletion of adipose tissue and peripheral nerve alpha-tocopherol in adult dogs

To assess the relationship between tissue alpha-tocopherol depletion and histopathologic or functional changes in nervous tissue, a longitudinal study of male 1-year-old beagle dogs, two fed a vitamin E-deficient diet (0.05 +/- 0.02 mg alpha-tocopherol/kg;--E dogs) and two fed a vitamin E-supplemented diet (114 +/- 14 mg alpha-tocopherol/kg; +E dogs), was carried out. Plasma and adipose tissue alpha-tocopherol concentrations, neurological examinations, and sensory and motor nerve conduction velocities were determined at approximately 8-wk intervals over 109 wk. Tibial nerve alpha-tocopherol concentrations were measured at 65 and 109 wk; adjacent sections were examined for histologic changes. In the two -E dogs, plasma alpha-tocopherols declined linearly on a semilog plot to < 0.1 microgram/mL by 109 wk. Plasma alpha-tocopherol concentrations were depleted to half of the initial concentrations in approximately 87 d. Adipose tissue alpha-tocopherol concentrations (based on wet weight, cholesterol or triglyceride) also declined linearly on semilog plots, and were depleted to half of the initial concentrations in approximately 120 d. Tibial nerve alpha-tocopherols (ng/microgram cholesterol) in -E dogs decreased to 16% of average +E at 65 wk, and to 2% at 109 wk. Neurologic examinations, histologies and nerve conduction velocities were normal in all dogs throughout the study. Our results demonstrate in dogs that depletion of plasma, adipose tissue and nerve alpha-tocopherol precedes histologic and functional changes in peripheral nerves during vitamin E deficiency.

Alpha-tocopherol concentrations of the nervous system and selected tissues of adult dogs fed three levels of vitamin E

The effects of dietary vitamin E levels on tissue alpha-tocopherol (alpha-T) concentrations in different parts of the nervous system are largely unknown. Therefore, we measured the alpha-T contents of nervous and other tissues obtained from beagle dogs fed for two years a vitamin E-deficient diet (-E, 0.05 +/- 0.02 mg vitamin E/kg diet, n = 2), a vitamin E-supplemented diet (+E, 114 +/- 14 mg/kg, n = 2), or a standard chow diet (En, 74 +/- 6 mg/kg, n = 3). Brain regions and spinal cords of +E dogs contained about double the alpha-T concentrations of En dogs, and about 10-fold those of -E dogs. The various brain regions of -E dogs, compared with En dogs, retained 12-18% of the alpha-T concentrations, with the exception of the caudal colliculus, which retained 48%. Peripheral nerve alpha-T concentrations in +E dogs (67 ng/mg wet weight) were nearly 5-fold higher than in En dogs (13.4 +/- 5.9 ng/mg) and 80-fold higher than in -E dogs (0.8 ng/mg). Within each dietary group, the lowest alpha-T concentrations in the central nervous system (CNS) were in the spinal cord. Peripheral nerves were the most susceptible to vitamin E repletion or depletion: in +E dogs, nerves contained higher concentrations of alpha-T than most brain regions; in En dogs, they contained similar concentrations; but in -E dogs, they contained less alpha-T than most brain regions. Muscles and other tissues of -E dogs retained from 1 to 10% of En values.(ABSTRACT TRUNCATED AT 250 WORDS)

Interrelationships of alpha-tocopherol with plasma lipoproteins in African green monkeys: effects of dietary fats

The distributions of plasma lipoprotein alpha-tocopherol and lipids were studied in African green monkeys consuming diets enriched in saturated, monounsaturated, or polyunsaturated fatty acids. Plasma total alpha-tocopherol concentrations were not different among the animals fed the three diets, whereas plasma total cholesterol concentrations were significantly different among the diet groups. The alpha-tocopherol: total lipid molar ratio in plasma high density lipoproteins (HDL) was significantly higher compared to low density lipoproteins (LDL) and very low plus intermediate-sized low density lipoprotein (VLDL + ILDL) of each diet group, suggesting that HDL may exhibit a greater affinity for alpha-tocopherol. The presence of a positive correlation between HDL alpha-tocopherol and plasma apoA-I concentration and the absence of a correlation between HDL alpha-tocopherol and total lipid in HDL suggested that alpha-tocopherol associates with the protein moiety of HDL on the surface of the particle. A direct relationship between the plasma apoA-I: apoB molar ratio and the percentage of alpha-tocopherol found in the HDL fraction indicated that a greater proportion of alpha-tocopherol associates with HDL as the number of HDL particles in plasma increases relative to LDL particles. LDL from monkeys fed diets high in saturated fat contained 40% and 33% fewer alpha-tocopherol molecules per particle than LDL from monkeys fed polyunsaturated and monounsaturated fats, respectively. The phase transition temperature of LDL cholesteryl esters, indicative of the physical state of the lipids in the particle core, was well above body temperature in LDL from saturated fat-fed monkeys and was significantly higher compared to the other diet groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in plasma alpha- and gamma-tocopherol concentrations in response to intravenous infusion of lipid emulsions in humans

To study the fate of intravenously infused vitamin E, we infused lipid emulsions rich in gamma-tocopherol (Intralipid, Kabi, Stockholm, Sweden), or in both alpha- and gamma-tocopherols (Lipidem, Hausmann Laboratories, St Gallen, Switzerland); in normal human volunteers. Plasma gamma-tocopherol levels increased in four subjects infused with Intralipid 10% (0.3 g triglyceride [TG]/kg/h for 6 hours) from 3 +/- 1 to 25 +/- 2 nmol/mL, but by 24 hours they decreased to 5 +/- 1 nmol/mL. Although eight times more gamma-tocopherol was infused, plasma alpha-tocopherol levels also increased from 26 +/- 7 to 39 +/- 9 nmol/mL at 8 hours and decreased to 24 +/- 5 nmol/mL at 24 hours. Increases of alpha-tocopherol in the very-low-density lipoprotein (VLDL) density range occurred at 6 and 8 hours, while decreases occurred in low-density lipoprotein (LDL) and high-density lipoprotein (HDL) density ranges at 3, 6, 8, and 24 hours. Infusion of both emulsions in random order to six subjects at therapeutic rates (0.1 g/kg/h for 6 hours) resulted in (1) a threefold increase in plasma gamma-tocopherol concentrations at 6 hours, (2) increases in plasma alpha-tocopherol concentrations only with Lipidem (from 14.3 +/- 1.0 nmol/mL at 0 hours to 18.4 +/- 2.7 at 6 hours and 18.9 +/- 1.1 at 24 hours), and (3) no decreases in lipoprotein alpha-tocopherol levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired discrimination between stereoisomers of alpha-tocopherol in patients with familial isolated vitamin E deficiency

We assessed whether patients with familial isolated vitamin E deficiency could discriminate between natural (RRR-) and synthetic (SRR-) stereoisomers of alpha-tocopherol labeled with six (d6) or three (d3) deuterium atoms, respectively. After oral administration of 20 mg of each of the stereoisomers, patients (seven) and controls (seven) had similar concentrations of both in chylomicrons, similar initial increases of both, and similar rates of decrease of d3-SRR-alpha-tocopherol in plasma. Patients and controls differed in their abilities to maintain plasma d6-RRR-alpha-tocopherol concentrations. Controls maintained plasma d6-RRR-alpha-tocopherol concentrations by preferentially secreting it in very low density lipoprotein (VLDL). Three of seven patients did not discriminate between the two stereoisomers and their plasma and lipoprotein d6-RRR-alpha-tocopherol concentrations declined rapidly. The remaining patients were intermediate between non-discriminators and controls in their ability to discriminate and maintain plasma d6-RRR-alpha-tocopherol concentrations. The degree of discrimination between the two stereoisomers in the patients was correlated with the age of onset of the neurologic disability (r2 = 0.64, P < 0.03). Estimates based on the rate of decrease of plasma d6-RRR-alpha-tocopherol in non-discriminators suggest that the entire plasma alpha-tocopherol pool of normal subjects is replaced daily. We suggest 1) that a hepatic alpha-tocopherol binding protein, which preferentially incorporates RRR-alpha-tocopherol into VLDL, is required to maintain plasma RRR-alpha-tocopherol concentrations; 2) that non-discriminators are lacking this protein, or have a marked defect in the RRR-alpha-tocopherol binding region of the protein; and 3) that patients who discriminate, but have difficulty maintaining plasma RRR-alpha-tocopherol concentrations, have a less severe defect, or perhaps a defect in the transfer function of the protein.

Comparison of four erythrocyte fragility tests as indicators of vitamin E status in adult dogs

Plasma alpha-tocopherol (alpha-T) concentrations, erythrocyte osmotic fragility and detergent sensitivity were measured at 8 week intervals in two 1-year-old male beagle dogs fed a vitamin E-deficient diet (< 0.08 mg per kg alpha-T) and in two control beagles fed the same diet supplemented with vitamin E (> 90 mg per kg alpha-T). Beginning at 24 weeks, dialuric acid haemolysis and spontaneous haemolysis were evaluated also. In the vitamin E-deficient dogs, plasma alpha-T concentrations declined progressively from baseline values of 20.5 and 31.3 micrograms per ml to 0.11 and 0.07 micrograms per ml, respectively, by 90 weeks. The supplemented dogs maintained alpha-T concentrations between 18.3 and 38.4 micrograms per ml. Both dialuric acid haemolysis (R = -0.89) and spontaneous haemolysis (R = -0.91) increased with declining plasma alpha-T concentration. In the dialuric acid haemolysis assay, 50 per cent haemolysis occurred when plasma alpha-T declined to 1.7 micrograms per ml, compared with spontaneous haemolysis in which 50 per cent haemolysis occurred when plasma alpha-T declined to 0.5 micrograms per ml. Osmotic fragility and detergent sensitivity remained unchanged in the vitamin E-deficient dogs throughout the study. Of the four tests, dialuric acid haemolysis was the most sensitive in-vitro assay for vitamin E deficiency in adult dogs.

Studies on the transfer of tocopherol between lipoproteins

The net transfer of labeled alpha-tocopherol from donor to acceptor lipoproteins at physiological concentrations was investigated. Labeled lipoproteins were isolated i) following in vitro addition of [3,4-3H] all rac-alpha-tocopherol to plasma, or ii) from plasma obtained 12-16 h after ingestion by normal subjects of an oral dose (100 mg each) of 2R,4'R,8'R-alpha-[5,7-(C2H3)2]tocopheryl acetate and 2S,4'R,'R-alpha-[5-C2H3]tocopheryl acetate. A constant amount (on a protein basis) of labeled lipoprotein was incubated with an increasing amount of unlabeled acceptor lipoprotein for 2 h at 37 degrees C. No discrimination between stereoisomers of alpha-tocopherol was detected. Labeled VLDL and labeled LDL (very low and low density lipoproteins, respectively) tended to retain their labeled tocopherol. Labeled high density lipoproteins (HDL) readily transferred the labeled tocopherol to VLDL (> 60% transferred), while the transfer to LDL was dependent upon the ratio of labeled HDL/LDL with a lower net transfer at higher ratios. This dependency of the distribution of tocopherol upon the ratio of HDL/LDL was also observed in vivo. The tocopherol/mg HDL protein was measured in 11 subjects with varying HDL levels. As the % HDL in the plasma increased from 14 to 50%, the tocopherol/HDL protein also increased (r2 = 0.37, P < 0.05).

Discrimination between forms of vitamin E by humans with and without genetic abnormalities of lipoprotein metabolism

To study the mechanisms of discrimination between various forms of vitamin E, four normal subjects, one patient with lipoprotein lipase deficiency, and three patients with abnormal apolipoprotein B-100 production were given an oral dose containing three tocopherols labeled with differing amounts of deuterium (2R,4'R,8'R-alpha-(5,7-(C2H3)2)tocopheryl acetate (d6-RRR-alpha-tocopheryl acetate), 2S,4'R,8'R-alpha-5-(C2H3)tocopheryl acetate (d3-SRR-alpha-tocopheryl acetate), and 2R,4'R,8'R-gamma-(3,4-2H)tocopherol (d2-RRR-gamma-tocopherol). The tocopherol contents of plasma, red cells, and lipoproteins were measured up to 76 h after the dose. In normal subjects all three tocopherols were absorbed and secreted in chylomicrons with equal efficiencies. Both d2-gamma- and d3-SRR-alpha-tocopherols peaked at similar concentrations in the other lipoprotein fractions, then decreased similarly, but 2-4 times more rapidly than did d6-RRR-alpha-tocopherol. A lipoprotein lipase-deficient patient and a patient with prolonged production of chylomicrons with absent apolipoprotein B-100 also demonstrated the lack of discrimination between tocopherols during absorption. Despite abnormal apolipoprotein B-100 production in two patients, the "VLDL" was preferentially enriched in d6-RRR-alpha-tocopherol. Our results show that there is no discrimination between the three tocopherols during absorption and secretion in chylomicrons, but subsequently there is a preferential enrichment of very low density lipoprotein (VLDL) with RRR-alpha-tocopherol. Catabolism of this VLDL results in the maintenance of plasma RRR-alpha-tocopherol concentrations.

Vitamin E status of patients receiving long-term parenteral nutrition: is vitamin E supplementation adequate?

Vitamin E status of eight patients receiving total parenteral nutrition (TPN), including 10 IU of all-racemic alpha-tocopheryl acetate daily and Intralipid 20% (500 mL; 12 mg of RRR-alpha- and 92 mg of RRR-gamma-tocopherols) two to three times per week for 69 +/- 45 (mean +/- SD) months was assessed by measuring plasma and adipose tissue tocopherol concentrations. Plasma alpha-tocopherols of TPN patients were similar to controls (17.5 +/- 6.6 mumol/L vs 22.4 +/- 5.1), whereas gamma-tocopherols were significantly reduced (6.0 +/- 3.1 vs 11.2 +/- 3.6, p less than 0.03). The adipose tissue alpha- and gamma-tocopherol/triglycerides (TG) were similar (369 +/- 215 nmol/mmol vs 452 +/- 228, and 125 +/- 102 vs 140 +/- 130, respectively), but cholesterol/TG were increased in the TPN patients (7.8 +/- 2.5 mumol/mmol vs 5.1 +/- 3.5, p less than 0.05), suggesting that adipose tissue was relatively TG-depleted and tocopherol/cholesterol measurements better reflect vitamin E status. The mean alpha-tocopherol/cholesterol ratios were significantly lower in the TPN patients than the controls (55 +/- 36 vs 106 +/- 63, p less than 0.04). Thus, current vitamin E supplementation of TPN patients seems insufficient for maintenance of adequate tissue stores.

RRR- and SRR-alpha-tocopherols are secreted without discrimination in human chylomicrons, but RRR-alpha-tocopherol is preferentially secreted in very low density lipoproteins

Five subjects ingested in a single oral dose containing 50 mg each of 2R,4'R,8'R-alpha-(5,7-(C2H3)2)tocopheryl acetate (d6-RRR-alpha-tocopheryl acetate) with natural stereochemistry, and of 2S,4'R,8'R-alpha-(5-C2H3)tocopheryl acetate (d3-SRR-alpha-tocopheryl acetate). These are two of eight stereoisomers in synthetic vitamin E. By day 1 the plasma and red blood cells were enriched fourfold with d6-RRR-alpha-tocopherol (P less than 0.004). The ratio of d6-RRR-/d2-SRR- further increased over the succeeding 4 days, because the d3-SRR- decreased at a faster rate than did the d6-RRR-stereoisomer. Plasma and lipoproteins were isolated at intervals during the first day, and daily for 3 days, from four additional subjects fed a mixture of equal amounts of the deuterated tocopherols. The plasma contained similar concentrations of the two forms until 11 h, when the d6-RRR-alpha-tocopherol concentration became significantly greater (P less than 0.05). The chylomicrons contained similar concentrations of the two deuterated tocopherols, but the VLDL (very low density lipoproteins) became preferentially enriched in d6-RRR-alpha-tocopherol by 11 h. The pattern of the deuterated tocopherols shows that during chylomicron catabolism all of the plasma lipoproteins were labeled equally with both tocopherols, but that during the subsequent VLDL catabolism the low and high density lipoproteins became enriched in d6-RRR-alpha-tocopherol. These results suggest the existence of a mechanism in the liver for assembling VLDL preferentially enriched in RRR- relative to SRR-alpha-tocopherol.

Nascent VLDL from liver perfusions of cynomolgus monkeys are preferentially enriched in RRR- compared with SRR-alpha-tocopherol: studies using deuterated tocopherols

The transport and secretion of vitamin E in lipoproteins have been studied in cynomolgus monkeys fed tocopherols labeled with different amounts of deuterium. The animals were fed a single dose of vitamin E containing 60 mumol of each 2R,4'R,8'R-alpha-(5,7-(C2H3)2)tocopheryl acetate (d6-RRR-alpha-tocopheryl acetate; alpha-tocopherol with natural stereochemistry), 2S,4'R,8'R-alpha-5-(C2H3)tocopheryl acetate (d3-SRR-alpha-tocopheryl acetate; alpha-tocopherol with unnatural stereochemistry), and 2R,4'R,8'R-gamma-(3,4-2H)tocopherol (d2-RRR-gamma-tocopherol; gamma-tocopherol with natural stereochemistry). Chylomicrons, as well as the other plasma lipoproteins, contained equal concentrations of all three tocopherols at the earliest time points after feeding suggesting that all three tocopherols were absorbed equally. At later times plasma lipoproteins became preferentially enriched in d6-RRR-alpha-tocopherol. This is likely to be due to hepatic secretion of VLDL (very low density lipoproteins) and other lipoproteins, which were enriched in d6-RRR-alpha-tocopherol, as demonstrated in the lipoproteins isolated from perfused livers that had been obtained 24 h following the administration of the deuterated tocopherols. Taken together these data demonstrate that the liver, not the intestine, is the likely site of discrimination between tocopherol isomers and that the liver secretes nascent lipoproteins preferentially enriched in d6-RRR-alpha-tocopherol.

Impaired ability of patients with familial isolated vitamin E deficiency to incorporate alpha-tocopherol into lipoproteins secreted by the liver

Plasma and lipoprotein alpha-tocopherol concentrations of four patients with familial isolated vitamin E deficiency and six control subjects were observed for 4 d after an oral dose (approximately 15 mg) of RRR-alpha-tocopheryl acetate labeled with six deuterium atoms (d6-tocopherol). Chylomicron d6-tocopherol concentrations were similar in the two groups. d6-Tocopherol concentrations of plasma, very low (VLDL), low (LDL), and high (HDL) density lipoproteins were similar in the two groups only during the first 12 h; then these were significantly lower, and the rate of disappearance faster, in the patients. The times (tmax) of the maximum chylomicron d6-tocopherol concentrations were similar for the two groups, but tmax values in the controls increased in the order: chylomicrons less than VLDL less than or equal to LDL approximately HDL, while the corresponding values in the patients were similar to the chylomicron tmax. Thus, plasma d6-tocopherol in controls increased during chylomicron and VLDL catabolism, whereas in patients it increased only during chylomicron catabolism, thereby resulting in a premature and faster decline in the plasma tocopherol concentration due to a lack of d6-tocopherol secretion from the liver. We suggest that these patients are lacking or have a defective liver "tocopherol binding protein" that incorporates alpha-tocopherol into nascent VLDL.

Vitamin E uptake by human intestinal cells during lipolysis in vitro

Vitamin E uptake by Caco-2 cells, a human intestinal cell line, was studied by incubating the cells with alpha-tocopherol/triglyceride emulsions with or without bile activated lipase or lipoprotein lipase. During a 1-h incubation, vitamin E was transferred to Caco-2 cells only in the presence of triglyceride hydrolysis by bile activated lipase and not by lipoprotein lipase. Incubation with either lipase resulted in hydrolysis of approximately 20% of the medium [3H]-triolein to free fatty acids and a 3-5-fold increase in cellular radioactivity. In the absence of lipases but the presence of taurocholate, addition of oleic acid in an amount equal to the molar concentration of triglyceride (5.7 mM) to triglyceride emulsions containing either alpha-tocopherol or cholesteryl ester resulted in an increase in cellular [3H]-triglyceride and alpha-tocopherol or cholesteryl ester. We suggest that the absorption of hydrophobic molecules such as vitamin E may occur in the presence of bile and amphipathic lipids via the uptake of micellar neutral lipids by the intestine.

Preferential incorporation of alpha-tocopherol vs gamma-tocopherol in human lipoproteins

Approximately 12 h after the ingestion of a single dose containing 1000 mg each of all-rac-alpha-tocopherol and RRR-gamma-tocopherol, the plasma and lipoproteins of normal subjects contained equal increases of both tocopherols; by 24 h the concentration of gamma-tocopherol, but not the alpha-tocopherol, decreased sharply. Similar studies in hyperlipidemic subjects demonstrated that the plasma and the chylomicron fraction from lipoprotein lipase-deficient patients (with elevated chylomicrons) contained both tocopherols up to 24 h, whereas plasma from a patient with dysbetalipoproteinemia (with elevated beta very-low-density lipoproteins) displayed the decrease in gamma-tocopherol at 24 h. These studies demonstrate that both alpha- and gamma-tocopherols are absorbed and secreted by the intestine in chylomicrons, and suggest that alpha-tocopherol is preferentially secreted by the liver in nascent lipoproteins. Furthermore, studies in post-gall bladder surgery patients suggest a preferential secretion of gamma-tocopherol in bile. Thus, the liver rather than the intestine appears to discriminate between alpha- and gamma-tocopherols.

Uptake of intact TPGS (d-alpha-tocopheryl polyethylene glycol 1000 succinate) a water-miscible form of vitamin E by human cells in vitro

The mechanism by which TPGS (alpha-tocopheryl succinate esterified to polyethylene glycol 1000 [PEG 1000]) delivers tocopherol (vitamin E) was studied in human fibroblasts and erythrocytes and a human intestinal cell line, Caco-2. The total cellular tocopherol content of saponified samples of fibroblasts or Caco-2 incubated for 4 h with TPGS (4 mumol/L) increased 10-fold without an increase in the free tocopherol content of nonsaponified samples. A 24-h incubation resulted in a free tocopherol content of approximately 20%, suggesting that intracellular hydrolysis of ester bonds had occurred. The increase in total tocopherol content after a 4-h incubation with TPGS was temperature dependent; no change was measurable at 4 degrees C. Addition of metabolic inhibitors during incubation with TPGS at 37 degrees C did not prevent the increase. [14C]TPGS (synthesized from [14C]PEG 1000) was taken up by Caco-2 cells but [14C]PEG 1000 was not. The intracellular total tocopherol (pmol) equaled the [14C]TPGS (pmol), unequivocally demonstrating uptake of the intact TPGS molecule.

Absorption and transport of deuterium-substituted 2R,4'R,8'R-alpha-tocopherol in human lipoproteins

Oral administration of a single dose of tri- or hexadeuterium substituted 2R,4'R,8'R-alpha-tocopheryl acetate (d3- or d6-alpha-T-Ac) to humans was used to follow the absorption and transport of vitamin E in plasma lipoproteins. Three hr after oral administration of d3-alpha-T-Ac (15 mg) to 2 subjects, plasma levels of d3-alpha-T were detectable; these increased up to 10 hr, reached a plateau at 24 hr, then decreased. Following administration of d6-alpha-T-Ac (15-16 mg) to 2 subjects, the percentage of deuterated tocopherol relative to the total tocopherol in chylomicrons increased more rapidly than the corresponding percentage in whole plasma. Chylomicrons and plasma lipoproteins were isolated from 2 additional subjects following administration of d3-alpha-T-Ac (140 or 60 mg). The percentage of deuterated tocopherol relative to the total tocopherol increased most rapidly in chylomicrons, then in very low density lipoproteins (VLDL), followed by essentially identical increases in low and high density lipoproteins (LDL and HDL, respectively) and lastly, in the red blood cells. This pattern of appearance of deuterated tocopherol is consistent with the concept that newly absorbed vitamin E is secreted by the intestine into chylomicrons; subsequently, chylomicron remnants are taken up by the liver from which the vitamin E is secreted in VLDL. The metabolism of VLDL in the circulation results in the simultaneous delivery of vitamin E into LDL and HDL.

Effect of high doses of dietary vitamin E on the concentrations of vitamin E in several brain regions, plasma, liver, and adipose tissue of rats

The object of this study was to assess the influence of high levels of dietary vitamin E on vitamin E concentrations in specific areas of the brain. Four-week-old male rats were fed vitamin E-deficient, control, and high-vitamin E (1,000 IU/kg) diets for 4 months. Concentrations of alpha-tocopherol in serum, adipose tissue, liver, cerebrum, cerebellum, and striatum were determined by liquid chromatography with fluorescence detection. In the high-vitamin E group, alpha-tocopherol concentrations in cerebrum, cerebellum, and striatum increased uniformly to 1.4-fold of values in controls; serum, adipose tissue, and liver attained even higher concentrations: 2.2-, 2.2-, and 4.6-fold, respectively, of control values. As observed before, brain levels of alpha-tocopherol were somewhat resistant to vitamin E deficiency, in contrast to the peripheral tissues.

Isolated vitamin E deficiency in the absence of fat malabsorption--familial and sporadic cases: characterization and investigation of causes

We observed four young adults, including three siblings, with a progressive neurologic disorder that developed over the first two decades. Electrophysiologic studies revealed mildly delayed nerve conduction, decreased amplitudes of sensory action potentials, and sensory delay in the posterior columns. Known causes of similar neurologic disorders were excluded. Although vitamin E deficiency was well documented, intestinal absorption and plasma lipoprotein transport of vitamin E were normal. Incubation studies in vitro failed to identify a plasma factor causing destruction of circulating vitamin E. There was no clinical or laboratory evidence of steatorrhea caused by gastrointestinal, hepatic, or pancreatic disease. Plasma lipoproteins, apolipoprotein B, and adipose tissue fatty acid composition were normal. Oral vitamin E therapy restored serum levels to normal and caused neurologic improvement in two patients. We postulate that an inherited defect in hepatocyte secretion of vitamin E into lipoproteins may account for this disorder, which occurs in sporadic cases as well as in siblings.

Polarized secretion of newly synthesized lipoproteins by the Caco-2 human intestinal cell line

Lipoprotein secretion by Caco-2 cells, a human intestinal cell line, was studied in cells grown on inserts containing a Millipore filter (0.45 micron), separating secretory products from the apical and basolateral membranes into separate chambers. Under these conditions, as observed by electron microscopy, the cells formed a monolayer of columnar epithelial cells with microvilli on the apical surface and tight junctions between cells. The electrical resistances of the cell monolayers were 250-500 ohms/cm2. Both 14C-labeled lipids and 35S-labeled proteins were used to assess lipoprotein secretion. After a 24-hr incubation with [14C]oleic acid, 60-80% of the secreted triglyceride (TG) was in the basolateral chamber; 40% of the TG was present in the d less than 1.006 g/ml (chylomicron + VLDL) fraction and 50% in the 1.006 less than d less than 1.063 g/ml (LDL) fraction. After a 4-hr incubation with [35S]methionine, apolipoproteins were found to be major secretory products with 75-100% secreted to the basolateral chamber. Apolipoproteins B-100, B-48, E, A-I, A-IV, and C-III were identified by immunoprecipitation. The d less than 1.006 g/ml fraction was found to contain all of the major apolipoproteins, while the LDL fraction contained primarily apoB-100 and apoE; the HDL (1.063 less than d less than 1.21 g/ml) fraction principally contained apoA-I and apoA-IV. Mn-heparin precipitated all of the [35S]methionine-labeled apoB-100 and B-48 and a majority of the other apolipoproteins, and 80% of the [14C]oleic acid-labeled triglyceride, but only 15% of the phospholipid, demonstrating that Caco-2 cells secrete triglyceride-rich lipoproteins containing apoB. Secretion of lipoproteins was dependent on the lipid content of the medium; prior incubation with lipoprotein-depleted serum specifically reduced the secretion of lipoproteins, while addition of both LDL and oleic acid to the medium maintained the level of apoB-100, B-48, and A-IV secretion to that observed in the control cultures.

Tocopherol distribution and intracellular localization in human adipose tissue

Adipocytes, isolated from human adipose tissue, were disrupted and then centrifuged, resulting in three fractions: oil, interface between the oil and the hypotonic buffer (containing lipid droplets surrounded by unilamellar membranes), and pelleted portion (containing bilayer membranes, intracellular organelles, and debris). The ratios of tocopherol:cholesterol and tocopherol:triglyceride were similar for adipose tissue, adipocytes, oil, and interface. The bulk lipid (oil plus interface fractions) of the adipocytes contained 99% of the tocopherol, 98% of the triglyceride, and 91% of the cholesterol. The tocopherol:cholesterol ratio was nearly eightfold greater in the bulk lipid than in the bilayer membrane fraction. By use of radioactively labeled tocopherol and cholesterol, it was demonstrated that the isolation protocol did not alter the distribution of the intracellular lipids; furthermore, the tocopherol in the lipid droplet did not readily exchange. Thus, the majority of tocopherol in adipose tissue is located in the bulk-lipid stores.