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

Vitamin E and immunity

Vitamin E is the most important chain-breaking, lipid-soluble antioxidant present in body tissues of all cells and is considered the first line of defense against lipid peroxidation and it is important for normal function of the immune cells. However, vitamin E deficiency is rare in well-nourished healthy subjects and is not a problem, even among people living on relatively poor diets, both T- and B-cell functions are impaired by vitamin E deficiency. While immune cells are particularly enriched in vitamin E because of their high polyunsaturated fatty acid content, this point puts them at especially high risk for oxidative damage. Besides its immunomodulatory effects, vitamin E also plays an important role in carcinogenesis with its antioxidant properties against cancer, and ischemic heart disease with limiting the progression of atherosclerosis. Supplementation of vitamin E significantly enhances both cell mediated and humoral immune functions in humans, especially in the elderly and animals.

Vitamin E transfer from lipid emulsions to plasma lipoproteins: mediation by multiple mechanisms

The present study determined alpha-tocopherol mass transfer from an alpha-tocopherol-rich emulsion to LDL and HDL, and assessed the potential of different mechanisms to modulate alpha-tocopherol transfers. Emulsion particles rich in alpha-tocopherol were incubated in vitro with physiological concentrations of LDL or HDL. The influence of plasma proteins was assessed by adding human lipoprotein poor plasma (LPP) fraction with intact vs heat inactivated PLTP, or with a specific cholesteryl ester transfer protein (CETP) inhibitor, or by adding purified PLTP or pig LPP which lacks CETP activity. After 4 h incubation in absence of LPP, alpha-tocopherol content was increased by ~80% in LDL and ~160% in HDL. Addition of LPP markedly enhanced alpha-tocopherol transfer leading to 350-400% enrichment in LDL or HDL at 4 h. Higher (~10 fold) enrichment was achieved after 20 h incubation with LPP. Facilitation of alpha-tocopherol transfer was (i) more than 50% higher with human vs pig LPP (despite similar PLTP phospholipid transfer activity), (ii) reduced by specific CETP activity inhibition, (iii) not fully suppressed by heat inactivation, and (iv) not restored by purified PLTP. In conclusion, alpha-tocopherol content in LDL and HDL can be markedly raised by rapid transfer from an alpha-tocopherol-rich emulsion. Our results indicate that alpha-tocopherol mass transfer between emulsion particles and lipoproteins is mediated by more than one single mechanism and that this transfer may be facilitated not only by PLTP but likely also by other plasma proteins such as CETP.

Effects of a medium-chain triacylglycerol/long-chain triacylglycerol fat emulsion containing a reduced ratio of phospholipid to triacylglycerol in pediatric surgical patients

OBJECTIVE

Medium-chain triacylglycerol (MCT) has been shown to provide better nutritional support than long-chain triacylglycerol (LCT). We compared the efficacy of MCT/LCT fat emulsions containing a usual (0.12) or a decreased (0.06) ratio of phospholipid to triacylglycerol (PL:TG) in pediatric patients under surgical stress.

METHODS

Three patient groups (n=10 in each) received equivalent amounts of glucose (12 g.kg(-1).d(-1)) and amino acids (2 g.kg(-1).d(-1)), but group A received a 10% MCT fat emulsion (PL:TG 0.06), group B received a 20% MCT fat emulsion (PL:TG 0.06), and group C received a 10% MCT/LCT fat emulsion (PL:TG 0.12) in amounts of 1.5 g.kg(-1).d(-1) in a randomized study. Total parenteral nutrition was given for 7 d. Blood samples were collected before total parenteral nutrition administration and on days 4 and 7 for determination of various biochemical indexes.

RESULTS

Serum phospholipid concentrations were significantly higher in group C than in group A or B on days 4 and 7 (P<0.05). Serum triacylglycerol and cholesterol concentrations and the very-low-density lipoprotein percentage were also significantly higher in group C than in group A or B on days 4 and 7 (P<0.05). The high-density lipoprotein percentage was significantly higher in group B on days 4 and 7 (P<0.05).

CONCLUSIONS

In pediatric patients under surgical stress, a total parenteral nutrition regimen containing an MCT/LCT fat emulsion with a decreased PL:TG ratio (0.06) is likely to result in partly better lipid and lipoprotein metabolism than an emulsion containing the usual ratio (0.12).

Vitamin E: absorption, plasma transport and cell uptake

PURPOSE OF REVIEW

Large-scale clinical trials have failed to demonstrate a benefit for vitamin E supplementation in cardiovascular prevention. This contrasts with previous epidemiological studies indicating that individuals with high vitamin E status benefit from protection against chronic illnesses, including cardiovascular diseases. These conflicting results suggest that the metabolism of supplemental versus naturally delivered vitamin E and their potential roles, other than a potent antioxidant action, are not fully understood. The purpose of this review is to provide an update on current knowledge on the intestinal absorption of vitamin E, its plasma transport and its supply to cells. The review will also discuss the intravascular metabolism of intravenously delivered vitamin E.

RECENT FINDINGS

Although the luminal digestion of vitamin E is fairly well understood, several pathways regulating net vitamin E absorption remain to be elucidated. In several cell types, cholesterol and vitamin E share common mechanisms for cellular uptake (scavenger receptor B type I and LDL receptors) and efflux (ABCA1 transporters). The role of specific binding proteins in alpha-tocopherol intracellular trafficking is increasingly being understood, leading to new insights into the non-antioxidant functions of vitamin E.

SUMMARY

Substantial progress has been made in characterizing the plasma transport of vitamin E and its delivery to cells. Mechanisms regulating the balance between the cellular uptake and efflux of vitamin E are under investigation. Vitamin E is not only an antioxidant but may also modulate pathways of cell signalling and gene expression. The translation of this new knowledge into clinical studies will help define future indications for vitamin E supplementation.

Tocopherol isoforms in parenteral lipid emulsions and neutrophil activation

BACKGROUND AND AIMS

Tocopherol is a lipid-soluble anti-oxidant that exists in several isoforms. Patients on total parenteral nutrition depend on lipid emulsions for their tocopherol intake. In the present study, we analysed the content of tocopherol isoforms in various lipid emulsions. We also tested the hypothesis that immune-modulating effects of lipid emulsions could be attributed to different concentrations of alpha-tocopherol (alpha-toc) or peroxidation products.

METHOD

alpha-, beta-, gamma- and delta-toc were measured in emulsions containing long-chain triglycerides (LCT), mixed long- and medium-chain triglycerides (LCT/MCT), structured lipids (SL), olive oil (OO) or fish oil (FO). As a measure for cellular activation, neutrophil membrane adhesion markers were assessed after exposure to two LCT/MCT emulsions that differ only in alpha-toc content.

RESULTS

Various emulsions differed widely in tocopherol contents, especially with respect to the alpha- isoform. The latter isomer also was subject to considerable degradation despite adequate storage conditions. The previously observed activation of neutrophils by LCT/MCT was not influenced by the increased concentration of alpha-toc or the decreased concentration of lipid peroxidation products in a new LCT/MCT emulsion.

CONCLUSIONS

Tocopherol supplementation by parenteral lipid emulsions strongly depends on the lipid source and the storage lifetime of the emulsion. The effects of LCT/MCT on the immune system are not modulated by alpha-toc or by lipid peroxidation products.

Effects of intravenous supplementation with alpha-tocopherol in patients receiving total parenteral nutrition containing medium- and long-chain triglycerides

OBJECTIVE

To compare the effects of a lipid emulsion containing medium-chain triglycerides (MCT) and supplemented with alpha-tocopherol to a conventional long-chain triglyceride (LCT) emulsion.

DESIGN

Randomised double blind study.

SETTING

Department of Internal Medicine, Antwerp University Hospital.

SUBJECTS AND INTERVENTIONS

Twenty-four patients with an indication for total parenteral nutrition for a minimum of 10 days were randomly assigned to two groups: group E received as lipid source MCT/LCT (50/50) suplemented with 100 mg DL-alpha-tocopherol/day and group C received LCT. Blood samples were analysed at inclusion, after 4-6 and after 9-11 days.

RESULTS

In group E, serum alpha-tocopherol doubled from 11.4+/-6.9 at inclusion to 20.9+/-7.9 and to 23.8+/-8.8 microg/ml after 4 and 9 days, respectively, but did not change in group C (P=0.008). Production of thiobarbituric acid-reacting substances (TBARS) after 120 min incubation with copper decreased from 66+/-34 at inclusion to 29+/-25 nmol MDA/mg LDL and VLDL-cholesterol after 4 and to 42+/-17 after 9 days (P=0.022 when compared to group C, which underwent no significant changes). Velocity of production of fluorescent products decreased in group E but not in group C (P=0.026).

CONCLUSIONS

Supplementation of TPN containing MCT/LCT with 100 mg DL-alpha-tocopherol/day leads to a doubling in serum alpha-tocopherol and to a decrease in the susceptibility of LDL and VLDL to peroxidation in vitro.

SPONSORSHIP

This study was partly financed by B Braun Medical NVSA, Diegem, Belgium.

Effect of vitamin supplementation on cytokine response and on muscle damage after strenuous exercise

The present double-blinded, placebo-controlled study investigated whether antioxidant vitamin supplementation was able to modulate the cytokine and lymphocyte responses after strenuous eccentric exercise. Furthermore, muscle enzyme release was examined to see whether antioxidant treatment could reduce muscle damage. Twenty male recreational runners randomly received either antioxidants (500 mg of vitamin C and 400 mg of vitamin E) or placebo for 14 days before and 7 days after a 5% downhill 90-min treadmill run at 75% V˙o 2 max. Although the supplemented group differed significantly with regard to plasma vitamin concentration before and after exercise when compared with the placebo group, the two groups showed identical exercise-induced changes in cytokine, muscle enzyme, and lymphocyte subpopulations. The plasma level of interleukin (IL)-6 and IL-1 receptor antagonist increased 20- and 3-fold after exercise. The plasma level of creatine kinase was increased sixfold the day after exercise. The concentrations of CD4+ memory T cells, CD8+ memory and naı̈ve T cells, and natural killer cells increased at the end of exercise. The total lymphocyte concentration was below prevalues in the postexercise period. In conclusion, the present study does not support the idea that exercise-induced inflammatory responses are induced by free oxygen radicals.

Antioxidant status of surgical patients receiving TPN with an omega-3-fatty acid-containing lipid emulsion supplemented with alpha-tocopherol

BACKGROUND

LCT lipid emulsions and even more fish oil-containing lipid emulsions are under debate regarding their tocopherol and PUFA content as well as their effect on the antioxidative status especially in patients with oxidative stress.

METHODS

Thirty-three patients undergoing major abdominal surgery were randomly assigned to receive either an alpha-tocopherol-supplemented (562 micromol/l) MCT/LCT/omega-3-acid triglycerides (MLF, 5/4/1 w/w/w, 20%) emulsion or a soybean oil-based LCT emulsion (20%). The TPN regimen continuously provided 1.4 g fat kg bw(-1)d(-1)over 5 days.

RESULTS

Plasma antioxidant concentrations were strongly reduced by surgical treatment. Following 5 days of TPN with the MLF emulsion, mean plasma alpha-tocopherol increased by 20.0 micromol/l (1.98 micromol/mmol lipid), while nearly no change was observed in the LCT emulsion group. In both groups, plasma concentrations of all non-supplemented antioxidants (vitamin C, carotenoids, selenium) as well as serum total antioxidant capacity further decreased during TPN. The concentrations of plasma cholesterol oxidation products as a measure of in vivo lipid peroxidation revealed no changes over the TPN period in either group.

CONCLUSION

In contrast to the LCT emulsion, administration of the a-tocopherol supplemented MLF lipid emulsion normalized a-tocopherol plasma concentrations. Despite its high long-chain PUFA content, no hint for increased lipid peroxidation was found.

Phagocyte-induced lipid peroxidation of different intravenous fat emulsions and counteractive effect of vitamin E

Unsaturated fatty acids, a major component of fat emulsions used in parenteral nutrition, are prone to peroxidation which is an important feature of oxygen-associated tissue damage. We used the nitroblue tetrazolium (NBT) reduction test to measure the production of superoxide radicals by stimulated polymorphonuclear neutrophils (PMN) in the presence of different fat emulsions: Intralipid (containing 100% long-chain triacylglycerols, LCT), Vasolipid (a physical mixture of 50% LCT and 50% medium-chain triacylglycerols, MCT) and Structolipid (structured triacylglycerols containing 63% LCT and 37% MCT). We measured the amount of malonaldehyde (MDA) and 4-hydroxyalkenal to determine the lipid peroxidation of the three fat emulsions in the presence of stimulated neutrophils. Further, we investigated the role of vitamin E (alpha-tocopherol) in preventing lipid peroxidation in vitro. The results showed that the values of NBT reduction of PMN were significantly decreased in each of the three fat emulsions and that increasing concentrations of fat emulsions were associated with decreased values of NBT reductions, in a dose-dependent way (P<0.001). There were, however, no statistically significant differences between the values of the three different types of fat emulsions (P>0.05). Lipid peroxidation increased significantly in the presence of all three types of fat emulsions, and was more pronounced for Intralipid than for Vasolipid and Structolipid after 1 and 2 h of incubation with resting as well as with stimulated phagocytes. The increased lipid peroxidation of the fat emulsions was markedly reduced by vitamin E, and the inhibition was concentration dependent. In conclusion, lipid peroxidation in vitro is more pronounced when PMNs are incubated with fat emulsions. This increase in lipid peroxidation can be reduced by adding vitamin E to the fat emulsions.

Clinical use of lipid emulsions

Substantial progress has been made in the understanding of the metabolism of intravenous lipid emulsions and the delivery of their various components to specific tissues or cells. Lipid emulsions should be considered not only as a means of providing energy substrates but also specific compounds that participate in the regulation of key metabolic functions. Such improved knowledge should find applications in the metabolic care of different types of patients.

Effect of infusion of a triacylglycerol emulsion on low-density lipoprotein composition and oxidizability

Six, healthy, male volunteers aged 19-48 years received a 4-h infusion of a triacylgycerol emulsion (Intralipid 10%) after an overnight fast. Plasma triacylglycerol (TAG) and low-density lipoprotein (LDL) -TAG, -protein and -phospholipid concentrations all rose significantly during the course of the infusion and remained elevated 90 min after its end. The weight/weight ratio of LDL-TAG to -protein also increased significantly (from 0.43+/-0.14 to 0.79+/-0.30 at 4 h and 0.63+/-0.31 90 min later), indicating TAG-enrichment of LDL particles. There were no significant changes in LDL particle density. Susceptibility of LDL to copper-induced oxidation, as measured by the lag time for conjugated diene formation, was less at the end of and 90 min after the end of the infusion than in the fasting state (59.3+/-16.5, 47.4+/-17.4 and 34.8+/-19.6 min, respectively). Lag time was positively correlated with LDL TAG in the fasting state (rS=0.900, P < 0.0001) and the correlation continued with the post-infusion TAG-enriched LDL (rs=0.886, P < 0.05). Acute hypertriglyceridaemia induced by infusion of Intralipid therefore causes TAG-enrichment and a decrease in the susceptibility to oxidation of LDL. TAG-enrichment of LDL may lead to subsequent potentially atherogenic changes in LDL following TAG hydrolysis.

Lipid peroxidation of i.v. lipid emulsions in TPN bags: the influence of tocopherols

Four commercial i.v. lipid emulsions containing soybean oil were investigated to determine the tocopherol content. A sensitive high-performance liquid chromatography (HPLC) on a diol column was established to quantitate the tocopherol isomers in lipid emulsions. A previously described iodometric titration was used to assess the peroxide value (mmol peroxides/L). The pH was measured also. The initial tocopherol concentration ranges in three of the four commercial soybean oil-based 20% lipid emulsions studied were compared ([mg/L]: alpha: 17-23, beta: 4, gamma: 88-129, delta: 40-44). One product showed an increased alpha-tocopherol content (172 mg/L) due to supplementation during manufacture. During storage in an ethylvinyl acetate (EVA) bag at 40 degrees C under light-protection (LP) for 34 d, a lipid emulsion 20% with a natural alpha-tocopherol content showed a peroxide value (PV) of 9.18 (about 450 times the value of controls in glass bottles) with a concomittant reduction of the tocopherol isomers to 61.6% (alpha), 86.5% (gamma), and 88.9% (delta) compared to the initial values. Comparison of two lipid emulsions with different amounts of alpha-tocopherol (Lipidem 20%, B. Braun, Switzerland: 156.29 mg/L vs. Intralipid 20%, Pharmacia Upjohn, Switzerland: 8.75 mg/L) for their antioxidative capacity using the same stress conditions revealed for the emulsion with the high alpha-tocopherol content a significantly higher PV over the whole test period (after 5 wk: 33.63 vs. 6.23; P < 0.001) and an increased alpha-tocopherol decomposition (51.6% vs. 8.7%). The drop in pH was higher, also (1.9 vs. 1.0 pH units). In contrast to ordinary concentrations of about 20 mg/L, alpha-tocopherol in 20% lipid emulsions showing antioxidative properties, a supplementation with about 160 mg/L showed a prooxidative effect when exposed to ambient atmosphere in an EVA bag.

gamma-Tocopherol: an efficient protector of lipids against nitric oxide-initiated peroxidative damage

Nitric oxide released by macrophages during inflammation reacts with active oxygen to form peroxynitrite. Peroxynitrite nitrates protein and peroxidizes lipids. gamma-Tocopherol traps peroxynitrite and is more effective than alpha-tocopherol in protecting lipids against such peroxidation.

Human plasma phospholipid transfer protein accelerates exchange/transfer of alpha-tocopherol between lipoproteins and cells

alpha-Tocopherol (alpha-T), an important anti-oxidant of plasma lipoproteins and cell membranes, is secreted from liver together with very-low-density lipoproteins into the blood stream. Other serum lipoprotein classes gain alpha-T by exchange and transfer processes. We show here that the lipoprotein-free d > 1.22 g/ml fraction of human or pig serum increases the exchange rate of alpha-T by a factor of 2-4 as compared with spontaneous exchange/transfer. The alpha-T exchange/transfer (alpha-TET) activity was purified by multiple-step column chromatography. It gave a single band in PAGE with an apparent molecular mass of 75 kDa, and was found to be identical with the phospholipid transfer protein (PLTP). PLTP catalysed alpha-T exchange between different lipoprotein classes, as well as the transfer of alpha-T from artificial liposomes to high-density lipoproteins. The alpha-TET activity measured with a newly developed assay in ten healthy people was 2.45 +/- 0.88 nmol.ml-1.h-1.alpha-TET activity was negatively correlated with plasma low-density lipoprotein-cholesterol (r = -0.75; P < 0.01). It is concluded that human PLTP catalyses exchange/transfer processes of alpha-T between lipid compartments. This factor may be of relevance in atherogenesis and tumour initiation and growth.