Vitamin A metabolism: alpha-tocopherol modulates tissue retinol levels in vivo, and retinyl palmitate hydrolysis in vitro

The BCO2 Genotype and the Expression of BCO1, BCO2, LRAT, and TTPA Genes in the Adipose Tissue and Brain of Rabbits Fed a Diet with Marigold Flower Extract

This study was undertaken to evaluate the effect of the BCO2 genotype and dietary supplementation with marigold flower extract on the expression of BCO1, BCO2, LRAT, and TTPA genes in the adipose tissue and brain of rabbits. The concentrations of lutein, zeaxanthin, β-carotene, retinol, and α-tocopherol were determined in samples collected from rabbits. Sixty young male Termond White rabbits were allocated to three groups based on their genotype at codon 248 of the BCO2 gene (ins/ins, ins/del, and del/del). Each group comprised two subgroups; one subgroup was administered a standard diet, whereas the diet offered to the other subgroup was supplemented with 6 g/kg of marigold flower extract. The study demonstrated that the BCO2 genotype may influence the expression levels of the BCO2, LRAT, and TTPA genes in adipose tissue, and TTPA and BCO1 genes in the brain. Moreover, an increase in the amount of lutein in the diet of BCO2 del/del rabbits may increase the expression of BCO1, LRAT, and TTPA genes in adipose tissue, and the expression of the BCO2 gene in the brain. Another finding of the study is that the content of carotenoids and α-tocopherol increases in both the adipose tissue and brain of BCO2 del/del rabbits.

Hepatic retinaldehyde dehydrogenases are modulated by tocopherol supplementation in mice with hepatic steatosis

OBJECTIVES

An altered retinol metabolism might play a role in the development of nonalcoholic fatty liver disease (NAFLD). Tocopherols (TF) modulate metabolic pathways and have been proposed as a complementary treatment of obesity-induced metabolic alterations. Moreover, there is evidence suggesting that TF may modulate retinol metabolism. The aim of this study was to evaluate whether the dietary supplementation of α- and γ-TF modulates the expression of hepatic retinaldehyde dehydrogenases, RALDH1, RALDH2, and RALDH3 (involved in retinol metabolism) and, lipogenic factors sterol regulatory element binding protein-1c (SREBP-1c) and cluster differentiation 36 (CD36) in an animal model of diet-induced NAFLD.

METHODS

Male C57BL/6J mice were divided into four groups: a control diet (CD) group (10% fat, 20% protein, 70% carbohydrates); a CD + TF group (α-tocopherol: 0.7 mg·kg·d^-1, γ-tocopherol: 3.5 mg·kg·d^-1); a high-fat diet (HFD) group (60% fat, 20% protein, 20% carbohydrates); and a HFD + TF group (0.01 mL·g body weight·d^-1), for 12 wk. General parameters (body-adipose tissue weight, glucose-triacylglyceride serum levels), liver steatosis (histology, liver triacylglycerides content), and hepatic RALDH1, RALDH2, RALDH3, SREBP-1c and CD36 (qPCR, quantitative polymerase chain reaction; IHQ, immunohistochemistry) were measured.

RESULTS

TF supplementation in HFD-fed mice decreased the presence of lipid vesicles (90%) and total lipid content (75%) and downregulated the expression of RALDH1, RALDH3, SREBP-1c, and CD36.

CONCLUSIONS

The present study demonstrated that α- and γ-TF (1:5 ratio) might play a role in modulating retinol metabolism in the prevention of NAFLD induced by a HFD.

Interactions between endogenous and dietary antioxidants against Pb-induced oxidative stress in wild ungulates from a Pb polluted mining area

Certain physiologic disorders, attributed to lead (Pb) exposure are related to the generation of oxidative stress. Organisms rely on a complex antioxidant system, composed of endogenously produced compounds such as glutathione (GSH), superoxide dismutase (SOD) and GSH peroxidase (GPX); and dietary antioxidants such as vitamins A and E. The aim of this work was to study the interactions between both groups of antioxidants and to evaluate their role in fighting Pb-induced oxidative stress in wild ungulates living in a Pb mining area. We studied red deer (n=54) and wild boar (n=60) from mining and control sites. Liver Pb, copper (Cu) and selenium (Se) levels were measured in liver and bone. Levels of GSH, oxidized GSH (GSSG), lipid peroxidation (TBARS), α-tocopherol, free retinol and retinyl esters, and the activities of SOD and GPX were measured in liver. Wild boar and red deer from the mining sites had higher bone (geometric means: 7.36 vs. 1.23μg/g dw for boar; and 0.46 vs. 0.11μg/g dw for deer) and liver Pb levels (0.81 vs. 0.24μg/g dw for boar; and 0.35 vs. 0.11μg/g dw for deer) than the controls. Pb exposure in deer was associated with lower GSH, α-tocopherol and retinyl esters, and higher free retinol and TBARS. A similar effect on vitamin A balance was observed in wild boar, but this was accompanied with an increase in GSH and α-tocopherol. GPX activity and Se levels were higher in wild boar (mean in controls: 670IU/mg protein and 1.3μg/g dw, respectively) than in red deer (150IU/mg protein and 0.3μg/g). These differences may indicate that red deer is more sensitive to Pb-induced oxidative stress than wild boar. Both endogenous and nutritional antioxidants may be negatively affected by Pb exposure, and their interactions are essential to fight against Pb-mediated oxidative damage.

Uptake of all-trans retinoic acid-containing aerosol by inhalation to lungs in a guinea pig model system--a pilot study

Systemic therapies with retinoic acid (RA) can result in toxic side effects without yielding biologically effective levels in target tissues such as lung. The authors adapted a PARI LC Star nebulizer to create a tubular system for short-term inhalation treatment of guinea pigs using a water-miscible formulation of all-trans RA (ATRA) or vehicle. Based on the initial average weight, animals received an estimated average ATRA doses of either 0.32 mg·kg(-1) (low dose, 1.4 mM), or 0.62 mg·kg(-1) (medium dose, 2.8 mM), or 1.26 mg·kg(-1) (high dose, 5.6 mM) 20 minutes per day for 6 consecutive days. This system led to a rise of ATRA levels in lung, but not liver or plasma. Cellular lung levels of retinol, retinyl palmitate, and retinyl stearate also appeared to be unaffected (245.6 ± 10.7, 47.4 ± 3.4, and 132.8 ± 7.7 ng·g(-1) wet weight, respectively). The application of this aerosolized ATRA also induced a dose-dependent protein expression of the cellular retinol-binding protein 1 (CRBP-1) in lung, without apparent harmful side effects.

Qualitative and quantitative analysis of retinol, retinyl esters, tocopherols and selected carotenoids out of various internal organs form different species by HPLC

We report the validation of a reversed-phase gradient HPLC method allowing simultaneous quantification of retinol, retinyl esters, tocopherols and selected carotenoids in lung, liver and plasma of mouse, rat and guinea pig (gp) using a diode array detector. A significant species difference was observed regarding the distribution of retinol and retinyl esters. The levels of total retinol in lung, liver and plasma were in the following order: mouse >> rat > gp; rat >mouse > gp; and gp >> rat > mouse, respectively. Furthermore, comparison studies revealed similarities between the vitamin A profiles of human and gp lung samples.

Vitamin A concentrations in piglet extrahepatic tissues respond differently ten days after vitamin A treatment

Periodic supplementation to infants and young children is encouraged in developing countries by the WHO. We investigated vitamin A (VA) in extrahepatic tissues of piglets after supplementation with retinyl acetate to determine long-term storage. 3, 4-Didehydroretinyl acetate (DRA) as a tracer was used to evaluate uptake from chylomicra in 4 h. Sows were fed a VA-depleted diet throughout pregnancy and lactation. Male castrated piglets (n = 28, 11.6 +/- 0.5 d) from these sows were weaned onto a VA-free diet for 1 wk, assigned to 4 groups, and dosed orally with 0, 26.2, 52.4, or 105 micromol VA. After 10 d, 5.3 micromol DRA was administered to determine short-term uptake of 3, 4-didehydroretinol (DR). Four hours later, piglets were killed; adrenal glands, kidney, lung, and spleen were collected and analyzed for retinol and DR. Retinol concentrations of kidney and adrenal gland were higher than control, but treated groups did not differ. Retinol concentration was highest in kidney (1.70-2.52 nmol/g), followed by adrenal gland (0.30-0.48 nmol/g), lung (0.15-0.21 nmol/g), and spleen (0.11-0.15 nmol/g). Total retinol in kidney and spleen was different among the groups (P < 0.05). Unesterified retinol was the major VA form; the percent retinol of total VA was lowest in adrenal glands. DR did not differ among the groups. In 4 h, the minimum estimated chylomicron contribution to tissue DR was 63-280% higher than the maximum DR exposure from retinol-binding protein. Constant dietary intake may be important in maintaining VA concentrations in extrahepatic tissues.

Study of potential systemic oxidative stress animal models for the evaluation of antioxidant activity: status of lipid peroxidation and fat-soluble antioxidants

Although many compounds have already been tested in-vitro to determine their antioxidant profile, it is necessary to investigate the in-vivo effect of potential antioxidants. However, representative models of systemic oxidative stress have been poorly studied. Here, different potential systemic oxidative stress animal models have been investigated. These included a vitamin E-deficient rat, a diabetic rat and an atherosclerotic rabbit model. Plasma/serum malondialdehyde was measured as a parameter of oxidative damage. Plasma/serum fat-soluble antioxidants were determined as markers of antioxidant defence. We demonstrated that vitamin E-deficient rats were not suitable as a model of systemic oxidative stress, whereas diabetic and atherosclerotic animals showed increased systemic oxidative damage, as reflected by significantly augmented plasma/serum malondialdehyde. Moreover, plasma coenzyme Q9 increased by 80% in diabetic rats, confirming systemic oxidative stress. In view of these observations and economically favouring factors, the diabetic rat appeared to be the most appropriate systemic oxidative stress model. These findings have provided important information concerning systemic oxidative stress animal models for the in-vivo study of antioxidants.

Vitamins A and E: metabolism, roles and transfer to offspring

Vitamins A and E are essential, naturally occurring, fat-soluble nutrients that are involved in several important biological processes such as immunity, protection against tissue damage, reproduction, growth and development. They are extremely important during the early stages of life and must be transferred adequately to the young during gestation and lactation. The present article presents an overview of their biological functions, metabolism and dynamics of transfer to offspring in mammals. Among other topics, the review focuses on the biochemical aspects of their intestinal absorption, blood transport, tissue uptake, storage and catabolism. It also describes their different roles as well as their use as preventive and therapeutic agents. Finally, the mechanisms involved in their transfer during gestation and lactation are discussed.

Serum nutritional status of tocopherol and retinol normalized to lipids of persons living in the southern rural Terai region in Nepal

OBJECTIVE

The present study examined the levels of serum α-Tocopherol (Toc), retinol (Ret), cholesterol (Chol) and triglycerides (TG), and their correlations in the sera of people in Nepal.

METHODS

The survey was conducted on the general populace in the agricultural Terai region in southern Nepal. The study population consisted of 93 males and 83 females aged 10-68 years. Serum Toc and Ret were measured by high-performance liquid chromatography.

RESULTS

No significant differences were observed between the genders for the average of total Chol (T-Chol) (140 and 145 mg/100 ml, respectively), HDL-C (45 and 47 mg/100 ml), LDL-C (94 and 97 mg/100 ml), and TG (106 and 110 mg/100 ml), and the ratio of LDL/HDL (2.16). The levels of mean Toc (4.32 and 4.27 μg/ml) were about the same for both genders, while the mean Ret levels were significantly higher for males (624 ng/ml) than for females (535 ng/ml) (p<0.001). A direct relationship was found between the levels of Toc and Ret (r=0.46, p<0.001 and r=0.28, p<0.05 for males and females, respectively). Serum levels of Toc and Ret were positively related to the levels of Chol (r=0.48 and r=0.58, p<0.001 for males and r=0.49, p<0.01 and r-0.28, p<0.05 for females, respectively). The ratio of Toc/TG normalized to serum TG was directly correlated to the ratio of Ret/TG (r=0.79 for males, and r=0.72 for females, p<0.001, respectively) and the ratios of Toc/TG and Ret/TG were negatively related to the LDL/HDL levels (r=-0.49 and r=-0.43, for males, and r=-0.46 and r=-0.57 for females, p<0.001, respectively).

CONCLUSION

The levels of Toc and Ret were low in the sera of people living in the southern rural Terai region in Nepal, and it was found that lower levels of Toc and Ret normalized to TG increased the ratio of LDL/HDL. These results suggest that greater intake of foods rich in Toc and Ret should be encouraged to reduce the erisk of coronary heart disease.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Retinoic acid: its biosynthesis and metabolism

This article presents a model that integrates the functions of retinoid-binding proteins with retinoid metabolism. One of these proteins, the widely expressed (throughout retinoid target tissues and in all vertebrates) and highly conserved cellular retinol-binding protein (CRBP), sequesters retinol in an internal binding pocket that segregates it from the intracellular milieu. The CRBP-retinol complex appears to be the quantitatively major form of retinol in vivo, and may protect the promiscuous substrate from nonenzymatic degradation and/or non-specific enzymes. For example, at least seven types of dehydrogenases catalyze retinal synthesis from unbound retinol in vitro (NAD+ vs. NADP+ dependent, cytosolic vs. microsomal, short-chain dehydrogenases/reductases vs. medium-chain alcohol dehydrogenases). But only a fraction of these (some of the short-chain de-hydrogenases/reductases) have the fascinating additional ability of catalyzing retinal synthesis from CRBP-bound retinol as well. Similarly, CRBP and/or other retinoid-binding proteins function in the synthesis of retinal esters, the reduction of retinal generated from intestinal beta-carotene metabolism, and retinoic acid metabolism. The discussion details the evidence supporting an integrated model of retinoid-binding protein/metabolism. Also addressed are retinoid-androgen interactions and evidence incompatible with ethanol causing fetal alcohol syndrome by competing directly with retinol dehydrogenation to impair retinoic acid biosynthesis.

Influence of caesarean delivery and maternal factors on fat-soluble vitamins in blood from cord and neonates

We measured plasma and erythrocyte vitamin E (VE) and plasma vitamin A (VA) profiles in 48 full-term and 8 preterm pairs of neonates and their mothers at birth and we determined whether there is any relationship between maternal and umbilical cord for the nutrients measured. At the same time, we assessed the influence of the delivery type and neonate anthropometric measurements on maternal and cord blood VA and VE levels. We measured vitamin levels in vein and arterial blood in order to establish differences due to fetal metabolism. To determine the influence of pregnancy on vitamin levels, we compared the maternal results with data from a group of 13 non-pregnant women. Cord blood had lower plasma VE (arterial 275.8+/-71.7 microg/dl and vein 282.89+/-64.4 microg/dl values), erythrocyte VE (arterial 256.96+/-50.41 microg/dl packet cells and vein 257.41+/-44.35 microg/dl values), and VA levels (arterial 26.72+/-11.83 microg/dl and 27.15+/-10.05 microg/dl values) and a lower vitamin E/total lipids ratio (VE/LT) (arterial 1.60+/-0.4 and vein 1.62+/-0.3 values) than maternal blood (1474.62+/-424.51 microg/dl, 305.94+/-54.75 microg/dl packet cells, 41.03+/-18.83 microg/dl, 2.34+/-0.5, respectively). VA levels were higher in preterm than full-term neonates (P<0.05). Plasma and erythrocyte VE levels were not correlated in maternal blood but were correlated in neonates and infants (r>0.40; P<0.01). We found a good correlation between erythrocyte tocopherol of maternal and cord blood (r>0.40; P<0.01), although there was no correlation with plasma VE values. Cord vein plasma VE levels were higher than cord arterial blood measurements (P<0.01). The plasma VE and VE/LT of the mother and cord following vaginal delivery were higher than measurements from caesarean delivery (P<0.05), although erythrocyte levels were similar. The plasma VE level was higher in mothers at delivery than non-pregnant women.

Purification and characterization of a neutral, bile salt-independent retinyl ester hydrolase from rat liver microsomes. Relationship To rat carboxylesterase ES-2

A neutral, bile salt-independent retinyl ester hydrolase (NREH) has been purified from a rat liver microsomal fraction. The purification procedure involved detergent extraction, DEAE-Sepharose ion exchange, Phenyl-Sepharose hydrophobic interaction, Sephadex G-100 and Sephacryl S-200 gel filtration chromatographies, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The isolated enzyme has an apparent molecular mass of approximately 66 kDa under denaturing conditions on SDS-PAGE. Analysis of the amino acid sequences of four peptides isolated after proteolytic digestion revealed that the enzyme is highly homologous with other rat liver carboxylesterases. In particular, the sequences of the four peptides of the NREH (60 amino acids total) were identical to those of a rat carboxylesterase expressed in the liver (Alexson, S. E. H., Finlay, T. H., Hellman, U., Svensson, L. T., Diczfalusy, U., and Eggertsen, G. (1994) J. Biol. Chem. 269, 17118-17124). Antibodies against this enzyme also react with the purified NREH. Purified NREH shows a substrate preference for retinyl palmitate over triolein and did not catalyze the hydrolysis of cholesteryl oleate. With retinyl palmitate as substrate, the enzyme had a pH optimum of 7 and showed apparent saturation kinetics, with half-maximal activity achieved at substrate concentrations (Km) of approximately 70 microM.

Vitamin E enhances the lymphatic transport of beta-carotene and its conversion to vitamin A in the ferret

BACKGROUND/AIMS

beta-Carotene and alpha-tocopherol may have either antagonistic or synergistic effects on each other's absorption and metabolism. The effects of both physiological and pharmacological concentrations of alpha-tocopherol on the absorption and metabolism of beta-carotene in ferret intestine were determined.

METHODS

A high concentration of beta-carotene was perfused through the upper portion of the small intestine of ferrets in vivo with varying levels of alpha-tocopherol. The effluent of a mesenteric lymph duct cannulation, the intestinal mucosal scraping, and portal vein blood were sampled and analyzed by high-performance liquid chromatography.

RESULTS

The lymphatic transport of beta-carotene was enhanced 4-fold by alpha-tocopherol at a physiological dose and 12-21-fold at a pharmacological dose. The lymphatic transport of alpha-tocopherol was linearly (r = 0.8; P < 0.05) related to the luminal alpha-tocopherol concentration even in the presence of a high concentration of beta-carotene. Furthermore, alpha-tocopherol increased the conversion of beta-carotene into retinol in the intestine in a dose-dependent manner.

CONCLUSIONS

alpha-Tocopherol has a positive effect on the intestinal absorption of intact beta-carotene and may modulate the metabolic conversion of beta-carotene into retinoids.

Hydrolysis of retinyl esters in rat liver. Description of a lysosomal activity

When incorporated into liposomes made of phospholipids, retinyl palmitate is an adequate substrate for an acidic REH (aREH). In rat liver, this activity is mainly localized in the lysosomal fraction. Kinetic parameters have been determined for retinyl palmitate (Km = 315 microM; maximal rate = 22.1 nmol retinol/h per mg protein). The aREH activity is different from the lysosomal acidic cholesteryl ester hydrolase (aCEH): cholesteryl oleate does not inhibit aREH activity, neither do some aCEH specific inhibitors, and aREH does not hydrolyse cholesteryl ester. Involvement of aREH in the hydrolysis of lipid droplets retinyl esters in fat storing cells is discussed.

Simultaneous determination of retinol and tocopherols by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) method to determine retinol and all four tocopherols (alpha-, beta-, gamma- and delta-) simultaneously was established using a reversed-phase column (YMC-PACK A-302 S-5 120A ODS). The HPLC conditions were mobile phase 65% isopropanol, sample solvent 99.5% methanol and temperature 30 degrees C. Retinol and tocopherols were measured in rat liver.

Effects of chronic beta-carotene supplementation on vitamin K status in adults

Plasma vitamin K concentrations and prothrombin coagulation activity were determined in 26 normal adults who had received daily beta-carotene supplementation (0, 15, 30, or 60 mg) for six months. Neither plasma vitamin K nor coagulation activity were significantly decreased at any supplementation level. Thus, chronic beta-carotene supplementation, even at high daily doses, is not expected to result in clinical vitamin K deficiency. The data suggest separate mechanisms for intestinal absorption of beta-carotene and vitamin K.

Cholate effects on all-trans-retinyl palmitate hydrolysis in tissue homogenates: solubilization of multiple kidney membrane hydrolases

Retinyl ester hydrolysis was observed in the absence of cholate in homogenates of rat lung, liver, kidney, intestine, and testes. Eighty-four percent of the activity in kidney was membrane-associated. The kidney microsomal fraction contained 19% of the total activity and was the only subcellular fraction that had increased specific activity relative to the homogenate (about 1.5-fold). In contrast, the cytosol was the only fraction that was decreased in specific activity (about 3-fold). Cholate (18 mM), reportedly required to observe hydrolysis of all-trans-retinyl esters by rat liver preparations, was not obligatory for activity in kidney homogenates or microsomes. The microsomal activity was solubilized efficiently and with a twofold increase in specific activity by the synthetic detergent 1-S-octyl-beta-D-thioglucopyranoside. Gel-permeation chromatography of the solubilizate suggested that at least two pools of activity existed, with molecular weights in the ranges 70-95 and 30-40 kDa. Neither hydrolyzed cholesteryl oleate. Both were more active in hydrolyzing retinyl palmitate than trioleoylglycerol. The higher mass pool had decreased trioleoylglycerol hydrolase activity relative to the solubilizate. Anion-exchange chromatography separated the lower mass pool into two major peaks. A major peak, distinct from the two peaks observed with the lower mass pool, was observed upon anion-exchange chromatography of the higher mass pool. These data demonstrate that multiple retinyl ester hydrolases, more efficient at hydrolyzing retinyl esters than cholesteryl esters and triacylglycerol, occur in a retinoid target tissue.

Retinoic acid synthesis by cytosol from the alcohol dehydrogenase negative deermouse

Cytosolic alcohol dehydrogenase in the deermouse is coded by a single genetic locus and a strain of the deermouse which is alcohol dehydrogenase negative exists. These two strains of the deermouse were used to extend insight into the role of cytosolic alcohol dehydrogenases in the conversion of retinol into retinoic acid. Retinoic acid synthesis from physiological concentrations of retinol (7.5 microM) with cytosol from the alcohol dehydrogenase negative deermouse was 13% (liver), 14% (kidney), 60% (testes), 78% (lung), and 100% (small intestinal mucosa) of that observed with cytosol from the positive deermouse. The rates in the negative strain ranged from 0.3 to 0.7 nmol/h/mg protein: sufficient to fulfill cellular needs for retinoic acid. Ten millimolar 4-methylpyrazole inhibited retinoic acid synthesis 92, 94, 26, and 30% in kidney, liver, lung, and testes of the positive deermouse, respectively, but only 50, 30, 0, and 0% in the same tissues from the negative deermouse. Ethanol (300 mM) did not inhibit retinoic acid synthesis in kidney cytosol from the negative strain. Therefore multiple cytosolic dehydrogenases, including alcohol dehydrogenases, contribute to retinol metabolism in vitro. The only enzyme(s) likely to be physiologically significant to retinoic acid synthesis in vivo, however, is the class of dehydrogenase, distinct from ethanol dehydrogenase, that is common to both the positive and the negative deermouse. This conclusion is supported by the data described above, the kinetics of retinoic acid synthesis and retinal reduction in kidney cytosol from the negative deermouse, and the very existence of the alcohol dehydrogenase negative deermouse. This work also shows that microsomes inhibit the cytosolic conversion of retinol into retinoic acid and that the synthesis of retinal, a retinoid that has no known function outside of the eye, does not reflect the ability or capacity of a sample to synthesize retinoic acid.

In vitro stimulation of rat liver retinyl ester hydrolase by ethanol

Retinyl ester hydrolase (REH), the enzyme which converts retinyl esters to retinol, was partially characterized from whole liver homogenates of rats using an HPLC method with quantitation of retinol product. Optimal results were obtained by incubation of 1 mg of whole homogenate protein with 900 microM all-trans-retinyl palmitate and 275 mM 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate in a 0.1 M Tris-maleate buffer, pH 7.0, for 1 h at 37 degrees C. The enzyme assay proved to be sensitive and reproducible, with an interanimal coefficient of variation of 13% (n = 7). Because ethanol has been shown to mobilize vitamin A from the liver, we tested its effect on REH activity at several concentrations. In concentrations ranging from 0.01 to 0.5 M, ethanol added in vitro caused a concentration related increase in REH activity (from 20 to 86% above baseline activity). This increase was specific to ethanol as acetaldehyde, 1-propanol, and t-butanol either did not change or significantly decreased REH activity over the range of concentrations tested. The range of concentrations of ethanol causing stimulation in our assays was within the range of concentrations seen in the blood of rats after acute ethanol ingestion. Stimulation of REH activity could explain, in part, the well-known effects of ethanol on mobilization of vitamin A from liver stores.

Effect of fatty acid supplementation on cholesterol and retinol esterification in J774 macrophages

J774 macrophages exposed to medium containing cholesterol-rich phospholipid dispersions accumulate cholesteryl ester. Supplementing this medium with 100 micrograms oleate/ml increased cellular cholesteryl ester contents 3-fold. Cell retinyl ester contents increased 8-fold when medium containing retinol dispersed in dimethyl sulfoxide was supplemented with oleate. These increases were not the result of increases in total lipid uptake by the cells but rather of redistribution of cholesterol and retinol into their respective ester pools. Effective oleate concentration of 15-30 micrograms/ml increased cellular retinyl and cholesteryl ester contents. The effective oleate concentration was reduced to 5 micrograms/ml when the fatty acid/albumin molar ratio was increased. The oleate-stimulated increase in cholesterol esterification was blocked by incubating cells with Sandoz 58-035, a specific inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT), indicating that the effect of fatty acid exposure is mediated through changes in ACAT activity. When cholesterol or retinol was added to cells which had been exposed to oleate for 24 h to provide a triacylglycerol store, the cellular contents of cholesteryl or retinyl ester were also significantly increased compared to cells not previously exposed to oleate. The oleate-stimulated increase in the esterification of cholesterol and/or retinol was also observed in P388D1 macrophages, human (HepG2) and rat (Fu5AH) hepatomas, human fibroblasts, rabbit aortic smooth muscle cells and MCF-7 breast carcinoma cells. In addition to oleate, a number of other fatty acids increased retinol esterification in J774 macrophages; however, cellular cholesterol esterification in these cells was increased only by unsaturated fatty acids and was inhibited in the presence of saturated fatty acids. Although the cellular uptake of radiolabeled oleate and palmitate was similar, a significant difference in the distribution of these fatty acids among the lipid classes was observed. These data demonstrate that exogenous fatty acids are one factor that regulate cellular cholesteryl and retinyl ester contents in cultured cells.

Hydrolysis of cis and trans isomers of retinyl palmitate by retinyl ester hydrolase of pig liver

Relative retinyl ester hydrolase activities of pig liver homogenates (n = 4) toward 9,13-cis-, 13-cis-, 9-cis-, and all-trans-retinyl palmitate were 6.8 +/- 0.5 (SE), 5.7 +/- 0.5, 2.4 +/- 0.1, and 1, respectively. The range of apparent Km values for the four isomers was 142 to 268 microM, and the pH optima were 8-9 in all cases. Peak activities of retinyl ester hydrolase activities in pig liver cytosol toward 13-cis- and all-trans-retinyl palmitate were found in the 20 to 40% and in the 60 to 80% saturated ammonium sulfate (AS) fractions, respectively. By use of size-exclusion chromatography in 2 M KCl, hydrolase activity eluted at volumes corresponding to greater than 2000, 180, and 15 kDa from the 20-40% AS fraction, and at 180 kDa from the 60-80% AS fraction. On the basis of molecular size, different substrate specificities, detergent effects, and susceptibilities to inhibition by phenylmethylsulfonyl fluoride, we conclude that at least three distinct retinyl ester hydrolases are present in pig liver cytosol.

Vitamin A metabolism in rats chronically treated with 3,3',4,4',5,5'-hexabromobiphenyl

Chronic dietary administration of 3,3',4,4',5,5'-hexabromobiphenyl (HBB), 1 mg/kg diet, caused a decrease in retinol (20-fold) and retinyl esters (23-fold) in the livers of female rats, but resulted in a 6.4-fold increase in retinol and 7.4-fold increase in retinyl esters in the kidneys. Liver acyl-CoA:retinol acyltransferase and retinyl palmitate hydrolase activities were reduced while serum concentration of retinol was unaffected by HBB feeding. Metabolism of a physiological dose of [11-3H]retinyl acetate (10 micrograms), was examined in rats fed either vitamin A-adequate diet, or marginal amounts of vitamin A, or vitamin A-adequate diet containing HBB. A 13-fold greater amount of the administered vitamin A was found in kidneys of HBB-treated rats. In rats fed adequate or low amounts of vitamin A, kidney radioactivity was primarily in the retinol fraction, while in HBB-fed rats the radioactivity was associated mostly with retinyl esters. Fecal and urinary excretion of radioactivity was greatly increased in HBB-treated rats. Chronic HBB feeding results in a loss of ability of liver to store vitamin A, and severely alters the uptake and metabolism of vitamin A in the kidneys. We conclude that HBB causes major disturbances in the regulation of vitamin A metabolism.

Properties of liver retinyl ester hydrolase in young pigs

A new sensitive method for the assay of retinyl ester hydrolase in vitro was developed and applied to liver homogenates of 18 young pigs with depleted-to-adequate liver vitamin A reserves. Radioactive substrate was not required, because the formation of retinol could be adequately quantitated by reversed-phase high-performance liquid chromatography. Optimal hydrolase activity was observed with 500 microM retinyl palmitate, 100 mM 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, and 2 mg/ml Triton X-100 at pH 8.0. The relative rates of hydrolysis of six different retinyl esters by liver homogenate were: retinyl linolenate (100%), myristate (99%), palmitate (47%), oleate (38%), linoleate (31%), and stearate (29%). The enzyme was found primarily in the membrane-containing fractions of liver (59 +/- 3%, S.E.) and kidney (76 +/- 3%), with considerably lower overall activity in kidney (57-375 nmol/h per g of tissue) than in liver (394-1040 nmol/h per g). Retinyl ester hydrolase activity in these pigs was independent of serum retinol values, which ranged from 3 to 24 micrograms/dl, and of liver vitamin A concentrations from 0 to 32 micrograms/g. Pig liver retinyl ester hydrolase differs from the rat liver enzyme in its substrate specificity, bile acid stimulation, and interanimal variability.

Retinyl ester hydrolase of bovine retina and pigment epithelium: comparisons to the rat liver enzyme

The hydrolysis of 3H-retinyl ester was examined in the retinal pigment epithelium (RPE) and neural retina of cattle eyes and compared to that in homogenates of rat liver. The optimum pH for hydrolysis was 4.0-4.5 for RPE and 7.5-8.0 for liver. The RPE activity, which shows no variability between individual animals, is localized mainly in the lysosomal fraction of the cell. It is strongly inhibited by bile salts at concentrations as low as 0.2-0.5% and conversely, is strongly activated by Triton X-100, with maximum stimulation found at a concentration of approximately 1%. The apparent Vmax for hydrolysis of labeled retinyl ester in the RPE is 2.7 nmoles/hr/mg protein, a value approximately 1/150 to 1/200 of the rate of retinol esterification in these cells. Little or no hydrolytic activity could be detected in neural retina or in rod outer segments. Studies on the specificity of the RPE retinyl ester hydrolase activity revealed unexpectedly high hydrolytic activity toward both cholesteryl oleate and triolein, approximately 20 and 5 times greater, respectively, than in rat liver. The hydrolytic activity for cholesteryl oleate in the RPE was mainly at pH 3.5, while that for triolein showed three pH maxima, one at pH 4.5-5.0, a second near neutral pH and the third at pH 8. These findings reflect an active and complex pattern of fatty acyl ester lipid-metabolizing enzymes in cattle RPE whose interrelationships to one another require further clarification.

Speculations on the origin of the ill-effects associated with the use of visual display terminals

There are increasing reports of a wide variety of unexplained ill-effects associated with the occupational use of visual display terminals (VDTs); these include psychological complaints such as headache, irritability, and fatigue, musculoskeletal pains, dry or burning eyes, deteriorating eyesight, cataracts, facial dermatitis, and pregnancy abnormalities. Many VDTs emit near-ultraviolet (UV-A) radiation in amounts ranging from 200 to 1500 times less than the present U.S. safety standard of 1.0 X 10(-3) W/cm2. Although the possibility of a radiation hazard from VDTs is widely discounted, various points of circumstantial evidence are marshalled in this paper in support of the hypothesis that prolonged exposure to even these low amounts of UV-A radiation may result in progressive increases in sensitivity to UV-A and in cumulative biologic damage. It is further proposed that UV-A produces the ill-effects by catabolizing vitamin A in skin and plasma into highly active metabolites, the gradual accumulation of which results in an endogenous form of vitamin A intoxication. In addition to producing the skin and systemic effects of hypervitaminosis A, these metabolites presumably enter the fetal circulation, leading to pregnancy abnormalities similar to those associated with high doses of the vitamin for the treatment of skin disorders.

Alpha-tocopherol and phylloquinone as non-competitive inhibitors of retinyl ester hydrolysis

Inhibition of rat intestinal retinyl ester hydrolase by alpha-tocopherol (vitamin E) and phylloquinone (vitamin K1) was non-competitive. Maximum inhibition occurred within 10 min, and, particularly with alpha-tocopherol, was substantially reversible. Consequently, increasing tissue concentrations of retinyl esters, which might occur with advancing age or changes in diet, would not diminish the effects of the inhibitors. These data further support the notion that alpha-tocopherol may, at physiological concentrations, influence the concentration of vitamin A and its ester in tissues.