Retinyl Ester Concentrations Research Articles

Human Plasma all-trans-, 13-cis- and 13-cis-4-Oxoretinoic Acid Profiles during Subchronic Vitamin A Supplementation: Comparison to Retinol and Retinyl Ester Plasma Levels

Plasma concentrations of retinyl esters, retinol, retinol-binding protein and the polar retinol metabolites all-trans-retinoic acid, 13-cis-retinoic acid, all-trans-4-oxoretinoic acid and 13-cis-4-oxoretinoic acid were measured for six male volunteers who received 0.46 mg retinyl palmitate per kilogram body weight as oily drops (equivalent to 0.25 mg retinol per kilogram body weight) once daily over a 20-d period. Retinol and retinol-binding protein levels remained virtually constant throughout the study. Following absorption of vitamin A, retinyl esters as well as all-trans-retinoic acid and 13-cis-retinoic acid were transiently increased in plasma. 13-cis-4-Oxoretinoic acid increased gradually to a steady state level present on d 10 or 20. All-trans-4-oxoretinoic acid was not detected in plasma of the volunteers, with the exception of one on d 10 of the study. Plasma pharmacokinetic profiles of retinyl esters and polar metabolites of retinol displayed great interindividual differences (peak concentrations, time to peak, area-under-the-concentration-time curve values) among the volunteers. Because of the relatively high and consistent steady state concentrations of plasma 13-cis-4-oxoretinoic acid, we suggest that this compound be further investigated as a biochemical marker of vitamin A uptake in humans.

The effect of fish oil supplementation on plasma α-tocopherol, retionol, lipid and lipoprotein levels in normolipidemic subjects

The effects of supplementation with MaxEPA fish oil (FO) on the plasma concentrations of α-tocopherol, retinol, lipids and lipoproteins in young male subjects were studied. Ten normolipidemic adult male volunteers (25–39 y) supplemented their usual Western diets for 6 wk with 18 g MaxEPA FO per d. Blood samples were collected prior to supplementation, after 6 wk of supplementation, and 10 wk following cessation of the supplement. There was a significant increase (21%) in plasma α-tocopherol concentration after 6 wk of FO supplementation which returned to baseline 10 wk after cessation of the supplements. Plasma retinol increased by 14% and plasma triglyceride (TG) concentration decreased by 28% after 6 wk of supplementation. When FO was taken as a supplement to a typical Western diet significant increases in the concentrations of total cholesterol (TC) (7%), low-density-lipoprotein cholesterol (LDL-C) (10%) and high-density-lipoprotein (HDL-C) (10%) in normolipidemic subjects were observed. Despite animal data indicating an increased need for dietary vitamin E with FO supplementation, in this clinical trial, the FO supplement administed in this study increased plasma α-tocopherol. This increase in due to the 1.35 mg of α-tocopherol added by the manufacturer which appears sufficient to maintain plasma vitamin E levels in the presence of increased PUFA intake, at least over a 6 wk period. Retinyl ester content in eight commercial FO products other than MaxEPA ranged from 2 to 2450 μg/g FO, with a mean of 315 μ/g. The α-tocopherol content in 12 other FO capsules available to consumers for over the counter purchase ranged from 18 to 2241 μg/g FO, with a mean of 1025 μg/g. The effects of ingesting FO supplements with a low content of α-tocopherol on the plasma levels of α-tocopherol needs to be further examined.

Postprandial lipoprotein metabolism in normolipidemic men with and without coronary artery disease.

A delayed clearance of postprandial lipoproteins from the plasma may play a role in the etiology of premature coronary atherosclerosis. To address this hypothesis, we studied chylomicron (remnant) metabolism in two groups of 20 selected normolipidemic men aged 35-65 years, a group of coronary artery disease (CAD) patients, and a matched control group with documented minimal coronary atherosclerosis. Subjects received an oral fat load supplemented with cholesterol and retinyl palmitate. Plasma samples obtained during the next 24-hour period were analyzed for total as well as d less than 1.019 g/ml and d greater than 1.019 g/ml triacylglycerol, cholesterol, and retinyl ester concentrations. Although both groups of patients responded identically in terms of the appearance of gut-derived lipids in the plasma, CAD patients showed a marked delay in the clearance of retinyl esters as well as in the normalization of plasma triacylglycerol concentrations. Postheparin plasma hepatic lipase activity was significantly lower in the CAD group. Apolipoprotein E phenotype measurements did not reveal marked differences in frequency between both groups. The frequency distribution was not unusual in comparison with the normal Dutch population. The magnitude of the postprandial responses of triacylglycerol and retinyl esters was correlated positively with the fasting levels of plasma triacylglycerol and negatively with high density lipoprotein subfraction 2 cholesterol concentrations. These data indicate that the clearance of postprandial lipoproteins in normolipidemic CAD patients as selected in the present study is delayed as compared with that of controls without coronary atherosclerosis and suggest that postprandial lipoproteins may play a role in the etiology of their disease.

Uptake of chylomicron remnant retinyl ester via the low density lipoprotein receptor: implications for the role of vitamin A as a possible preventive for some forms of cancer

The anti-cancer action of retinoids (vitamin A and its metabolites) has been most successfully demonstrated in cell culture systems. In most of these investigations retinoic acid was found to be the most active retinoid [l-21. In such studies, however, the fat-soluble retinoic acid is dissolved in an organic solvent before it is added to the cells. Therefore, these in vitro systems do not take into account transport to and uptake by the target cells that must occur in vivo. The body has a specific and high-capacity transport system for retinol (bound to retinol-binding protein) and retinyl esters (bound to lipoproteins), but not for retinoic acid and its synthetic analogues [3]. Recent studies have shed some light on the in vivo transport of retinoids to cancer cells. Promising results have been obtained from an initial clinical trial in which children with acute myeloid leukaemia in remission were treated with high doses of retinol [4]. This treatment produces a significant increase in the plasma concentration of chylomicron remnant retinyl esters. Furthermore, in vitro studies have shown that physiological concentrations of retinyl palmitate bound to chylomicron remnants effectively induce differentiation and inhibit proliferation of human myeloid leukaemic cells in culture [S]. Thus the anti-cancer effect of vitamin A may be mediated via retinyl ester bound to chylomicron remnants, rather than by retinol bound to retinol-binding protein or free retinoic acid. The role of vitamin A as a possible preventive for various forms of cancer should be re-examined in the light of these findings.

Postprandial plasma vitamin A metabolism in humans: A reassessment of the use of plasma retinyl esters as markers for intestinally derived chylomicrons and their remnants

We investigated postprandial vitamin A metabolism by measuring retinyl ester, triglyceride, and apolipoprotein (apo)B-48 in the plasma lipoproteins of human subjects before and after fat-feeding. Following a 14-hour fast, eight healthy subjects (two men, six women, 28 to 79 years) were given a fat-rich meal (1 g fat/kg body weight) containing vitamin A (40 retinol equivalents per kilogram body weight). Blood was collected every 3 hours for 12 hours and lipoproteins were isolated by sequential ultracentrifugation. Mean plasma retinyl ester concentration peaked 6 hours after the fat-rich meal, whereas mean plasma triglyceride peaked at 3 hours. Data obtained from hourly samples in 3 subjects showed that changes in the postprandial plasma concentration of retinyl ester occurred 1 to 2 hours after changes in the plasma triglyceride concentration. In triglyceride-rich lipoproteins (TRL) of d<1.006 g/mL, retinyl ester similarly peaked at 6 hours, whereas triglyceride as well as apoB-48 peaked at 3 hours. Although retinyl esters were found mainly in TRL in the initial postprandial period (84%, 3 hours; 83%, 6 hours), in fasting and postprandial plasma, particularly 9 or more hours after fat-feeding, a large percentage of plasma retinyl esters were in low-density lipoproteins (LDL) (44%, fasting; 9%, 3 hours; 9%, 6 hours; 19%, 9 hours; 32%, 12 hours). A small percentage of retinyl esters were also found in postprandial high-density lipoproteins (HDL) (2% to 7%). ApoB-48 was not detected in LDL of fasting or postprandial plasma. We conclude from these data that retinyl esters do not always serve as markers for intestinal apoB-48-containing TRL in fasting or postprandial plasma.

Postprandial plasma retinyl ester response is greater in older subjects compared with younger subjects. Evidence for delayed plasma clearance of intestinal lipoproteins.

Postprandial vitamin A and intestinal lipoprotein metabolism was studied in 86 healthy men and women, aged 19-76 yr. Three independent experiments were carried out. In the first experiment, a supplement dose of vitamin A (3,000 retinol equivalents [RE]) was given without a meal to 59 subjects, aged 22-76 yr. In the second experiment, 20 RE/kg body wt was given with a fat-rich meal (1 g fat/kg body wt) to seven younger subjects (aged less than 50 yr) and seven older subjects (aged greater than or equal to 50 yr). In both experiments, postprandial plasma retinyl ester response increased significantly with advancing age (P less than 0.05). In the third experiment, retinyl ester-rich plasma was infused intravenously into nine young adult subjects (aged 18-30 yr) and nine elderly subjects (aged greater than or equal to 60 yr), and the rate of retinyl ester disappearance from plasma during the subsequent 3 h was determined. Mean (+/- SE) plasma retinyl ester residence time was 31 +/- 4 min in the young adult subjects vs. 57 +/- 8 min in the elderly subjects (P less than 0.05). These data are consistent with the concept that increased postprandial plasma retinyl ester concentrations in older subjects are due to delayed plasma clearance of retinyl esters in triglyceride-rich lipoproteins of intestinal origin.

The effect of dietary retinyl palmitate on the retinyl ester content of human or rat liver tumours

(1990). The effect of dietary retinyl palmitate on the retinyl ester content of human or rat liver tumours. Food Additives & Contaminants: Vol. 7, No. sup1, pp. S69-S72.

Marked alterations in retinoid homeostasis of Sprague—Dawley rats induced by a single i.p. dose of 10 μg/kg of 2,3,7,8-tetrachlorodibenzo- p-dioxin

Interference of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) in retinoid homeostasis was investigated in Sprague—Dawley rats with a low (dietary induced) retinoid status, that were fed a [ 3H]retinol-containing diet (37 MBq, 10 000 IU/kg diet) for 21 days to facilitate determination of retinoid concentrations in various tissues. The rats were exposed to a single i.p. dose of 10 μg TCDD/kg body weight in corn oil, or to corn oil at day 7 of [ 3H]retinol supplementation. TCDD induced significant reductions in retinol and retinyl ester concentrations and [ 3H]retinol-derived radioactivity in the liver, the lung, the intestine and the adrenals to 3–5%, 40–45%, 37%, and 56% of control values, respectively, at 14 days after exposure. In contrast, the retinoid concentrations and the amount of [ 3H]retinol-derived radioactivity in the kidney and serum of TCDD-treated rats was increased to 440% and 140% of corn oil-treated controls, respectively, at the termination time of the experiment. Analysis of the amount of serum retinol binding protein (RBP) by gel-permeation chromatography revealed an 150% increase in the free fraction of retinol-RBP, i.e., uncoupled to transthyretin (TTR), in serum of TCDD-treated rats. In addition, urinary excretion of [ 3H]retinol-derived radioactivity was significantly enhanced (to 140% of controls) by TCDD. These data indicate that TCDD induces an increased mobilization of retinoids from hepatic and extrahepatic storage sites into serum accompanied by an enhanced elimination via the kidney into the urine of rats.

Role of triglyceride-rich lipoproteins from the liver and intestine in the etiology of postprandial peaks in plasma triglyceride concentration

Plasma triglyceride concentration in human subjects peaks once, twice or three times in the twelve-hour period following the ingestion of a fat-rich meal. Triglyceride-rich lipoproteins (TRL) containing apolipoprotein (apo)B-48 (of intestinal origin), and TRL containing apoB-100 (predominantly of hepatic origin) both contribute to postprandial changes in plasma triglyceride concentration. To test the hypothesis that earlier peaks in postprandial triglyceridemia are due predominantly to the secretion of TRL from the intestine, while later peaks are due to the secretion of TRL from the liver, TRL apoB-48, TRL apoB-100 and retinyl ester (a marker of intestinal lipoproteins) were measured in plasma samples from subjects fed a fat-rich meal (1 g fat/kg body wt). Data from seven subjects (four fed 40 retinol equivalents vitamin A/kg body wt, three fed 20 retinol equivalents vitamin A/kg body wt, with the fat meal), showed that postprandial peaks in plasma triglyceride were always associated with increases in plasma retinyl ester concentration. In four subjects, who were selected because they had two clearly defined postprandial triglyceride peaks, the plasma concentration of TRL triglyceride, apoB-48, apoE and apoC increased in conjunction with both the earlier (three hour) and later (nine hour) peaks in plasma triglyceride. Increase in TRL apoB-100 was associated with both peaks in two of the four subjects. Our data suggest that 1) TRL from the liver and intestine contribute to both earlier and later peaks in postprandial triglyceridemia; and 2) the rate of appearance of TRL from the intestine is not constant after dietary fat absorption.

A relationship between retinol and cellular retinol-binding protein concentrations in human squamous cell carcinomas

The retinol and retinyl ester concentrations in human xenografted squamos cell carcinomas, with various concentrations of cellular retinol-binding protein (CRBP), were studied, as well as the in vivo uptake and esterification in these tumours of labelled retinol, presented as a complex with plasma RBP. The mean retinol concentration in the different tumours was in the range 3.7–6.2 nmol/g protein, and the mean CRBP concentration was between 16 and 69 nmol/g protein. There was a statistically significant correlation between the retinol and the CRBP concentrations in the same tumour ( P < 0.001; r = 0.622). Calculation of the maximal extent of retinol-saturation of CRBP showed low values (range: 9–26%). Retinyl palmitate, the predominant retinyl ester, comprised approx. 70% of the retinyl esters in the tumours. There was no correlation between the concentration of CRBP and that of retinyl palmitate. The uptake of [ 3H]retinol from intravenously injected retinol-RBP complex was similar in the four human squamous cell carcinomas studied, and not related to their CRBP concentration. 20% of the radioactivity in tumour specimens was lipid soluble, as compared to 96% in liver specimens, showing that in the former a higher fraction metabolised to polar compounds. Taken together, our results suggest that in these squamous carcinoma cells, factors other than cellular CRBP content are the major determinants of net cellular uptake and esterification of retinol. The cellular retinol concentration, on the other hand, appears proportional to CRBP content.

Changes in Hepatic Parenchymal and Nonparenchymal Cell Vitamin A Content During Vitamin A Depletion in the Rat

Levels of total, unesterified and esterified retinol were determined in liver, liver parenchymal cells (PC) and liver nonparenchymal cells (NPC) during vitamin A depletion in rats. Liver vitamin A levels decreased from 113 to 4 micrograms over a 97-d experimental period; plasma retinol concentrations did not change significantly during this time. Initially, greater than 90% of hepatic vitamin A was in the esterified form and most (93%) was localized in NPC. During vitamin A depletion, there were significant declines in retinyl ester content of both PC and NPC, but unesterified retinol levels were not significantly affected. Plasma retinol concentrations were significantly correlated with unesterified retinol mass in PC and NPC, but not with retinyl ester mass. Although 94% of the liver's negative vitamin A balance was due to changes in NPC retinyl ester levels, the fractional rate of retinyl ester loss from PC and NPC was almost identical. Since unesterified retinol levels in plasma, PC and NPC appeared to be conserved even when liver retinyl ester stores were virtually depleted, and since the retinol utilization rate was apparently not decreasing during this stage of vitamin A depletion, these data support the hypotheses that homeostatic mechanisms controlling the three pools of unesterified retinol are linked, and that vitamin A utilization rate is maintained as long as unesterified retinol levels in plasma, PC and NPC are normal.

Alterations in retinoid concentrations in several extrahepatic organs of rats by 3,4,3′,4′-tetrachlorobiphenyl

In this study, the effect of polychlorinated biphenyls on retinoid homeostasis was investigated in Sprague—Dawley rats, by analysing [ 3H]retinoid concentrations in peripheral organs, following exposure to 3,4,3′,4′-tetrachlorobiphenyl (TCB). The rats were rendered retinoid-deficient through dietary restriction, followed by dietary supplementation with [ 3H]retinol for 14 days, in order to facilitate determination of retinoid concentrations in various tissues. At day 7 of [ 3H]retinol supplementation the rats were exposed to a single i.p. dose of 15 mg TCB dissolved in corn oil/kg body weight. In corn oil-treated control rats, the highest concentrations of [ 3H]retinoid radioactivity, consisting mainly of retinol and several retinylesters, were obtained in the liver (> 10 6 cpm/g), followed by the kidney and the lung, while only minor concentrations were found in skin and heart. Exposure to TCB resulted in a significant reduction of both retinol and retinylester concentrations in the liver (to 25% of controls) and the lung (to 44% of controls), while in the heart a reduction of retinol to 35% of controls was observed. No significant alterations in retinoid concentrations were observed in the skin and kidney. It is suggested that the reductions in retinoid concentrations might contribute to the toxicological alterations reported in these organs upon exposure to TCB.

Hepatic stores of retinol and retinyl esters in elderly people.

Post-mortem concentrations of hepatic retinol and retinyl esters were determined in 40 subjects aged over 65 years to assess the effects of disease and malnutrition on vitamin A reserves. Three groups of patients (mean age 79.6 years) were studied: (1) previously healthy, (2) chronically ill, (3) chronically ill and wasted. There was no significant difference in height or age between the groups, but group 3 was lighter than both group 1 (P less than 0.001) and group 2 (P less than 0.05). Free retinol and retinyl esters were measured by high pressure liquid chromatography, and the total hepatic retinol calculated. Analysis of variance showed that the three groups differed significantly (P less than 0.02) with regard to total retinol, retinyl palmitate and total retinyl ester content.

Novel aspects of vitamin A metabolism in the dog: distribution of lipoprotein retinyl esters in vitamin A-deprived and cholesterol-fed animals

Retinyl ester concentrations in plasma from fasting humans, rabbits and rats are usually negligible. In contrast, plasma from fasting dogs contains appreciable amounts of retinyl esters, associated almost entirely with the low-density lipoproteins. This study was undertaken to gather additional information about the nature and origin of canine retinyl ester-containing lipoproteins. We examined the metabolism of endogenous lipoprotein retinyl esters in adult mongrel dogs with moderate vitamin A deficiency. Four animals were fed a diet of oatmeal and tunafish that provided only 4% of the vitamin A contained in their control rations (15 vs. 367% of the canine recommended daily intake). There was an initial rapid decline in plasma retinyl esters. However, measurable concentrations persisted in plasma for up to 1 year of restricted vitamin A intake. Total plasma retinyl ester concentrations after 6 months of vitamin A deprivation, extrapolated from best-fit monoexponential decay curves for each animal, ranged from 11 to 89% of control, suggesting that there was sustained secretion of retinyl esters from endogenous stores. Density gradient ultracentrifugation of plasma from fasting vitamin A-deprived dogs showed retinyl esters in the very-low- and low-density lipoproteins. After fat and vitamin A feeding retinyl esters appeared among the very-low-, intermediate- and low-density lipoproteins, consistent with the suggestion that chylomicron retinyl esters are first taken up by the liver, and then resecreted as density < 1.006–1.063 g/ml lipoproteins. Maximal incorporation of dietary retinyl esters into low-density lipoproteins was not reached until 24–48 h. Intermediate-density and β-migrating low-density lipoprotein retinyl esters were increased markedly in fasting animals maintained on cholesterol- and saturated fat-enriched diets. These observations provide further evidence for the proposal that the canine liver secretes retinyl ester-containing particles, in amounts governed by dietary composition and vitamin A content. What selective advantage this unusual transport pathway might provide is not apparent.

Dietary vitamins A and E influence retinyl ester composition and content of the retinal pigment epithelium

Experiments were conducted to determine the influence of dietary levels of vitamin levels A and α-tocopherol on the amounts and composition of retinyl esters in the retinal pigment epithelium of light-adapted albino rats. Groups of rats were fed diets containing α-tocopherol and either no retinyl palmitate, adequate retinyl palmitate, or excessive retinyl palmitate. Other groups of rats received diets lacking α-tocopherol and containing the same three levels of retinyl palmitate. Retinoic acid was added to diets lacking retinyl palmitate. After 27 weeks, the animals were light-adapted to achieve essentially total visual pigment bleaches, and the neural retinas and retinal epithelium-eyecups were then dissected from each eye for vitamin A ester determinations. Almost all of the retinyl were found in the retinal pigment epithelium-eyecup portions of the eyes, mainly as retinyl palmitate and retinyl stearate. Maintaining rats on a vitamin A-deficient, retinoic acid-containing diet led to a significant reductions in retinal pigment epithelial retinyl ester levels in rats fed both the vitamin E-supplemented and vitamin E-deficient diets; contrary to expectations, the effect of dietary vitamin A deficiency was more pronounced in the vitamin E-supplemented rats. Vitamin A deficiency in retonic acid-maintained animals also led to significant reductions in retinyl palmitate-to-stearate ester ratios in the retinal pigment epithelia of both vitamin E-supplemented and vitamin E-deficient rats. Excessive dietary intake of vitamin A had little, if any, effect on retinal pigment epithelial retinyl ester content or composition. Vitamin E deficiency resulted in significant increases in retinal pigment epithelial retinyl palmitate content and in palmitate-to-stearate ester ratios in rats fed all three levels of vitamin A, but had little effect on retinal pigment epithelial retinyl stearate content. In other tissues, vitamin E deficiency has been shown to lower vitamin A levels, and its widely accepted that this effect is due to autoxidative destruction of vitamin A. The increase in retinal pigment epithelial vitamin A ester levels in response to vitamin E deficiency indicates that vitamin E does not regulate vitamin A levels in this tissue primarily by acting as antioxidant, but rather may act as in inhibitor of vitamin A uptake and/or storage. The effect of vitamin E on pigment epithelial vitamin A levels may be mediated by the vitamin E-induced change in retinyl palmitate-to-stearate ratios.

Effects of estrogen/progestin agents on plasma retinoids and chylomicron remnant metabolism

Women using estrogen-progestin oral contraceptive agents have a marked decrease in the activity of hepatic triglyceride lipase, an enzyme believed to be involved in the catabolism of lipoprotein remnants. The hypothesis that women receiving these agents have defective remnant processing resulting in elevated chylomicron remnant concentrations in plasma was tested. Retinyl esters, which in humans are transported by intestinally-derived lipoproteins, were used to estimate chylomicron and chylomicron remnant concentrations. Women on a variety of oral contraceptive agents had increased plasma triglyceride concentrations, but only minimally increased fasting retinyl ester concentrations. Retinol and retinyl binding protein were elevated to about 150% of controls (p < 0.001). Retinyl ester concentrations during active fat absorption were then measured in a second group of women taking a single preparation. Three-hour retinyl ester levels were lower in the treated group than control (p < 0.05), but the difference had disappeared at six hours. Postheparin plasma hepatic triglyceride lipase was reduced by 46% in the treated group (p < 0.05). Thus despite reduction in hepatic lipase activity, there was no accumulation of retinyl esters in the contraceptive-treated women to suggest impaired remnant processing.

Placental Transport of Retinol in Ewes Fed High Intakes of Vitamin A

Twelve ewes were fed retinyl propionate equivalent to 12,000 µg retinol/kg body weight per day for the last two trimesters of pregnancy (group fed high vitamin A). Four ewes received a control level of 120 µg/kg body weight per day. Indwelling catheters were implanted surgically in fetal jugular veins and carotid arteries. Ewes or fetal lambs received [3H]retinol intravenously, and blood was sampled until parturition. The ewes fed large amounts of vitamin A maintained viable fetal lambs for 6 d less than did controls. Plasma retinyl ester concentrations were elevated in the ewes fed a high level of vitamin A but not in their fetal lambs. Rates of plasma retinol transport and clearance increased with vitamin A intake in ewes and their fetal lambs. Efficiency of placental retinol transport in the group fed high levels of vitamin A was less than one-quarter that for controls. However, placental transport rate was approximately 100 µg/d greater than that of controls with an equivalent amount retained by the fetoplacental unit in the group fed high vitamin A. These data indicate that placental transport of retinol is partially regulated. High maternal vitamin A intake results in high retinol transport to the fetus.

Postchallenge plasma lipoprotein retinoids: Chylomicron remnants in endogenous hypertriglyceridemia

This study was conducted to determine whether chylomicron remnants accumulate in the plasma of patients with “endogenous” hypertriglyceridemia. Retinyl esters were used as markers of chylomicrons and chylomicron remnants since they are carried mainly if not exclusively by lipoproteins of intestinal origin. Seventy-six fasting normotriglyceridemic and hypertriglyceridemic patients were studied 12 to 15 hours after ingesting 25,000 IU of vitamin A. Plasma retinol and retinyl esters were measured by reversed-phase high-performance liquid chromatography. Chylomicronemia was assessed by flotation at unit gravity and by chylomicron flocculation in 3% polyvinylpyrrolidone. Plasma lipids, retinoids, lipoprotein cholesterol, and the electrophoretic mobility of very-low density lipoproteins were determined in a subset of 36 subjects. Progressively elevated plasma retinyl ester concentrations were observed among patients with mild, moderate, and severe hypertriglyceridemia. All subjects with fasting chylomicronemia had retinyl ester retention. The majority of subjects with mild or moderate hypertriglyceridemia (predominantly type IV hyperlipoproteinemia) also had elevated plasma concentrations of retinyl esters. Total plasma retinyl ester and plasma triglyceride concentrations correlated significantly ( r s = 0.721, P < 0.001) in nonchylomicronemic subjects. In addition, total plasma retinol concentrations were mildly elevated among hypertriglyceridemic subjects because retinol, as well as retinyl esters, is transported by triglyceride-rich lipoproteins. If lipoprotein remnants are atherogenic in man, then chylomicron remnant retention may accelerate atherogenesis in hypertriglyceridemic individuals.

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.

Retinol homeostasis in lambs given low and high intakes of vitamin A.

Four groups of lambs were fed on a low-carotene basal diet. One group received no supplemental vitamin A (mildly deficient). Remaining groups were supplemented daily with vitamin A acetate equivalent to 100 (control) 9000 (mildly intoxicated) and 18000 (severely intoxicated) microgram retinol/kg body-weight. After 16 weeks lambs received a bolus of [15-3H]retinol intravenously; blood, urine and faeces were sampled for 48 h. Plasma retinol was complexed to a protein of 20000 molecular weight (MW), which in turn was complexed to a protein of 65000 MW; these proteins correspond respectively to retinol-binding protein and prealbumin. Plasma retinol concentration reached plateau values in intoxicated lambs, but plasma retinyl ester concentrations increased rapidly when liver contents of both retinol and retinyl esters exceeded approximately 10 and 100 mg respectively and kidney contents of both retinol and retinyl esters exceeded 30 micrograms. Labelled compounds, more polar than retinol, were found in plasma; their concentration increased tenfold in intoxicated lambs within 48 h. Plasma retinol transport rates were 0.1, 10.5 and 11.8 times control values, and clearance rates were 0.3, 14.1 and 14.3 times control values in mildly-deficient, and mildly- or severely-intoxicated lambs respectively. Turnover of retinol increased rapidly when liver contents of retinol and retinyl esters exceeded approximately 10 and 100 mg respectively and kidney contents of both retinol and retinyl esters exceeded approximately 30 micrograms. Plasma clearance of retinyl esters was unchanged with intake. Faecal excretion of tracer increased linearly with plasma retinol clearance. Our findings identify several variables that appear to be involved in retinol homeostasis, including plasma retinol clearance and excretion.