Liver Retinyl Ester Research Articles

β-Carotene, Retinol and Retinyl Ester Concentrations in Serum and Selected Tissues of Ferrets Fed β-Carotene

The concentrations of β-carotene, retinol and retinyl esters in serum and selected tissues of ferrets fed diets supplemented with β-carotene (80 μg/g wet diet) for 3 wk were determined. The initial concentration of serum β-carotene was 0.011 ± 0.006 μmol/L (mean ± SEM); at the end of the experimental period it was 5.75 ± 1.60 μmol/L. No significant differences in serum retinol and total retinyl esters were observed between β-carotene-fed and control ferrets that had been fed an unsupplemented diet. The predominant retinyl esters in serum were retinyl stearate (53%) and retinyl palmitate (35%). Of the tissues analyzed after β-carotene feeding, the liver contained the highest concentration of β-carotene (78.8 ± 18.8 nmol/g). Other tissues that contained β-carotene in amounts ranging from 17 to 20 nmol/g were adrenals, small intestine, stomach and colon; lesser amounts (6.9 nmol/g) were found in kidneys. Amounts ranging from 1.2 to 2.3 nmol/g were found in muscle, bladder, adipose tissue, lungs and skin; only 0.37 and 0.34 nmol/g were present in brain and eyes, respectively. Thus, like humans, ferrets have the capacity to absorb intact β-carotene and to store this compound in tissues, especially the liver. However, compared with humans, ferrets have elevated concentrations of retinyl esters in serum, liver and other tissues.

Retinol Storage in the Rat Liver after Daily Intramuscular Administration of Physiological Doses of a Vitamin A Oil Emulsion

We have recently shown the kinetic behavior of liver retinyl esters in rats with adequate vitamin A levels receiving oral vitamin supplementation. In the present work we have studied the effects of intramuscular administration of a vitamin A preparation on the metabolism of vitamin A in the rat. Retinol administered intramuscularly to rats in the form of an emulsion brought about a significant increase in the serum and liver concentration of vitamin A; this increase was slightly less than in orally treated rats. In each group, retinyl palmitate constituted 80-85% of the total retinyl esters, followed by stearate (9-13%), laurate, palmitoleate, myristate, linoleate and pentadecanoate making up 3-10%. The subcellular localization of all retinyl esters is similar and dependent on age but not on the route of administration. These results indicate that although the best hepatic storage is achieved with an orally administered vitamin A emulsion, the intramuscular administration of a physiological dose might provide an effective supplementation method if oral vitamin A is contraindicated.

The Effect of Dietary Fatty Acid Composition on Liver Retinyl Ester (Vitamin A Ester) Composition in the Rat

The dependence of liver retinyl ester (vitamin A ester) composition on dietary fatty acid composition was studied in rats. Weanling male Sprague-Dawley rats were fed purified test diets containing 8% of one of 10 test triglycerides (either tributyrin, tricaproin, tricaprylin, tricaprin, trilaurin, trimyristin, tripalmitin, tristearin, triolein, trilinolein) or corn oil (a mixed triglyceride) for 23 d after an initial 1-wk stabilization period. Essential fatty acids were provided by safflower oil (8 g/kg diet). Total liver fatty acid composition showed the expected responses to dietary fatty acid intake; short-chain and medium-chain fatty acids were readily elongated to palmitate and stearate, and palmitate and stearate were readily desaturated. Consumption of oleate or linoleate (as their triglycerides) markedly enriched their concentrations in liver lipids. The proportions of palmitate were generally greater in liver retinyl esters than in total liver fatty acids, and the proportions of oleate were less. However, retinyl ester composition was significantly affected by dietary triglyceride, particularly by diets providing long-chain fatty acids. Total liver vitamin A levels were depressed in the group fed trilinolein.

The effect of high vitamin a intake on fatty acid composition of rat liver retinyl esters and total liver fatty acids

Female Fischer 344 rats fed AIN-76 diet plus 7 μmoles (2.3 mg) retinyl acetate in corn oil daily for 17 d (vitamin A-supplemented) showed markedly enhanced liver vitamin A levels compared with control animals fed AIN-76 diet plus corn oil (6.6 μmol/g liver vs. 0.21 μmole/g, p<0.001) and lower serum retinol (0.55 μM vs. 0.99 μM, p<0.001). As mole per cent of liver retinyl esters, retinyl palmitate increased on vitamin A supplementation (from 61.1% to 75.5%, p<0.001), retinyl stearate and laurate plus arachidonate (unresolved) decreased (from 19.2% to 10.4%, p<0.001, and 5.5% to 1.8%, p<0.001, resp.), and retinyl oleate did not change (5.0% to 4.8%). The fatty acid composition of total liver lipids showed similar changes. In general, the ratios of more unsaturated to less unsaturated fatty acids increased, while those of elongated to unelongated fatty acids fell. Thus, high intakes of vitamin A clearly influence the overall fatty acid metabolism of the liver, possibly by stimulating desaturation reactions and by inhibiting elongation.

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 μ m all- trans-retinyl palmitate and 275 m m 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate in a 0.1 m Trismaleate buffer, pH 7.0, for 1 h at 37 °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.

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.

Fatty acid composition and kinetic behaviour of liver retinyl esters in vitamin A sufficient and deficient rats

Weanling rats were fed vitamin A deficient diets (− A) or diets supplemented with vitamin A (+ A) (4.4 mg retinol equivalents/kg diet) for a period of 7 or 6 wk, respectively. In liver tissues of these two groups of animals both the subcellular localization as well as the fatty acid composition of the retinyl esters was studied. During vitamin A supplementation or deprivation, the kinetics of the different ester forms were investigated. Results indicate that the subcellular localization of all retinyl esters is similar and dependent on age. Two pools exist, ie one consisting of the nuclear/cell debris and mitochondrial-lysosomal fractions and the other containing the microsomal and cytosol fractions. HPLC analysis showed retinyl palmitate as the predominating (80%) form of the various retinyl esters. By supplementation clearly two kinetic behaviours can be demonstrated: one being a relatively stable storage of the palmitate and stearate, increasing with time and the second one being a more labile pattern for the ester forms with other saturated and unsaturated fatty acids. By vitamin A depletion all retinyl esters are affected indicating that the ester forms other than palmitate and stearate are also storage forms of vitamin A.

Quantitative determination of vitamin A in liver and liverwurst using high pressure liquid chromatography (HPLC)

Samples of liver sausage and liver tissue from slaughtered animals were analysed for pre-formed vitamin A. A sensitive reversed phase HPLC method with fluorescence detection was used to determine the amount of free and esterified retinol in extracts of these samples. Under our chromatographic conditions retinyl myristate and linoleate run together as do retinyl palmitate and oleate. The contents of all forms of vitamin A were expressed in milligrams of all-trans retinol per 100 g wet weight. The average concentrations of total vitamin A for turkey, pig, calf, beef, and chicken liver were 42.31, 38.15, 22.74, 14.98, and 13.22, respectively. In addition to these differences, the fatty acid composition of liver retinyl esters, as well as the amount of retinol, also varied considerably among species. Approximately 95% of the hepatic vitamin A was present as retinyl ester with retinyl palmitate/oleate as the main ester fraction. The second most abundant fatty acid in the retinyl ester fraction of calf, beef, and chicken liver was stearic acid followed by myristic/linoleic acid. An increment of retinol, however, was also found in these tissue samples. By contrast, in turkey liver the retinol fraction was increased and therefore both retinyl stearate and retinol were present in nearly equal amounts, whereas the retinyl ester fatty acids from pig liver contained relatively more myristate/linoleate and less stearate than those from liver of the other domestic animals (apparently a hallmark of pig liver). The retinol and retinyl ester pattern present in liver sausage was similar to, but not identical with, that of pig liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Liver retinoids and retinyl esters in patients with alcoholic liver disease

Liver retinoids and retinyl esters in patients with alcoholic liver disease

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 μM 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 streate (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 μg/dl, and of liver vitamin A concentrations from 0 to 32 μg/g. Pig liver retinyl ester hydrolase from the rat liver enzyme in its substrate specificity, bile acid stimulation, and interanimal variability.

Effects of retinoic acid on the metabolism of vitamin a in rat liver

Sprague-Dawley male rats with adequate storage of vitamin A were fed vitamin A deficient diet either alone (RA-) or supplemented with retinoic acid (RA+). The rats were sacrificed at different days for the measurement of free retinol and the composition of retinyl esters in liver. At the same time the plasma retinol levels of these rats were also determined. Supplementation of retinoic acid significantly lowered the plasma retinol levels with a concomitant slower depletion of the total vitamin A in the liver of RA+ rats as compared. to RA-rats. The ratio of retinyl esters to free retinol in the liver tissue of RA- rats decreased with time, whereas in RA+ rat livers the ratio remained constant. The percentage distribution of retinyl esters in the liver tissue revealed a significant decrease in saturated retinyl esters with an increase in unsaturated retinyl esters in the RA+ group compared to RA- group. These data support the earlier findings that retinoic acid spares the utilization of retinol and suggest that it may affect the release of retinol from retinyl esters in the liver.

Acyl coenzyme A:Retinol acyltransferase activity and the vitamin A content of polar bear ( Ursus maritimus) liver

1. 1. Acyl coenzyme A: retinol acyltransferase activity was identified in the microsomes from a polar bear liver. 2. 2. The highest rate of in vitro retinol esterification was 821 pmol/min/mg microsomal protein. 3. 3. The in vitro esterification rate displayed a small dependence upon the concentration of exogenous protein (bovine serum albumin) and even less on the concentration of sulfhydryl-reducing agent (dithiothreitol). 4. 4. Vitamin A was present in the liver at a concentration of 8050 μg/g tissue, with 98% of the vitamin in its ester form. 5. 5. Retinyl palmitate was 37.3% of the total liver retinyl esters, while retinyl oleate represented 20.9%, stearate 12.8%, and linoleate 7.7%.

Biochemical Indicators of Vitamin A Depletion in Children With Cholestasis

Biochemical indicators of vitamin A status were measured in 24 children (1 month to 6 years old) with severe cholestasis starting early in life and in 21 children (3 months to 13 years old) with liver disease but without cholestasis. Liver vitamin A concentrations, expressed as micrograms of retinol per gram of liver (mean +/- S.D.), were 6.3 +/- 7.1 (range: 0.14 to 28) and 143 +/- 108 (range: 18 to 424), respectively, in cholestatic and non-cholestatic children. In infants less than 6 months of age, liver vitamin A values less than 10 micrograms per gm were found in 14 of 17 cholestatic children but in none of 3 non-cholestatic subjects. Plasma vitamin A values, expressed as micrograms of retinol per deciliter (mean +/- S.D.), were 23 +/- 18 (range: 3 to 62) and 46 +/- 33 (range: 14 to 125), respectively, for the two groups. Plasma retinol values less than 10 micrograms per dl were always associated with liver concentrations less than 10 micrograms per gm. Plasma retinol-binding protein was only reduced to 71% of control values in cholestatic children. The fatty acid composition of liver retinyl esters was unaffected by any condition studied. Infants with chronic cholestasis are in a precarious nutritional status very early in life relative to liver reserves of vitamin A. Plasma vitamin A values, unless less than 10 micrograms retinol per dl, are poor indicators of inadequate vitamin A status.

Multiple functions of vitamin A.

Multiple functions of vitamin A.

Relation of the relative dose response to liver concentrations of vitamin A in generally well-nourished surgical patients

In 12 adult, generally well-nourished, surgical patients, liver vitamin A concentrations obtained by the analysis of liver biopsies were compared with the relative dose response (RDR). RDR is defined as the percentage increase in plasma retinol level relative to the plasma retinol level 5 h after the oral administration of a standard oral dose (450 µg) of retinyl acetate. Liver vitamin A values, expressed as retinol, and corresponding RDR values were 14 µg/g (28%), 30 µg/g (15%), and 58 to 434 µg/g (0 to 12%). Other indicators of vitamin A nutriture, which did not correlate with liver vitamin A concentrations in this group, were serum retinol level, serum retinol-binding protein concentration, and the percentage saturation of serum retinol-binding protein. The relative distribution of fatty acyl esters in liver retinyl ester did not change with total liver reserves. The relative percentage of retinol to total liver vitamin A, however, was higher (>5%) when reserves were ≤30 µg/g. Although the RDR cutoff point of >14% suggested earlier by others is in reasonable accord with our results, a somewhat higher RDR cutoff value (possibly 20%) may agree better with the suggested criterion of a liver vitamin A concentration of 20 µg/g, expressed as retinol, as a minimally adequate reserve.

Retinyl esterase activity of purified rat liver retinyl ester lipoprotein complex

Retinyl ester lipoprotein complex from rat liver was shown to possess a retinyl esterase activity toward its own ligand complement. In the presence of serum albumin the retinyl esterase activity at 30 °C was about fivefold larger than the activity at 4 °C, while higher temperatures than 30 °C led to some degradation of retinyl compounds. The pH optimum was 7.8. The esterase activity was markedly enhanced by serum albumin although the serum albumin as such had no retinyl esterase activity. In the presence of serum albumin and under optimal conditions, some 75 to 80% of the total retinyl ester complement of the lipoprotein was hydrolyzed in 24 h. The retinyl esterase activity was totally abolished by treatment with the serine esterase inhibitor diisopropyl fluorophosphate (1.4 × 10 −4 M), by treatment with sulfhydryl reagents, and by detergents (0.2% of Tween 80 and sodium deoxycholate). From this series of experiments it was concluded that the retinyl ester lipoprotein complex possesses the additional physiological function of hydrolyzing its own retinyl ester complement to unesterified retinol which may then combine with serum retinol-binding protein.

Rate of Metabolism of Retinol in Retinoic Acid-Maintained Rats after a Single Dose of Radioactive Retinol

The half-life and metabolism of vitamin A were determined in a group of vitamin A-deficient retinoic acid supplemented rats after a single dose of 340 mug of [6,7-14C2] retinol. The total daily urinary radioactivity, plotted semilogarithmically as a function of days after injection, revealed three pools for retinol and/or metabolites in the rat: (1) a rapidly declining pool with half-life of 0.75 day; (2) a slowly declining pool with a constant rate of decrease and (3) a pool with a half-life of 13 days which begins at approximately 6 weeks after dose. The total daily fecal radioactivity also indicated three pools with half-lives of 2,28.5 and 11.5 days. The effect of retinoic acid feeding was observed on the fifth day after supplementation, as indicated by a decrease in the total daily urinary radioactivity. Thus, retinoic acid is probably in the metabolic pathway of retinol. The half-life and metabolism time of liver vitamin A in these rats were determined as 7 and 10 days, respectively. The specific activities of liver retinyl esters and retinol determined at different intervals after dose indicated continuous mixing of radioactive retinol with a pool of endogenous retinol. Blood retinol levels indicated normal values at 1 week after dose. However, they decreased at 2 weeks after dose and remained constant until the sixth week. The specific activity of blood retinol did not change indicating rapid equilibration after initial mixing and no further dilution from endogenous source.

Retinal in the blood and liver of the fowl in relation to sex and maturity.

1. Concentrations of retinal (vitamin A(1) aldehyde) in the plasma and liver of laying hens, mature cockerels, immature pullets and pullets undergoing sexual maturation have been measured. 2. The plasma of laying hens contained about 8mug. of retinal/100ml., about ten times that found in the plasma of mature cockerels and immature pullets. In laying hens that had received large doses of retinyl palmitate 8-4 weeks previously, the mean concentration of retinal was 18.3mug./100ml. of plasma. 3. The appearance of significant amounts of retinal in the plasma of maturing pullets coincided with hypertrophy of the oviduct, increase in concentration of plasma lipid and onset of egg-laying. 4. Retinal was present in the livers of all types of fowl examined and the concentrations, which ranged from 0.2 to 5.8mug./g. wet wt., were highly correlated (r=0.79) with the concentrations of liver retinyl esters, which ranged from 92 to 1530mug./g. wet wt.