Composition Of Retinyl Esters Research Articles

Fatty-acyl esters of retinol (vitamin A) in the liver of the harp seal (Phoca groenlandica), hooded seal (Cystophora cristata), and California sea lion (Zalophus californianus).

The fatty-acid composition of retinyl esters in the livers of two species of phocid seal, the harp seal (Phoca groenlandica, n = 20) and the hooded seal (Cystophora cristata, n = 15), and one species of otariid seal, the California sea lion (Zalophus californianus, n = 6), was determined. Vitamin A ranged in concentration from 4 to 1024 nmol retinol/g liver for the phocids and from 381 to 979 nmol/g liver for the otariids. In most of the livers, retinyl palmitate was not the principal ester, and the palmitate + stearate + oleate trio of retinyl esters represented less than 50% of the total. In all samples, the retinyl esters contained 20:1, 20:4, 20:5, and 22:6 in unusually large amounts. Retinyl esters tended to be richer than whole-liver lipids in 20:5 + 22:6, whereas whole-liver lipids were richer in 18:0 and 18:2. Therefore, the pool of acyl donors used for the esterification of retinol may be distinct from that used for other lipids. Birth-to-weaning changes were seen only in the harp seals. In the pups, the hepatic vitamin-A concentration increased 454%, while the proportion of 18:0 and 20:1 in the retinyl esters rose and that of 14:0 + 16:1 and 20:4 fell. Concomitantly, in their mothers, the proportion of 20:4 increased but that of 16:0 and 18:0 decreased.

The selectivity of acyl coenzyme A: Retinol O-acyltransferase toward its fatty-acyl-CoA substrate

Although vitamin A is known to be stored in the liver primarily as retinyl palmitate, the selectivity of one of the microsomal enzymes thought to esterify retinol in vivo, acyl CoA: retinol O-acyltransferase (ARAT), has not been defined with respect to the acyl-CoA substrate. Therefore, retinol and liver microsomes were incubated with several exogenous CoA-thioester derivatives, present together and at equimolar amounts in the reaction mixture, and the observed formation rates were corrected for acyl-donor endogenous to the microsomes. CoA thioesters of oleic and C 12−C 20 saturated fatty acids were effective substrates for rat-liver ARAT in vitro, whereas polyunsaturated derivatives were virtually ineffective. ARAT from polar-bear liver displayed a similar specificity. ARAT selectivity observed in vitro fails to account for the fatty-acid composition of retinyl esters in vivo. Thus, either the concentration of palmitoyl CoA at the intracellular site of the ARAT-catalyzed transacylation is uniquely high, or the esters of retinol that occur in vivo are synthesized by another enzyme.

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.

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.

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.

A Marginal Vitamin A Status Alters the Distribution of Vitamin A among Parenchymal and Stellate Cells in Rat Liver

The aim of this study was to determine whether the distribution of vitamin A between parenchymal and stellate cells in rats with low vitamin A liver reserves is different from that observed in rats with adequate reserves. Retinol and retinyl esters were quantitated in parenchymal and stellate cells of the livers of three groups (n=3) of adult Sprague-Dawley rats with mean total liver vitamin A reserves of 1.2, 14.5 and 28.9 µg retinol/g fresh liver for groups 1, 2 and 3, respectively. The amount of vitamin A per 106 cells was similar in parenchymal and stellate cells of group 1, whereas in groups 2 and 3, a much higher amount was found in stellate cells (11-fold and 27-fold higher, respectively). In group 1, 83% of the liver vitamin A was present in parenchymal cells. As liver stores rose, however, a progressively greater amount was stored in stellate cells. In group 3, 82% of the liver vitamin A was found in stellate cells. In groups 2 and 3, 87–91% of the vitamin A was found as retinyl ester, whereas in group 1, 52% was found as retinol. The retinyl ester composition was similar in parenchymal and stellate cells, major esters being palmitate (84%) and stearate (10%), with smaller amounts of oleate (2.5%) and other esters. Thus, in rats with low liver vitamin A reserves, most of the liver vitamin A is found in parenchymal cells.

Relative Amount and Ester Composition of Vitamin A in Rat Hepatocytes as a Function of the Method of Cell Preparation and of Total Liver Stores

The aim of this study was to determine the cellular distribution of vitamin A in the liver of adult, well-nourished Sprague-Dawley rats with different stores of this vitamin. Three groups of three rats each were fed AIN-76 diet. Each group received an oral supplement of either 0, 2, or 12 mg of retinol. Retinol and retinyl esters were measured in the whole liver, in hepatocytes isolated either by gravity sedimentation or by centrifugal elutriation, and in the supematant solution recovered after hepatocyte sedimentation. Dietary supplementation with vitamin A increased the liver stores from 138 to 390 μg/g. A mean of 5% of the liver vitamin A was found in hepatocytes isolated by centrifugal elutriation. This percentage tended to decrease as liver vitamin A increased. In contrast, a mean of 38% of total liver vitamin A was found in hepatocytes isolated by gravity sedimentation over a threefold range of liver reserves. Whereas the retinol concentration in hepatocytes remained constant, the percentage of total vitamin A as retinyl esters, which accounted for more than 96% of the total vitamin A, increased as liver stores increased. The retinyl ester composition was similar in all three groups, major esters being palmitate (76–82%), stearate (9–12%), oleate (5–7%), and linoleate (3–4%). Over a threefold range of liver vitamin A concentrations in well-nourished rats, therefore, we estimate that only 10–20% of the total vitamin A in liver is present in the parenchymal cells.

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.

Vitamin A metabolism: analysis of steady-state neutral metabolites in rat tissues.

High-performance liquid chromatography systems were developed to rapidly separate retinol from its esters, analyze the total spectrum of neutral vitamin A compounds, and purify retinyl esters to homogeneity. Chemical ionization mass spectrometric techniques were used to identify vitamin A compounds; these techniques are also applicable to quantification of tissue vitamin A compounds. These methods provide rapid and sensitive techniques for separation and quantification of neutral retinol metabolites. Their utility was demonstrated by analysis of vitamin A metabolites in rat tissues under steady-state conditions. Tissue specificity was noted for the concentrations of retinol and its long-chain fatty acid esters, the ratio of retinol to retinyl esters, and the fatty acid composition of retinyl esters. Quantitatively minor amounts of several neutral polar retinol metabolites were detected, but neither 13-cis-retinol nor 4-hydroxyretinol was observed in vivo as metabolites of retinol in kidney.