Rapid synthesis and purification of vitamin a esters

Uptake and esterification of vitamin A by RCS rat retinal pigment epithelial cells in primary culture

We investigated the capacity of Royal College of Surgeons (RCS) rat retinal pigment epithelial (RPE) cells to take up all-trans-retinol (ROL) (vitamin A) and to metabolize it into retinyl esters (RE). Cultures of RPE cells were established from RCS and control newborn rats. All-trans-ROL was delivered to the apical surface of the RPE monolayer. Retinoids were analyzed by high-performance liquid chromatography. The cellular retinol-binding protein type I (CRBP-I) was assessed by Western blotting. Before supplementation with ROL, RE were lower in RCS rats. After ROL supplementation, esters increased and reached values that were similar in the two strains, but the increase, expressed relative to the initial value, was higher in RCS rats. The uptake of ROL and the level of CRBP-I were greater in RCS rats. Our results provide evidence of a functional retinol esterifying enzyme in cultured RCS RPE cells and suggest that CRBP-I could play a role in the uptake and esterification of ROL in the RPE cells.

Amounts and types of fatty acids in meals affect the pattern of retinoids secreted in human chylomicrons after a high-dose preformed vitamin A intake

High doses of preformed vitamin A are commonly used to correct vitamin A deficiency. Newly absorbed vitamin A is secreted mainly as retinyl esters in chylomicrons. The effect of changing types and amounts of fatty acids on fatty acid composition of chylomicron retinoid esters when a high dose of vitamin A is ingested have not been studied previously. In the present study, 10 healthy young men ingested, in a random order, mixed meals containing 15,000 retinol equivalents (RE) of vitamin A (as retinyl palmitate) and either no fat or 40 g of fat provided as butter, olive oil, or sunflower oil. Fasting and postprandial blood samples were obtained for 7 hours after meals. Free retinol and the main retinyl esters (retinyl palmitate/oleate, stearate, and linoleate) were measured in chylomicrons by high-performance liquid chromatography (HPLC). Chylomicron retinyl palmitate/oleate and retinyl stearate concentrations significantly increased after intake of the 4 test meals. Conversely, chylomicron retinyl linoleate and chylomicron free retinol significantly increased only after the sunflower and the fat-free meals, respectively. The main retinoid secreted in chylomicrons after the intake of the fat-rich meals was retinyl palmitate/oleate, accounting for 63% to 79% of total RE, but it was free retinol after the fat-free meal (51% of total RE). Thus, the retinoid pattern secreted in chylomicrons after the intake of a high dose of preformed vitamin A depends on type and amounts of fatty acids ingested. To explain this result we suggest that the esterification process of retinol in the enterocyte by lecithin:retinol acyltransferase can be overwhelmed by a high load of vitamin A. Consequently, a significant proportion of the retinol is esterified by acyl coenzyme A:retinol acyltransferase (ARAT) with ingested fatty acids, explaining the appearance of retinyl linoleate in chylomicrons after the sunflower oil meal. If a high dose of preformed vitamin A is ingested with a fat-free meal, a significant proportion of retinol is not esterified, owing to the lack of fatty acids for ARAT, which explains the appearance of free retinol in chylomicrons.

Processing of vitamin A and E in the human gastrointestinal tract

We aimed to provide basic data on the processing of vitamin A and E in the human gastrointestinal tract and to assess whether the size of emulsion fat globules affects the bioavailability of these vitamins. Eight healthy men received intragastrically two lipid formulas differing in their fat-globule median diameter (0.7 vs. 10. 1 microm. Formulas provided 28 mg vitamin A as retinyl palmitate and 440 mg vitamin E as all-rac alpha-tocopherol. Vitamins were measured in gastric and duodenal aspirates, as well as in chylomicrons, during the postprandial period. The gastric emptying rate of lipids and vitamin A and E was similar. The free retinol/total vitamin A ratio was not significantly modified in the stomach, whereas it was dramatically increased in the duodenum. The proportion of ingested lipid and vitamins was very similar in the duodenal content. The chylomicron response of lipids and vitamins was not significantly different between the two emulsions. Our main conclusions are as follows: 1) there is no significant metabolism of vitamin A and E in the human stomach, 2) the enzyme(s) present in the duodenal lumen is significantly involved in the hydrolysis of retinyl esters, and 3) the size of emulsion fat globules has no major effect on the overall absorption of vitamin A and E.

Chylomicron beta-carotene and retinyl palmitate responses are dramatically diminished when men ingest beta-carotene with medium-chain rather than long-chain triglycerides

The effect of the ingestion of beta-carotene with medium-chain triglycerides (MCT) or long-chain triglycerides (LCT) on the bioavailability and the provitamin A activity of beta-carotene was investigated in humans. Sixteen healthy young men ingested, on two different days, a test meal containing 120 mg beta-carotene incorporated into 40 g LCT (LCT meal) or 40 g MCT (MCT meal). This meal was followed 6 h later by a beta-carotene-free meal containing 40 g LCT. Chylomicron beta-carotene, retinyl palmitate and triglycerides were measured every hour for 12.5 h after the first meal. No significant increase in chylomicron triglycerides was detected for the 6 h after the MCT meal intake, whereas a significant increase in chylomicron triglycerides was observed after the LCT meal intake. The chylomicron beta-carotene and retinyl palmitate responses to the MCT meal (0-6 h area under the curves, AUC) were significantly (P < 0.05) lower [AUC = 68.1 +/- 26.8 and 43. 4 +/- 10.4 nmol/(L.h), for beta-carotene and retinyl palmitate, respectively] than those obtained after the LCT meal [301.4 +/- 64.0 and 166.0 +/- 29.0 nmol/(L.h), respectively]. The chylomicron beta-carotene and retinyl palmitate responses obtained after the beta-carotene-free meal (6-12.5 h AUC) were also significantly lower when the first meal provided MCT rather than LCT. The chylomicron (retinyl palmitate/beta-carotene) ratios were constant during the postprandial periods, whatever the meal ingested. We conclude that the chylomicron beta-carotene response is markedly diminished when beta-carotene is absorbed with MCT instead of LCT. This phenomenon is apparently due to the lack of secretion of chylomicrons in response to MCT; however, a lower intestinal absorption of beta-carotene or a higher transport of beta-carotene via the portal way in the presence of MCT cannot be ruled out. Finally, the data obtained show that MCT do not affect the rate of intestinal conversion of beta-carotene into vitamin A.

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.