Properties of liver retinyl ester hydrolase in young pigs

Metabolism of Neonatal Vitamin A Supplementation: A Systematic Review

A systematic review was conducted to summarize the absorption, transport, storage, and metabolism of oral neonatal vitamin A supplementation (NVAS). This review focused specifically on the neonatal period (first 28 d of life for humans) to inform guidance by WHO on recommendations related to NVAS. A systematic search of international and regional databases was conducted. Inclusion criteria were human or animal studies that gave oral vitamin A as a single or limited number of doses to apparently healthy neonates. Studies evaluating fortification or food-based approaches, dosing with retinoic acid, or studies of neonatal models of disease were excluded. The search retrieved 8847 unique records. After screening by title and abstract, 88 were screened using the full text, and 35 records met inclusion criteria: 13 human and 22 animal studies. Studies indicate that high-dose NVAS is absorbed well by neonates, typically mirroring fat absorption. Doses were primarily stored in the liver and transiently increased in the lung, kidney, spleen, adrenal glands, brain, skin, and adipose tissue, generally with a dose-response. Serum retinol and retinyl esters also transiently increased following NVAS. Although minimal acute adverse effects are noted, there is a lack of data supporting NVAS for improving organ maturation or sustained delivery to target organs. Research gaps include the physiological effects of the short-term increase of vitamin A concentrations in extrahepatic tissues, or whether there are unknown adverse effects over time.

A comparative study on the effects of oral amiodarone and trimeprazine, twoin vitroretinyl ester hydrolase inhibitors, on the metabolic availability of vitamin A in rats

Amiodarone, an antiarrhythmic drug, and trimeprazine, an antipsychotic drug, are bothin vitroinhibitors of retinyl ester hydrolase. To determine whether these agents have deleterious effects on aspects of vitamin A metabolism, Brown Norway rats (n18) were treated at clinically equivalent doses once daily for 26d with either oral drug. On day 27, a tolerance test was used to determine whether these agents interfered with vitamin absorption. During the first 8d, the plasma retinol level declined in all animals. Between days 12 and 27, it rose to near pre-treatment concentrations in the control and trimeprazine groups and remained relatively constant at low levels (P<0·001) in the amiodarone group. The intestinal absorption of vitamin A was reduced (P<0·05) in the amiodarone group compared with the placebo and trimeprazine groups, which did not differ significantly from each other. At the end of the 4-week treatment period, hepatic retinyl ester hydrolase activity was lower in the drug-dosed rats (P=0·06 for amiodarone) than in the controls. With regard to effects on liver reserves, drug treatment resulted in vitamin A depletion (P<0·019), and distinctive patterns of retinol and its esters were seen in response to dosing. In conclusion, amiodarone and trimeprazine have been shown to influence different aspects of retinoid metabolism, namely absorption, storage and transport. In clinical practice, the routine unmonitored use of these drugs and the suggestion that these agents be taken with meals are not recommended.

One-time vitamin A supplementation of lactating sows enhances hepatic retinol in their offspring independent of dose size

BACKGROUND

Single megadoses of vitamin A between 200,000 and 400,000 IU have been administered to lactating mothers to improve the vitamin A status of both mothers and breastfeeding infants. However, the most beneficial dosing regimen is not known.

OBJECTIVE

The effect of megadoses of vitamin A supplements given to lactating sows on hepatic vitamin A concentrations in their nursing offspring was examined.

DESIGN

Lactating sows were given a high (2.1 mmol), low (1.05 mmol), or control (0 mmol) dose of retinyl acetate in oil (n=3 sows per treatment). Piglets nursed for 3 or 14 d, consumed a vitamin A-free diet for the next 4 d, and were then killed. Liver and serum samples were analyzed for vitamin A.

RESULTS

After 3 d, piglets of the control, low-dose, and high-dose sows had different (P=0.034) hepatic vitamin A concentrations, ie, 0.078+/-0.004, 0.14+/-0.053, and 0.13+/-0.026 micromol/g, respectively. Liver vitamin A concentrations on day 18 were 0.069+/-0.004, 0.14+/-0.044, and 0.11+/-0.026 micromol/g in the control, low-dose, and high-dose piglets, respectively (P=0.017). Liver vitamin A concentrations in piglets of the low- and high-dose sows were not significantly different (day 3: P=0.97; day 18: P=0.59). Serum retinol concentrations were higher (P=0.02) at early kill (0.95+/-0.22 micromol/L) than at late kill (0.76+/-0.24 micromol/L) but were not significantly different between groups.

CONCLUSIONS

Maternal vitamin A supplementation enhances liver vitamin A concentrations in offspring. Larger one-time doses are not more effective than are smaller doses. Additional research is needed to determine the most effective maternal dosing regimens for improving infant vitamin A status.

Cellular retinol-binding protein I, a regulator of breast epithelial retinoic acid receptor activity, cell differentiation, and tumorigenicity

BACKGROUND

Retinoic acid receptor (RAR) activation induces cell differentiation and may antagonize cancer progression. Cellular retinol-binding protein I (CRBP-I) functions in retinol storage and its expression is lower in human cancers than in normal cells. We hypothesized that retinol storage might be linked to RAR activation and thus that lowered CRBP-I function might impair RAR activity and cell differentiation.

METHODS

Sarcoma virus 40-immortalized human mammary epithelial cells (MTSV1-7) devoid of CRBP-I were transfected with wild-type CRBP-I or CRBP-I point mutants with low RA binding affinity. The subcellular localization of CRBP-I was investigated in these cells and in wild-type or CRBP-I null mouse mammary epithelial cells (MECs), using indirect immunofluorescence and sucrose gradient fractionation. RAR activity was assessed using reporter gene assays. Acinar differentiation and in vivo tumor growth were assessed in reconstituted basement membrane and athymic mice, respectively.

RESULTS

In cells expressing wild-type CRBP-I but not the CRBP-I mutants, CRBP-I was found mainly in lipid droplets, the retinol storage organelle, and this localization was associated with promotion of retinol storage by wild-type CRBP-I only. RAR activity was higher and acinar differentiation was observed in cells expressing wild-type but not mutant CRBP-I. RAR antagonist treatment blocked and chronic RA treatment mimicked, the CRBP-I induction of cell differentiation. Finally, CRBP-I suppressed tumorigenicity in athymic mice.

CONCLUSIONS

Physiologic RAR activation is dependent on CRBP-I-mediated retinol storage, and CRBP-I downregulation chronically compromises RAR activity, leading to loss of cell differentiation and tumor progression.

Adjustments to the modified relative dose response (MRDR) test for assessment of vitamin A status minimize the blood volume used in piglets

The modified relative dose response (MRDR) test is widely used in public health research to assess vitamin A (VA) status of populations and individuals. However, method adjustments intended to make the test more useful in large field studies and/or less invasive have not been systematically verified. To compare the similarity between modified tests and the standard MRDR test, and validate both modified and standard tests against liver reserves of VA, we used a piglet model. Following the typical MRDR procedure, piglets (n = 10) were dosed with 5.3 micromol 3,4-didehydroretinyl acetate. Method adjustments were made to the postdose blood sample collection time to decrease both the amount of serum analyzed and sample throughput time. We collected 3 blood samples/piglet at 3, 5, and 7 h or 4, 6, and 8 h postdose. Postdose blood samples obtained between 4 and 7 h gave MRDR values that did not differ. Serum volumes as small as 200 microL, half the volume of the standard method, yielded accurate MRDR values. Method adjustments to reduce sample throughput time require further investigation. In conclusion, because 200 microL of serum can be used in the test, only 0.5 mL, as opposed to 1 mL of blood has to be collected from an individual. This adjustment allows for easier application of the test to individuals, especially infants, from whom it is difficult to obtain a large venous blood sample, thus increasing the utility of the test for researchers.

The hepatic retinyl ester hydrolase activity is depressed at the onset of diabetes in BB rats

Dietary vitamin A as retinyl ester is hydrolysed and re-esterified with long-chain fatty acids in the small intestine. The esterified vitamin A is subsequently stored in the liver, where it is hydrolysed to free retinol to be transported by carrier proteins to the target tissue. A decreased availability of retinol carrier proteins has been suggested to be responsible for affecting metabolic availability of vitamin A in type 1 diabetes. Using BB Wistar rats, the present study was undertaken to examine whether the presence of a hyperglycaemic state modifies retinyl ester hydrolase (REH) activity in the intestine and the liver. At the onset of diabetes, hepatic REH enzymatic activity was significantly decreased. However, REH activity remained unaffected in the small intestine, including both ileum and jejunum. Diabetes also resulted in decreased plasma and liver concentrations of retinol. An in vitro study was conducted to examine the effect of diabetes on the intestinal uptake of retinyl palmitate. Jejunum and ileum from diabetic and non-diabetic BB rats were incubated with labelled retinyl palmitate at different concentrations ranging from 32 to 256 nmol/l. The uptake of retinyl palmitate was increased in both diabetic and non-diabetic rats together with the increase of substrate concentration. However, no significant difference was observed in the uptake of retinyl palmitate between diabetic and non-diabetic rats. These present results suggest that the depressed hepatic REH activities may contribute to the diabetes-associated metabolic derangement of vitamin A.

Inhibition of purified pig and human liver retinyl ester hydrolase by pharmacologic agents

Identification of inhibitors of retinyl ester hydrolase (REH) would help to elucidate its role in vitamin A metabolism in vivo. By using standard incubation conditions, the effects of 215 drugs as potential inhibitors of purified pig and human liver REH when acting on micellar substrate retinyl palmitate were evaluated at 16.7, 167, and 1670 microM. Out of the compounds tested, 103 were inhibitors of the pig liver enzyme. The most potent compounds, in order of decreasing activity, were chloral hydrate, lovastatin, phytomenadione, alimemazine, physostigmine, thioridazine, phenoxybenzamine, probucol, cinnarizine, cyclandelate, amiodarone, flupenthixol, and naftidrofuryl; this order is roughly similar to that of their inhibition of human liver REH. Of the 10 tricyclic ring-containing drugs tested, alimemazine was the most potent enzyme inhibitor. The concentrations necessary for 50% enzyme inhibition ranged from <2.6 up to >540 microM. Moreover, inhibitory kinetic studies showed that at least two pharmaceuticals, chloral hydrate and amiodarone, are potent REH inhibitors at therapeutically achievable serum concentrations. First-pass metabolites were inactive as REH inhibitors compared to that of the parent compounds, in the cases of chloral hydrate, lovastatin, and cyclandelate.

Lipases and carboxylesterases: possible roles in the hepatic metabolism of retinol

The formation of hydrolysis of retinyl esters are key processes in the metabolism of the fat-soluble micronutrient vitamin A. Long-chain acyl esters of retinol are the major chemical form of vitamin A (retinoid) stored in the body. Retinyl esters are found in a variety of tissues and cell types, but most of the total body retinoid is accounted for by the retinyl esters stored in the liver. Thus, these esters represent the major endogenous source of retinoid that can be delivered to peripheral tissues for conversion to biologically active forms. This review summarizes current knowledge about the identity, function, and regulation of the hepatic enzymes potentially involved in catalyzing the hydrolysis of retinyl esters. These enzymes include several known and characterized lipases and carboxylesterases. Although there is accumulating evidence that these enzymes function as retinyl ester hydrolases in vitro, it is not clear which play important physiological roles in hepatic retinyl ester metabolism.

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.

Retinyl ester hydrolysis in the rabbit lacrimal gland

The lacrimal gland stores retinyl esters which are synthesized by the enzyme acyl CoA:retinyl acyl transferase. Retinol is released from retinyl ester reserves by retinyl ester hydrolase (REH). Since the lacrimal gland secretes retinol, this gland should also contain this enzyme. To identify bile salt-dependent REH activity, rabbit lacrimal glands were homogenized in 0.05 M Trismaleate buffer, and enzyme activity was determined in the tissue homogenate, in the membrane fraction and in the cytosolic fraction by measurement of production of retinol from retinyl palmitate (nmol retinol produced/mg protein/h). In the lacrimal gland, production of retinol was optimal in the presence of 200 mM CHAPS at pH 7. The REH activity in the presence of 1000 microM retinyl palmitate was 2.38 +/- 0.18 nmol/mg/h in the homogenate, 1.13 +/- 0.16/nmol/mg/h in membranes and 3.25 +/- 0.26 nmol/mg/h in cytosol. By comparison, REH activity in rabbit liver was 6.58 +/- 0.75 nmol/mg/h. The REH activity in lacrimal gland was not affected by vitamin A deficiency. These data are consistent with the presence of retinyl ester hydrolase activity in the lacrimal gland and provide further evidence that this gland is adapted for metabolism and secretion of retinol.

Bile salt independent retinyl ester hydrolases in the bovine eye

Homogenates of bovine neuroretina and retinal pigment epithelium (RPE) were incubated with 11-cis and all-trans retinyl palmitate to study retinyl ester hydrolysis. The highest activity was found in RPE when 11-cis retinyl palmitate served as substrate (Km = 7.8 microM and Vmax = 44.8 pmol/min/mg). This retinyl ester hydrolase (REH) had an optimum activity at acidic pH (pH 5), which is in contrast to the neutral hydrolase (pH 8) found in the neuroretina. Similar to REH in the liver, REH activities in the bovine eye are not stimulated by bile salts because sodium cholate, taurocholate and deoxycholate did not enhance retinyl ester hydrolysis. Most REH activities in retinal homogenate were soluble, whereas in the RPE, these activities were membrane-bound. Divalent cations such as zinc and cadmium completely inhibited REH activities in the neuroretina. Our results show that bovine ocular tissues contain several retinyl ester hydrolases with distinct biochemical properties.

Retinyl ester hydrolase and vitamin A status in rats treated with 3,3',4, 4'-tetrachlorobiphenyl

Previous studies have shown that rats exposed to 3,3',4,4'-tetrachlorobiphenyl (TCB) exhibit decreased liver vitamin A stores. The activity of retinyl ester hydrolase (REH), the enzyme responsible for the hydrolysis of the storage form of vitamin A (retinyl esters) into free retinol, may therefore be altered by TCB. This study was carried out to investigate the effect of TCB on vitamin A distribution and on REH activity in the rat. REH activity was measured in liver homogenates and microsomes (650 micrograms protein), in Tris-maleate buffer 0.1 M at pH 7.2 in the presence of 150 mM CHAPS and 1.5 mM retinyl palmitate dispersed in Triton X-100 0.2%. Using these conditions, the kinetic parameters of the enzyme were determined and the inter-animal variation coefficient (10%) allowed statistical comparisons between experimental groups. Male Wistar rats of sufficient or deficient vitamin A status were treated IP with 340 mumol of TCB/kg. Vitamin A levels were significantly depressed in liver. REH activity was decreased about 20%, and serum retinol was decreased about 50%, independent of the initial vitamin A status of the animals. Vitamin A levels in lungs and testes were also decreased, suggesting that TCB could interfere with vitamin A delivery to target organs. The negative effect of TCB on REH activity in vivo was also observed when TCB was added in vitro to the incubation medium at concentrations near to those expected after in vivo treatment. TCB is a non-competitive inhibitor of retinyl palmitate hydrolase.

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.

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.

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.

Severe hypervitaminosis A in siblings: evidence of variable tolerance to retinol intake

A 2-year-old boy had signs and symptoms of chronic hypervitaminosis A. A course of increasing severity led to eventual death. A younger brother later had similar clinical features. Chicken liver spread containing up to 420 IU/g vitamin A was the likely source of intoxication. Markedly elevated circulating retinyl ester levels have persisted in the surviving sibling for 3 subsequent years despite severe restriction of vitamin A intake. A therapeutic trial of the carbohydrate-derived complexing agent 2-hydroxypropyl-beta-cyclodextrin was initiated. Circulating retinyl esters transiently increased during the infusion (from 407 to 4791 micrograms/dL), and urinary total vitamin A excretion, undetectable before infusion, increased to 23 micrograms/dL after infusion. The frequency of hypervitaminotic episodes has decreased somewhat in the 2 years since the infusion, probably related to dietary vitamin A restriction. The occurrence of this syndrome in two brothers, while a sister ingesting the same diet remains completely healthy, suggests an inherited variance in tolerance to vitamin A intake.