Esterification by rat liver microsomes of retinol bound to cellular retinol-binding protein

Acitretin (Neotigason). A review of pharmacokinetics and teratogenicity and hypothesis on metabolic pathways

Acitretin was introduced as a replacement for etretinate, the ethyl ester of acitretin. Acitretin is eliminated at a much faster rate than etretinate. Although both drugs are teratogens, the replacement was important especially as it allowed for a much shorter post-medication period in which pregnancy should be precluded. Recent findings showed the presence of etretinate in the plasma of acitretin-treated patients. This article gives a review of known metabolic pathways of the retinoids and tries to elucidate the possible conversion of acitretin into etretinate after acitretin ingestion.

Microsomal retinal synthesis: retinol vs. holo-CRBP as substrate and evaluation of NADP, NAD and NADPH as cofactors

Holo-CRBP (cellular retinol binding protein) is recognized specifically by an NADP-dependent microsomal retinol dehydrogenase and protects retinol from conversion into retinal by NAD and NADPH dependent dehydrogenases. The synthesis of retinal from free retinol is catalyzed by both NADP- and NAD-dependent pathways, with the former being the preferred one (Km of 4 vs. 22 microM for retinol, and Vmax/Km of 33 vs. 9, respectively). NADPH does not support quantitatively significant retinal synthesis from physiological concentrations of retinol or holo-CRBP, if an NADPH regenerating system is used to prevent NADP formation.

Fatty acyl CoA-dependent and -independent retinol esterification by rat liver and lactating mammary gland microsomes

Retinol esterification was examined in microsomes from rat liver and lactating mammary gland as a function of the form of retinol substrate, dependence on fatty acyl CoA, and sensitivity to phenylmethylsulfonyl fluoride (PMSF). Retinol bound to cellular retinol-binding protein (CRBP) or dispersed in solvent was esterified in a fatty acyl CoA-independent, PMSF-sensitive reaction, consistent with lecithin:retinol acyltransferase (LRAT) activity. LRAT activity exhibited the same Km (2 microM retinol) between tissues but a higher Vmax in liver as compared to that in mammary gland (47 vs 8 pmol/min/mg microsome protein, respectively). Solvent-dispersed retinol was also esterified in a fatty acyl CoA-dependent, PMSF-resistant reaction, consistent with acyl CoA:retinol acyltransferase (ARAT) activity. Retinol bound to CRBP was not a good substrate for this reaction. ARAT activity displayed a similar Vmax (300 pmol/min/mg microsome protein) between tissues but Km values of 15 and 5 microM for retinol and fatty acyl CoA in mammary gland as compared to 30 and 25 microM, respectively, in the liver. Thus, when substrate was near or below Km, retinol esterification occurred predominantly by LRAT in the liver and ARAT in the mammary gland, respectively. The concentration of CRBP in the cytosol, determined by Western blotting, was approximately 2 microM in the liver but was almost nondetectable in the mammary gland. These data suggest that retinol esterification is regulated via different mechanisms in liver and mammary gland and support a specific role for CRBP in the liver.

Holocellular retinol binding protein as a substrate for microsomal retinal synthesis

Holocellular retinol binding protein (holo-CRBP) was substrate for retinal synthesis at physiological pH with microsomes prepared from rat liver, kidney, lung, and testes. Four observations indicated that retinal synthesis was supported by holo-CRBP directly, rather than by the unbound retinol in equilibrium with CRBP. First, the rate of retinal synthesis with holo-CRBP exceeded the rate that was observed from the concentration of unbound retinol in equilibrium with CRBP. Second, NADP was the preferred cofactor only with holo-CRBP, supporting a rate about 3-fold greater than that of NAD. In contrast, with unbound retinol as substrate, similar rates of retinal formation were supported by either NAD or NADP. Third, the rate of retinal synthesis was not related to the decrease in the concentration of unbound retinol in equilibrium with holo-CRBP caused by increasing the concentration of apo-CRBP. Fourth, the rate of retinal synthesis increased with increases in the concentration of holo-CRBP as a fixed concentration of unbound retinol was maintained. This was achieved by increasing both apo-CRBP and holo-CRBP, but keeping constant the ratio apo-CRBP/holo-CRBP. Retinal formation from holo-CRBP displayed typical Michaelis-Menten kinetics with a Km about 1.6 microM, less than the physiological retinal concentration of 4-10 microM in the livers of rats fed diets with recommended vitamin A levels. The Vmax for retinal formation from holo-CRBP was 14-17 pmol min-1 (mg of protein)-1, a rate sufficiently high to generate adequate retinal to contribute significantly to retinoic acid synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological occurrence, biosynthesis and metabolism of retinoic acid: evidence for roles of cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP) in the pathway of retinoic acid homeostasis

This article will address recent work on the physiological occurrence, biogenesis and metabolism of retinoic acid and summarize the data that retinoic acid is synthesized in situ in multiple tissues and cell types via enzymes or enzyme complexes that are distinct from the alcohol dehydrogenases. There is now considerable evidence that retinoic acid is an activated metabolite of retinol that supports the systemic functions of vitamin A in vivo. Many studies in vitro, for example, have shown that retinoic acid is the most potent naturally-occurring retinoid with an ED-50 in the range of 1 pM to 10 nM, depending on the assay system. This is below the tissue concentrations of retinoic acid which range from approximately 20-600 nM. Retinoic acid synthesis from retinol in the dog kidney cell line MDCK maintained in serum-free medium is inhibited by the prostanoid, PGE, and the phorbol ester, TPA. In tissues, one pathway of retinoic acid synthesis begins with apo-CRBP stimulating retinyl ester hydrolysis by a microsomal, cholate-independent retinyl ester hydrolase to form holo-CRBP. The holo-CRBP itself is used as substrate by an NADP-dependent, microsomal retinol dehydrogenase to generate retinal, which is converted into retinoic acid by a cytosolic NAD-dependent retinal dehydrogenase. Therefore, cellular retinol-binding protein (CRBP) apparently has at least 2 functions in retinoic acid synthesis: the apo form stimulates retinol mobilization from retinyl ester stores; the holo form delivers the retinol via direct transfer to dehydrogenase(s). Retinoic acid is converted into a mixture of at least 4 metabolites by testes microsomes which migrate closely on reverse-phase HPLC with 4-hydroxyretinoic acid, and may be mistaken for either 4-hydroxy or 4-oxo-retinoic acid. More rigorous analysis, however, shows that only one of them is 4-hydroxyretinoic acid, and another is 18-hydroxyretinoic acid. Two others remain unidentified. These metabolites are also formed in the presence of excess cellular retinoic acid-binding protein (CRABP), which increases the elimination half-life of retinoic acid, but does not prevent retinoic acid catabolism, suggesting that holo-CRABP may be a substrate for retinoic acid catabolism that modulates the steady-state concentrations of retinoic acid. Thus, both retinoid binding proteins, CRBP and CRABP, may each have direct roles as substrate in the biosynthesis and metabolism of retinoic acid, respectively.

Distribution of lecithin-retinol acyltransferase activity in different types of rat liver cells and subcellular fractions

It is now well documented that lecithin-retinol acyltransferase (LRAT) is the physiologically important enzyme activity involved in the esterification of retinol in the liver. However, no information regarding the cellular distribution of this enzyme in the liver is presently available. This study characterizes the distribution of LRAT activity in the different types of rat liver cells. Purified preparations of isolated parenchymal, fat-storing, and Kupffer + endothelial cells were isolated from rat livers and the LRAT activity present in microsomes prepared from each of these cell fractions was determined. The fat-storing cells were found to contain the highest level of LRAT specific activity (383 +/- 54 pmol retinyl ester formed min-1.mg-1 versus 163 +/- 22 pmol retinyl ester formed min-1.mg-1 for whole liver microsomes). The level of LRAT specific activity in parenchymal cell microsomes (158 +/- 53 pmol retinyl ester formed min-1.mg-1) was very similar to LRAT levels in whole liver microsomes. The Kuppfer + endothelial cell microsome fractions were found to contain LRAT, at low levels of activity. These results indicate that the fat-storing cells are very enriched in LRAT but the parenchymal cells also posses significant levels of LRAT activity.

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.

Transport and storage of vitamin A

The requirement of vitamin A (retinoids) for vision has been recognized for decades. In addition, vitamin A is involved in fetal development and in the regulation of proliferation and differentiation of cells throughout life. This fat-soluble organic compound cannot be synthesized endogenously by humans and thus is an essential nutrient; a well-regulated transport and storage system provides tissues with the correct amounts of retinoids in spite of normal fluctuations in daily vitamin A intake. An overview is presented here of current knowledge and hypotheses about the absorption, transport, storage, and metabolism of vitamin A. Some information is also presented about a group of ligand-dependent transcription factors, the retinoic acid receptors, that apparently mediate many of the extravisual effects of retinoids.

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.