beta-carotene 15,15'-dioxygenase activity and cellular retinol-binding protein type II level are enhanced by dietary unsaturated triacylglycerols in rat intestines

Evaluation of the carotenoid and fat-soluble vitamin profile of industrial hemp inflorescence by liquid chromatography coupled to mass spectrometry and photodiode-array detection

Industrial hemp (Cannabis sativa L.) is a plant matrix whose use is recently spreading for pharmaceutical and nutraceutical purposes. Detailed characterization of hemp composition is needed for future research that further exploits the beneficial effects of hemp compounds on human health. Among minor constituents, carotenoids and fat-soluble vitamins have largely been neglected to date despite carrying out several biological activities and regulatory functions. In the present paper, 22 target carotenoids and fat-soluble vitamins were analyzed in the inflorescences of seven Italian industrial hemp varieties cultivated outdoor. The analytes were extracted by cold saponification to avoid artifacts and analyzed by high-performance liquid chromatography coupled with Selected reaction monitoring mass spectrometry. Phytoene, phytofluene, and all-trans-β-carotene were the most abundant in all analyzed samples (31-55 µg g^-1, 11.6-29 µg g^-1, and 7.3-53 µg g^-1, respectively). Besides the target analytes, liquid chromatography coupled with photodiode-array detection allowed us to tentatively identify several other carotenoids based on their retention behavior and UV-vis spectra with the support of theoretical rules and data in the literature. To the best of our knowledge, this is the first comprehensive characterization of carotenoids and fat-soluble vitamins in industrial hemp inflorescence.

Dietary Level of the Omega-3 Fatty Acids EPA and DHA Influence the Flesh Pigmentation in Atlantic Salmon

Atlantic salmon with a start weight of 53 g were fed diets with different levels of EPA and DHA or a diet with 1 : 1 EPA+DHA (0%, 1.0%, and 2.0% of the diet). At 400 g, all fish groups were mixed and equally distributed in new tanks and fed three diets with 0.2%, 1.0%, or 1.7% of EPA+DHA. At 1200 g, the fish were transferred to seawater pens where they were fed the same three diets until they reached a slaughter size of 3.5 kg. The fillet concentration of astaxanthin and its metabolite idoxanthin was analysed before transfer to seawater pens at 1200 g and at slaughter. The fatty acid composition in the fillet was also analysed at the same time points. Salmon fed low levels of EPA and DHA had lower fillet astaxanthin concentration and higher metabolic conversion of astaxanthin to idoxanthin compared to salmon fed higher dietary levels of EPA and/or DHA. DHA had a more positive effect on fillet astaxanthin concentrations than EPA. There were positive correlations between fillet DHA, EPA, sum N-3 fatty acids, and fillet astaxanthin concentration. A negative correlation was found between the concentration of N-6 fatty acids in the fillet and the astaxanthin concentration.

Mechanisms of carotenoid intestinal absorption and the regulation of dietary lipids: lipid transporter-mediated transintestinal epithelial pathways

Dietary lipids are key ingredients during cooking, processing, and seasoning of carotenoid-rich fruits and vegetables, playing vitals in affecting the absorption and utilization of carotenoids for achieving their health benefits. Besides, dietary lipids have also been extensively studied to construct various delivery systems for carotenoids, such as micro/nanoparticles, micro/nanoemulsions, and liposomes. Currently, the efficacies of these techniques on improving carotenoid bioavailability are often evaluated using the micellization rate or "bioaccessibility" based on in vitro models. However, recent studies have found that dietary lipids may also affect the carotenoid uptake via intestinal epithelial cells and the efflux of intracellular chyle particles via lipid transporters. An increasing number of studies reveal the varied impact of different dietary lipids on the absorption of different carotenoids and some lipids may even have an inhibitory effect. Consequently, it is necessary to clarify the relationship between the addition of dietary lipids and the intestinal absorption of carotenoid to fully understand the role of lipids during this process. This paper first introduces the intestinal absorption mechanism of carotenoids, including the effect of bile salts and lipases on mixed micelles, the types and regulation of lipid transporters, intracellular metabolizing enzymes, and the efflux process of chyle particles. Then, the regulatory mechanism of dietary lipids during intestinal carotenoid absorption is further discussed. Finally, the importance of selecting the dietary lipids for the absorption and utilization of different carotenoids and the design of an efficient delivery carrier are emphasized. This review provides suggestions for precise dietary carotenoid supplementation and offere an important reference for constructing efficient transport carriers for liposoluble nutrients.

Retinol-binding protein 2 (RBP2): biology and pathobiology

Retinol-binding protein 2 (RBP2; originally cellular retinol-binding protein, type II (CRBPII)) is a 16 kDa cytosolic protein that in the adult is localized predominantly to absorptive cells of the proximal small intestine. It is well established that RBP2 plays a central role in facilitating uptake of dietary retinoid, retinoid metabolism in enterocytes, and retinoid actions locally within the intestine. Studies of mice lacking Rbp2 establish that Rbp2 is not required in times of dietary retinoid-sufficiency. However, in times of dietary retinoid-insufficiency, the complete lack of Rbp2 gives rise to perinatal lethality owing to RBP2 absence in both placental (maternal) and neonatal tissues. Moreover, when maintained on a high-fat diet, Rbp2-knockout mice develop obesity, glucose intolerance and a fatty liver. Unexpectedly, recent investigations have demonstrated that RBP2 binds long-chain 2-monoacylglycerols (2-MAGs), including the canonical endocannabinoid 2-arachidonoylglycerol, with very high affinity, equivalent to that of retinol binding. Crystallographic studies establish that 2-MAGs bind to a site within RBP2 that fully overlaps with the retinol binding site. When challenged orally with fat, mucosal levels of 2-MAGs in Rbp2 null mice are significantly greater than those of matched controls establishing that RBP2 is a physiologically relevant MAG-binding protein. The rise in MAG levels is accompanied by elevations in circulating levels of the hormone glucose-dependent insulinotropic polypeptide (GIP). It is not understood how retinoid and/or MAG binding to RBP2 affects the functions of this protein, nor is it presently understood how these contribute to the metabolic and hormonal phenotypes observed for Rbp2-deficient mice.

Avocado consumption enhances human postprandial provitamin A absorption and conversion from a novel high-β-carotene tomato sauce and from carrots

Dietary lipids have been shown to increase bioavailability of provitamin A carotenoids from a single meal, but the effects of dietary lipids on conversion to vitamin A during absorption are essentially unknown. Based on previous animal studies, we hypothesized that the consumption of provitamin A carotenoids with dietary lipid would enhance conversion to vitamin A during absorption compared with the consumption of provitamin A carotenoids alone. Two separate sets of 12 healthy men and women were recruited for 2 randomized, 2-way crossover studies. One meal was served with fresh avocado (Persea americana Mill), cultivated variety Hass (delivering 23 g of lipid), and a second meal was served without avocado. In study 1, the source of provitamin A carotenoids was a tomato sauce made from a novel, high-β-carotene variety of tomatoes (delivering 33.7 mg of β-carotene). In study 2, the source of provitamin A carotenoids was raw carrots (delivering 27.3 mg of β-carotene and 18.7 mg of α-carotene). Postprandial blood samples were taken over 12 h, and provitamin A carotenoids and vitamin A were quantified in triglyceride-rich lipoprotein fractions to determine baseline-corrected area under the concentration-vs.-time curve. Consumption of lipid-rich avocado enhanced the absorption of β-carotene from study 1 by 2.4-fold (P < 0.0001). In study 2, the absorption of β-carotene and α-carotene increased by 6.6- and 4.8-fold, respectively (P < 0.0001 for both). Most notably, consumption of avocado enhanced the efficiency of conversion to vitamin A (as measured by retinyl esters) by 4.6-fold in study 1 (P < 0.0001) and 12.6-fold in study 2 (P = 0.0013). These observations highlight the importance of provitamin A carotenoid consumption with a lipid-rich food such as avocado for maximum absorption and conversion to vitamin A, especially in populations in which vitamin A deficiency is prevalent. This trial was registered at clinicaltrials.gov as NCT01432210.

Co-cultures of enterocytes and hepatocytes for retinoid transport and metabolism

Dietary retinoid bioavailability involves the interplay of the intestine (transport and metabolism) and the liver (secondary metabolism). To reproduce these processes in vitro, differentiated human intestinal Caco-2/TC7 cells were co-cultured with two hepatocyte cell lines. Murine 3A cells and the more highly differentiated human HepaRG hepatocytes were both shown to respond to β-carotene (BC) and retinol (ROH) treatment by secreting Retinol Binding Protein 4 (RBP4). In co-culture experiments, Caco-2/TC7 were differentiated on filter inserts and transferred for the time of the experiment to culture wells containing confluent 3A or differentiated HepaRG cells. Functionality of the co-cultures was assayed using as endpoints the retinol-dependent secretion of RBP4 and the retinoic acid-dependent induction of CYP26A1 in hepatocytes. BC and ROH added to intestinal Caco-2/TC7 induced a reduction in intracellular RBP4 levels in the underlying hepatocytes and its secretion into the medium. HepaRG hepatocytes were also shown to up-regulate the expression of CYP26A1 mRNA in response to retinoid treatment. This in vitro model represents a useful tool to analyze the absorption and metabolism of retinoids and could be further developed to investigate other dietary compounds and molecules of pharmacological interest.

Mechanisms involved in the intestinal absorption of dietary vitamin A and provitamin A carotenoids

Vitamin A is an essential nutrient for humans and is converted to the visual chromophore, 11-cis-retinal, and to the hormone, retinoic acid. Vitamin A in animal-derived foods is found as long chain acyl esters of retinol and these are digested to free fatty acids and retinol before uptake by the intestinal mucosal cell. The retinol is then reesterified to retinyl esters for incorporation into chlylomicrons and absorbed via the lymphatics or effluxed into the portal circulation facilitated by the lipid transporter, ABCA1. Provitamin A carotenoids such as β-carotene are found in plant-derived foods. These and other carotenoids are transported into the mucosal cell by scavenger receptor class B type I (SR-BI). Provitamin A carotenoids are partly converted to retinol by oxygenase and reductase enzymes and the retinol so produced is available for absorption via the two pathways described above. The efficiency of vitamin A and carotenoid intestinal absorption is determined by the regulation of a number of proteins involved in the process. Polymorphisms in genes for these proteins lead to individual variability in the metabolism and transport of vitamin A and carotenoids. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Importance of β,β-carotene 15,15'-monooxygenase 1 (BCMO1) and β,β-carotene 9',10'-dioxygenase 2 (BCDO2) in nutrition and health

In humans, varying amounts of absorbed β-carotene are oxidatively cleaved by the enzyme β,β-carotene 15,15'-monooxygenase 1 (BCMO1) into two molecules of all-trans-retinal. The other carotenoid cleavage enzyme β,β-carotene 9',10'-dioxygenase (BCDO2) cleaves β-carotene at the 9',10' double bond forming β-apo-10'-carotenal and β-ionone. Although the contribution of BCDO2 to vitamin A formation has long been debated, BCMO1 is currently considered the key enzyme for retinoid metabolism. Furthermore, BCMO1 has limited enzyme activity towards carotenoids other than provitamin A carotenoids, whereas BCDO2 exhibits a broader specificity. Both enzymes are located at different sites within the cell, with BCMO1 being a cytosolic protein and BCDO2 being located in the mitochondria. Expression of BCMO1 in tissues other than the intestine has recently revealed its function for tissue-specific retinoid metabolism with importance in embryogenesis and lipid metabolism. On the other hand, biological activity of BCDO2 metabolites has been shown to be important in protecting against carotenoid-induced mitochondrial dysfunction. Single-nucleotide polymorphisms (SNPs) such as R267S and A379V in BCMO1 can partly explain inter-individual variations observed in carotenoid metabolism. Advancing knowledge about the physiological role of these two enzymes will contribute to understanding the importance of carotenoids in health and disease.

Microarray assessment of the influence of the conceptus on gene expression in the mouse uterus during decidualization

During pregnancy in several species including humans and rodents, the endometrium undergoes decidualization. This process of differentiation from endometrial to decidual tissue occurs only after the onset of implantation in mice. It can also be artificially induced causing the formation of deciduomal tissue. The purpose of this study was to compare the gene expression profile of the developing decidua in pregnant mice with the deciduoma formed after artificial induction in an effort to identify conceptus-influenced changes in uterine gene expression during decidualization. We induced decidualization artificially by transferring blastocyst-sized ConA-coated agarose beads into the uterus on day 2.5 of pseudopregnancy. Recently published work has found this model to be more 'physiological' than other methods. Total RNA was isolated from blastocyst and bead-induced 'implantation' sites of the uteri of day 7.5 pregnant (decidua) and pseudopregnant (deciduoma) mice respectively. This RNA was then used for microarray analysis using Mouse Illumina BeadArray chips. This analysis revealed potential differential mRNA levels of only 45 genes between the decidua and bead-induced deciduoma tissues. We confirmed the differential mRNA levels of 31 of these genes using quantitative RT-PCR. Finally, the level and localization of some of the mRNAs for select genes (Aldh3a1, Bcmo1, Guca2b, and Inhbb) identified by our microarray analysis were examined in more detail. This study provides the identity of a small set of genes whose expression in the uterus during decidualization may be influenced by molecular signals from the conceptus.

In vitro and in vivo characterization of retinoid synthesis from beta-carotene

Retinoids are indispensable for the health of mammals, which cannot synthesize retinoids de novo. Retinoids are derived from dietary provitamin A carotenoids, like beta-carotene, through the actions of beta-carotene-15,15'-monooxygenase (BCMO1). As the substrates for retinoid-metabolizing enzymes are water insoluble, they must be transported intracellularly bound to cellular retinol-binding proteins. Our studies suggest that cellular retinol-binding protein, type I (RBP1) acts as an intracellular sensor of retinoid status that, when present as apo-RBP1, stimulates BCMO1 activity and the conversion of carotenoids to retinoids. Cellular retinol-binding protein, type II (RBP2), which is 56% identical to RBP1 does not influence BCMO1 activity. Studies of mice lacking BCMO1 demonstrate that BCMO1 is responsible for metabolically limiting the amount of intact beta-carotene that can be absorbed by mice from their diet. Our studies provide new insights into the regulation of BCMO1 activity and the physiological role of BCMO1 in living organisms.

Differences in expression and activity of ?,?'-carotene-15,15'- oxygenase in liver and duodenum of cattle with yellow or white fat

Pasture-fed cattle show yellow pigmentation of their fat due to beta-carotene stored in this tissue. beta,beta'-Carotene-15,15'-oxygenase (betaCO) is an enzyme expressed in different tissues, and it cleaves beta-carotene into retinal. We compared the expression and activity of betaCO in duodenum and liver of cattle with pigmented or non-pigmented fat. In the duodenum, in situ hybridizations showed expression of betaCO in epithelial cells and crypts of the mucosa that was similar in animals from pigmented and non-pigmented fat; liver showed diffuse signal at lobules, but pigmented animals showed higher signals near the portal space. Analyses by real-time reverse-transcription polymerase chain reaction also showed amplification of mRNA for betaCO in duodenum and liver, with no difference between pigmented or non-pigmented animals. Enzyme activity was similar in the duodenum, but pigmented animals had higher enzyme activity (p = 0.004) in liver. Cattle with pigmented fat had higher expression and activity of betaCO in liver, but its level was not high enough to prevent the storage of beta-carotene in adipose tissues.

Effects of dietary retinoids and carotenoids on immune development

Carotenoids and retinoids are groups of nutritionally-relevant compounds present in many foods of plant origin (carotenoids) and animal origin (mainly retinoids). Their levels in human subjects vary depending on the diversity and amount of the individual's nutrient intake. Some carotenoids and retinoids have been investigated for their effects on the immune system bothin vitroandin vivo. It has been shown that retinoids have the potential to mediate or induce proliferative and differentiating effects on several immune-competent cells, and various carotenoids are known to be inducers of immune function. The immune-modulating effects of retinoids have been well documented, while the effects of carotenoids on the immune system have not been investigated as extensively, because little is known about their molecular mechanism of action. The present review will mainly focus on the molecular mechanism of action of retinoids and particularly carotenoids, their nutritional origin and intake, their transfer from the maternal diet to the child and their effects or potential effects on the developing immune system.

Intestinal β-carotene 15,15′-dioxygenase activity is markedly enhanced in copper-deficient rats fed on high-iron diets and fructose

The purpose of the present work was to examine effects of the Cu–Fe interaction on intestinal β-carotene 15,15′-dioxygenase activity when a wide range of dietary Fe (deficiency to excess) was used in relation to Cu status of rats. The effect of dietary carbohydrates was also examined since they play a role in the Cu–Fe interactionin vivo. Weanling male Sprague-Dawley rats (n72) were divided into twelve dietary groups, which were fed on either low-, normal-, or high-Fe levels (0·9, 9·0, and 90·0 mmol Fe/kg diet respectively) combined with Cu-adequate or -deficient levels (0·94 and 0·09 mmol Cu/kg diet respectively) and with starch or fructose in the diets. The data showed that both Fe concentration and β-carotene 15,15′-dioxygenase activity in small intestinal mucosa were enhanced with increasing dietary Fe and with Cu deficiencyv. Cu adequacy. Dietary fructose did not aggravate the Fe-enhancement, related to Cu deficiency, in the small intestine; however, fructose increased the intestinal dioxygenase activity in rats fed on normal- or high-Fe diets when compared with starch controls. Thus, the highest intestinal dioxygenase activity associated with the lowest hepatic retinol (total) concentration was found in rats fed on the Cu-deficient, high-Fe, fructose-based diet. Finally, a positive linear relationship was found between the dioxygenase activity and Fe concentration in intestinal mucosa. In conclusion, the data indicate that β-carotene 15,15′-dioxygenase activity requires Fe as cofactorin vivoand the enzyme is modulated by the three dietary components: Cu, Fe, and fructose.

Micellar oleic and eicosapentaenoic acid but not linoleic acid influences the β-carotene uptake and its cleavage into retinol in rats

Improving the bioavailability of beta-carotene is vital to manage vitamin A deficiency. The influence of micellar oleic (OA), linoleic (LA) and eicosapentaenoic (EPA) acids on plasma beta-carotene response and its conversion to retinol has been studied in rats employing single (9 h time course) and repeated (10 days) dose administrations. After a single dose, the levels (area under the curve) of plasma beta-carotene and retinyl palmitate in OA and EPA groups were higher (p < 0.05) by 13, 7 and 11, 6 folds than LA group. The liver beta-carotene level in OA and EPA groups were higher (p < 0.05) by 3 and 1.2 folds than LA group. After repeated dose, the plasma beta-carotene and retinyl palmitate levels in OA (6.2%, 51.7%) and EPA (25.4%, 17.23%) groups were higher (p < 0.05) than LA group. The liver beta-carotene level in OA (21.2%) and EPA (17.6%) groups were higher (p < 0.05) than LA group. In both the experiments, the activity of beta-carotene 15,15'-dioxygenase in the intestinal mucosa and plasma triglyceride levels were also higher in OA and EPA groups than LA group. beta-Carotene excreted through urine and feces of OA and EPA groups was lower than the LA group. These results demonstrate an improved absorption and metabolism of beta-carotene when fed mixed micelles with OA or EPA compared with LA. Although the mechanism involved in selective absorption of fatty acids needs further studies, intestinal beta-carotene uptake and its conversion to vitamin A can be modulated using specific fatty acids.

Molecular cloning of the rat beta-carotene 15,15'-monooxygenase gene and its regulation by retinoic acid

BACKGROUND

beta-Carotene exhibits biological activity as provitamin A. Key step in vitamin A formation is the cleavage of beta-carotene to retinal by an enzyme designated as beta-carotene 15,15'-monooxygenase (BCM). Recently, it is reported that expression of BCM gene in the intestine is under feedback regulation by retinoic acid (RA). However, the regulation of BCM gene expression in various other tissues is still unknown.

AIM OF THE STUDY

In the present study, we identified the full-length cDNA encoding the rat BCM gene and investigated the regulation of its expression in several tissues by RA in the presence of vitamin A deficiency.

METHODS

We cloned the full-length cDNA encoding BCM gene from a rat intestinal cDNA library by hybridization screening. The BCM gene expression was examined using Northern blotting and reverse transcription-PCR analysis. We also investigated whether BCM gene expression was regulated by retinoids in several tissues of vitamin A-deficient rats.

RESULTS

Sequence analysis of this clone revealed an open reading frame of 1,701 bases encoding a protein of 566 amino acids. The predicted polypeptide showed 94%, 81%, and 66% identity with mouse, human, and chicken BCM, respectively. Rat BCM mRNA was highly expressed in the intestine and liver, while there was weak expression in the testes, kidneys, and lungs. Immunoblotting revealed that rat BCM is a 64-kDa protein. BCM gene expression was increased in the small intestine by vitamin A deficiency compared with that in rats on a control diet, while this upregulation was suppressed by all-trans RA (ATRA) or 9-cis RA (9-cis RA). BCM gene expression in the lungs and testes was also suppressed by ATRA or 9-cis RA in rats with vitamin A deficiency. However, hepatic BCM gene expression was only decreased by ATRA and renal expression was not affected by either retinoid. As the small intestine is the major site of beta-carotene conversion, intestinal BCM gene expression may be more tightly regulated.

CONCLUSION

These data suggest that BCM gene expression in several tissues may be down-regulated by RA at the level of conversion of beta-carotene to retinal. To prevent an excess of retinol, homeostasis may occur at the level of conversion of beta-carotene to retinal in several tissues.

Intestinal absorption and metabolism of carotenoids: insights from cell culture

Cell culture models are useful for studying intestinal absorption and metabolism of carotenoids. The human intestinal cell line, Caco-2, has been the most widely used model for these studies. The PF11 and TC7 clones of Caco-2 exhibit beta-carotene-15,15'-oxygenase activity, a key enzyme in the conversion of carotenoids to vitamin A. Studies on the recent cloning of this enzyme are discussed. An in vitro cell culture system used to study intestinal absorption of carotenoids is presented. Under conditions mimicking the postprandial state, Caco-2 cells on membranes take up carotenoids and secrete them incorporated into chylomicrons. Both the cellular uptake and secretion of beta-carotene are saturable, concentration-dependent processes. The selective absorption of all-trans beta-carotene versus its cis isomers, the differential absorption of individual carotenoids, and the specific interactions between carotenoids during their absorption are discussed. The participation of a specific epithelial transporter in the intestinal absorption of carotenoids is proposed.

Oxidative conversion of carotenoids to retinoids and other products

In vertebrates, provitamin A carotenoids are converted to retinal by beta-carotene-15,15'-dioxygenase. The enzyme activity is expressed specifically in intestinal epithelium and in liver. The intestinal enzyme not only plays an important role in providing animals with vitamin A, but also determines whether provitamin A carotenoids are converted to vitamin A or circulated in the body as intact carotenoids. We have found that a high fat diet enhanced the beta-carotene dioxygenase activity together with the cellular retinol binding protein type II level in rat intestines. Flavonols with a catechol structure in the B-ring and 2,6-di-tert-butyl-4-methylphenol inhibited the dioxygenase activity of pig intestinal homogenates and the conversion of beta-carotene to retinol in Caco-2 human intestinal cells. Thus, the bioavailability of dietary provitamin A carotenoids might be modulated by the other food components ingested. Regulation of the dioxygenase activity and its relation to the retinoid metabolism as well as to lipid metabolism deserve further study. In contrast to enzymatic cleavage, it is known that both retinal and beta-apocarotenals are formed in vitro from beta-carotene by chemical transformation, which cleaves conjugated double bonds at random positions under various oxidative conditions. Moreover, recent studies have indicated that the oxidation products formed by chemical transformation might have specific actions on the proliferation of certain cancer cells. We have found that lycopene, a typical nonprovitamin A carotenoid, was cleaved in vitro to acycloretinal, acycloretinoic acid and apolycopenals in a nonenzymatic manner, and that the mixture of oxidation products of lycopene induced apoptosis of HL-60 human promyelocytic leukemia cells. Thus, it is worth evaluating the formation of oxidation products and their biological actions, in order to elucidate the underlying mechanisms of the beneficial effects of carotenoids on human health.

Carotenoid bioavailability and bioconversion

The possible role of carotenoids and their metabolites in disease prevention is far from fully understood, because the bioavailabilities of carotenoids are complicated by multiple factors that affect their absorption, breakdown, transport, and storage. Rapid progress in developing sophisticated methodologies, including use of stable-isotope dilution methods, now allows for an accurate determination of the true vitamin A activity of provitamin A carotenoids. The recent identification of specific enzymes, which catalyze the breakdown of beta-carotene as well as nonprovitamin A carotenoids, is providing a better understanding of the functions of carotenoids at the molecular level. The pathways and possible mechanisms of carotenoid breakdown and factors affecting the bioavailability of carotenoids, such as carotenoid type, food matrix, interaction with other carotenoids and other food components, nutritional status, aging, and infection, are discussed in this review.

A high retinol dietary intake increases its apical absorption by the proximal small intestine of juvenile sunshine bass (Morone chrysops x M. saxatilis)

The relationship between dietary intake and systemic availability of retinol is likely to be complex because although retinol is an essential nutrient, it is toxic at high levels. The present study determined whether rates of transapical retinol absorption are modulated so that availability is increased at low dietary levels, but decreased when dietary intake is excessive. Juvenile hybrid striped bass were fed for 6 wk diets with 568 (below), 1657 (approximating the requirement) and 40,244 (excessive) micro g/kg dry diet of trans retinol. Proximal small intestine segments were used to measure rates of retinol absorption and tissue concentrations. Initial and final body mass did not differ among groups; deficiency and toxicity symptoms were not observed. Uptake of tracer retinol was inhibited by unlabeled retinol, indicating the presence of saturable, carrier-mediated absorption. Increasing dietary levels of retinol increased the rates of absorption measured at 0.05 mmol/L [8.04 +/- 0.65; 15.2 +/- 1.53; 25.1 +/- 3.4 pmol/(mg. min) for below, approximating and exceeding the retinol requirement; P < 0.0001]; this resulted in higher tissue concentrations of all-trans retinol (0.21 +/- 0.03, 0.49 +/- 0.21 and 338 +/- 89 pmol/g; P < 0.0001) and dehydro-retinol (0.11 +/- 0.04, 0.91 +/- 0.04, and 454 +/- 109 pmol/g; P < 0.001). These findings suggest that the systemic availability of various dietary levels of retinol is modulated after transapical absorption.

Feeding of red palm oil-supplemented diets to rats may impact positively on vitamin A status

The impact of feeding of dietary palm oil supplements on plasma vitamin A profile was investigated in animals. Four-week-old Wistar albino rats (n = 8 per group) were maintained for 28 days on standard rat food (4.7% fat by weight) supplemented (10%, 20% and 30% by weight) with red palm oil (RPO) and refined palm olein (REFPO). Plasma beta-carotene and vitamin A concentrations of rats fed RPO-based diets were higher than in rats fed REFPO and control diets. Animals fed 30% RPO-containing diets had lower plasma beta-carotene concentrations than those fed 20% RPO-containing diets. The results suggest that consumption of palm oil in moderate amounts enhances growth of tissues and bioavailability of beta-carotene, which may combat vitamin A deficiency in developing countries, in view of the fact that performed vitamin A in animal products (namely meat, liver, eggs and fatty fish) is out of the reach of economically deprived people.

Expression and characterization of a murine enzyme able to cleave beta-carotene. The formation of retinoids

Because animals are not able to synthesize retinoids de novo, ultimately they must derive them from dietary provitamin A carotenoids through a process known as carotene cleavage. The enzyme responsible for catalyzing carotene cleavage (beta-carotene 15,15'-dioxygenase) has been characterized primarily in rat intestinal scrapings. Using a recently reported cDNA sequence for a carotene cleavage enzyme from Drosophila, we identified a cDNA encoding a mouse homolog of this enzyme. When the cDNA was expressed in either Escherichia coli or Chinese hamster ovary cells, expression conferred upon bacterial and Chinese hamster ovary cell homogenates the ability to cleave beta-carotene to retinal. Several lines of evidence obtained upon kinetic analyses of the recombinant enzyme suggested that carotene cleavage enzyme interacts with other proteins present within cell or tissue homogenates. This was confirmed by pull-down experiments upon incubation of recombinant enzyme with tissue 12,000 x g supernatants. Matrix-assisted laser desorption ionization-mass spectrometry analysis of pulled-down proteins indicates that an atypical testis-specific isoform of lactate dehydrogenase associates with recombinant carotene cleavage enzyme. mRNA transcripts for the carotene cleavage enzyme were detected by reverse transcription-polymerase chain reaction in mouse testes, liver, kidney, and intestine. In situ hybridization studies demonstrated that carotene cleavage enzyme is expressed prominently in maternal tissue surrounding the embryo but not in embryonic tissues at 7.5 and 8.5 days postcoitus. This work offers new insights for understanding the biochemistry of carotene cleavage to retinoids.

The enzymatic cleavage of beta-carotene: end of a controversy

The central cleavage of dietary beta-carotene to retinal was found to be the predominant mechanism whereby retinoids were formed in vivo in rats; apo-carotenals, indicative of eccentric cleavage of beta-carotene, were only a minor component (<5% of retinoids). A gene from maize that codes for a plant carotenoid cleavage enzyme was used to isolate a homologous gene from Drosophila. This gene, when transfected into an E Coli strain capable of synthesizing and accumulating beta-carotene, caused the central cleavage of beta-carotene, forming exclusively retinoids. The enzyme that the gene codes for, beta-carotene-15,15'-dioxygenase, was purified and characterized.

Expression pattern and localization of beta,beta-carotene 15,15'-dioxygenase in different tissues

Beta,beta-carotene 15,15'-dioxygenase cleaves beta,beta-carotene into two molecules of retinal, and is the key enzyme in the metabolism of beta,beta-carotene to vitamin A. The enzyme has been known for more than 40 years, yet all attempts to purify the protein to homogeneity have failed. Recently, the successful cloning and sequencing of an enzyme with beta,beta-carotene 15,15'-dioxygenase activity from chicken, as well as from Drosophila, has been reported. Here, we describe in detail our attempt to enrich the chicken beta,beta-carotene 15,15'-dioxygenase to such an extent as to allow determination of partial amino acid sequences, which were then used to design degenerate oligonucleotides. Screening of a chicken duodenal expression library yielded a full-length clone containing a coding sequence of 1578 bp. Functional expression in Escherichia coli and in eukaryotic cell lines confirmed that we had cloned the first vertebrate dioxygenase that cleaves beta,beta-carotene at the central 15,15'-double bond. By performing a sequence homology search, the cDNA sequence of the mouse homologue was found as an expressed sequence tag (EST) in the gene bank. At the amino-acid level, the degree of homology between the chicken and mouse sequences is 81%. Thus beta,beta-carotene 15,15'-dioxygenase can be considered as being an enzyme that is evolutionarily rather well conserved. We established the expression pattern of beta,beta-carotene 15,15'-dioxygenase in chicken and mouse tissues with a combination of Northern blots and in situ hybridization. The mRNA for beta,beta-carotene 15,15'-dioxygenase was localized primarily in duodenal villi, as well as in liver and in tubular structures of lung and kidney. These new findings demonstrate that beta,beta-carotene 15,15'-dioxygenase is also expressed in epithelial structures, where it serves to provide the tissue-specific vitamin A supply.

The effect of alpha-tocopherol on the oxidative cleavage of beta-carotene

Two cleavage pathways of beta-carotene have been proposed, one by central cleavage and the other by random (excentric) cleavage. The central cleavage pathway involves the metabolism of beta-carotene at the central double bond (15, 15') to produce retinal by beta-carotene 15, 15'-dioxygenase (E.C.888990988). The random cleavage of beta-carotene produces beta-apo-carotenoids, but the mechanism is not clear. To understand the various mechanisms of beta-carotene cleavage, beta-carotene was incubated with the intestinal postmitochondrial fractions of 10-week-old male rats for 1 h, and cleavage products of beta-carotene were analyzed using reverse-phase, high-performance liquid chromatography (HPLC). We also studied the effects of alpha-tocopherol and NAD(+)/NADH on beta-carotene cleavage. In addition to beta-carotene, we used retinal and beta-apo-14'-carotenoic acid as substrates in these incubations. Beta-apo-14'-carotenoic acid is the two-carbon longer homologue of retinoic acid. In the presence of alpha-tocopherol, beta-carotene was converted exclusively to retinal, whereas in the absence of alpha-tocopherol, both retinal and beta-apo-carotenoids were formed. Retinoic acid was produced from both retinal and beta-apo-14'-carotenoic acid incubations only in the presence of NAD(+). Our data suggest that in the presence of an antioxidant such as alpha-tocopherol, beta-carotene is converted exclusively to retinal by central cleavage. In the absence of an antioxidant, beta-carotene is cleaved randomly by enzyme-related radicals to produce beta-apo-carotenoids, and these beta-apo-carotenoids can be oxidized further to retinoic acid via retinal.

A comparison of lycopene and astaxanthin absorption from corn oil and olive oil emulsions

The effect of different oils on the absorption of carotenoids was investigated in mesenteric lymph duct cannulated rats. Sixteen treatment emulsions containing increasing concentrations of either lycopene (LYC) or astaxanthin (AST) (5, 10, 15, 20 micromol/L) were prepared with olive oil or corn oil and continuously infused into the duodenum of the rat. Absorption of carotenoids into the mesenteric lymph duct was determined. Absorption of LYC and AST from both oils increased with the amount infused into the duodenum. The average recovery of AST in the lymph from the olive oil emulsion was 20% but was decreased to 13% from emulsions containing corn oil. Lycopene was not as well absorbed as AST. The average recovery of LYC was 6% from olive oil emulsions but only 2.5% when infused with corn oil. The LYC used in this study was isolated from tomato paste and was primarily in the all-trans form. We did not observe any significant isomerization of all-trans LYC to 9-cis LYC during absorption. We conclude that the type of oil with which a carotenoid is consumed can influence its absorption.

Inhibition of beta-carotene-15,15'-dioxygenase activity by dietary flavonoids

Beta-carotene-15,15'-dioxygenase is an enzyme responsible for providing vertebrates with vitamin A by catalyzing oxidative cleavage of beta-carotene at its central double bond to two molecules of retinal in intestinal cells. However, little data have been reported regarding regulation of the enzyme activity. We have evaluated the effects of antioxidants and dietary flavonoids on the beta-carotene dioxygenase activity in vitro using a pig intestinal homogenate as the enzyme source. 2,6-Di-tert-butyl-4-methylphenol (BHT), a synthetic antioxidant, strongly inhibited the activity at the level of 10(-6) M (a mixed-type inhibition), whereas butylated hydroxyanisole, nor-dihydroguaiaretic acid, n-propyl gallate, and curcumin were moderately inhibitory. Flavonoids such as luteolin, quercetin, rhamnetin, and phloretin remarkably inhibited the dioxygenase activity noncompetitively, whereas flavanones, isoflavones, catechins, and anthocyanidins were less inhibitory. The structure-activity relationship indicated that catechol structure of ring B and a planar flavone structure were essential for inhibition. The enzyme inhibition was also indicated in the cultured Caco-2 cells by the significantly reduced conversion of beta-carotene to retinol when incubated with BHT and rhamnetin at 2 microM and 5 microM, respectively. The results suggest that some dietary antioxidants derived from food sources modulate conversion of beta-carotene to vitamin A in intestinal cells.

Amount of fat in the diet affects bioavailability of lutein esters but not of alpha-carotene, beta-carotene, and vitamin E in humans

BACKGROUND

Fat-soluble vitamin E and carotenoids are regarded as being protective against chronic diseases. Little is known about the effect of dietary fat on the bioavailability of these compounds.

OBJECTIVE

The objective of this study was to assess the effect of the amount of dietary fat on plasma concentrations of vitamin E and carotenoids after supplementation with these compounds.

DESIGN

During two 7-d periods, 4 groups of 14-15 volunteers received daily, with a low-fat hot meal, 1 of 4 different supplements: vitamin E (50 mg), alpha- plus beta-carotene (8 mg), lutein esters (8 mg lutein), or placebo. The supplements were provided in a low- or high-fat spread supplied in random sequence during either of the 2 experimental periods.

RESULTS

As anticipated, plasma concentrations of vitamin E, alpha- and beta-carotene, and lutein were significantly higher in the supplemented groups than in the placebo group. The amount of dietary fat consumed with the hot meal (3 or 36 g) did not affect the increases in plasma concentrations of vitamin E (20% increase with the low-fat spread and 23% increase with the high-fat spread) or alpha- and beta-carotene (315% and 139% with the low-fat spread and 226% and 108% with the high-fat spread). The plasma lutein response was higher when lutein esters were consumed with the high-fat spread (207% increase) than with the low-fat spread (88% increase).

CONCLUSION

Optimal uptake of vitamin E and alpha- and beta-carotene requires a limited amount of fat whereas the amount of fat required for optimal intestinal uptake of lutein esters is higher. 2000;71:-93.

Nutrient regulation of intestinal gene expression

This review examines recent advances in the dietary modulation of gene expression in the gastrointestinal tract. We have chosen to concentrate on individual genes and examine what is known about their regulation, attempting to link different studies together.