Dietary intake of vitamin A, lung function and incident asthma in childhood

BACKGROUND

Longitudinal epidemiological data are scarce on the relationship between dietary intake of vitamin A and respiratory outcomes in childhood. We investigated whether a higher intake of preformed vitamin A or pro-vitamin β-carotene in mid-childhood is associated with higher lung function and with asthma risk in adolescence.

METHODS

In the Avon Longitudinal Study of Parents and Children, dietary intakes of preformed vitamin A and β-carotene equivalents were estimated by food frequency questionnaire at 7 years of age. Post-bronchodilator forced expiratory volume in 1 s (FEV₁), forced vital capacity (FVC) and forced expiratory flow at 25-75% of FVC (FEF_25-75%) were measured at 15.5 years and transformed to z-scores. Incident asthma was defined by new cases of doctor-diagnosed asthma at age 11 or 14 years.

RESULTS

In multivariable adjusted models, a higher intake of preformed vitamin A was associated with higher lung function and a lower risk of incident asthma: comparing top versus bottom quartiles of intake, regression coefficients for FEV₁ and FEF_25-75% were 0.21 (95% CI 0.05-0.38; p_trend=0.008) and 0.18 (95% CI 0.03-0.32; p_trend=0.02), respectively; odds ratios for FEV₁/FVC below the lower limit of normal and incident asthma were 0.49 (95% CI 0.27-0.90; p_trend=0.04) and 0.68 (95% CI 0.47-0.99; p_trend=0.07), respectively. In contrast, there was no evidence for association with β-carotene. We also found some evidence for modification of the associations between preformed vitamin A intake and lung function by BCMO1, NCOR2 and SCGB1A1 gene polymorphisms.

CONCLUSION

A higher intake of preformed vitamin A, but not β-carotene, in mid-childhood is associated with higher subsequent lung function and lower risk of fixed airflow limitation and incident asthma.

Key role of conserved histidines in recombinant mouse beta-carotene 15,15'-monooxygenase-1 activity

Alignment of sequences of vertebrate beta-carotene 15,15'-monooxygenase-1 (BCMO1) and related oxygenases revealed four perfectly conserved histidines and five acidic residues (His172, His237, His308, His514, Asp52, Glu140, Glu314, Glu405, and Glu457 in mouse BCMO1). Because BCMO1 activity is iron-dependent, we propose that these residues participate in iron coordination and therefore are essential for catalytic activity. To test this hypothesis, we produced mutant forms of mouse BCMO1 by replacing the conserved histidines and acidic residues as well as four histidines and one glutamate non-conserved in the overall family with alanines by site-directed mutagenesis. Our in vitro and in vivo data showed that mutation of any of the four conserved histidines and Glu405 caused total loss of activity. However, mutations of non-conserved histidines or any of the other conserved acidic residues produced impaired although enzymatically active proteins, with a decrease in activity mostly due to changes in V(max). The iron bound to protein was determined by inductively coupled plasma atomic emission spectrometry. Bound iron was much lower in preparations of inactive mutants than in the wild-type protein. Therefore, the conserved histidines and Glu405 are absolutely required for the catalytic mechanism of BCMO1. Because the mutant proteins are impaired in iron binding, these residues are concluded to coordinate iron required for catalytic activity. These data are discussed in the context of the predicted structure for the related eubacterial apocarotenal oxygenase.

Metalloporphyrines as active site analogues--lessons from enzymes and enzyme models

Research at the interface of enzyme chemistry and organic chemistry of metal complexes is particularly rewarding employing metal porphyrins as cofactor surrogates. Three examples are discussed: active site analogues of cytochrome P450 and chloroperoxidase (CPO), both heme-thiolate proteins, and enzyme models of beta-carotene monooxygenase, a non-heme iron protein. In all cases, catalytically active synthetic systems could be established displaying chemical reactivity close to the native proteins. Further, it is demonstrated that enzymatic reaction mechanisms can be elucidated by means of active site analogues (CPO) and information can be obtained from enzyme models that is useful to explain certain aspects of Nature's sophisticated approach to develop very efficient catalysts.

Towards a better understanding of carotenoid metabolism in animals

Vitamin A derivatives (retinoids) are essential components in vision; they contribute to pattern formation during development and exert multiple effects on cell differentiation with important clinical implications. All naturally occurring vitamin A derives by enzymatic oxidative cleavage from carotenoids with pro-vitamin A activity. To become biologically active, these plant-derived compounds must first be absorbed, then delivered to the site of action in the body, and metabolically converted to the real vitamin. Recently, molecular players of this pathway were identified by the analysis of blind Drosophila mutants. Similar genome sequences were found in vertebrates. Subsequently, these homologous genes were cloned and their gene products were functionally characterized. This review will summarize the advanced state of knowledge about the vitamin A biosynthetic pathway and will discuss biochemical, physiological, developmental and medical aspects of carotenoids and their numerous derivatives.

Cell type-specific expression of beta-carotene 15,15'-mono-oxygenase in human tissues

We studied the cell type-specific expression of human beta-carotene 15,15'-mono-oxygenase (BCO1), an enzyme that catalyzes the first step in the conversion of dietary provitamin A carotenoids to vitamin A. Immunohistochemical analysis using two monoclonal antibodies against different epitopes of the protein revealed that BCO1 is expressed in epithelial cells in a variety of human tissues, including mucosa and glandular cells of stomach, small intestine, and colon, parenchymal cells in liver, cells that make up the exocrine glands in pancreas, glandular cells in prostate, endometrium, and mammary tissue, kidney tubules, and in keratinocytes of the squamous epithelium of skin. Furthermore, BCO1 is detected in steroidogenic cells in testis, ovary, and adrenal gland, as well as skeletal muscle cells. Epithelia in general are structures that are very sensitive to vitamin A deficiency, and although the extraintestinal function of BCO1 is unclear, the finding that the enzyme is expressed in all epithelia examined thus far leads us to suggest that BCO1 may be important for local synthesis of vitamin A, constituting a back-up pathway of vitamin A synthesis during times of insufficient dietary intake of vitamin A.

A Potential Role for beta-Carotene in Avian Embryonic Development

Vitamin A is essential for vertebrate embryonic development; dietary carotenoids are the primary source of vitamin A since animals cannot synthesize it de novo. To study the role of beta-carotene during embryonic development, we analyzed in chick embryos the expression of beta,beta-carotene 15,15'-oxygenase (beta-oxy) which cleaves beta-carotene to produce two molecules of retinal. beta-oxy transcripts were detected in one-and-a-half- to five-day-old embryo homogenates and in situ hybridization in five-day-old embryos, revealing their presence in tissues including the central nervous system, lungs, limbs, and cardiovascular system. Moreover, we detected beta-oxy enzymatic activity in extracts from five-day-old embryos as well as small amounts of beta-carotene in the egg yolk. These results indicate that beta-oxy is present during early developmental stages, raising the possibility that yolk-stored beta-carotene is utilized as a source of vitamin A. Thus, our results suggest that beta-carotene could play an important role in early avian embryonic development as a local source of vitamin A in specific tissues.

Drosophila ninaB and ninaD act outside of retina to produce rhodopsin chromophore

The Drosophila ninaB gene encodes a beta,beta-carotene-15,15'-oxygenase responsible for the centric cleavage of beta-carotene that produces the retinal chromophore of rhodopsin. The ninaD gene encodes a membrane receptor required for efficient use of beta-carotene. Despite their importance to the synthesis of visual pigment, we show that these genes are not active in the retina. Mosaic analysis shows that ninaB and ninaD are not required in the retina, and exclusive retinal expression of either gene, or both genes simultaneously, does not support rhodopsin biogenesis. In contrast, neuron-specific expression of ninaB and ninaD allows for rhodopsin biogenesis. Additional directed expression studies failed to identify other tissues supporting ninaB activity in rhodopsin biogenesis. These results show that nonretinal sites of NinaB beta,beta-carotene-15,15'-oxygenase activity, likely neurons of the central nervous system, are essential for production of the visual chromophore. Retinal or another C(20) retinoid, not members of the beta-carotene family of C(40) carotenoids, are supplied to photoreceptors for rhodopsin biogenesis.

Vitamin A formation in animals: molecular identification and functional characterization of carotene cleaving enzymes

Vitamin A and its derivatives (retinoids) are essential components in vision; they contribute to pattern formation during development and exert multiple effects on cell differentiation. It has been known for 70 y that the key step in vitamin A biosynthesis is the oxidative cleavage of a carotenoid with provitamin A activity. While a detailed biochemical characterization of the respective enzymes could be achieved in cell-free homogenates, their molecular nature has remained elusive for a long time. Recent research led to the identification of genes encoding two different types of carotene oxygenases from animal species. The molecular cloning of these different types of animal carotene oxygenases establishes the existence of a family of carotenoid metabolizing enzymes in animals heretofore described in plants. With these tools in hands, old questions in vitamin A research can be definitively addressed on the molecular levels contributing to a mechanistic understanding of the regulation of vitamin A homeostasis or tissue specificity of vitamin A formation, with impact on animal physiology and human health.

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.

Vitamin A: overlapping delivery pathways to tissues from the circulation

Although retinol bound to retinol-binding protein (RBP) is the most abundant retinoid form present in the circulations of humans and most mammals, other retinoid and proretinoid forms are also present in the blood. We are interested in understanding to what extent each of these circulating retinoid forms contributes towards retinoid actions within cells and tissues. Here we report two studies focused on this question. First, we examined retinoid transport and storage in RBP-deficient mice that lack circulating RBP. These mice under normal laboratory conditions are phenotypically normal except for a visual impairment early in life that is corrected if the mice are maintained on a vitamin A-sufficient diet throughout life. The RBP-deficient mice take up vitamin A from the diet into most tissues at least as well as wild type mice. Compared to wild type mice, mice lacking RBP accumulate excess vitamin A in the liver, since there is no RBP to facilitate mobilization of stored retinol from hepatic stores. In a second study, we explored in vitro the actions of carotene cleavage enzyme (CCE) in facilitating beta-carotene cleavage to retinoid in the testis. CCE is most highly expressed in the testis. Pull-down experiments coupled with MALDI-MS analysis showed that mouse testis CCE is able to interact with the testis-specific lactate dehydrogenase-C (LDH-C) isoform. This may suggest that CCE and LDH-C act in concert to catalyze beta-carotene cleavage.

Carotene oxygenases: a new family of double bond cleavage enzymes

Beta,beta-carotene 15,15'-monooxygensae (betaCMOOX) is the key enzyme involved in the metabolism of provitamin A carotenoids to retinal. Although the enzyme has been known for >40 y, it has been only within the last 2 y that the cloning and the molecular characterization of the betaCMOOX from several species was reported in literature. New clones of the carotene metabolizing enzyme have emerged, all belonging to the family of double bond cleavage enzymes, suggesting common ancestry. BetaCMOOX cleaves beta,beta-carotene to retinal in an in vitro activity assay; no apo-carotenals were identified. The second enzyme involved in carotenoid metabolism, beta,beta-carotene 9',10'-dioxygenase, is responsible for the excentric cleavage pathway of carotenoids, cleaving beta,beta-carotene to 10'-apo-carotenal and beta-ionone. In an expression overview, the betaCMOOX was detected in duodenum, liver, kidney and in the lungs of chickens. In mice, the mRNA for the central cleavage enzyme was highly expressed in liver, testes, small intestine, and kidney. betaCMOOX expression was highest in epithelial and endothelial structures in both species. These results suggest that the source of vitamin A originates from carotenoids in the corresponding tissues, in addition to retinol supplied from liver stores.

Alpha and omega of carotenoid cleavage

In early 1900s, based on indirect evidence, Steenbock and Morton independently predicted that beta-carotene could be the biological precursor of vitamin A, although this notion was contested by others. In the 1930s, Thomas Moore showed the in vivo formation of vitamin A from beta-carotene. But it was not until Jim Olson and DeWitt Goodman independently showed in 1965 the formation of retinal, the aldehyde form of vitamin A from beta-carotene in cell-free extracts of liver and intestine, that this vital pathway of beta-carotene was recognized. Despite compelling evidence in several experimental systems for the central cleavage of beta-carotene to retinal by many investigators, there were some careful independent studies by Glover et al., Ganguly et al., Hansen and Meret and Krinsky et al. showing the eccentric cleavage of beta-carotene resulting in the formation of apocarotenoids both in vivo and in vitro. In an attempt to resolve this controversial issue, we revisited this problem in 1989 and showed beyond doubt the formation of retinal as the sole enzymatic product of a cytosolic enzyme from rabbit and rat intestinal mucosa by mass spectrometry and tracer analysis of the crystallized product. This was confirmed in 1996 by Nagao using the pig intestinal extract. Yeum et al. confirmed in 2000 that retinal is the sole product of beta-carotene cleavage in the presence of alpha-tocopherol, and that the observed formation of apocarotenoids occurs only in the absence of an antioxidant like alpha-tocopherol. In the same year, Barua and Olson also concluded from their in vivo studies in rats that central cleavage is by far the major pathway for the formation of vitamin A from beta-carotene. Beta, beta-carotene 15,15'-dioxygenase (EC 1.13.11.21) is the key enzyme that cleaves beta-carotene into two molecules of retinal. It is a cytosolic enzyme primarily localized in the duodenal mucosa although it has been found in liver. It is a 66 kDa sulfhydryl protein, requires molecular oxygen and is activated by ferrous ions. It is highly specific for 15:15' ethylenic bond of carotenoids although it has fairly broad specificity towards a number of carotenoids with at least one intact beta-ionone ring. The dioxygenase was recently cloned from Drosophila melanogaster and from the chicken intestine. The recombinant protein was found to form retinal as the sole cleavage product of beta-carotene. No apo-carotenoids were formed. Therefore, it is unequivocally proven that the major, if not the sole, pathway of beta-carotene cleavage to vitamin A is by oxidative cleavage of the central ethylenic bond of beta-carotene to yield two molecules of retinal. Most recently, human dioxygenase has also been cloned. Thus, the wisdom, vision and epoch-making mission of Jim Olson in the science of beta-carotene metabolism have been accomplished. I have no doubt that the impact of his original discovery of the dioxygenase and its importance in vitamin A nutriture should be forthcoming in the near future.

Identification of beta-carotene 15, 15'-monooxygenase as a peroxisome proliferator-activated receptor target gene

Beta-carotene 15,15'-monooxygenase (BCM) catalyzes the first step of vitamin A biosynthesis from provitamin A carotenoids. We wished to determine the factors underlying the transcriptional regulation of this gene. After cloning of the 40 kilobase pair (kbp) mouse Bcm gene and determination of its genomic organization, analysis of the 2 kb 5'-flanking region showed several putative transcription factor binding sites including TATA box, a peroxisome proliferator response element (PPRE), AP2, and bHLH. The 2 kb fragment drove specific luciferase gene expression in vitro only in cell lines that express BCM (TC7, PF11, and monkey retinal pigment epithelium). Nucleotides -41 to +163, and -60 to +163 drove basal and specific Bcm transcriptional activity, respectively. Site-directed mutagenesis and gel shift experiments demonstrate that PPRE was essential for Bcm promoter specificity and that the peroxisome proliferator activated receptor (PPAR) gamma (PPARgamma) specifically binds to this element. Furthermore, cotransfection experiments and pharmacological treatments in vitro, using the specific PPARgamma agonists LY17883 and ciglitazone, demonstrate that the PPRE element confers peroxisome proliferator responsiveness via the PPARgamma and retinoid X receptor-alpha heterodimer. Treatment of mice with the PPARalpha/gamma agonist WY14643 increases BCM protein expression in liver. Thus PPAR is a key transcription factor for the transcriptional regulation of the Bcm gene, suggesting a broader function for PPARs in the regulation of carotenoid metabolism metabolism that is consistent with their established role in neutral lipid metabolism and transport.

Provitamin A conversion to retinal via the beta,beta-carotene-15,15'-oxygenase (bcox) is essential for pattern formation and differentiation during zebrafish embryogenesis

The egg yolk of vertebrates contains carotenoids, which account for its characteristic yellow color in some species. Such plant-derived compounds, e.g. beta-carotene, serve as the natural precursors (provitamins) of vitamin A, which is indispensable for chordate development. As egg yolk also contains stored vitamin A, carotenoids have so far been solely discussed as pigments for the coloration of the offspring. Based on our recent molecular identification of the enzyme catalyzing provitamin A conversion to vitamin A, we address a possible role of provitamin A during zebrafish (Danio rerio) development. We cloned the zebrafish gene encoding the vitamin A-forming enzyme, a beta,beta-carotene-15,15'-oxygenase. Analysis of its mRNA expression revealed that it is under complex spatial and temporal control during development. Targeted gene knockdown using the morpholino antisense oligonucleotide technique indicated a vital role of the provitamin A-converting enzyme. Morpholino-injected embryos developed a morphological phenotype that included severe malformation of the eyes, the craniofacial skeleton and pectoral fins, as well as reduced pigmentation. Analyses of gene expression changes in the morphants revealed that distinct retinoic acid-dependent developmental processes are impaired, such as patterning of the hindbrain and differentiation of hindbrain neurons, differentiation of neural crest derivatives (including the craniofacial skeleton), and the establishment of the ventral retina. Our data provide strong evidence that, for several developmental processes, retinoic acid generation depends on local de novo formation of retinal from provitamin A via the carotene oxygenase, revealing an unexpected, essential role for carotenoids in embryonic development.

Measurement of retinoids and beta-carotene 15,15'-dioxygenase activity in HR-1 hairless mouse skin with UV exposure

We investigated the vitamin A status and beta-carotene 15,15'-dioxygenase activity in hairless mice with UV exposure to assess the regulation of vitamin A metabolism after UV irradiation. HR-1 hairless mice were irradiated with UV at 3 J/cm2 for 5 d. After UV irradiation, the mice were sacrificed and samples were obtained to analyze the retinoid concentration, expression of RXR-alpha, and beta-carotene 15,15'-dioxygenase activity. After UV exposure, the skin retinoid concentration was significantly lower as well as the expression of RXR-alpha. Higher skin beta-carotene dioxygenase activity was observed in the UV group as compared to the control group. We found no significant differences in the alpha-tocopherol concentration or acrolein levels in the skins of the two groups. In conclusion, the elevation of beta-carotene 15,15'-dioxygenase activity in hairless mice after UV exposure may be a response to reduction of the skin retinoid concentration.

Expression of beta-carotene 15,15' monooxygenase in retina and RPE-choroid

PURPOSE

Beta-carotene 15,15' monooxygenase (beta-CM) catalyzes the central cleavage of beta-carotene to all-trans-retinal, the first step in vitamin A synthesis. This study was conducted to determine the expression of beta-CM in the mammalian retina and RPE, to assess its relevance in carotenoid-retinoid metabolism in the retina and RPE.

METHODS

RT-PCR was used to detect expression of beta-CM mRNA in the retina and RPE-choroid of the mouse, cow, human, and monkey and in RPE cells and other cell lines. Immunofluorescence microscopy was used to localize beta-CM in mouse and monkey retina with an anti-peptide antibody specific for beta-CM.

RESULTS

By RT-PCR, beta-CM mRNA was detected at a low level in mouse and monkey retina and in the RPE-choroid of the monkey but not of the mouse. Conversely, beta-CM mRNA was expressed at a low level in both human and bovine RPE-choroid, but not in the retina of either. RPE primary cultured cells of the monkey also showed beta-CM mRNA expression, although the three human lines did not. In addition, of nine other cell lines tested, only COS-7 was positive for beta-CM. Immunofluorescence microscopy showed weak immunoreactivity in the inner retina in both the mouse and monkey. beta-CM immunoreactivity was not detectable in RPE of the mouse. Use of a long-wavelength exciting and emitting secondary probe to mitigate lipofuscin autofluorescence, facilitated the detection of a low level of beta-CM immunoreactivity in monkey RPE.

CONCLUSIONS

Beta-CM mRNA and protein are expressed at low levels in the mammalian retina and RPE-choroid. Given the low and variable expression of beta-CM in the retina and RPE, it can be concluded that beta-CM is not necessary for a conserved retina or RPE-specific function, but may be necessary for a species-specific function.

Feedback regulation of beta,beta-carotene 15,15'-monooxygenase by retinoic acid in rats and chickens

beta,beta-Carotene 15,15'-monooxygenase (formerly termed beta,beta-carotene 15,15'-dioxygenase, EC 1.13.11.21) catalyzes the conversion of provitamin A carotenoids to retinal in vertebrate tissues. In the present study, we investigated whether preformed vitamin A or beta-carotene and its direct metabolites can regulate the enzyme activity in vivo. We found dose-dependent decreases in intestinal beta,beta-carotene monooxygenase activity after oral administration to rats of retinyl acetate (up to -79%), beta-carotene (up to -79%), apo-8'-carotenal (up to -56%), all-trans retinoic acid (up to -88%), and 9-cis retinoic acid (up to -67%). Liver beta,beta-carotene 15,15'-monooxygenase (betaCMOOX) activity was not affected. Apo-12'carotenal and the retinoic acid receptor (RAR) alpha antagonist Ro 41-5253 significantly increased the intestinal enzyme activity by 55 and 94%, respectively. When beta-carotene was administered to rats pretreated with the two cytochrome P(450) (CYP) inducers, pentobarbital and naphthoflavone, the intestinal betaCMOOX activity increased by 39%. In a transcriptional study in chickens, treatment with retinoic acid resulted in low expression of the intestinal betaCMOOX. Our data suggest that retinoids and carotenoids might regulate betaCMOOX expression by a transcriptional feedback mechanism via interaction with members of the RAR family.

Biochemical properties of purified recombinant human beta-carotene 15,15'-monooxygenase

Beta-carotene 15,15'-monooxygenase (BCO), formerly known as beta-carotene 15,15'-dioxygenase, catalyzes the first step in the synthesis of vitamin A from dietary carotenoids. We have biochemically and enzymologically characterized the purified recombinant human BCO enzyme. A highly active BCO enzyme was expressed and purified to homogeneity from baculovirus-infected Spodoptera frugiperda 9 insect cells. The K(m) and V(max) of the enzyme for beta-carotene were 7 microm and 10 nmol retinal/mg x min, respectively, values that corresponded to a turnover number (k(cat)) of 0.66 min(-1) and a catalytic efficiency (k(cat)/K(m)) of approximately 10(5) m(-1) x min(-1). The enzyme existed as a tetramer in solution, and substrate specificity analyses suggested that at least one unsubstituted beta-ionone ring half-site was imperative for efficient cleavage of the carbon 15,15'-double bond in carotenoid substrates. High levels of BCO mRNA were observed along the whole intestinal tract, in the liver, and in the kidney, whereas lower levels were present in the prostate, testis, ovary, and skeletal muscle. The current data suggest that the human BCO enzyme may, in addition to its well established role in the digestive system, also play a role in peripheral vitamin A synthesis from plasma-borne provitamin A carotenoids.

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.

Molecular analysis of vitamin A formation: cloning and characterization of beta-carotene 15,15'-dioxygenases

Beta-carotene 15,15'-dioxygenase cleaves beta-carotene into two molecules of retinal and is the key enzyme in the metabolism of carotene to vitamin A. Although the enzyme has been known for more than 40 years, all attempts to purify the protein to homogeneity or to clone its gene have failed until recently, when the successful cloning and sequencing of cDNAs encoding enzymes with beta-carotene 15,15'-dioxygenase activity from Drosophila (J. von Lintig and K. Vogt, 2000, J. Biol. Chem. 275, 11915-11920) and chicken (A. Wyss et al., 2000, Biochem. Biophys. Res. Commun. 271, 334-336) were reported. Very soon it became clear, that we have cloned two members of a new family of carotenoid cleaving enzymes. Overall homologies are very high, certain amino acid stretches almost identical. Thus, beta-carotene 15,15'-dioxygenase can be considered as evolutionarily well conserved. These findings open up wide perspectives for further analysis of this important biosynthetic pathway, concerning basic and medical research as well as biotechnological aspects related to vitamin A supply, which are discussed here.

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.

Free radical lipid peroxidation inhibits enzymatic conversion of beta-carotene into vitamin A

Free radical oxidation of arachidonic acid with soybean lipoxygenase was accompanied by inhibition of retinal synthesis from beta-carotene catalyzed by enzyme preparation from rabbit intestinal mucosa. Lipoxygenase inhibitor salicylhydroxamic acid and antioxidants suppressing free radical reactions (ethyl gallate, alpha-tocopherol, astaxanthine, and quercetin) promoted conversion of beta-carotene into retinal catalyzed by beta-carotene-15,15'-dioxygenase. These results indicate that lipoperoxides and/or products of their homolysis attenuate enzymatic conversion of beta-carotene and confirm the important role of natural antioxidants in the maintenance of stable vitamin A content in mammals.

Identification and characterization of a mammalian enzyme catalyzing the asymmetric oxidative cleavage of provitamin A

In vertebrates, symmetric versus asymmetric cleavage of beta-carotene in the biosynthesis of vitamin A and its derivatives has been controversially discussed. Recently we have been able to identify a cDNA encoding a metazoan beta,beta-carotene-15,15'-dioxygenase from the fruit fly Drosophila melanogaster. This enzyme catalyzes the key step in vitamin A biosynthesis, symmetrically cleaving beta-carotene to give two molecules of retinal. Mutations in the corresponding gene are known to lead to a blind, vitamin A-deficient phenotype. Orthologs of this enzyme have very recently been found also in vertebrates and molecularly characterized. Here we report the identification of a cDNA from mouse encoding a second type of carotene dioxygenase catalyzing exclusively the asymmetric oxidative cleavage of beta-carotene at the 9',10' double bond of beta-carotene and resulting in the formation of beta-apo-10'-carotenal and beta-ionone, a substance known as a floral scent from roses, for example. Besides beta-carotene, lycopene is also oxidatively cleaved by the enzyme. The deduced amino acid sequence shares significant sequence identity with the beta,beta-carotene-15,15'-dioxygenases, and the two enzyme types have several conserved motifs. To establish its occurrence in different vertebrates, we then attempted and succeeded in cloning cDNAs encoding this new type of carotene dioxygenase from human and zebrafish as well. As regards their possible role, the apocarotenals formed by this enzyme may be the precursors for the biosynthesis of retinoic acid or exert unknown physiological effects. Thus, in contrast to Drosophila, in vertebrates both symmetric and asymmetric cleavage pathways exist for carotenes, revealing a greater complexity of carotene metabolism.

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.

Identification, expression, and substrate specificity of a mammalian beta-carotene 15,15'-dioxygenase

We have identified from mouse the first mammalian beta-carotene 15,15'-dioxygenase (beta-CD), a crucial enzyme in development and metabolism that governs the de novo entry of vitamin A from plant-derived precursors. beta-CD is related to the retinal pigment epithelium-expressed protein RPE65 and belongs to a diverse family that includes the plant 9-cis-epoxycarotenoid dioxygenase and bacterial lignostilbene dioxygenases. beta-CD expression in Escherichia coli cells engineered to produce beta-carotene led to the accumulation of all-trans-retinal at the expense of beta-carotene, confirming that beta-CD catalyzed the central cleavage of this vitamin A precursor. Purified recombinant beta-CD protein cleaves beta-carotene in vitro with a V(max) of 36 pmol of retinal/mg of enzyme/min and a K(m) of 6 microm. Non-provitamin A carotenoids were also cleaved, although with much lower activity. By Northern analysis, a 2.4-kilobase (kb) message was observed in liver, kidney, small intestine, and testis, tissues important in retinoid/carotenoid metabolism. This message encoded a 63-kDa cytosolic protein expressed in these tissues. A shorter transcript of 1.8 kb was found in testis and skin. Developmentally, the 2.4-kb mRNA was abundant at embryonic day 7, with lower expression at embryonic days 11, 13, and 15, suggesting a critical role for this enzyme in gastrulation. Identification of beta-CD in an accessible model organism will create new opportunities to study vitamin A metabolism.

Cloning and characterization of a human beta,beta-carotene-15,15'-dioxygenase that is highly expressed in the retinal pigment epithelium

Retinoids play a critical role in vision, as well as in development and cellular differentiation. beta,beta-Carotene-15,15'-dioxygenase (Bcdo), the enzyme that catalyzes the oxidative cleavage of beta,beta-carotene into two retinal molecules, plays an important role in retinoid synthesis. We report here the first cloning of a mammalian Bcdo. Human BCDO encodes a protein of 547 amino acid residues that demonstrates 68% identity with chicken Bcdo. It is expressed highly in the retinal pigment epithelium (RPE) and also in kidney, intestine, liver, brain, stomach, and testis. The gene spans approximately 20 kb, is composed of 11 exons and 10 introns, and maps to chromosome 16q21-q23. A mouse orthologue was also identified, and its predicted amino acid sequence is 83% identical with human BCDO. Biochemical analysis of baculovirus expressed human BCDO demonstrates the predicted beta,beta-carotene-15,15'-dioxygenase activity. The expression pattern of BCDO suggests that it may provide a local supplement to the retinoids available to photoreceptors, as well as a supplement to the retinoid pools utilized elsewhere in the body. In addition, the finding that many of the enzymes involved in retinoid metabolism are mutated in retinal degenerations suggests that BCDO may also be a candidate gene for retinal degenerative disease.

Co-ordinated induction of beta-carotene cleavage enzyme and retinal reductase in the duodenum of the developing chicks

The developmental patterns of expression of beta-carotene cleavage enzyme activity were compared with those of retinal reductase and NAD-dependent retinol dehydrogenase activities in chick duodenum during the perinatal period. The beta-carotene cleavage enzyme activity was not detected in the duodenum before hatching, but it increased rapidly during 24 h after hatching. On the other hand, a considerable level of beta-carotene cleavage enzyme activity was observed in the liver of embryonic stages and its activity gradually rose during the perinatal period. Comparison of kinetic constants for the beta-carotene cleavage enzyme activities in the duodenum and the liver indicated that the enzyme in the duodenum possessed a lower affinity for beta-carotene than that in the liver. The retinal reductase activity was detected in the microsomes of the duodenum at the earliest time examined, i.e. day 16 of embryogenesis and its activity began to rise on the last day of embryogenesis, which was followed by a gradual increase until 1 day of age. The NAD-dependent retinol dehydrogenase activity was also seen in the microsomes of the duodenum in embryonic stages and its activity increased in parallel with the retinal reductase activity around the hatching period. These developmental inductions of beta-carotene cleavage enzyme and retinal reductase activities in the duodenum coincided with those of cellular retinol-binding protein, type II (CRBPII) and lecithin: retinol acyltransferase (LRAT). These results suggest that a co-ordinated induction mechanism should be operative for beta-carotene cleavage enzyme and retinal reductase, both of which are inevitable in the process of beta-carotene absorption and metabolism.

Analysis of the blind Drosophila mutant ninaB identifies the gene encoding the key enzyme for vitamin A formation invivo

Visual pigments (rhodopsins) are composed of a chromophore (vitamin A derivative) bound to a protein moiety embedded in the retinal membranes. Animals cannot synthesize the visual chromophore de novo but rely on the uptake of carotenoids, from which vitamin A is formed enzymatically by oxidative cleavage. Despite its importance, the enzyme catalyzing the key step in vitamin A formation resisted molecular analyses until recently, when the successful cloning of a cDNA encoding an enzyme with beta,beta-carotene-15,15'-dioxygenase activity from Drosophila was reported. To prove its identity with the key enzyme for vitamin A formation in vivo, we analyzed the blind Drosophila mutant ninaB. In two independent ninaB alleles, we found mutations in the gene encoding the beta,beta-carotene-15,15'-dioxygenase. These mutations lead to a defect in vitamin A formation and are responsible for blindness of these flies.

Beta-carotene 15,15'-dioxygenase activity in streptozotocin-induced diabetic rats

We measured retinol levels and beta-carotene 15,15'-dioxygenase activity in rats with streptozotocin-induced diabetes mellitus to assess the relationship between the disease and the conversion of beta-carotene to retinol. The plasma retinol level was significantly lower in diabetic rats than in control rats, but the hepatic retinol level was significantly higher than the control. The hepatic dioxygenase activity, but not that of the intestinal mucosa, was significantly lower in diabetic rats than in control rats. The hepatic dioxygenase activity showed a significant negative correlation with the hepatic retinol levels. The results suggest the disturbed secretion of retinol from the liver and suppression of hepatic dioxygenase activity by the retinol increased in the liver in diabetic rats.

Low cleavage activity of 15,15'dioxygenase to convert beta-carotene to retinal in cattle compared with goats, is associated with the yellow pigmentation of adipose tissue

Two experiments (one with twelve heifers and the other with 14 goats) were designed to evaluate the effect of increasing dietary beta-carotene concentration on the activity of the cleaving enzyme 15,15' beta-carotene dioxygenase and the concentration of beta-carotene and retinol in selected tissues. The experiments lasted 120 days. During the first 90 days animals were offered a beta-carotene unsupplemented diet. In the following 30 days, they were distributed to one of three treatments: 0, 5.5 or 352 mg of beta-carotene/kg of dry matter intake. All animals were slaughtered at the end. In heifers the concentration of beta-carotene in plasma, reflected the level of beta-carotene fed. Goats had detectable levels of beta-carotene only on day 10 of supplementation. In the liver, beta-carotene concentrations were highest with the 352 dose in both species. Heifers had the highest concentration of beta-carotene in the adipose tissue. In bovines, no interaction between beta-carotene treatment and intestinal sampling site was found (P > 0.2) for the activity of 15,15 dioxygenase. Across beta-carotene levels, results of the enzyme assay were: 0.19, 0.32 and 0.45 nmol retinal/(mg S-9 protein/h) (P < 0.01) for 0, 5.5 and 352 mg beta-carotene.kg dry matter intake -1.d-1, respectively; across intestinal sampling sites results of the enzyme assay were 0.45, 0.43 and 0.08 nmol retinal/(mg S-9 protein/h) (P < 0.01) for duodenum, jejunum and ileum, respectively. Caprine data showed an interaction between beta-carotene treatment and intestinal sampling site (P < 0.05) for the activity of 15,15 dioxygenase. The results for treatment 0 were: 1.4, 1.4 and 0; for treatment 5.5: 1.41, 1.42 and 0.13; and for treatment 352: 1.46, 1.99 and 0.48 nmol retinal/mg S-9 protein/h for duodenum, jejunum and ileum, respectively. The lower levels of duodenal and jejunal 15,15'dioxygenase activity in cattle compared with goats, may explain the greater pigmentation of adipose tissue in the former ruminant specie.

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.

Intestinal beta-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 beta-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 interaction in vivo. Weanling male Sprague-Dawley rats (n 72) 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 beta-carotene 15,15'-dioxygenase activity in small intestinal mucosa were enhanced with increasing dietary Fe and with Cu deficiency v. 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 beta-carotene 15,15'-dioxygenase activity requires Fe as cofactor in vivo and the enzyme is modulated by the three dietary components: Cu, Fe, and fructose.

Cloning and expression of beta,beta-carotene 15,15'-dioxygenase

beta,beta-Carotene 15,15'-dioxygenase cleaves beta-carotene into two molecules of retinal and is therefore the key enzyme in beta-carotene metabolism to vitamin A. In the present study, it was possible to enrich the chicken beta,beta-carotene 15,15'-dioxygenase to such an extent that partial amino acid sequence information could be obtained to design degenerate oligonucleotides. With RT-PCR a cDNA fragment could be obtained and used subsequently in a radioactive screening of a chicken duodenal expression library. We cloned the first eukaryotic beta,beta-carotene 15,15'-dioxygenase which symmetrically cleaves beta-carotene at the 15,15'-double bond.

Filling the gap in vitamin A research. Molecular identification of an enzyme cleaving beta-carotene to retinal

Vitamin A and its derivatives (retinoids) are essential components in vision; they contribute to pattern formation during development and exert multiple effects on cell differentiation with important clinical implications. It has been known for 50 years that the key step in the formation of vitamin A is the oxidative cleavage of beta-carotene; however, this enzymatic step has resisted molecular analysis. A novel approach enabled us to clone and identify a beta-carotene dioxygenase from Drosophila melanogaster, expressing it into the background of a beta-carotene (provitamin A)-synthesizing and -accumulating Escherichia coli strain. The carotene-cleaving enzyme, identified here for the first time on the molecular level, is the basis of the numerous branches of vitamin A action and links plant and animal carotene metabolism.

Nutritional status affects intestinal carotene cleavage activity and carotene conversion to vitamin A in rats

Validation of an in vivo method we developed recently and its application to assess the role of dietary factors in carotene conversion were tested in rats. We compared the ratio of area under plasma vitamin A time-curves (AUC(0-12h)) obtained after a dose of beta-carotene to that after a dose of vitamin A, with the in vitro intestinal supernatant beta-carotene dioxygenase activity. In separate experiments, vitamin A (AD) and protein deficiencies (PD) were produced in male WNIN weanling rats. Corresponding food-restricted (AR and PR) and unrestricted rats (AA and PA) served as controls. Three rats in each of the AD, AR and AA groups received oral doses of 50-300 microgram beta-carotene or 25-150 microgram vitamin A and four rats in each of the PD, PR and PA groups received only 100 microg beta-carotene or vitamin A. The plasma vitamin A AUC(0-12h) with beta-carotene or vitamin A were significantly and positively correlated (r = 0.714-0.918, n = 9-12, P < 0.05) with the dose in AD, AR and AA groups. The AUC(0-12h) slope ratios in AD, AR and AA rats were 0.33, 0.20 and 0.26, respectively. The beta-carotene dioxygenase activity (pmol retinal. h(-1). mg protein(-1)) was significantly higher in the AD group (14.9 +/- 2.43) compared to both AR (6.7 +/- 0.62) and AA (6.3 +/- 1.37) groups and was parallel with in vivo conversion of beta-carotene to vitamin A. The AUC(0-12h) ratio was lower in PD rats (0.13) compared to PR (0.26) and PA (0.5) groups. Similarly, the in vitro enzyme activity (pmol retinal. h(-1). mg protein(-1)) in PD rats was significantly lower (3.6 +/- 1.30) compared to PR (13.7 +/- 0.92) and PA groups (13.8 +/- 1.6). Thus the results validate the methodology and confirm the role of nutritional factors in carotene conversion to vitamin A.

Beta-carotene 15,15'-dioxygenase activity is responsive to copper and iron concentrations in rat small intestine

OBJECTIVE

Previous in vitro studies have suggested that beta-carotene 15,15'-dioxygenase is an iron-dependent enzyme. However, in vivo, it is difficult to alter iron tissue concentration by varying dietary iron because of homeostatic control. On the other hand, an interaction between iron and copper has been shown, i.e., copper-deficiency results in an increase of iron in rat liver. Therefore, we hypothesized that intestinal iron concentration could be increased by copper-deficiency. Our objective was to examine the effects of iron as affected by dietary copper on beta-carotene 15,15'-dioxygenase activity in the small intestine.

METHODS

Weanling male Sprague-Dawley rats (40 to 45g) were divided into four dietary groups: two copper-adequate groups (6.0 microg Cu/g diet) and two copper-deficient groups (0.6 microg Cu/g) combined with either normal iron (44 microg Fe/g) or high iron (87 microg Fe/g). Iron and copper concentrations were determined by atomic absorption spectrophotometry and the dioxygenase activity by reverse phase HPLC.

RESULTS

Intestinal copper concentration was significantly reduced (40%) by the consumption of the copper-deficient diets, but intestinal iron was not changed by doubling dietary iron in rats fed either copper-adequate or copper-deficient diets. However, as hypothesized, the two copper-deficient groups exhibited higher intestinal iron concentration (> or =137%, p<0.001) than the copper-adequate controls. In addition, intestinal beta-carotene 15,15'-dioxygenase activity was increased by 27% and 106%, respectively, for copper-deficient rats fed either normal or high iron diets, compared to the respective copper-adequate controls (p<0.01). The dioxygenase activity was not significantly affected by dietary iron in either copper-adequate or copper-deficient groups. Finally, the enzyme activity was positively correlated (r=0.67, p<0.0001) with iron concentration and negatively correlated (r=-0.49, p<0.01) with copper concentration in small intestine.

CONCLUSIONS

Intestinal beta-carotene 15,15'-dioxygenase may be an iron-dependent enzyme sensitive to copper status in vivo.

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

The purpose of this study was to examine effects of dietary triacylglycerols on beta-carotene 15,15'-dioxygenase (EC 1.13.11.21) activity and cellular retinol-binding protein [CRBP (II)] in rats. Six groups of eight rats (7-wk old) were fed one of the following diets: standard (STD; 2.5% soybean oil), saturated (SFA; 15% hydrogenated soybean oil), monounsaturated (MUFA; 15% olive oil), polyunsaturated (PUFA; 15% soybean oil) or clofibrate (CLF; 2.5% soybean oil + 0.2% clofibrate) for 3 wk. The dioxygenase specific activities of the intestinal homogenates in the MUFA and PUFA groups fed the high fat diets were 2.4 times that of the STD group fed a low fat diet (P < 0.01), whereas the activities of the SFA and CLF groups were not significantly different from that of the STD group. The level of CRBP (II) in the intestine of the PUFA group was 1. 3-fold that of the STD group (P < 0.05), whereas there were no significant differences among the other groups. In a second experiment, the dioxygenase activity of rat intestine was followed over 3 wk of feeding the STD and PUFA diets. After the PUFA diet was consumed for 1 d, the activity was enhanced to 2.7 times the baseline level and remained thereafter at that high level, whereas the activity of the STD group remained at the low baseline level. Thus, dietary polyunsaturated triacylglycerols enhanced both beta-carotene 15,15'-dioxygenase activity and CRBP (II) level in rat intestine. These results suggest that the dioxygenase and CRBP (II) are regulated by the same mechanism involving long-chain fatty acids and their metabolites.

Characterization of beta-carotene 15,15'-dioxygenase activity in TC7 clone of human intestinal cell line Caco-2

The purpose of this study was to identify mammalian cell line(s) which possess intrinsic enzymatic activity of beta-carotene 15, 15'-dioxygenase. This enzyme (EC1.13.11.21) converts beta-carotene to retinal (precursor of retinol and retinoic acid). To assess activity, cellular enzyme preparations were incubated with beta-carotene for 60 min; retinal formed was quantified by HPLC. Activity was not detected in IPEC-1, HepG2, HL60, Wurzburg, or parent Caco-2 cell lines. However, two subclones of Caco-2, PF11 and TC7, possessed activity (2.5 and 14.7 pmol/h.mg, respectively). Using the enzyme preparation of TC7 cells, retinal formation was linear with incubation time and protein concentration; Km and Vm values were 1.6 microM and 23.8 pmol/h.mg, respectively. In addition, when TC7 cells were maintained in serum-free medium, activity was increased 8.2-fold after 19 days of postconfluency. Finally, 48 h incubation with beta-carotene (delivered to TC7 cells in Tween 40) resulted in a 1.7-fold increase of dioxygenase activity and the appearance of vitamin A (9.3 pmol/mg protein). However, retinoic acid was not detected under our experimental conditions. In sum, the TC7 subclone of the Caco-2 cell line possesses beta-carotene 15, 15'-dioxygenase activity and thus can be useful in future investigations of human carotenoid metabolism.

Some natural and synthetic antioxidants as stabilizers of beta-carotene conversion into vitamin A

The effect of antioxidants (vitamins C and E, quercetin, probucol, butylated hydroxytoluene) on the oxidation of beta-carotene and its conversion into retinal under the influence of beta-carotene 15,15'- dioxygenase (CDO) from rat intestinal mucosa was studied. The activity of CDO decreased in the presence of oxidants. Antioxidants protected both the substrate and the enzyme. The extent of the protection depended on the antioxidant type. The combined injection of antioxidants and beta-carotene to animals completely or partially prevented the inhibition of the intestinal CDO which was caused by products of non-enzymatic oxidation of beta-carotene. Vitamins C and E, which protected the enzyme--substrate complex in vivo and in vitro, were found to be the most efficient protectors of beta-carotene conversion into retinal.

In vitro and in vivo inhibition of beta-carotene dioxygenase activity by canthaxanthin in rat intestine

beta-Carotene dioxygenase catalyzes the conversion of provitamin A carotenoids to vitamin A in mammalian tissues. Whether the enzyme can also cleave non-provitamin A carotenoids to retinoid analogs with biological activities is still unclear. We investigated (i) substrate specificities of beta-carotene dioxygenase toward provitamin A and non-provitamin A carotenoids and (ii) potential antagonistic effects of non-provitamin A carotenoids on beta-carotene conversion to vitamin A. Provitamin A substrates were 8 to 23% as active as beta-carotene. No polar metabolites were detected with canthaxanthin or zeaxanthin as substrates; these compounds efficiently inhibited the beta-carotene cleavage reaction by 71 and 40%, respectively. Kinetic studies indicated mixed inhibition for canthaxanthin (Ki = 1.6 microM) and non-competitive for zeaxanthin (Ki = 7.8 microM), suggesting that both compounds do not interact significantly with the active site of the enzyme. In vivo, dietary combinations of canthaxanthin and beta-carotene resulted in lower liver levels of both carotenoids and vitamin A and in a higher beta-carotene/vitamin A ratio as compared to groups supplemented with the compounds separately. This supports the view that canthaxanthin at high doses competes with beta-carotene for intestinal absorption and inhibits the conversion of beta-carotene to vitamin A. Thus, we suggest that although canthaxanthin is not a substrate for beta-carotene dioxygenase, it is likely to affect the activity of provitamin A carotenoids by direct interaction with the enzyme.

Enzymatic oxidation of beta-apo-8'-carotenol to beta-apo-14'-carotenal by an enzyme different from beta-carotene-15,15'-dioxygenase

Extracts of rat and rabbit intestinal mucosa were fractionated with ammonium sulfate. The precipitate contained beta-apocarotenoid-14',13'-dioxygenase (ADO) activity. beta-Apo-14'-carotenal was found to be the product of enzymatic cleavage of beta-apo-8'-carotenol. Activities of ADO and beta-carotene-15,15'-dioxygenase (CDO) were detected in the presence of thiols and were inactivated by 1,10-phenanthroline. Optimal pH values were 7.0 for ADO and 8.0 for CDO. Heating at 52 degrees C inhibited ADO by 70% and produced no effect on CDO. ADO activity was maximal in the presence of sodium cholate or 3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate (CHAPS). Sodium dodecylsulfate was required for maximal CDO activity. Proteins with ADO activity were not retained by phenyl-Sepharose CL-4B. CDO activity was eluted from columns only in the presence of detergents. The data suggest that the enzymes that catalyze the oxidative cleavage of beta-carotene to yield retinal are different from those that cleave beta-apo-8'-carotenol to yield beta-apo-14'-carotenal.

Assay of beta-carotene 15,15'-dioxygenase activity by reverse-phase high-pressure liquid chromatography

beta-Carotene 15,15'-dioxygenase catalyzes the conversion of beta-carotene into two molecules of retinal. Although this enzyme reaction is the first step in providing animals with vitamin A, there is little knowledge about its regulation in mammals. In order to facilitate studies on this enzyme, we have developed a rapid and simple assay method for the measurement of retinal formation by beta-carotene dioxygenase. All-trans-beta-carotene solubilized in aqueous solution with Tween 40 was incubated with an enzyme preparation of rat tissue at 37 degrees C for 30 min. Then, the reaction was stopped with a formaldehyde treatment followed by addition of acetonitrile. After centrifugation, the supernatant was directly subjected to high-pressure liquid chromatographic analysis of retinal. This assay method did not involve any vigorous extraction or concentration procedure. All-trans- and 13-cis-retinals, major geometric isomers found in the reaction mixture, were coeluted at 7.5 min, but they were well separated from other possible metabolites of beta-carotene such as retinoic acid, retinol, and apocarotenals. Moreover, the recovery of retinal reached more than 93% and the detection limit of standard retinal was 0.2 pmol/0.2 ml of reaction mixture. Enzyme activities of rat tissues were 694, 180, 16, 8, and approx 1 pmol retinal/mg protein/h in the intestine, liver, brain, lung, and kidney homogenates, respectively.

Rat intestinal beta-carotene dioxygenase activity is located primarily in the cytosol of mature jejunal enterocytes

The purposes of this study were to determine the location of beta-carotene dioxygenase (EC 1.13.11.21) activity within the rat gastrointestinal tract, within the villus and within enterocytes, and to identify the metabolites produced in each intestinal fraction. In Wistar female rats, maximal activity was detected in the cytosol (74-93% of the total cellular activity) of mature functional enterocytes harvested from the jejunum (67% of the intestinal activity). The specific activity, expressed in pmol of retinoids/(h x mg protein) rose from 49 +/- 3 in the stem cells to 199 +/- 12 in the mature functional cells (P < 0.05). Thus the intestinal beta-carotene cleavage activity might be regulated during the enterocyte maturation process. By using HPLC with diode array and radioactive detectors, retinal, and in the presence of NAD+, retinoic acid, were identified as the only metabolites produced. No beta-12'-, 10'-, and 8'-apo-carotenals were detected, even when various enzyme sources were tested. These results suggest that the major, if not the sole, pathway for the formation of vitamin A from beta-carotene in the rat intestine is central cleavage.

The enzymatic cleavage of beta-carotene: still controversial

The enzyme beta-carotene-15,15'-dioxygenase from guinea pig intestinal mucosa was found to cleave beta-carotene and produce 2 mol of retinal for 1 mol of beta-carotene. However, extensive evidence exists also for random (excentric) cleavage, resulting in retinoic acid and retinal, with a preponderance of apocarotenals formed as intermediates.

Stabilization and competitive inhibition of beta-carotene 15,15'-dioxygenase by carotenoids

Carotenoids stabilize beta-carotene 15,15'-dioxygenase (CDO) during the isolation of the enzyme from the rabbit small intestinal mucosa using a thiol protector and protease inhibitors and in the course of the enzyme purification. The inhibition of CDO by lycopene, lutein and astaxanthin is competitive. Carotenoids partly protect CDO from the action of both iodacetate and trypsin due to enzyme-pseudosubstrate interaction.

beta-Carotene-15,15'-dioxygenase (EC 1.13.11.21) isolation reaction mechanism and an improved assay procedure

beta-Carotene-15,15'-dioxygenase (EC 1.13.11.21; beta-carotene dioxygenase) activity in extracts from guinea-pig intestinal mucosa was assayed by supplying [15,15'-14C2]- or [15,15'-3H2] beta-carotene dissolved in Tween 80. Methods were developed to minimize the breakdown of labelled beta-carotene and beta-carotene cleavage products during the isolation procedure. Antioxidants and unlabelled carriers were added to extracting solvents and C18 Sep-Pak cartridges were used to isolate the remaining beta-carotene and retinaldehyde, which was the only cleavage product detected. The labelled material produced by the enzyme was analysed by either normal-phase TLC or reversed-phase HPLC and characterized chemically as retinaldehyde. The lack of other labelled apo-carotenals isolated in these experiments and the formation of between 1.5 and 2 mol retinaldehyde/mol beta-carotene consumed confirm the central cleavage mechanism for the enzyme's action. More beta-carotene dioxygenase activity was obtained from guinea-pig mucosa than from chicken or pig intestinal mucosa. The beta-carotene dioxygenase was obtained as a soluble enzyme which was partially purified by gel filtration and ion-exchange chromatography to a specific activity of 0.6 nmol retinaldehyde formed/mg protein per h. The formation of a lipid-protein aggregate containing the beta-carotene dioxygenase activity, which has been reported to be present in the exclusion volume of Sephadex columns, was avoided if the mucosal scrapings were homogenized in buffer at a proportion of 1:4 (w/v).