Retinal is not formed in vitro by enzymatic central cleavage of beta-carotene

Fate of carotenoids in yeasts: synthesis and cleavage

Carotenoids and their cleavage products (norisoprenoids) have excellent functional properties with diverse applications in foods, medicaments, cosmetics, etc. Carotenoids can be oxidatively cleaved through nonspecific reactions or by carotenoid cleavage oxygenases (CCOs), the product of which could further modify food flavor. This review provides comprehensive information on both carotenoid synthesis and cleavage processes with emphasis on enzyme characterization and biosynthetic pathway optimization. The use of interdisciplinary approaches of bioengineering and computer-aided experimental technology for key enzyme modification and systematic pathway design is beneficial to monitor metabolic pathways and assess pathway bottlenecks, which could efficiently lead to accumulation of carotenoids in microorganisms. The identification of CCOs spatial structures isolated from different species has made a significant contribution to the current state of knowledge. Current trends in carotenoid-related flavor modification are also discussed. In particular, we propose the carotenoid-synthesizing yeast Rhodotorula spp. for the production of food bioactive compounds. Understanding the behavior underlying the formation of norisoprenoids from carotenoids using interdisciplinary approaches may point toward other areas of investigation that could lead to better exploiting the potential use of autochthonous yeast in flavor enhancement.

The human mitochondrial enzyme BCO2 exhibits catalytic activity toward carotenoids and apocarotenoids

The enzyme β-carotene oxygenase 2 (BCO2) converts carotenoids into more polar metabolites. Studies in mammals, fish, and birds revealed that BCO2 controls carotenoid homeostasis and is involved in the pathway for vitamin A production. However, it is controversial whether BCO2 function is conserved in humans, because of a 4-amino acid long insertion caused by a splice acceptor site polymorphism. We here show that human BCO2 splice variants, BCO2a and BCO2b, are expressed as pre-proteins with mitochondrial targeting sequence (MTS). The MTS of BCO2a directed a green fluorescent reporter protein to the mitochondria when expressed in ARPE-19 cells. Removal of the MTS increased solubility of BCO2a when expressed in Escherichia coli and rendered the recombinant protein enzymatically active. The expression of the enzymatically active recombinant human BCO2a was further improved by codon optimization and its fusion with maltose-binding protein. Introduction of the 4-amino acid insertion into mouse Bco2 did not impede the chimeric enzyme's catalytic proficiency. We further showed that the chimeric BCO2 displayed broad substrate specificity and converted carotenoids into two ionones and a central C14-apocarotendial by oxidative cleavage reactions at C9,C10 and C9',C10'. Thus, our study demonstrates that human BCO2 is a catalytically competent enzyme. Consequently, information on BCO2 becomes broadly applicable in human biology with important implications for the physiology of the eyes and other tissues.

Molecular components affecting ocular carotenoid and retinoid homeostasis

The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.

Expression and Characterization of Mammalian Carotenoid Cleavage Dioxygenases

Carotenoid cleavage dioxygenases (CCDs) are nonheme iron enzymes that catalyze double bond processing of carotenoids and their apocarotenoid metabolites. Mammalian genomes encode three members of this protein family, namely BCO1, BCO2, and RPE65. Mutations and genetic polymorphism in the corresponding genes are associated with inherited blinding diseases, vitamin A deficiency, and high carotenoid plasma levels. Here we describe a method for the heterologous expression of mammalian BCO1 and BCO2 in E. coli and the biochemical characterization of these recombinant enzymes. Dissecting the enzymatic properties of CCDs will advance our knowledge of the biochemical processes that are govern by these disease-associated enzymes and may assist the design of interventions directed against these disease states.

Apocarotenoids: Emerging Roles in Mammals

Apocarotenoids are cleavage products of C40 isoprenoid pigments, named carotenoids, synthesized exclusively by plants and microorganisms. The colors of flowers and fruits and the photosynthetic process are examples of the biological properties conferred by carotenoids to these organisms. Mammals do not synthesize carotenoids but obtain them from foods of plant origin. Apocarotenoids are generated upon enzymatic and nonenzymatic cleavage of the parent compounds both in plants and in the tissues of mammals that have ingested carotenoid-containing foods. The best-characterized apocarotenoids are retinoids (vitamin A and its derivatives), generated upon central oxidative cleavage of provitamin A carotenoids, mainly β-carotene. In addition to the well-known biological actions of vitamin A, it is becoming apparent that nonretinoid apocarotenoids also have the potential to regulate a broad spectrum of critical cellular functions, thus influencing mammalian health. This review discusses the current knowledge about the generation and biological activities of nonretinoid apocarotenoids in mammals.

Evidence for compartmentalization of mammalian carotenoid metabolism

The critical role of retinoids (vitamin A and its derivatives) for vision, reproduction, and survival has been well established. Vitamin A is produced from dietary carotenoids such as β-carotene by centric cleavage via the enzyme BCO1. The biochemical and molecular identification of a second structurally related β-carotene metabolizing enzyme, BCO2, has led to a prolonged debate about its relevance in vitamin A biology. While BCO1 cleaves provitamin A carotenoids, BCO2 is more promiscuous and also metabolizes nonprovitamin A carotenoids such as zeaxanthin into long-chain apo-carotenoids. Herein we demonstrate, in cell lines, that human BCO2 is associated with the inner mitochondrial membrane. Different human BCO2 isoforms possess cleavable N-terminal leader sequences critical for mitochondrial import. Subfractionation of murine hepatic mitochondria confirmed the localization of BCO2 to the inner mitochondrial membrane. Studies in BCO2-knockout mice revealed that zeaxanthin accumulates in the inner mitochondrial membrane; in contrast, β-carotene is retained predominantly in the cytoplasm. Thus, we provide evidence for a compartmentalization of carotenoid metabolism that prevents competition between BCO1 and BCO2 for the provitamin and the production of noncanonical β-carotene metabolites.

Substrate specificity of purified recombinant human β-carotene 15,15'-oxygenase (BCO1)

Humans cannot synthesize vitamin A and thus must obtain it from their diet. β-Carotene 15,15'-oxygenase (BCO1) catalyzes the oxidative cleavage of provitamin A carotenoids at the central 15-15' double bond to yield retinal (vitamin A). In this work, we quantitatively describe the substrate specificity of purified recombinant human BCO1 in terms of catalytic efficiency values (kcat/Km). The full-length open reading frame of human BCO1 was cloned into the pET-28b expression vector with a C-terminal polyhistidine tag, and the protein was expressed in the Escherichia coli strain BL21-Gold(DE3). The enzyme was purified using cobalt ion affinity chromatography. The purified enzyme preparation catalyzed the oxidative cleavage of β-carotene with a Vmax = 197.2 nmol retinal/mg BCO1 × h, Km = 17.2 μM and catalytic efficiency kcat/Km = 6098 M(-1) min(-1). The enzyme also catalyzed the oxidative cleavage of α-carotene, β-cryptoxanthin, and β-apo-8'-carotenal to yield retinal. The catalytic efficiency values of these substrates are lower than that of β-carotene. Surprisingly, BCO1 catalyzed the oxidative cleavage of lycopene to yield acycloretinal with a catalytic efficiency similar to that of β-carotene. The shorter β-apocarotenals (β-apo-10'-carotenal, β-apo-12'-carotenal, β-apo-14'-carotenal) do not show Michaelis-Menten behavior under the conditions tested. We did not detect any activity with lutein, zeaxanthin, and 9-cis-β-carotene. Our results show that BCO1 favors full-length provitamin A carotenoids as substrates, with the notable exception of lycopene. Lycopene has previously been reported to be unreactive with BCO1, and our findings warrant a fresh look at acycloretinal and its alcohol and acid forms as metabolites of lycopene in future studies.

The formation, occurrence, and function of β-apocarotenoids: β-carotene metabolites that may modulate nuclear receptor signaling

β-Carotene is the major dietary source of provitamin A. Central cleavage of β-carotene yields 2 molecules of retinal followed by further oxidation to retinoic acid. Eccentric cleavage of β-carotene occurs at double bonds other than the central double bond, and the products of these reactions are β-apocarotenals and β-apocarotenones. We reviewed recent developments in 3 areas: 1): the enzymatic production of β-apocarotenoids in higher animals; 2) the occurrence of β-apocarotenoids in foods and animal tissues; and 3) the biological activity of β-apocarotenoids, particularly on retinoid receptors. HPLC-mass spectrometry techniques were developed to quantify these compounds in mouse serum and tissues and in foods. β-Apo-10'- and -12'-carotenals were detected in mouse serum and liver. β-Apo-8'-, β-apo-10'-, β-apo-12'-, and β-apo-14'-carotenals and β-apo-13-carotenone were detected in orange-fleshed melons. Transactivation assays were performed to see whether apocarotenoids activate or antagonize retinoid X receptor (RXR) α. Reporter gene constructs and retinoid receptor (RXRα) were transfected into cells, which were used to perform quantitative assays for the activation of this ligand-dependent transcription factor. None of the β-apocarotenoids significantly activated RXRα. However, β-apo-13-carotenone antagonized the 9-cis-retinoic acid activation of RXRα. Competitive radioligand binding assays showed that this antagonist competes directly with the agonist for binding to purified receptor, a finding confirmed by molecular modeling studies. These findings suggest that a possible biological function of β-apocarotenoids is their ability to interfere with nuclear receptor signaling. Recent work showed that β-apo-13-carotenone is also a high-affinity antagonist of all 3 retinoic acid receptors (RARα, RARβ, and RARγ).

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.

A supramolecular enzyme model catalyzing the central cleavage of carotenoids

Several bis-beta-cyclodextrin porphyrins have been prepared as supramolecular receptors of carotenoids. The binding constants of carotenoids to receptors were determined by quenching the fluorescence of the porphyrins on hydrophobic binding of carotenoids within the cavities of cyclodextrins. K(a)=8.3 x 10(6) M(-1) was calculated for binding of beta,beta-carotene to bis-beta-cyclodextrin Zn porphyrin. The corresponding Ru complex catalyzes the central cleavage of carotenoids in the presence of tert-butyl hydroperoxide in a biphasic system.

The Reaction Mechanism of the Enzyme-Catalyzed Central Cleavage of β-Carotene to Retinal

Seeing things as they really are: The enzyme catalyzing the central cleavage of β-carotene (1) to retinal (2) is not, as previously thought, a dioxygenase. Incubation of the substrate analogue α-carotene in the presence of highly enriched ¹⁷ O₂ and H₂¹⁸ O revealed a monooxygenase mechanism.

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).

Review: absorption and metabolism of beta-carotene

To better understand the potential function of beta-carotene (beta-C) in the prevention of cancer, greater knowledge of beta-C metabolism and a suitable animal model to mimic human beta-C metabolism are necessary. The small intestinal mucosa contains beta-C cleavage enzyme(s), thereby playing an important role in both the provitamin A activity and anti-cancer properties of beta-C. The ability of the ferret (Mustela putorius furo) to absorb intact beta-C makes it an appropriate model for studying human beta-C absorption. This article reviews the absorption and cleavage mechanisms of beta-C in both the human and the ferret. The biosynthesis of retinoic acid (RA) from beta-C via central and eccentric cleavage pathways is reviewed. The possible significance of the conversion of beta-C to RA as an anticancer mechanism is discussed.

Retinoid dynamics in chicken eye during pre- and postnatal development

Changes in the steady state level of retinols, retinaldehydes and retinyl esters in the trans and 11-cis forms and trans retinoic acid were measured in whole chicken eye during development from day 6 in ovo to day 3 post-hatch. These retinoids, quantified by different HPLC systems, were detected in this time sequence: trans-retinol and trans-retinyl esters in the first week in ovo, 11-cis-retinol in the second week. The highest level of 11-cis-retinaldehyde and 11-cis-retinyl esters was reached at the end of development in ovo; however, their levels increased further after hatching. The retinoic acid level decreased at the end of the first week, rising again at the end of the second week. The enzyme activities involved in the metabolism of these retinoids-acyl-CoA: retinol acyltransferase, trans-retinol dehydrogenase, 11-cis-retinol dehydrogenase, trans-retinyl ester hydrolase and trans: 11-cis-retinol isomerase were also estimated and they were detectable already in the first week of development in ovo. At day 6 of the biosynthesis of retinoic acid by the retinaldehyde dehydrogenase activity from retina cytosol was also shown.

Differential depletion of carotenoids and tocopherol in liver disease

Carotenoids and tocopherols are major natural protective agents against free radical-mediated liver damage, but their levels in diseased liver are largely uncharted. Therefore we carried out measurements with high-pressure liquid chromatography of alpha- and beta-carotene, lycopene, cryptoxanthin, lutein and zeaxanthin, total retinoids and alpha- and gamma-tocopherol. Liver tissue was obtained from percutaneous needle biopsies, livers of transplant recipients or a donor bank. Compared with controls (transplant donors; n = 13), levels of all carotenoids and retinoids were extremely low at all stages of liver disease. Patients with alcoholic cirrhosis (n = 11) had 20- and 25-fold decreases of levels of lycopene (p < 0.001) and alpha- and beta-carotene (p < 0.005), respectively. Even in subjects with less severe alcoholic liver disease (steatosis, perivenular fibrosis, portal fibrosis; n = 14) and in patients with nonalcoholic liver disease (n = 13), levels were four to six times lower than those in normal subjects. By contrast, levels of alpha-tocopherol were decreased significantly only in patients with cirrhosis, who displayed a threefold reduction. In the serum of most patients, lycopene and tocopherol concentrations were not depressed, whereas one third of alpha- and beta-carotene levels were low, probably reflecting poor dietary intake. A significant correlation was observed between serum and liver alpha- and beta-carotene levels (p < 0.0001; r = 0.715). However, of the patients with extremely low liver alpha- and beta-carotene concentrations, more than half had blood levels in the normal range, suggesting that liver disease interferes with the uptake, excretion or, perhaps, metabolism of alpha- and beta-carotene. In the cirrhotic livers of eight candidates for liver transplantation, the ratios of alpha- and beta-carotene to total retinoids and of beta-carotene to retinoids were much higher than those in normal livers, suggesting some impairment in the conversion of alpha- and beta-carotene to retinoids. In most cases, even with high ratios, absolute levels of hepatic alpha- and beta-carotene and retinoids were severely depressed. We concluded that, even in the presence of normal serum levels alpha- and beta-carotene, tocopherol and lycopene, patients with cirrhosis have extremely low hepatic levels.

Interaction of ethanol with beta-carotene: delayed blood clearance and enhanced hepatotoxicity

Because we had found that ethanol interacts with retinol, we investigated whether it also affects its precursor, beta-carotene. In 14 baboons fed ethanol (50% of total energy) for 2 to 5 yr with a standard amount of beta-carotene (one 200-gm carrot/day), levels of beta-carotene were much higher than in controls fed isocaloric carbohydrate, both in plasma (122.5 +/- 30.9 nmol/dl vs. 6.3 +/- 1.4 nmol/dl; p less than 0.005) and in liver (7.9 +/- 1.1 nmol/gm vs. 1.8 +/- 0.5 nmol/gm; p less than 0.001). Even 20 days after withdrawal of the carrots, plasma beta-carotene levels remained higher in alcohol-fed baboons than in controls (10.1 +/- 3.8 nmol/dl vs. less than 0.1 nmol/dl). Next, the diet was supplemented with beta-carotene beadlets: in four pairs of baboons given a low dose of beta-carotene (3 mg/1,000 kcal), plasma levels were significantly higher in alcohol-fed animals than in controls, even when expressed per cholesterol (although the latter increased with alcohol intake). Seven pairs of animals were given a higher dose (30 mg/1,000 kcal) of beta-carotene for 1 mo, followed, in four pairs, by 45 mg for another month. On cessation of beta-carotene treatment, plasma levels decreased more slowly in the alcohol-fed baboons than in the controls. Percutaneous liver biopsy specimens revealed that liver concentrations of beta-carotene correlated with plasma levels but were higher in the alcohol-fed baboons than in the control baboons, whereas the beta-carotene-induced increase in liver retinoids was lower (p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Retinoic acid can be produced from excentric cleavage of beta-carotene in human intestinal mucosa

The hypothesis that retinoic acid (RA) is produced from the excentric cleavage of beta-carotene was tested in human intestinal homogenates in vitro. Significant amounts of RA were identified by HPLC and derivatization after incubation of intestinal mucosal homogenates with retinal, beta-carotene, or beta-apocarotenals at 37 degrees C for 60 min. RA formation was inhibited, in a dose-dependent fashion, when retinal was incubated in the presence of 0.1-3.0 mM citral (3,7-dimethyl-2,6-octadienal) under identical experimental conditions. The formation of RA from both beta-carotene and beta-apocarotenals was dose and time dependent and RA was the major metabolite of both beta-apo-8'-carotenal and beta-apo-12'-carotenal after the incubation. However, citral (0.1 to 4 mM) did not inhibit the formation of beta-apocarotenals and RA from 2 microM beta-carotene (P greater than 0.05), which proves the existence of an excentric cleavage mechanism for beta-carotene conversion into retinoids. Furthermore, RA formation from both beta-apo-8'-carotenal and beta-apo-12'-carotenal in human intestinal homogenate occurred in the presence of citral, which demonstrates that RA can be produced from excentric cleavage of beta-carotene via a series of beta-apocarotenals as intermediates.

Antimutagenic and anticarcinogenic effects of carotenoids and dietary palm oil

Four carotenoids, canthaxanthin, beta-carotene, 8H-apo-beta-carotenal, and 8'-apo-beta-carotene methylester were tested for their ability to suppress the mutagenicity of 1-methyl-3-nitro-1-nitrosoguanidine and benzo[a]pyrene (BP) in Salmonella typhimurium tester strain TA 100. The anticarcinogenic efficacy of the four carotenoids was further assessed in the BP-induced forestomach tumor model in female Swiss mice. The effect of dietary palm oil was also examined in BP-induced neoplasia in the female Haffkine Swiss mouse strain. Canthaxanthin, beta-carotene, 8'-apo-beta-carotenal, and 8'-apo-beta-carotene methylester showed a dose-dependent decrease in the mutagenicity compared with 1-methyl-3-nitro-1-nitrosoguanidine and BP in strain TA 100. In the BP-induced forestomach tumor model, all four carotenoids showed a similar significant anticarcinogenic effect. Dietary administration of palm oil showed a dose-dependent antitumor activity in the animals. Our results show that the intrinsic antimutagenic and anticarcinogenic properties of the carotenoids are not significantly influenced by their conversion to vitamin A.

Determination of several retinoids, carotenoids and E vitamers by high-performance liquid chromatography. Application to plasma and tissues of rats fed a diet rich in either beta-carotene or canthaxanthin

A method, using two different systems, is described for the high-performance liquid chromatographic analysis of retinol, retinal, retinoic acid, retinyl acetate, retinyl palmitate, alpha-, beta- and gamma-carotene, beta-apo-6'-, beta-apo-8', beta-apo-10'- and beta-apo-12'-carotenal, ethyl beta-apo-8'-carotenoate, alpha-tocopherol and alpha-tocopheryl acetate. The first system consists of a laboratory-packed Hypersil-ODS 3-microns column and a mobile phase of acetonitrile-methylene chloride-methanol-water (70:10:15:5, v/v). The second system consists of a laboratory-packed Hypersil-ODS 3-microns column and a mobile phase of acetonitrile-methylene chloride-methanol-water (70:10:15:5, v/v). The second system consists of a laboratory-packed Nucleosil C18 3-microns column and a mobile phase of acetonitrile-0.1 M ammonium acetate (80:20, v/v). The detection limits in standard solutions were 10 ng/ml for retinoids and carotenoids and 60 ng/ml for the E vitamers. Analysis of the tissues and plasma of rats, after 2 weeks on a diet supplemented with either beta-carotene or canthaxanthin (both 2 mg/g), led to the conclusion that the rats were able both to transport and store beta-carotene and canthaxanthin and to convert beta-carotene to retinol. Incubation of cytosol preparations from the mucosa of the small intestine of rat with 1 microgram of beta-carotene resulted in the formation of 10-20 ng of retinal within 1 h.

Enzymatic conversion of beta-carotene into beta-apo-carotenals and retinoids by human, monkey, ferret, and rat tissues

Whether the conversion of beta-carotene into retinoids involves an enzymatic excentric cleavage mechanism was examined in vitro with homogenates prepared from human, monkey, ferret, and rat tissue. Using high-performance liquid chromatography, significant amounts of beta-apo-12'-, -10'-, and -8'-carotenals, retinal, and retinoic acid were found after incubation of intestinal homogenates of the four different species with beta-carotene in the presence of NAD+ and dithiothreitol. No beta-apo-carotenals or retinoids were detected in control incubations done without tissue homogenates. The production of beta-apo-carotenals was linear for 30 min and up to tissue protein concentrations of 1.5 mg/ml. The rate of formation of beta-apo-carotenals from 2 microM beta-carotene was about 7- to 14-fold higher than the rate of retinoid formation in intestinal homogenates, and the rate of beta-apo-carotenal production was fivefold greater in primate intestine vs rat or ferret intestine (P less than 0.05). The amounts of beta-apo-carotenals and retinoids formed were markedly reduced when NAD+ was replaced by NADH, or when dithiothreitol and cofactors were deleted from the incubation mixture. Both beta-apo-carotenal and retinoid production from beta-carotene were inhibited completely by adding disulfiram, an inhibitor of sulfhydryl-containing enzymes. Incubation of beta-carotene with liver, kidney, lung, and fat homogenates from each species also resulted in the appearance of beta-apo-carotenals and retinoids. The identification of three unknown compounds which might be excentric cleavage products is ongoing. These data support the existence of an excentric cleavage mechanism for beta-carotene conversion.

Enzymatic conversion of all-trans-beta-carotene to retinal by a cytosolic enzyme from rabbit and rat intestinal mucosa

Enzymatic conversion of all-trans-beta-carotene to retinal by a partially purified enzyme from rabbit and rat intestinal mucosa was demonstrated. The enzymatic product was characterized based on the following evidence: (i) The product gave rise to its O-ethyloxime by treatment with O-ethylhydroxylamine with an absorption maximum at 363 nm in ethanol characteristic of authentic retinal O-ethyloxime. High-pressure liquid chromatography (HPLC) of this derivative yielded a sharp peak with a retention time of 7.99 min corresponding to the authentic compound. The enzyme blank and boiled enzyme blank failed to show any significant HPLC peaks corresponding to retinal O-ethyloxime, retinal, or retinol. (ii) The mass spectrum of the O-ethyloxime of the enzymatic product was identical to that of authentic retinal O-ethyloxime (m/z 327: 45%, M+. and m/z 282: 100%, M--ethoxy). (iii) The specific activity of the enzymatically formed [14C]retinal O-ethyloxime remained constant even after repeated crystallization. (iv) The enzymatic product exhibited an absorption maximum at 370 nm in light petroleum characteristic of authentic retinal. Furthermore, it was reduced by horse liver alcohol dehydrogenase to retinol with an absorption maximum at 326 nm in light petroleum. This retinol was enzymatically esterified to retinyl palmitate by rat pancreatic esterase with a retention time of 10 min on HPLC corresponding to authentic retinyl palmitate. Thus, the enzymatic product of beta-carotene cleavage by the partially purified intestinal enzyme was unequivocally confirmed to be retinal.