Excentral cleavage of beta-carotene in vivo in a healthy man

Analysis of the current vitamin A terminology and dietary regulations from vitamin A1 to vitamin A5

Dietary recommendations on vitamin intake for human food fortification concerning vitamin A in various countries, larger economic zones and international organizations are mainly based on the Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) "Codex Alimentarius standards". The general vitamin A terminology is based on regulations of the International Union of Pure and Applied Chemistry (IUPAC) that are used to describe the involved derivatives. These regulations and terminology were set up in the middle of the last century. Starting with the decade of the 80ies in the 20th century a large improvement of molecular biological methodologies, background physiological mechanisms as well as analytical techniques contributed to a large diversification of this simply claimed vitamin A terminology. Unfortunately, the following terminology and governmental regulations for food fortification are imprecise and non-harmonized. In this article we tried to unravel this terminology for updating terminology, nutritional suggestions and governmental regulations for vitamin A, which are currently based on various uncertainties. According to the current regulations, the newly found vitamin A5/X can be included in the current vitamin A terminology as "vitamin A5" or alternatively or even in parallel as a new vitamin A-independent terminology as "vitamin X". Based on the detailed knowledge of research from the early beginning of general vitamin A pathway identification towards detailed research of the last decades the commonly used and simplified term vitamin A with relevance for governmental recommendations on vitamin intake and food fortification advice was now more correctly sub-categorized to further vitamin A1, and A5 sub-categories with vitamin A1-alcohol as retinol, vitamin A2-alcohol as 3,4-didehydroretinol and vitamin A5-alcohol as 9-cis-13,14-dihydroretinol as their mainly relevant vitamin forms present in the human organism. Here we suggest and advise how the vitamin A terminology and further governmental regulations should be organized depending on a successful unraveling of the organization of the current vitamin A terminology.

Carotenoid metabolites, their tissue and blood concentrations in humans and further bioactivity via retinoid receptor-mediated signalling

Many epidemiological studies have emphasised the relation between carotenoid dietary intake and their circulating concentrations and beneficial health effects, such as lower risk of cardiometabolic diseases and cancer. However, there is dispute as to whether the attributed health benefits are due to native carotenoids or whether they are instead induced by their metabolites. Several categories of metabolites have been reported, most notably involving (a) modifications at the cyclohexenyl ring or the polyene chain, such as epoxides and geometric isomers, (b) excentric cleavage metabolites with alcohol-, aldehyde- or carboxylic acid-functional groups or (c) centric cleaved metabolites with additional hydroxyl, aldehyde or carboxyl functionalities, not counting their potential phase-II glucuronidated / sulphated derivatives. Of special interest are the apo-carotenoids, which originate in the intestine and other tissues from carotenoid cleavage by β-carotene oxygenases 1/2 in a symmetrical / non-symmetrical fashion. These are more water soluble and more electrophilic and, therefore, putative candidates for interactions with transcription factors such as NF-kB and Nrf2, as well as ligands for RAR-RXR nuclear receptor interactions. In this review, we discuss in vivo detected apo-carotenoids, their reported tissue concentrations, and potential associated health effects, focusing exclusively on the human situation and based on quantified / semi-quantified carotenoid metabolites proven to be present in humans.

Uptake and metabolism of β-apo-8'-carotenal, β-apo-10'-carotenal, and β-apo-13-carotenone in Caco-2 cells

β-Apocarotenoids are eccentric cleavage products of carotenoids formed by chemical and enzymatic oxidations. They occur in foods containing carotenoids and thus might be directly absorbed from the diet. However, there is limited information about their intestinal absorption. The present research examined the kinetics of uptake and metabolism of β-apocarotenoids. Caco-2 cells were grown on 6-well plastic plates until a differentiated cell monolayer was achieved. β-Apocarotenoids were prepared in Tween 40 micelles, delivered to differentiated cells in serum-free medium, and incubated at 37°C for up to 8 h. There was rapid uptake of β-apo-8'-carotenal into cells, and β-apo-8'-carotenal was largely converted to β-apo-8'-carotenoic acid and a minor metabolite that we identified as 5,6-epoxy-β-apo-8'-carotenol. There was also rapid uptake of β-apo-10'-carotenal into cells, and β-apo-10'-carotenal was converted into a major metabolite identified as 5,6-epoxy-β-apo-10'-carotenol and a minor metabolite that is likely a dihydro-β-apo-10'-carotenol. Finally, there was rapid cellular uptake of β-apo-13-carotenone, and this compound was extensively degraded. These results suggest that dietary β-apocarotenals are extensively metabolized in intestinal cells via pathways similar to the metabolism of retinal. Thus, they are likely not absorbed directly from the diet.

Identification of Enzymatic Cleavage Products of -Carotene-Rich Extracts of Kale and Biofortified Maize

Provitamin A carotenoids in plant foods provide more than 80% of vitamin A intake for people in developing countries. Therefore, the conversion efficiency of β-carotene to vitamin A is important, as it determines the effectiveness of plant foods as sources of vitamin A in humans. The objective of this study was to determine the effect of plant food antioxidants such as α-tocopherol, γ-tocopherol, α-tocotrienol, γ-tocotrienol and total γ-oryzanol on the cleavage of β-carotene in vitro. Rat intestinal mucosa post mitochondrial fractions were incubated with β-carotene-rich extracts of kale and biofortified maize for an hour at 37°C. Rat intestinal mucosa post mitochondrial fractions were also incubated with β-carotene in the presence of either α-tocopherol, γ-tocopherol, α-tocotrienol, γ-tocotrienol or γ-oryzanol for 60 min at 37°C. The β-carotene cleavage products were extracted and analyzed by an HPLC equipped with a C18 column at 340nm and 450nm. When β-carotene alone was incubated without intestinal mucosa homogenate (control), no cleavage products were detected. When β-carotene alone was incubated with intestinal mucosa homogenate, β-apo-13-carotenone, β-apo-14-carotenal, retinal, retinol and retinoic acid were formed. However, incubation of β-carotene with either α-tocopherol, γ-tocopherol or α-tocotrienol resulted in a 10 fold inhibition of β-apo-14-carotenal and β-apo-13-carotenone formation. Antioxidant rich biofortified maize extract incubated with postmitochondrial fraction produced less β-apo-13-carotenone compared to the kale extract. These results suggest that antioxidants inhibit the cleavage of β-carotene and the formation of excentric cleavage products (β-apo-13-carotenone, β-apo-14-carotenal).

Limited appearance of apocarotenoids is observed in plasma after consumption of tomato juices: a randomized human clinical trial

Background

Nonvitamin A apocarotenoids occur in foods. Some function as retinoic acid receptor antagonists in vitro, though it is unclear if apocarotenoids are absorbed or accumulate to levels needed to elicit biological function.

Objective

The aim of this study was to quantify carotenoids and apocarotenoids (β-apo-8'-, -10'-, -12'-, and -14'-carotenal, apo-6'-, -8'-, -10'-, -12'-, and -14'-lycopenal, retinal, acycloretinal, β-apo-13-carotenone, and apo-13-lycopenone) in human plasma after controlled consumption of carotenoid-rich tomato juices.

Design

Healthy subjects (n = 35) consumed a low-carotenoid diet for 2 wk, then consumed 360 mL of high-β-carotene tomato juice (30.4 mg of β-carotene, 34.5 μg total β-apocarotenoids/d), high-lycopene tomato juice (42.5 mg of lycopene, 119.2 μg total apolycopenoids/d), or a carotenoid-free control (cucumber juice) per day for 4 wk. Plasma was sampled at baseline (after washout) and after 2 and 4 wk, and analyzed for carotenoids and apocarotenoids using high-pressure liquid chromatography (HPLC) and HPLC-tandem mass spectrometry, respectively. The methods used to analyze the apocarotenoids had limits of detection of ∼ 100 pmol/L.

Results

Apocarotenoids are present in tomato juices at 0.1-0.5% of the parent carotenoids. Plasma lycopene and β-carotene increased (P < 0.001) after consuming high-lycopene and β-carotene tomato juices, respectively, while retinol remained unchanged. β-Apo-13-carotenone was found in the blood of all subjects at every visit, although elevated (P < 0.001) after consuming β-carotene tomato juice for 4 wk (1.01 ± 0.27 nmol/L) compared with both baseline (0.37 ± 0.17 nmol/L) and control (0.46 ± 0.11 nmol/L). Apo-6'-lycopenal was detected or quantifiable in 29 subjects, while β-apo-10'- and 12'-carotenal were detected in 6 and 2 subjects, respectively. No other apolycopenoids or apocarotenoids were detected.

Conclusions

β-Apo-13-carotenone was the only apocarotenoid that was quantifiable in all subjects, and was elevated in those consuming high-β-carotene tomato juice. Levels were similar to previous reports of all-trans-retinoic acid. Other apocarotenoids are either poorly absorbed or rapidly metabolized or cleared, and so are absent or limited in blood. β-Apo-13-carotenone may form from vitamin A and its presence warrants further investigation. This trial was registered at clinicaltrials.gov as NCT02550483.

Production of asymmetric oxidative metabolites of [13C]-β-carotene during digestion in the gastrointestinal lumen of healthy men

Background

Asymmetric β-apo-carotenoids (nonvitamin A-active metabolites) of provitamin A carotenoids have been observed in humans, but no study has investigated their formation during digestion.

Objective

The aim of this study was to follow the formation and absorption of asymmetric β-apo-carotenoids during digestion.

Design

Healthy men were intragastrically and intraduodenally intubated, and randomly assigned to consume a lipid-rich control meal (n = 3) or a lipid-rich test meal containing 20 mg [13C-10]-β-carotene (n = 7). Digesta samples were collected over 5 h, and blood collected over 7 h. The triglyceride-rich lipoprotein (TRL) fractions of plasma were also isolated. Lipophilic extracts of digesta, plasma, and TRL were analyzed via a high-performance liquid chromatography-tandem mass spectrometry method developed to identify [13C]-labeled β-apo-carotenals/carotenone, [13C]-β-apo-carotenols, and [13C]-β-apo-carotenoic acids.

Results

Relative to [13C]-β-carotene, [13C]-β-apo-carotenal levels remained ∼3 orders of magnitude lower throughout digestion (no [13C]-β-apo-carotenols, or [13C]-β-apo-carotenoic acids were observed). A mixed model determined relative influence of digesta type and time on digesta metabolite level. Increasing time significantly increased the model levels of digesta [13C]-β-apo-10',12',14',15-carotenal and [13C]-β-apo-13-carotenone (P < 0.05) and trended toward decreased [13C]-β-apo-8'-carotenal (P = 0.0876). Gastric digesta were associated with a significantly higher level of [13C]-β-apo-8'-carotenal (P = 0.0289), and lower levels of [13C]-β-apo-12',14',15-carotenal (P < 0.05), relative to duodenal digesta. Anticipated retinoids, but no asymmetric [13C]-β-apo-carotenals, [13C]-β-apo-carotenols, or [13C]-β-apo-carotenoic acids, were observed in the blood or TRL samples.

Conclusions

β-Carotene appears to be robust to digestion, with minor amounts of β-apo-carotenals/carotenone formed. Absence of asymmetric [13C]-β-apo-carotenals in plasma and TRL suggests lack of absorption, levels below the limit of detection, lack of stability, or further conversion during the digestive process to as-yet unidentified products. Lack of asymmetric [13C]-β-apo-carotenals in plasma also suggests a lack of postprandial intestinal BCO2 activity in healthy humans. This trial was registered at clinicaltrials.gov as NCT03492593.

Biomarkers of carotenoid bioavailability

The use of biomarkers constitutes an essential tool to assess the bioavailability of carotenoids in humans. The present article aims to review several methodological, host-related and modulating factors relevant on assessing and interpreting carotenoid bioavailability. Markers for carotenoid bioavailability can be broadly divided into direct, biochemical or "analytical" markers and indirect, physiological or "functional" indicators. Analytical markers usually refer to biochemical indicators of intake and/or status (short and long term exposure) while functional measures may be interpreted in terms of cumulative exposure, biological effect (bioactivity) or modification of risk factors. Both types of markers display advantages and limitations but, in general, a relationship exists among the type of marker, the biological specimen needed and the time required for a change. Humans may absorb a wide range of carotenes and xanthophylls and many of them may be found in serum and tissues. However, under physiological conditions, the several classes of dietary carotenoids may behave unequally leading to a different systemic profile and, moreover, they can be selectively accumulated at target tissues. In addition, some carotenoids may be chemically and enzymatically modified generating different oxidative metabolites and apocarotenoids. Quantitatively, the biological response upon carotenoid intervention (assessed by analytical and functional markers) is highly variable but the use of large doses and long-term protocols may lead to saturation effects and the loss of linearity in the response. Also, despite carotenoid exposition is considered to be safe, markers of overexposure include clinical signs (i.e. carotenodermia, corneal rings and retinopathy) and biochemical indicators (hypercarotenemia, xanthophyll esters). Overall, both host-related and methodological factors may influence analytical and functional markers to assess carotenoid bioavailability although the different subclasses of carotenoids may not be equally affected.

Meeting the Vitamin A Requirement: The Efficacy and Importance of β-Carotene in Animal Species

Vitamin A is essential for life in all vertebrate animals. Vitamin A requirement can be met from dietary preformed vitamin A or provitamin A carotenoids, the most important of which is β-carotene. The metabolism of β-carotene, including its intestinal absorption, accumulation in tissues, and conversion to vitamin A, varies widely across animal species and determines the role that β-carotene plays in meeting vitamin A requirement. This review begins with a brief discussion of vitamin A, with an emphasis on species differences in metabolism. A more detailed discussion of β-carotene follows, with a focus on factors impacting bioavailability and its conversion to vitamin A. Finally, the literature on how animals utilize β-carotene is reviewed individually for several species and classes of animals. We conclude that β-carotene conversion to vitamin A is variable and dependent on a number of factors, which are important to consider in the formulation and assessment of diets. Omnivores and herbivores are more efficient at converting β-carotene to vitamin A than carnivores. Absorption and accumulation of β-carotene in tissues vary with species and are poorly understood. More comparative and mechanistic studies are required in this area to improve the understanding of β-carotene metabolism.

Absorption of Carotenoids and Mechanisms Involved in Their Health-Related Properties

Carotenoids participate in the normal metabolism and function of the human body. They are involved in the prevention of several diseases, especially those related to the inflammation syndrome. Their main mechanisms of action are associated to their potent antioxidant activity and capacity to regulate the expression of specific genes and proteins. Recent findings suggest that carotenoid metabolites may explain several processes where the participation of their parent carotenoids was unclear. The health benefits of carotenoids strongly depend on their absorption and transformation during gastrointestinal digestion. The estimation of the 'bioaccessibility' of carotenoids through in vitro models have made possible the evaluation of the effect of a large number of factors on key stages of carotenoid digestion and intestinal absorption. The bioaccessibility of these compounds allows us to have a clear idea of their potential bioavailability, a term that implicitly involves the biological activity of these compounds.

β-Apo-13-carotenone regulates retinoid X receptor transcriptional activity through tetramerization of the receptor

Retinoid X receptor (RXRα) is activated by 9-cis-retinoic acid (9cRA) and regulates transcription as a homodimer or as a heterodimer with other nuclear receptors. We have previously demonstrated that β-apo-13-carotenone, an eccentric cleavage product of β-carotene, antagonizes the activation of RXRα by 9cRA in mammalian cells overexpressing this receptor. However, the molecular mechanism of β-apo-13-carotenone's modulation on the transcriptional activity of RXRα is not understood and is the subject of this report. We performed transactivation assays using full-length RXRα and reporter gene constructs (RXRE-Luc) transfected into COS-7 cells, and luciferase activity was examined. β-Apo-13-carotenone was compared with the RXRα antagonist UVI3003. The results showed that both β-apo-13-carotenone and UVI3003 shifted the dose-dependent RXRα activation by 9cRA. In contrast, the results of assays using a hybrid Gal4-DBD:RXRαLBD receptor reporter cell assay that detects 9cRA-induced coactivator binding to the ligand binding domain demonstrated that UVI3003 significantly inhibited 9cRA-induced coactivator binding to RXRαLBD, but β-apo-13-carotenone did not. However, both β-apo-13-carotenone and UVI3003 inhibited 9-cRA induction of caspase 9 gene expression in the mammary carcinoma cell line MCF-7. To resolve this apparent contradiction, we investigated the effect of β-apo-13-carotenone on the oligomeric state of purified recombinant RXRαLBD. β-Apo-13-carotenone induces tetramerization of the RXRαLBD, although UVI3003 had no effect on the oligomeric state. These observations suggest that β-apo-13-carotenone regulates RXRα transcriptional activity by inducing the formation of the "transcriptionally silent" RXRα tetramer.

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.

Bioavailability of iron, zinc, and provitamin A carotenoids in biofortified staple crops

International research efforts, including those funded by HarvestPlus, a Challenge Program of the Consultative Group on International Agricultural Research (CGIAR), are focusing on conventional plant breeding to biofortify staple crops such as maize, rice, cassava, beans, wheat, sweet potatoes, and pearl millet to increase the concentrations of micronutrients that are commonly deficient in specific population groups of developing countries. The bioavailability of micronutrients in unfortified staple crops in developing regions is typically low, which raises questions about the efficacy of these crops to improve population micronutrient status. This review of recent studies of biofortified crops aims to assess the micronutrient bioavailability of biofortified staple crops in order to derive lessons that may help direct plant breeding and to infer the potential efficacy of food-based nutrition interventions. Although reducing the amounts of antinutrients and the conduction of food processing generally increases the bioavailability of micronutrients, antinutrients still possess important benefits, and food processing results in micronutrient loss. In general, biofortified foods with relatively higher micronutrient density have higher total absorption rates than nonbiofortified varieties. Thus, evidence supports the focus on efforts to breed plants with increased micronutrient concentrations in order to decrease the influence of inhibitors and to offset losses from processing.

Validation and application of sub-2 μm core-shell UHPLC-UV-ESI-Orbitrap MS for identification and quantification of β-carotene and selected cleavage products with preceding solid-phase extraction

A validated ultrahigh-performance liquid chromatography method using 1.7 μm core–shell particles is presented for the identification and quantification of β-carotene (BC) and related cleavage products (CPs) in primary cell culture media. Besides BC, apo-4′-, apo-8′-, apo-10′-, and apo-12′-carotenals, as well as 5,6-epoxy-β-carotene, were selected as target analytes. Detection was performed via an 80-Hz diode array detector and an electrospray ionization–linear quadrupole ion trap–Orbitrap XL mass spectrometer, both hyphenated in series. Total analysis time was below 6 min with peak widths <12 s. Addition of trifluoroacetic acid and tetrahydrofuran to the mobile phase allowed for the mass spectrometric detection of BC and related CPs and reduced peak tailing due to improved solubility of hydrophobic analytes. Intra-day and inter-day precision for UV and mass spectrometric detection were ≤1.5 % for retention times and ≤5.1 % for peak areas. Instrumental linearity was confirmed by Mandel’s fitting test between 0.25 (or 1.00 μg/mL) and 5.00 μg/mL for UV detection. The higher sensitivity of mass spectrometric detection allowed for the coverage of three concentration domains between 0.025 and 5.00 μg/mL in linearity testing. Homoscedasticity was confirmed between 0.10 and 5.00 μg/mL for Orbitrap XL MS. The limits of quantification were between 52.6 and 889.4 ng/mL for UV detection and between 19.3 and 102.4 ng/L for mass spectrometric detection. Offline solid-phase extraction from culture media fortified with BC and CPs provided intra- and inter-day recoveries between 65.8 and 102.4 % with coefficients of variation ≤6.2 %. Primary rat hepatocyte cultures treated with BC and subjected to different oxidative stress conditions contained 5,6-epoxy-BC and apo-4′-carotenal besides residual BC. Apparently, 5,6-epoxy-BC was formed in the medium via autoxidation of BC by ambient oxygen.

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.

Carotenoid metabolism in mammals, including man: formation, occurrence, and function of apocarotenoids

Vitamin A was recognized as an essential nutrient 100 years ago. In the 1930s, it became clear that dietary β-carotene was cleaved at its central double to yield vitamin A (retinal or β-apo-15'-carotenal). Thus a great deal of research has focused on the central cleavage of provitamin A carotenoids to form vitamin A (retinoids). The mechanisms of formation and the physiological role(s) of noncentral (eccentric) cleavage of both provitamin A carotenoids and nonprovitamin A carotenoids has been less clear. It is becoming apparent that the apocarotenoids exert unique biological activities themselves. These compounds are found in the diet and thus may be absorbed in the intestine, or they may form from enzymatic or nonenzymatic cleavage of the parent carotenoids. The mechanism of action of apocarotenoids in mammals is not fully worked out. However, as detailed in this review, they have profound effects on gene expression and work, at least in part, through the modulation of ligand-activated nuclear receptors. Understanding the interactions of apocarotenoids with other lipid-binding proteins, chaperones, and metabolizing enzymes will undoubtedly increase our understanding of the biological roles of these carotenoid metabolites.

Biological and biomedical (14)C-accelerator mass spectrometry and graphitization of carbonaceous samples

Accelerator mass spectrometry (AMS) is the ultimate technique for measuring rare isotopes in small samples. Biological and biomedical applications of (14)C-AMS (bio-(14)C-AMS) commenced in the early 1990s and are now widely used in many research fields including pharmacology, toxicology, food, and nutrition. For accurate, precise, and reproducible bio-(14)C-AMS analysis, the graphitization step in sample preparation is the most critical step. So, various sample preparation methods for a process called graphitization have been reported for specific applications. Catalytic graphitization using either a flame-sealed borosilicate tube or a septa-sealed vial is a popular sample preparation method for bio-(14)C-AMS. In this review, we introduce the AMS system, especially for bio-(14)C-AMS. In addition, we also review the graphitization method for bio-(14)C-AMS to promote further understanding and improvement of sample preparation for this technique. Examples of catalytic graphitization methods over the past two decades are described.

The challenge to reach nutritional adequacy for vitamin A: β-carotene bioavailability and conversion--evidence in humans

β-Carotene is an important dietary source of vitamin A for humans. However, the bioavailability and vitamin A equivalency of β-carotene are highly variable and can be affected by food- and diet-related factors, including the food matrix, food-processing techniques, size of the dose of β-carotene, and the amounts of dietary fat, fiber, vitamin A, and other carotenoids in the diet as well as by characteristics of the target population, such as vitamin A status, nutrient deficiencies, gut integrity, and genetic polymorphisms associated with β-carotene metabolism. The absorption of β-carotene from plant sources ranges from 5% to 65% in humans. Vitamin A equivalency ratios for β-carotene to vitamin A from plant sources range from 3.8:1 to 28:1, by weight. Vitamin A equivalency ratios for β-carotene from biofortified Golden Rice or biofortified maize are 3.8:1 and 6.5:1, respectively, and are lower than ratios for vegetables that have more complex food matrices (10:1 to 28:1). The vitamin A equivalency of β-carotene is likely to be context-specific and dependent on specific food- and diet-related factors and the health, nutritional, and genetic characteristics of human populations. Although the vitamin A equivalency of β-carotene is highly variable, the provision of vegetable and fruit sources of β-carotene has significantly increased vitamin A status in women and children in community settings in developing countries; these results support the inclusion of dietary interventions with plant sources of β-carotene as a strategy for increasing vitamin A status in populations at risk of deficiency.

Lycopene metabolism and its biological significance

The beneficial effects of a high intake of tomatoes and tomato products on the risk of certain chronic diseases have been presented in many epidemiologic studies, with the suggestion that lycopene (a major carotenoid in tomatoes) is a micronutrient with important health benefits. Within the past few years, we have gained greater knowledge of the metabolism of lycopene and the biological effects of lycopene derivatives. In particular, the characterization and study of β-carotene 9',10'-oxygenase has shown that this enzyme can catalyze the excentric cleavage of both provitamin and non-provitamin A carotenoids to form apo-10'-carotenoids, including apo-10'-lycopenoids from lycopene. This raised an important question of whether the effect of lycopene on various cellular functions and signaling pathways is a result of the direct actions of intact lycopene or its derivatives. Several reports, including our own, support the notion that the biological activities of lycopene can be mediated by apo-10'-lycopenoids. More research is clearly needed to identify and characterize additional lycopene metabolites and their biological activities, which will potentially provide invaluable insights into the mechanisms underlying the effects of lycopene in humans.

Techniques for measuring vitamin A activity from β-carotene

Dietary β-carotene is the most important precursor of vitamin A. However, the determination of the efficiency of in vivo conversion of β-carotene to vitamin A requires sensitive and safe techniques. It presents the following challenges: 1) circulating β-carotene concentration cannot be altered by eating a meal containing ≤6 mg β-carotene; 2) because retinol concentrations are homeostatically controlled, the conversion of β-carotene into vitamin A cannot be estimated accurately in well-nourished humans by assessing changes in serum retinol after supplementation with β-carotene. In the past half-century, techniques using radioisotopes of β-carotene and vitamin A, depletion-repletion with vitamin A and β-carotene supplements, measurement of postprandial chylomicron fractions after consumption of a β-carotene dose, and finally, stable isotopes as tracers to follow the absorption and conversion of β-carotene in humans have been developed. The reported values for β-carotene to vitamin A conversion showed a wide variation from 2 μg β-carotene to 1 μg retinol (for synthetic pure β-carotene in oil) and 28 μg β-carotene to 1 μg retinol (for β-carotene from vegetables). In recent years, a stable isotope reference method (IRM) was developed that used labeled synthetic β-carotene. The IRM method provided evidence that the conversion of β-carotene to vitamin A is likely dose dependent. With the development of intrinsically labeled plant foods harvested from a hydroponic system with heavy water, vitamin A activity of stable isotope-labeled biosynthetic β-carotene from various foods consumed by humans was studied. The efficacy of plant foods rich in β-carotene, such as natural (spinach, carrots, spirulina), hybrid (high-β-carotene yellow maize), and bioengineered (Golden Rice) foods, to provide vitamin A has shown promising results. The results from these studies will be of practical importance in recommendations for the use of pure β-carotene and foods rich in β-carotene in providing vitamin A and ultimately in preventing either overconsumption or poor intake of vitamin A by humans.

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).

Naturally occurring eccentric cleavage products of provitamin A β-carotene function as antagonists of retinoic acid receptors

β-Carotene is the major dietary source of provitamin A. Central cleavage of β-carotene catalyzed by β-carotene oxygenase 1 yields two molecules of retinaldehyde. Subsequent oxidation produces all-trans-retinoic acid (ATRA), which functions as a ligand for a family of nuclear transcription factors, the retinoic acid receptors (RARs). Eccentric cleavage of β-carotene at non-central double bonds is catalyzed by other enzymes and can also occur non-enzymatically. The products of these reactions are β-apocarotenals and β-apocarotenones, whose biological functions in mammals are unknown. We used reporter gene assays to show that none of the β-apocarotenoids significantly activated RARs. Importantly, however, β-apo-14'-carotenal, β-apo-14'-carotenoic acid, and β-apo-13-carotenone antagonized ATRA-induced transactivation of RARs. Competitive radioligand binding assays demonstrated that these putative RAR antagonists compete directly with retinoic acid for high affinity binding to purified receptors. Molecular modeling studies confirmed that β-apo-13-carotenone can interact directly with the ligand binding site of the retinoid receptors. β-Apo-13-carotenone and the β-apo-14'-carotenoids inhibited ATRA-induced expression of retinoid responsive genes in Hep G2 cells. Finally, we developed an LC/MS method and found 3-5 nm β-apo-13-carotenone was present in human plasma. These findings suggest that β-apocarotenoids function as naturally occurring retinoid antagonists. The antagonism of retinoid signaling by these metabolites may have implications for the activities of dietary β-carotene as a provitamin A and as a modulator of risk for cardiovascular disease and cancer.

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

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

New frontiers in science and technology: nuclear techniques in nutrition

The use of nuclear techniques in nutrition adds value by the increased specificity and sensitivity of measures compared with conventional techniques in a wide range of applications. This article provides a brief overview of well-established stable-isotope techniques to evaluate micronutrient bioavailability and assess human-milk intake in breastfed infants to monitor the transfer of micronutrients from the mother to the infant. Recent developments are highlighted in the use of nuclear techniques to evaluate biological interactions between food, nutrition, and health to move the agenda forward.

Domestic cats convert [2H8]-β-carotene to [2H4]-retinol following a single oral dose

Many animals convert β-carotene to retinol to meet their vitamin A (VA) requirement. However, this pathway is inefficient in many carnivores. This study quantified the plasma response to a single oral dose of [(2) H(8)]-β-carotene in adult domestic cats, including measurement of [(2) H(4)]-retinol derived from the dose. Cats were fed with either a control diet containing adequate VA (n = 5) or a VA-devoid diet (n = 5) for 28 days. An oral dose of either 5 mg/kg body weight (BW) (n = 4) or 10 mg/kg BW (n = 6) of [(2) H(8) ]-β-carotene was administered on day 28. Plasma samples were collected prior to dosing and at 6, 12, 24, 32, 48, 72, 120, 168 and 216 h post-dose. Plasma retinoids and β-carotene were measured using HPLC and [(2) H(4)]-retinol by GC-ECNCI-MS (gas chromatography/electron capture negative chemical ionization/mass spectrometry). β-carotene was undetectable in plasma prior to dosing. Post-dose, mean peak plasma β-carotene was 0.37 ± 0.06 nmol/ml at 9.0 ± 1.8 h following the dose, while [(2) H(4) ]-retinol peaked at 3.71 ± 0.69 pmol/ml at 55.2 ± 16.3 h. The ratio per cent of total area under the curve for [(2) H(4)]-retinol compared with the β-carotene response was 4.6 ± 2.6%. There was little effect of diet or dose on the β-carotene or [(2) H(4)]-retinol responses. The appearance of [(2) H(4)]-retinol in plasma indicates that cats are capable of converting β-carotene to active VA. Conversion efficiency was not calculated in this study, but it is likely inadequate to meet cats' VA requirement without the inclusion of preformed VA in the diet.

Carbon isotopes profiles of human whole blood, plasma, red blood cells, urine and feces for biological/biomedical 14C-accelerator mass spectrometry applications

Radiocarbon ((14)C) is an ideal tracer for in vivo human ADME (absorption, distribution, metabolism, elimination) and PBPK (physiological-based pharmacokinetic) studies. Living plants peferentially incorporate atmospheric (14)CO(2) versus (13)CO(2) versus (12)CO(2), which result in unique signature. Furthermore, plants and the food chains they support also have unique carbon isotope signatures. Humans, at the top of the food chain, consequently acquire isotopic concentrations in the tissues and body fluids depending on their dietary habits. In preparation of ADME and PBPK studies, 12 healthy subjects were recruited. The human baseline (specific to each individual and their diet) total carbon (TC) and carbon isotope (13)C (δ(13)C) and (14)C (F(m)) were quantified in whole blood (WB), plasma, washed red blood cell (RBC), urine, and feces. TC (mg of C/100 μL) in WB, plasma, RBC, urine, and feces were 11.0, 4.37, 7.57, 0.53, and 1.90, respectively. TC in WB, RBC, and feces was higher in men over women, P < 0.05. Mean δ(13)C were ranked low to high as follows: feces < WB = plasma = RBC = urine, P < 0.0001. δ(13)C was not affected by gender. Our analytic method shifted δ(13)C by only ±1.0 ‰ ensuring our F(m) measurements were accurate and precise. Mean F(m) were ranked low to high as follows: plasma = urine < WB = RBC = feces, P < 0.05. F(m) in feces was higher for men over women, P < 0.05. Only in WB, (14)C levels (F(m)) and TC were correlated with one another (r = 0.746, P < 0.01). Considering the lag time to incorporate atmospheric (14)C into plant foods (vegetarian) and or then into animal foods (nonvegetarian), the measured F(m) of WB in our population (recruited April 2009) was 1.0468 ± 0.0022 (mean ± SD), and the F(m) of WB matched the (extrapolated) atmospheric F(m) of 1.0477 in 2008. This study is important in presenting a procedure to determine a baseline for a study group for human ADME and PBPK studies using (14)C as a tracer.

Accelerator mass spectrometry-enabled studies: current status and future prospects

Accelerator mass spectrometry is a detection platform with exceptional sensitivity compared with other bioanalytical platforms. Accelerator mass spectrometry (AMS) is widely used in archeology for radiocarbon dating applications. Early exploration of the biological and pharmaceutical applications of AMS began in the early 1990s. AMS has since demonstrated unique problem-solving ability in nutrition science, toxicology and pharmacology. AMS has also enabled the development of new applications, such as Phase 0 microdosing. Recent development of AMS-enabled applications has transformed this novelty research instrument to a valuable tool within the pharmaceutical industry. Although there is now greater awareness of AMS technology, recognition and appreciation of the range of AMS-enabled applications is still lacking, including study-design strategies. This review aims to provide further insight into the wide range of AMS-enabled applications. Examples of studies conducted over the past two decades will be presented, as well as prospects for the future of AMS.

Antioxidant activity of β-carotene compounds in different in vitro assays

β-Carotene (BC) is the most abundant carotenoid in human diet, almost solely as (all-E)-isomer. Significant amounts of (Z)-isomers of BC are present in processed food as well as in mammalian tissues. Differences are described for the activity of various BC isomers in forming retinal and protecting against cancer and cardiovascular diseases. Eccentric cleavage of BC leads to degradation products such as carotenals. A variety of negative consequences were published for the non-vitamin A active BC metabolites, such as inducing the carcinogenesis of benzo[a]pyrene, impairing mitochondrial function, or increasing CYP activity. To increase the knowledge on the antioxidant activity, a variety of BC isomers and metabolites were tested in various in vitro assays.

In the present study, no ferric reducing activity (FRAP assay) was observed for the BC isomers. Between the major BC isomers (all-E, 9Z, and 13Z) no significant differences in bleaching the ABTS^●+ (αTEAC assay) or in scavenging peroxyl radicals (ROO^●) generated by thermal degradation of AAPH (using a chemiluminescence assay) were detected. However, the (15Z)-isomer was less active, maybe due to its low stability. The degradation to β-apo-carotenoids increased FRAP activity and ROO^● scavenging activity compared to the parent molecule. Dependence on chain length and character of the terminal function was determined in αTEAC assay with following order of increasing activity: β-apo-8’-carotenal < β-apo-8’-carotenoic acid ethyl ester < 6’-methyl-β-apo-6’-carotene-6’-one (citranaxanthin). The results indicate that BC does not lose its antioxidant activity by degradation to long chain breakdown products.

Beta-carotene is an important vitamin A source for humans

Experts in the field of carotenoids met at the Hohenheim consensus conference in July 2009 to elucidate the current status of β-carotene research and to summarize the current knowledge with respect to the chemical properties, physiological function, and intake of β-carotene. The experts discussed 17 questions and reached an agreement formulated in a consensus answer in each case. These consensus answers are based on published valid data, which were carefully reviewed by the individual experts and are justified here by background statements. Ascertaining the impact of β-carotene on the total dietary intake of vitamin A is complicated, because the efficiency of conversion of β-carotene to retinol is not a single ratio and different conversion factors have been used in various surveys and following governmental recommendations within different countries. However, a role of β-carotene in fulfilling the recommended intake for vitamin A is apparent from a variety of studies. Thus, besides elucidating the various functions, distribution, and uptake of β-carotene, the consensus conference placed special emphasis on the provitamin A function of β-carotene and the role of β-carotene in the realization of the required/recommended total vitamin A intake in both developed and developing countries. There was consensus that β-carotene is a safe source of vitamin A and that the provitamin A function of β-carotene contributes to vitamin A intake.

Colors with functions: elucidating the biochemical and molecular basis of carotenoid metabolism

Carotenoids affect a rich variety of physiological functions in nature and are beneficial for human health, serving as antioxidants in lipophilic environments and blue light filters in the macula of human retina. These dietary compounds also serve as precursors of a unique set of apo-carotenoid cleavage products, including retinoids. Although knowledge about retinoid biology has tremendously increased, the metabolism of retinoids' parent precursors remains poorly understood. Recently, molecular players in carotenoid metabolism have been identified and biochemically characterized. Moreover, mutations in their corresponding genes impair carotenoid metabolism and induce various pathologies in animal models. Polymorphisms in these genes alter carotenoid and retinoid homeostasis in humans as well. This review summarizes our current knowledge about the molecular/biochemical basis of carotenoid metabolism and particularly the physiological role of carotenoids in retinoid-dependent physiological processes.

The eccentric cleavage product of β-carotene, β-apo-13-carotenone, functions as an antagonist of RXRα

In this study, we investigated the effects of eccentric cleavage products of β-carotene, i.e. β-apocarotenoids (BACs), on retinoid X receptor alpha (RXRα) signaling. Transactivation assays were performed to test whether BACs activate or antagonize RXRα. Reporter gene constructs (RXRE-Luc, pRL-tk) and RXRα were transfected into Cos-1 cells and used to perform these assays. None of the BACs tested activated RXRα. Among the compounds tested, β-apo-13-carotenone was found to antagonize the activation of RXRα by 9-cis-retinoic acid and was effective at concentrations as low as 1 nM. Molecular modeling studies revealed that β-apo-13-carotenone makes molecular interactions like an antagonist of RXRα. The results suggest a possible function of BACs on RXRα signaling.

{beta}-Apocarotenoids do not significantly activate retinoic acid receptors {alpha} or {beta}

beta-Carotene oxygenase 2 cleaves beta-carotene asymmetrically at non-central double bonds of the polyene chain, yielding apocarotenal molecules. The hypothesis tested was that apocarotenoids are able to stimulate transcription by activating retinoic acid receptors (RARs). The effects of long- and short-chain apocarotenals and apocarotenoic acids on the activation of RARalpha and RARbeta transfected into monkey kidney fibroblast cells (CV-1) were investigated. We synthesized or purified beta-apo-8'-carotenoic acid (apo-8'-CA), beta-apo-14'-carotenoic acid (apo-14'-CA), beta-cyclocitral (BCL), beta-cyclogernanic acid (BCA), beta-ionone (BI), beta-ionylideneacetaldehyde (BIA) beta-ionylideneacetic acid (BIAA) and a C13 ketone, beta-apo-13-carotenone (C13). None of the apocarotenoids tested showed significant transactivation activity for the RARs when compared with all-trans retinoic acid (RA). The results suggest that biological effects of these apocarotenoids are through mechanisms other than activation of RARalpha and beta.

Beta-carotene conversion to vitamin A decreases as the dietary dose increases in humans

It has been suggested that high doses of beta-carotene limit its conversion to vitamin A, yet this effect has not been well established in humans. A feeding study was conducted in a randomized crossover design in which volunteers consumed 2 doses of deuterium-labeled beta-carotene on 2 occasions, with beta-carotene and vitamin A response assessed by plasma area under the concentration time curve (AUC). Seven volunteers (4 men, 3 women) consumed each of 2 doses of beta-carotene-d8 and provided serial blood samples for 37 d after each dose. beta-Carotene doses were 20 and 40 mg. Plasma beta-carotene-d8 was assessed by HPLC-MS. Plasma retinol (ROH)-d4, which was derived from the beta-carotene-d8, was evaluated by GC-MS after saponification to convert retinyl esters to ROH prior to the formation of the trimethylsilylether. The plasma AUC for beta-carotene-d8 increased 2-fold from the 20-mg dose to the 40-mg dose. The plasma AUC for ROH-d4 increased 36% from the 20-mg dose to the 40-mg dose. These results establish that, in humans, beta-carotene conversion to vitamin A decreases as the dietary dose increases.

Calculating radiation exposures during use of (14)C-labeled nutrients, food components, and biopharmaceuticals to quantify metabolic behavior in humans

(14)C has long been used as a tracer for quantifying the in vivo human metabolism of food components, biopharmaceuticals, and nutrients. Minute amounts (< or =1 x 10 (-18) mol) of (14)C can be measured with high-throughput (14)C-accelerator mass spectrometry (HT (14)C-AMS) in isolated chemical extracts of biological, biomedical, and environmental samples. Availability of in vivo human data sets using a (14)C tracer would enable current concepts of the metabolic behavior of food components, biopharmaceuticals, or nutrients to be organized into models suitable for quantitative hypothesis testing and determination of metabolic parameters. In vivo models are important for specification of intake levels for food components, biopharmaceuticals, and nutrients. Accurate estimation of the radiation exposure from ingested (14)C is an essential component of the experimental design. Therefore, this paper illustrates the calculation involved in determining the radiation exposure from a minute dose of orally administered (14)C-beta-carotene, (14)C-alpha-tocopherol, (14)C-lutein, and (14)C-folic acid from four prior experiments. The administered doses ranged from 36 to 100 nCi, and radiation exposure ranged from 0.12 to 5.2 microSv to whole body and from 0.2 to 3.4 microSv to liver with consideration of tissue weighting factor and fractional nutrient. In comparison, radiation exposure experienced during a 4 h airline flight across the United States at 37000 ft was 20 microSv.

Mathematical modeling of serum 13C-retinol in captive rhesus monkeys provides new insights on hypervitaminosis A

Hypervitaminosis A is increasingly a public health concern, and thus noninvasive quantitative methods merit exploration. In this study, we applied the (13)C-retinol isotope dilution test to a nonhuman primate model with excessive liver stores. After baseline serum chemistries, rhesus macaques (Macaca mulatta; n = 16) were administered 3.5 mumol (13)C(2)-retinyl acetate. Blood was drawn at baseline, 5 h, and 2, 4, 7, 14, 21, and 28 d following the dose. Liver biopsies were collected 7 d before and 2 d after dosing (n = 4) and at 7, 14, and 28 d (n = 4/time) after dosing. Serum and liver were analyzed by HPLC and GC-combustion-isotope ratio MS for retinol and its enrichment, respectively. Model-based compartmental analysis was applied to serum data. Lactate dehydrogenase was elevated in 50% of the monkeys. Total body reserves (TBR) of vitamin A (VA) were calculated at 28 d. Predicted TBR (3.52 +/- 2.01 mmol VA) represented measured liver stores (4.56 +/- 1.38 mmol VA; P = 0.124). Predicted liver VA concentrations (13.3 +/- 9.7 micromol/g) were similar to measured liver VA concentrations (16.4 +/- 5.3 micromol/g). The kinetic models predict that 27-52% of extravascular VA is exchanging with serum in hypervitaminotic A monkeys. The test correctly diagnosed hypervitaminosis A in all monkeys, i.e. 100% sensitivity. Stable isotope techniques have important public health potential for the classification of VA status, including hypervitaminosis, because no other technique besides invasive liver biopsies, correctly identifies excessive liver VA stores.

Carotenoid maintenance handicap and the physiology of carotenoid-based signalisation of health

Despite a reasonable scientific interest in sexual selection, the general principles of health signalisation via ornamental traits remain still unresolved in many aspects. This is also true for the mechanism preserving honesty of carotenoid-based signals. Although it is widely accepted that this type of ornamentation reflects an allocation trade-off between the physiological utilisation of carotenoids (mainly in antioxidative processes) and their deposition in ornaments, some recent evidence suggests more complex interactions. Here, we further develop the models currently proposed to explain the honesty of carotenoid-based signalisation of heath status by adding the handicap principle concept regulated by testosterone. We propose that under certain circumstances carotenoids may be dangerous for the organism because they easily transform into toxic cleavage products. When reserves of other protective antioxidants are insufficient, physiological trade-offs may exist between maintenance of carotenoids for ornament expression and their removal from the body. Furthermore, we suggest that testosterone which enhances ornamentation by increasing carotenoid bioavailability may also promote oxidative stress and hence lower antioxidant reserves. The presence of high levels of carotenoids required for high-quality ornament expression may therefore represent a handicap and only individuals in prime health could afford to produce elaborate colourful ornaments. Although further testing is needed, this 'carotenoid maintenance handicap' hypothesis may offer a new insight into the physiological aspects of the relationship between carotenoid function, immunity and ornamentation.

A minute dose of 14C-{beta}-carotene is absorbed and converted to retinoids in humans

Our objective was to quantify the absorption and conversion to retinoids of a 1.01-nmol, 3.7-kBq oral dose of (14)C-beta-carotene in 8 healthy adults. The approach was to quantify, using AMS, the elimination of (14)C in feces for up to 16 d after dosing and in urine for up to 30 d after dosing. The levels of total (14)C in undiluted serial plasma samples were measured for up to 166 d after dosing. Also, the levels of (14)C in the retinyl ester (RE), retinol (ROH), and beta-carotene fractions that were isolated from undiluted plasma using HPLC were measured. The apparent digestibility of the (14)C was 53 +/- 13% (mean +/- SD), based on the mass balance data, and was generally consistent with the area under the curve for zero to infinite period of (14)C that was eliminated in the feces collections made up to 7.5 d after dosing. Metabolic fecal elimination, calculated as the slope per day (% (14)C-dose/collection from d 7.5 to the final day), was only 0.05 +/- 0.02%. The portion of the (14)C dose eliminated via urine was variable (6.5 +/- 5.2%). Participants [except participant 6 (P6)] had a distinct plasma peak of (14)C at 0.25 d post-dose, preceded by a shoulder at approximately 0.1 d, and followed by a broad (14)C peak that became indistinguishable from baseline at approximately 40 d. Plasma (14)C-RE accounted for most of the absorbed (14)C early after dosing and P1 had the longest delay in the first appearance of (14)C-RE in plasma. The data suggest that plasma RE should be considered in estimating the ROH activity equivalent of ingested beta-carotene.

beta-Carotene conversion products and their effects on adipose tissue

Recent epidemiological data suggest that beta-carotene may be protective against metabolic diseases in which adipose tissue plays a key role. Adipose tissue constitutes the major beta-carotene storage tissue and its functions have been shown to be modulated in response to beta-carotene breakdown products, especially retinal produced after cleavage by beta-carotene 15,15'-monooxygenase (BCMO1), and retinoic acid arising from oxidation of retinal. However, the possibility exists that beta-carotene in its intact form can also affect adipocyte function. Development of a knock out model and identification of a loss-of-function mutation have pointed out BCMO1 as being probably the sole enzyme responsible for provitamin A conversion into retinal in mammals. The utilisation of BCMO1(-/-)mice should provide insights on beta-carotene effect on its own in the future. In humans, intervention studies have highlighted the huge interindividual variation of beta-carotene conversion efficiency, possibly due to genetic polymorphisms, which might impact on response to beta-carotene. This brief review discusses the processes involved in beta-carotene conversion and the effect of cleavage products on body fat and adipose tissue function.

Identification of bacterial carotenoid cleavage dioxygenase homologues that cleave the interphenyl alpha,beta double bond of stilbene derivatives via a monooxygenase reaction

Carotenoid cleavage oxygenases (CCOs), which are also referred to as carotenoid cleavage dioxygenases (CCDs) are a new class of nonheme iron-type enzymes that oxidatively cleave double bonds in the conjugated carbon chain of carotenoids. The oxidative cleavage mechanism of these enzymes is not clear, and both monooxygenase and dioxygenase mechanisms have been proposed for different carotenoid cleavage enzymes. CCOs have been described from plants, animals, fungi, and cyanobacteria, but little is known about their distribution and activities in bacteria other than cyanobacteria. We surveyed bacterial genome sequences for CCO homologues and report the characterization of CCO homologues that were identified in Novosphingobium aromaticivorans DSM 12444 (NOV1 and NOV2) and in Bradyrhizobium sp. (BRA-J and BRA-S). In vitro and in vivo assays with carotenoid and stilbene compounds were used to investigate the cleavage activities of the recombinant enzymes. The NOV enzymes cleaved the interphenyl alpha-beta double bond of stilbenes that had an oxygen functional group at the 4' carbon atom (e.g., resveratrol, piceatannol, and rhaponticin) to the corresponding aldehyde products. Carotenoids and apocarotenoids were not substrates for these enzymes. The two homologous enzymes from Bradyrhizobium sp. did not possess carotenoid or stilbene cleavage oxygenase activities, but showed activity with farnesol. To investigate whether the oxidative cleavage of stilbenes proceeds via a monooxygenase or dioxygenase reaction, oxygen-labeling studies were conducted with NOV2. Our labeling studies show that the double-bond cleavage of stilbenes occurs via a monooxygenase reaction mechanism.

Carotenoid derived aldehydes-induced oxidative stress causes apoptotic cell death in human retinal pigment epithelial cells

Carotenoids have been advocated as potential therapeutic agents in treating age-related macular degeneration (AMD). In ocular tissues carotenoids may undergo oxidation and form carotenoid-derived aldehydes (CDA), which would be toxic to tissues. We have investigated the cytotoxic effects of CDA from beta-carotene, Lutein and Zeaxanthin on human retinal pigment epithelial cells (ARPE-19). The serum-starved ARPE-19 cells were treated with CDA without or with antioxidant, N-acetylcysteine (NAC) and cell viability, apoptosis, reactive oxygen species (ROS) levels, nuclear chromatin condensation as well as fragmentation, change in mitochondrial membrane potential (MMP) and activation of transcription factors NF-kappaB and AP-1 were determined. We observed a dose and time-dependent decline in cell viability upon incubation of ARPE-19 cells with CDA. The CDA treatment also led to elevation in ROS levels in a dose-dependent manner. Upon CDA treatment a significant number of apoptotic cells were observed. Also early apoptotic changes in ARPE-19 cells induced by CDA were associated with change in MMP. Increased nuclear chromatin condensation and fragmentation were also observed in cells treated with CDA. The cytotoxicity of CDA in ARPE-19 cells was significantly ameliorated by the antioxidant, NAC. Furthermore, CDA induced the activation of NF-kappaB and AP-1 which was significantly inhibited by NAC. Thus our results demonstrate that CDA could increase the oxidative stress in ARPE-19 cells by elevating ROS levels that would cause imbalance in cellular redox status, which could lead to cell death. This would suggest that high carotenoid supplementation for treatment of AMD should be used cautiously.