Guangwen Tang

Retinaldehyde represses adipogenesis and diet-induced obesity.

The metabolism of vitamin A and the diverse effects of its metabolites are tightly controlled by distinct retinoid-generating enzymes, retinoid-binding proteins and retinoid-activated nuclear receptors. Retinoic acid regulates differentiation and metabolism by activating the retinoic acid receptor and retinoid X receptor (RXR), indirectly influencing RXR heterodimeric partners. Retinoic acid is formed solely from retinaldehyde (Rald), which in turn is derived from vitamin A. Rald currently has no defined biologic role outside the eye. Here we show that Rald is present in rodent fat, binds retinol-binding proteins (CRBP1, RBP4), inhibits adipogenesis and suppresses peroxisome proliferator-activated receptor-γ and RXR responses. In vivo, mice lacking the Rald-catabolizing enzyme retinaldehyde dehydrogenase 1 (Raldh1) resisted diet-induced obesity and insulin resistance and showed increased energy dissipation. In ob/ob mice, administrating Rald or a Raldh inhibitor reduced fat and increased insulin sensitivity. These results identify Rald as a distinct transcriptional regulator of the metabolic responses to a high-fat diet.

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

Whether the conversion of β-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 β-apo-12′-, -10′-, and -8′-carotenals, retinal, and retinoic acid were found after incubation of intestinal homogenates of the four different species with β-carotene in the presence of NAD+ and dithiothreitol. No β-apo-carotenals or retinoids were detected in control incubations done without tissue homogenates. The production of β-apo-carotenals was linear for 30 min and up to tissue protein concentrations of 1.5 mg/ml. The rate of formation of β-apo-carotenals from 2 μm β-carotene was about 7- to 14-fold higher than the rate of retinoid formation in intestinal homogenates, and the rate of β-apo-carotenal production was fivefold greater in primate intestine vs rat or ferret intestine (P < 0.05). The amounts of β-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 β-apo-carotenal and retinoid production from β-carotene were inhibited completely by adding disulfiram, an inhibitor of sulfhydryl-containing enzymes. Incubation of β-carotene with liver, kidney, lung, and fat homogenates from each species also resulted in the appearance of β-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 β-carotene conversion.

Bioconversion of dietary provitamin A carotenoids to vitamin A in humans

Recent progress in the measurement of the bioconversion of dietary provitamin A carotenoids to vitamin A is reviewed in this article. Methods to assess the bioavailability and bioconversion of provitamin A carotenoids have advanced significantly in the past 10 y, specifically through the use of stable isotope methodology, which includes the use of labeled plant foods. The effects of the food matrix on the bioconversion of provitamin A carotenoids to vitamin A, dietary fat effects, and the effect of genotype on the absorption and metabolism of beta-carotene have been reported recently. A summary of the major human studies that determined conversion factors for dietary beta-carotene to retinol is presented here, and these data show that the conversion efficiency of dietary beta-carotene to retinol is in the range of 3.6-28:1 by weight. There is a wide variation in conversion factors reported not only between different studies but also between individuals in a particular study. These findings show that the vitamin A value of individual plant foods rich in provitamin A carotenoids may vary significantly and need further investigation.

β-Carotene in Golden Rice is as good as β-carotene in oil at providing vitamin A to children

BACKGROUND Golden Rice (GR) has been genetically engineered to be rich in β-carotene for use as a source of vitamin A. OBJECTIVE The objective was to compare the vitamin A value of β-carotene in GR and in spinach with that of pure β-carotene in oil when consumed by children. DESIGN Children (n = 68; age 6-8 y) were randomly assigned to consume GR or spinach (both grown in a nutrient solution containing 23 atom% ²H₂O) or [²H₈]β-carotene in an oil capsule. The GR and spinach β-carotene were enriched with deuterium (²H) with the highest abundance molecular mass (M) at M(β-C)+²H₁₀. [¹³C₁₀]Retinyl acetate in an oil capsule was administered as a reference dose. Serum samples collected from subjects were analyzed by using gas chromatography electron-capture negative chemical ionization mass spectrometry for the enrichments of labeled retinol: M(retinol)+4 (from [²H₈]β-carotene in oil), M(retinol)+5 (from GR or spinach [²H₁₀]β-carotene), and M(retinol)+10 (from [¹³C₁₀]retinyl acetate). RESULTS Using the response to the dose of [¹³C₁₀]retinyl acetate (0.5 mg) as a reference, our results (with the use of AUC of molar enrichment at days 1, 3, 7, 14, and 21 after the labeled doses) showed that the conversions of pure β-carotene (0.5 mg), GR β-carotene (0.6 mg), and spinach β-carotene (1.4 mg) to retinol were 2.0, 2.3, and 7.5 to 1 by weight, respectively. CONCLUSIONS The β-carotene in GR is as effective as pure β-carotene in oil and better than that in spinach at providing vitamin A to children. A bowl of ~100 to 150 g cooked GR (50 g dry weight) can provide ~60% of the Chinese Recommended Nutrient Intake of vitamin A for 6-8-y-old children.

Yellow maize with high β-carotene is an effective source of vitamin A in healthy Zimbabwean men

BACKGROUND The bioconversion efficiency of yellow maize β-carotene to retinol in humans is unknown. OBJECTIVE The objective of this study was to determine the vitamin A value of yellow maize β-carotene in humans. DESIGN High β-carotene-containing yellow maize was grown in a hydroponic medium with 23 atom% (2)H(2)O during grain development. Yellow maize β-carotene showed the highest abundance of enrichment as [(2)H(9)]β-carotene. Eight healthy Zimbabwean men volunteered for the study. On day 1 after a fasting blood draw, subjects consumed 300 g yellow maize porridge containing 1.2 mg β-carotene, 20 g butter, and a 0.5-g corn oil capsule. On day 8, fasting blood was drawn, and subjects consumed 1 mg [(13)C(10)]retinyl acetate in a 0.5-g corn oil capsule and 300 g white maize porridge with 20 g butter. Thirty-six blood samples were collected from each subject over 36 d. Concentrations and enrichments of retinol and β-carotene in labeled doses and serum were determined with the use of HPLC, gas chromatography-mass spectrometry, and liquid chromatography-mass spectrometry. RESULTS The area under the curve (AUC) of retinol from 1.2 mg yellow maize β-carotene was 72.9 nmol · d, and the AUC of retinol from 1 mg retinyl acetate (13)C(10) was 161.1 nmol · d. The conversion factor of yellow maize β-carotene to retinol by weight was 3.2 ± 1.5 to 1. CONCLUSION In 8 healthy Zimbabwean men, 300 g cooked yellow maize containing 1.2 mg β-carotene that was consumed with 20.5 g fat showed the same vitamin A activity as 0.38 mg retinol and provided 40-50% of the adult vitamin A Recommended Dietary Allowance. This trial was registered at clinicaltrials.gov as NCT00636038.

Serum carotenoids and retinoids in ferrets fed canthaxanthin

Abstract A high-performance liquid chromatographic method has been developed to analyze canthaxanthin (CX) with other carotenoids and retinoids in serum. Serum (100 μL) was extracted first with chloroform/methanol and then with hexane, using retinyl acetate and γ-carotene as the internal standards. The residue of the extract was resuspended into ethanol and injected onto an 8.3 × 0.46 cm 3-μm ODS column. The gradient system consisted of two solvents of acetonitrile/tetrahydrofuran/water with different ratios. The carotenoids and retinoids were measured at 450 nm and 340 nm, respectively. CX was detected in the serum of the ferrets fed CX (79% all- trans and 21% cis isomer, 50 mg/kg bodyweight by gavage for 1 month), but was not detected in the control group. The existence of CX in the serum was confirmed by its UV/visible absorption spectrum and by its negative chemical ionization/mass spectrum with an M − ion at m/z 564. The serum level of all- trans CX in the experimental ferrets was 33.34 ± 3.33 nmol/L ( n = 13) and that of cis CX was 72.48 ± 7.64 nmol/L ( n = 13). There was no difference between the CX-fed group and the control group in the serum levels of retinol, retinyl esters, lutein, cryptoxanthin, or β-carotene. Retinyl esters in ferret serum represent 93% of total vitamin A level, which is more than 10 times higher than the vitamin A level in human blood. High retinyl esters level in the circulation of the ferret is a general phenomenon in carnivores.

Stable isotope dilution techniques for assessing vitamin A status and bioefficacy of provitamin A carotenoids in humans

Vitamin A deficiency is a major global public health problem. Among the variety of techniques that are available for assessing human vitamin A status, evaluating the provitamin A nutritional values of foodstuffs and estimating human vitamin A requirements, isotope dilution provides the most accurate estimates. Although the relative expense of isotope dilution restricts its applications, it has an important function as the standard of reference for other techniques. Mathematical modelling plays an indispensable role in the interpretation of isotope dilution data. This review summarises recent applications of stable isotope methodology to determine human vitamin A status, estimate human vitamin A requirements, and calculate the bioconversion and bioefficacy of food carotenoids.

Deuterium enrichment of retinol in humans determined by gas chromatography electron capture negative chemical ionization mass spectrometry

Abstract A new application of electron capture negative chemical ionization mass spectrometry method has been developed for detecting vitamin A enrichment in human serum. Octadeuterated all-trans retinyl acetate 1.5 mg was fed to a volunteer and blood samples were collected over a period of 32 days. Serum samples were extracted and isolated by high performance liquid chromatography to collect the fraction containing retinol. The retinol fraction was derivatized to a trimethylsilyl ether, which was analyzed by gas chromatograph (GC)/mass spectrometry using a capillary column coated with DB-1 followed by methane electron capture negative chemical ionization/mass spectrometry (ECNCI-MS). Although ECNCI-MS of derivatized retinol produced no molecular ion, it produced a single negatively charged fragment ion at m/z 268 for natural retinol (m/z 276 for octadeuterated retinol) due to loss of the silyl group. The serum enrichment of labeled retinol was detectable at 7 hours, reached a maximum of 2.6% at 24 hours, and declined thereafter but was still at 0.07% at 32 days. In 200 μL of serum, the minimum detectable enrichment of retinol-d8 was 0.01%. The GC/ECNCI-MS method for detecting retinol in serum is at least 10 times more sensitive than any previously published mass spectrometry method.

Determination of Carotenoids in Yellow Maize, the Effects of Saponification and Food Preparations

Maize is an important staple food consumed by millions of people in many countries. Yellow maize naturally contains carotenoids which not only provide provitamin A carotenoids but also xanthophylls, which are known to be important for eye health. This study was aimed at 1) evaluating the effect of saponification during extraction of yellow maize carotenoids, 2) determining the major carotenoids in 36 genotypes of yellow maize by high-performance liquid chromatography with a C30 column, and 3) determining the effect of cooking on the carotenoid content of yellow maize. The major carotenoids in yellow maize were identified as all-trans lutein, cis-isomers of lutein, all-trans zeaxanthin, alpha- and beta-cryptoxanthin, all-trans beta-carotene, 9-cis beta-carotene, and 13-cis beta-carotene. Our results indicated that carotenoid extraction without saponification showed a significantly higher yield than that obtained using saponification. Results of the current study indicate that yellow maize is a good source of provitamin A carotenoids and xanthophylls. Cooking by boiling yellow maize at 100 degrees C for 30 minutes increased the carotenoid concentration, while baking at 450 degrees F for 25 minutes decreased the carotenoid concentrations by almost 70% as compared to the uncooked yellow maize flour.

Formation of all-trans-retinoic acid and 13-cis-retinoic acid from all-trans-retinyl palmitate in humans

Abstract Increments of levels of both 13-cis- and all-trans-retinoic acid in human plasma were observed after either a physiologic or a pharmacologic oral dose of all-trans-retinyl palmitate. Subjects receiving a physiologic dose showed mean ± SEM plasma rises over baseline as follows: all-trans-retinoic acid = 1.1 ± 0.3 nmol/L and 13-cis-retinoic acid = 4.7 ± 1.1 nmol/L, which represented increases in 1.3 fold and 1.9 fold over fasting plasma levels. Those receiving a pharmacologic dose showed mean ± SEM plasma rises over baseline as follows: all- trans -retinoic acid = 11.5 ± 2.6 nmol/L and 13-cis -retinoic acid = 37.5 ± 6.1 nmol/L, which represented increases of 3.9-fold and 8.4-fold over fasting plasma levels. Moreover, areas under the curve of the means of all-trans- and 13-cis-retinoic acid over 24 hours showed that larger amounts of 13-cis-retinoic acid appear in the circulation than all-trans-retinoic acid after feeding all-trans-retinyl palmitate. The increase in retinoic acid in the circulation may be an important source of retinoic acid for some organs.