[Show abstract][Hide abstract] ABSTRACT: Tumor suppressor p53 regulates the expression of p53-induced genes (PIG) that trigger apoptosis. PIG3 or TP53I3 is the only known member of the medium chain dehydrogenase/reductase superfamily induced by p53 and is used as a proapoptotic marker. Although the participation of PIG3 in the apoptotic pathway is proven, the protein and its mechanism of action were never characterized. We analyzed human PIG3 enzymatic function and found NADPH-dependent reductase activity with ortho-quinones, which is consistent with the classification of PIG3 in the quinone oxidoreductase family. However, the activity is much lower than that of zeta-crystallin, a better known quinone oxidoreductase. In addition, we report the crystallographic structure of PIG3, which allowed the identification of substrate- and cofactor-binding sites, with residues fully conserved from bacteria to human. Tyr-59 in zeta-crystallin (Tyr-51 in PIG3) was suggested to participate in the catalysis of quinone reduction. However, kinetics of Tyr/Phe and Tyr/Ala mutants of both enzymes demonstrated that the active site Tyr is not catalytic but may participate in substrate binding, consistent with a mechanism based on propinquity effects. It has been proposed that PIG3 contribution to apoptosis would be through oxidative stress generation. We found that in vitro activity and in vivo overexpression of PIG3 accumulate reactive oxygen species. Accordingly, an inactive PIG3 mutant (S151V) did not produce reactive oxygen species in cells, indicating that enzymatically active protein is necessary for this function. This supports that PIG3 action is through oxidative stress produced by its enzymatic activity and provides essential knowledge for eventual control of apoptosis.
Full-text · Article · May 2009 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Retinoic acid (RA), the most active retinoid, is synthesized in two steps from retinol. The first step, oxidation of retinol to retinaldehyde, is catalyzed by cytosolic alcohol dehydrogenases (ADHs) of the medium-chain dehydrogenase/reductase (MDR) superfamily and microsomal retinol dehydrogenases (RDHs) of the short-chain dehydrogenase/reductase (SDR) superfamily. The second step, oxidation of retinaldehyde to RA, is catalyzed by several aldehyde dehydrogenases. ADH1 and ADH2 are the major MDR enzymes in liver retinol detoxification, while ADH3 (less active) and ADH4 (most active) participate in RA generation in tissues. Several NAD+- and NADP+-dependent SDRs are retinoid active. Their in vivo contribution has been demonstrated in the visual cycle (RDH5, RDH12), adult retinoid homeostasis (RDH1) and embryogenesis (RDH10). K
values for most retinoid-active ADHs and RDHs are close to 1 μM or lower, suggesting that they participate physiologically in retinol/retinaldehyde interconversion. Probably none of these enzymes uses retinoids bound to cellular retinol-binding protein, but only free retinoids. The large number of enzymes involved in the two directions of this step, also including aldo-keto reductases, suggests that retinaldehyde levels are strictly regulated.
Full-text · Article · Dec 2008 · Cellular and Molecular Life Sciences CMLS
[Show abstract][Hide abstract] ABSTRACT: The medium-chain dehydrogenase/reductase (MDR) superfamily can be divided into Zn-containing and Zn-lacking proteins. Zn-containing MDRs are generally well-known enzymes, mostly acting as dehydrogenases. The non-Zn MDR are much less studied, and classified in several families of NADP(H)-dependent reductases, including quinone oxidoreductases (QOR). zeta-Crystallins are the best studied group of QOR, have a structural function in the lens of several mammals, exhibit ortho-quinone reductase activity, and bind to specific adenine-uracil-rich elements (ARE) in RNA. In the present work, we have further characterized human zeta-crystallin and Saccharomyces cerevisiae Zta1p, the only QOR in yeast. Subcellular localization using a fluorescent protein tag indicates that zeta-crystallin is distributed in the cytoplasm but not in nucleus. The protein may also be present in mitochondria. Zta1p localizes in both cytoplasm and nucleus. NADPH, but not NADH, competitively prevents binding of zeta-crystallin to RNA, suggesting that the cofactor-binding site is involved in RNA binding. Interference of NADPH on Zta1p binding to RNA is much lower, consistent with a weaker binding of NADPH to the yeast enzyme. Disruption of the yeast ZTA1 gene does not affect cell growth under standard conditions but makes yeast more sensitive to oxidative stress agents. Sequence alignments, phylogenetic tree analysis and kinetic properties reveal a close relationship between zeta-crystallin and Zta1p. Amino acid conservation, between the substrate-binding sites of the two proteins and that of an E. coli QOR, indicates that zeta-crystallins maintained their kinetic function throughout evolution. Quinones are toxic compounds and a relevant step in their detoxification is reduction to their corresponding hydroquinones. Many enzymes of several superfamilies can reduce quinones, including NAD(P)H:quinone oxidoreductase 1 (NQO1 or DT-diaphorase), aldo-keto reductases and short-chain dehydrogenases/reductases. In this context, the physiological role of zeta-crystallins is discussed.
No preview · Article · Nov 2008 · Chemico-biological interactions
[Show abstract][Hide abstract] ABSTRACT: NADP(H)-dependent cytosolic aldo-keto reductases (AKRs) have been added to the group of enzymes which contribute to oxidoreductive conversions of retinoids. Recently, we found that two members from the AKR1B subfamily (AKR1B1 and AKRB10) were active in the reduction of all-trans- and 9-cis-retinaldehyde, with K(m) values in the micromolar range, but with very different k(cat) values. With all-trans-retinaldehyde, AKR1B10 shows a much higher k(cat) value than AKR1B1 (18 min(-1)vs. 0.37 min(-1)) and a catalytic efficiency comparable to that of the best retinaldehyde reductases. Structural, molecular dynamics and site-directed mutagenesis studies on AKR1B1 and AKR1B10 point that subtle differences at the entrance of their retinoid-binding site, especially at position 125, are determinant for the all-trans-retinaldehyde specificity of AKR1B10. Substitutions in the retinoid cyclohexene ring, analyzed here further, also influence such specificity. Overall it is suggested that the rate-limiting step in the reaction mechanism with retinaldehyde differs between AKR1B1 and AKR1B10. In addition, we demonstrate here that enzymatic activity of AKR1B1 and AKR1B10 lowers all-trans- and 9-cis-retinoic acid-dependent trans-activation in living cells, indicating that both enzymes may contribute to pre-receptor regulation of retinoic acid and retinoid X nuclear receptors. This result supports that overexpression of AKR1B10 in cancer (an updated review on this topic is included) may contribute to dedifferentiation and tumor development.
No preview · Article · Nov 2008 · Chemico-biological interactions
[Show abstract][Hide abstract] ABSTRACT: AKR1B10 is a human aldo-keto reductase (AKR) found to be elevated in several cancer types and in precancerous lesions. In vitro, AKR1B10 exhibits a much higher retinaldehyde reductase activity than any other human AKR, including AKR1B1 (aldose reductase). We here demonstrate that AKR1B10 also acts as a retinaldehyde reductase in vivo. This activity may be relevant in controlling the first step of retinoic acid synthesis. Up-regulation of AKR1B10, resulting in retinoic acid depletion, may lead to cellular proliferation. Both in vitro and in vivo activities of AKR1B10 were inhibited by tolrestat, an AKR1B1 inhibitor developed for diabetes treatment. The crystal structure of the ternary complex AKR1B10-NADP(+)-tolrestat was determined at 1.25-A resolution. Molecular dynamics models of AKR1B10 and AKR1B1 with retinaldehyde isomers and site-directed mutagenesis show that subtle differences at the entrance of the retinoid-binding site, especially at position 125, are determinant for the all-trans-retinaldehyde specificity of AKR1B10. Substitutions in the retinaldehyde cyclohexene ring also influence the specificity. These structural features should facilitate the design of specific inhibitors, with potential use in cancer and diabetes treatments.
Full-text · Article · Jan 2008 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Zeta-crystallins constitute a family of proteins with NADPH:quinone reductase activity found initially in mammalian lenses but now known to be present in many other organisms and tissues. Few proteins from this family have been characterized, and their function remains unclear. In the present work, zeta-crystallins from human and yeast (Zta1p) were expressed, purified and characterized. Both enzymes are able to reduce ortho-quinones in the presence of NADPH but are not active with 2-alkenals. Deletion of the ZTA1 gene makes yeast more sensitive to menadione and hydrogen peroxide, suggesting a role in the oxidative stress response. The human and yeast enzymes specifically bind to adenine-uracil rich elements (ARE) in RNA, indicating that both enzymes are ARE-binding proteins and that this property has been conserved in zeta-crystallins throughout evolution. This supports a role for zeta-crystallins as trans-acting factors that could regulate the turnover of certain mRNAs.
No preview · Article · Jul 2007 · Cellular and Molecular Life Sciences CMLS
[Show abstract][Hide abstract] ABSTRACT: The metabolism of all-trans- and 9-cis-retinol/ retinaldehyde has been investigated with focus on the activities of human, mouse and rat alcohol dehydrogenase 2 (ADH2), an intriguing enzyme with apparently different functions in human and rodents. Kinetic constants were determined with an HPLC method and a structural approach was implemented by in silico substrate dockings. For human ADH2, the determined K(m) values ranged from 0.05 to 0.3 microM and k(cat) values from 2.3 to 17.6 min(-1), while the catalytic efficiency for 9-cis-retinol showed the highest value for any substrate. In contrast, poor activities were detected for the rodent enzymes. A mouse ADH2 mutant (ADH2Pro47His) was studied that resembles the human ADH2 setup. This mutation increased the retinoid activity up to 100-fold. The K(m) values of human ADH2 are the lowest among all known human retinol dehydrogenases, which clearly support a role in hepatic retinol oxidation at physiological concentrations.
No preview · Article · Mar 2007 · Cellular and Molecular Life Sciences CMLS
[Show abstract][Hide abstract] ABSTRACT: Retinoic acid biosynthesis in vertebrates occurs in two consecutive steps: the oxidation of retinol to retinaldehyde followed by the oxidation of retinaldehyde to retinoic acid. Enzymes of the MDR (medium-chain dehydrogenase/reductase), SDR (short-chain dehydrogenase/reductase) and AKR (aldo-keto reductase) superfamilies have been reported to catalyse the conversion between retinol and retinaldehyde. Estimation of the relative contribution of enzymes of each type was difficult since kinetics were performed with different methodologies, but SDRs would supposedly play a major role because of their low K(m) values, and because they were found to be active with retinol bound to CRBPI (cellular retinol binding protein type I). In the present study we employed detergent-free assays and HPLC-based methodology to characterize side-by-side the retinoid-converting activities of human MDR [ADH (alcohol dehydrogenase) 1B2 and ADH4), SDR (RoDH (retinol dehydrogenase)-4 and RDH11] and AKR (AKR1B1 and AKR1B10) enzymes. Our results demonstrate that none of the enzymes, including the SDR members, are active with CRBPI-bound retinoids, which questions the previously suggested role of CRBPI as a retinol supplier in the retinoic acid synthesis pathway. The members of all three superfamilies exhibit similar and low K(m) values for retinoids (0.12-1.1 microM), whilst they strongly differ in their kcat values, which range from 0.35 min(-1) for AKR1B1 to 302 min(-1) for ADH4. ADHs appear to be more effective retinol dehydrogenases than SDRs because of their higher kcat values, whereas RDH11 and AKR1B10 are efficient retinaldehyde reductases. Cell culture studies support a role for RoDH-4 as a retinol dehydrogenase and for AKR1B1 as a retinaldehyde reductase in vivo.
Full-text · Article · Nov 2006 · Biochemical Journal
[Show abstract][Hide abstract] ABSTRACT: (R)-all-trans-3-hydroxyretinal 1, (S)-all-trans-4-hydroxyretinal and (R)-all-trans-4-hydroxyretinal have been synthesized stereoselectively by Horner-Wadsworth-Emmons and Stille cross-coupling as bond-forming reactions. The CBS method of ketone reduction was used in the enantioface-differentiation step to provide the precursors for the synthesis of the 4-hydroxyretinal enantiomers. The kinetic constants of Xenopus laevis ADH8 with these retinoids have been determined.
No preview · Article · Feb 2006 · Organic & Biomolecular Chemistry
[Show abstract][Hide abstract] ABSTRACT: Human stomach mucosa contains a characteristic alcohol dehydrogenase (ADH) enzyme, σσ-ADH. Its cDNA has been cloned from a human stomach library and sequenced. The deduced amino acid sequence shows 59–70% identities with the other human ADH classes, demonstrating that the stomach enzyme represents a distinct structure, constituting class IV, coded by a separate gene, ADH7. The amino acid identity with the rat stomach class IV ADH is 88%, which is intermediate between constant and variable dehydrogenases. This value reflects higher conservation than for the classical liver enzymes of class I, compatible with a separate functional significance of the class IV enzyme. Its enzymic features can be correlated with its structural characteristics. The residues lining the substrate-binding cleft are bulky and hydrophobic, similar to those of the class I enzyme; this explains the similar specificity of both classes, compatible with the origin of class IV from class I. Position 47 has Arg, in contrast to Gly in the rat class IV enzyme, but this Arg is still associated with an extremely high activity (kcat= 1510 min-1) and weak coenzyme binding (KiaNAD+= 1.6 mM). Thus, the strong interaction with coenzyme imposed by Arg47 in class I is probably compensated for in class IV by changes that may negatively affect coenzyme binding: Glu230, His271, Asn260, Asn261, Asn363. The still higher activity and weaker coenzyme binding of rat class IV (kcat= 2600 min-1, KiaNAD = 4 mM) can be correlated to the exchanges to Gly47, Gln230 and Tyr363. An important change at position 294, with Val in human and Ala in rat class IV, is probably responsible for the dramatic difference in Km values for ethanol between human (37 mM) and rat (2.4 M) class IV enzymes.
[Show abstract][Hide abstract] ABSTRACT: This chapter discusses the methods for histological localization of alcohol dehydrogenase. Tissue localization of multiple enzyme forms of human Alcohol Dehydrogenase (ADH) is of paramount importance to understand their physiological involvement in ethanol metabolism and organ susceptibility to alcohol damage. Histological techniques-such as immunohistochemistry and in situ hybridization, provide powerful and complementary approaches to identify definite cell layers or cell types that may contribute to ethanol elimination and, for that matter, be subjected to the local toxic effects of ethanol. Immunohistochemical methods rely on the availability of specific antibodies against different ADH forms. When polyclonal antiserum is the starting material, antibodies specific for a given ADH should be affinity purified and thoroughly checked for cross-reactivity against other ADH forms and proteins by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis. In situ hybridization methods, being highly sensitive, should use specific homologous probes, which have been previously characterized by Northern blot analysis. With either technique, it is essential to perform adequate control experiments by using pre-immune serum, pre-absorbed antiserum or sense RNA probes. Experimental conditions should always be carefully optimized and it is advisable that results be validated with appropriate functional tests or activity assays.
[Show abstract][Hide abstract] ABSTRACT: Ring-oxidized retinoids have been synthesized stereoselectively using the Stille cross-coupling reaction. Kinetic constants of mouse class I alcohol dehydrogenase (ADH1) with these retinoids were determined.
No preview · Article · Dec 2004 · Organic & Biomolecular Chemistry
[Show abstract][Hide abstract] ABSTRACT: Human alcohol dehydrogenases (ADH1 and ADH4) actively use retinoids oxidized at the cyclohexenyl ring (4-oxo-, 4-hydroxy-, and 3,4-didehydro-retinoids), which are functional compounds in several cells and tissues (i.e., in human skin). Remarkably, activities with 4-oxo-retinal and 4-hydroxy-retinol (kcat = 2050 min(-1) for ADH4) are the highest among retinoids, similar to those of the best aliphatic alcohols. Thus, ADH1 and ADH4 provide a metabolic pathway for the synthesis of the corresponding retinoic acids. Tween 80, a widely used detergent in the retinoid activity assay, behaves as a competitive inhibitor. The Km values for all-trans-retinol (2-3 microM), estimated in the absence of detergent, are 10-fold lower than those obtained at the usual 0.02% Tween 80. This suggests a contribution of ADH to retinoid metabolism more relevant than previously expected. However, Tween 80 stabilizes retinoids in water solution and provides a reliable and reproducible assay, suitable for comparing different ADHs and different retinoid substrates.
No preview · Article · Nov 2004 · Archives of Biochemistry and Biophysics
[Show abstract][Hide abstract] ABSTRACT: The crystal structure of Saccharomyces cerevisiae ScAdh6p has been solved using the anomalous signal from the two zinc atoms found per subunit, and it constitutes the first structure determined from a member of the cinnamyl alcohol dehydrogenase family. ScAdh6p subunits exhibit the general fold of the medium-chain dehydrogenases/reductases (MDR) but with distinct specific characteristics. In the three crystal structures solved (two trigonal and one monoclinic), ScAdh6p molecules appear to be structural heterodimers composed of one subunit in the apo and the second subunit in the holo conformation. Between the two conformations, the relative disposition of domains remains unchanged, while two loops, Cys250-Asn260 and Ile277-Lys292, experience large movements. The apo-apo structure is disfavoured because of steric impairment involving the loop Ile277-Lys292, while in the holo-holo conformation some of the hydrogen bonds between subunits would break apart. These suggest that the first NADPH molecule would bind to the enzyme much more tightly than the second. In addition, fluorimetric analysis of NADPH binding demonstrates that only one cofactor molecule binds per dimer. Therefore, ScAdh6p appears to function according to a half-of-the-sites reactivity mechanism, resulting from a pre-existing (prior to cofactor binding) tendency for the structural asymmetry in the dimer. The specificity of ScAdh6p towards NADPH is mainly due to the tripod-like interactions of the terminal phosphate group with Ser210, Arg211 and Lys215. The size and the shape of the substrate-binding pocket correlate well with the substrate specificity of ScAdh6p towards cinnamaldehyde and other aromatic compounds. The structural relationships of ScAdh6p with other MDR structures are analysed.
No preview · Article · Sep 2004 · Journal of Molecular Biology
[Show abstract][Hide abstract] ABSTRACT: Studies in knockout mice support the involvement of alcohol dehydrogenases ADH1 and ADH4 in retinoid metabolism, although kinetics with retinoids are not known for the mouse enzymes. Moreover, a role of alcohol dehydrogenase (ADH) in the eye retinoid interconversions cannot be ascertained due to the lack of information on the kinetics with 11-cis-retinoids. We report here the kinetics of human ADH1B1, ADH1B2, ADH4, and mouse ADH1 and ADH4 with all-trans-, 7-cis-, 9-cis-, 11-cis- and 13-cis-isomers of retinol and retinal. These retinoids are substrates for all enzymes tested, except the 13-cis isomers which are not used by ADH1. In general, human and mouse ADH4 exhibit similar activity, higher than that of ADH1, while mouse ADH1 is more efficient than the homologous human enzymes. All tested ADHs use 11-cis-retinoids efficiently. ADH4 shows much higher k(cat)/K(m) values for 11-cis-retinol oxidation than for 11-cis-retinal reduction, a unique property among mammalian ADHs for any alcohol/aldehyde substrate pair. Docking simulations and the kinetic properties of the human ADH4 M141L mutant demonstrated that residue 141, in the middle region of the active site, is essential for such ADH4 specificity. The distinct kinetics of ADH4 with 11-cis-retinol, its wide specificity with retinol isomers and its immunolocalization in several retinal cell layers, including pigment epithelium, support a role of this enzyme in the various retinol oxidations that occur in the retina. Cytosolic ADH4 activity may complement the isomer-specific microsomal enzymes involved in photopigment regeneration and retinoic acid synthesis.
Full-text · Article · Jun 2004 · European Journal of Biochemistry
[Show abstract][Hide abstract] ABSTRACT: Gastric tissues from amphibian Rana perezi express the only vertebrate alcohol dehydrogenase (ADH8) that is specific for NADP(H) instead of NAD(H). In the crystallographic
ADH8-NADP+ complex, a binding pocket for the extra phosphate group of coenzyme is formed by ADH8-specific residues Gly223-Thr224-His225, and the highly conserved Leu200 and Lys228. To investigate the minimal structural determinants for coenzyme specificity, several ADH8 mutants involving residues 223
to 225 were engineered and kinetically characterized. Computer-assisted modeling of the docked coenzymes was also performed
with the mutant enzymes and compared with the wild-type crystallographic binary complex. The G223D mutant, having a negative
charge in the phosphate-binding site, still preferred NADP(H) over NAD(H), as did the T224I and H225N mutants. Catalytic efficiency
with NADP(H) dropped dramatically in the double mutants, G223D/T224I and T224I/H225N, and in the triple mutant, G223D/T224I/H225N
(kcat/KmNADPH = 760 mm-1 min-1), as compared with the wild-type enzyme (kcat/KmNADPH = 133,330 mm-1 min-1). This was associated with a lower binding affinity for NADP+ and a change in the rate-limiting step. Conversely, in the triple mutant, catalytic efficiency with NAD(H) increased, reaching
values (kcat/KmNADH = 155,000 mm-1 min-1) similar to those of the wild-type enzyme with NADP(H). The complete reversal of ADH8 coenzyme specificity was therefore
attained by the substitution of only three consecutive residues in the phosphate-binding site, an unprecedented achievement
within the ADH family.
No preview · Article · Nov 2003 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: To gain a better insight into the alterations of brain function after chronic ethanol, we measured the release of various neurotransmitters from nerve terminals of cortex and hippocampus isolated fm rats chronically fed with ethanol. The K+-evoked release of [3H]acetylcholine (ACh), f[H]dopamine (DA), [3H] glutamate(Glu) and [3H]noradrenaline (NA) was determined in superfused synaptosomes of brain cortex and hippocampus from rats exposed to the Lieber-DeCarli alcohol liquid diet for 5 weeks. In cortical synaptosomes, chronic ethanol administration did not affect the release of ACh and of DA, while significantly decreasing the release of Glu and NA. The endogenous levels of NA, DA and their metabolites were unchanged. In hippocampal synaptosomes the only effect of chronic alcohol was an increased release of Glu. It can be concluded that at presynaptic level chronic ethanol alters brain neurotransmitter systems selectively. Glutamatergic and noradrenergic nerve terminals from cortex are more vulnerable than those from hippocampus.
No preview · Article · Oct 2003 · Addiction Biology
[Show abstract][Hide abstract] ABSTRACT: The ADH2 gene codes for the Arabidopsis glutathione-dependent formaldehyde dehydrogenase (FALDH), an enzyme involved in formaldehyde metabolism in eukaryotes. In the present work, we have investigated the potential role of FALDH in detoxification of exogenous formaldehyde. We have generated a yeast (Saccharomyces cerevisiae) mutant strain (sfa1Delta) by in vivo deletion of the SFA1 gene that codes for the endogenous FALDH. Overexpression of Arabidopsis FALDH in this mutant confers high resistance to formaldehyde added exogenously, which demonstrates the functional conservation of the enzyme through evolution and supports its essential role in formaldehyde metabolism. To investigate the role of the enzyme in plants, we have generated Arabidopsis transgenic lines with modified levels of FALDH. Plants overexpressing the enzyme show a 25% increase in their efficiency to take up exogenous formaldehyde, whereas plants with reduced levels of FALDH (due to either a cosuppression phenotype or to the expression of an antisense construct) show a marked slower rate and reduced ability for formaldehyde detoxification as compared with the wild-type Arabidopsis. These results show that the capacity to take up and detoxify high concentrations of formaldehyde is proportionally related to the FALDH activity in the plant, revealing the essential role of this enzyme in formaldehyde detoxification.
[Show abstract][Hide abstract] ABSTRACT: Aldo-keto reductases (AKRs) are NAD(P)H-dependent oxidoreductases that catalyse the reduction of a variety of carbonyl compounds, such as carbohydrates, aliphatic and aromatic aldehydes and steroids. We have studied the retinal reductase activity of human aldose reductase (AR), human small-intestine (HSI) AR and pig aldehyde reductase. Human AR and HSI AR were very efficient in the reduction of all- trans -, 9- cis - and 13- cis -retinal ( k (cat)/ K (m)=1100-10300 mM(-1).min(-1)), constituting the first cytosolic NADP(H)-dependent retinal reductases described in humans. Aldehyde reductase showed no activity with these retinal isomers. Glucose was a poor inhibitor ( K (i)=80 mM) of retinal reductase activity of human AR, whereas tolrestat, a classical AKR inhibitor used pharmacologically to treat diabetes, inhibited retinal reduction by human AR and HSI AR. All- trans -retinoic acid failed to inhibit both enzymes. In this paper we present the AKRs as an emergent superfamily of retinal-active enzymes, putatively involved in the regulation of retinoid biological activity through the assimilation of retinoids from beta-carotene and the control of retinal bioavailability.