Cloning of a cDNA encoding an aldehyde dehydrogenase and its expression in Escherichia coli. Recognition of retinal as substrate

Estrogen receptor alpha regulates retinaldehyde dehydrogenase 1 expression in rat anterior pituitary cells

Retinoic acid (RA) plays a critical role in embryonic development, growth, and reproduction. RA is synthesized from retinoids via oxidation processes, and the oxidation of retinal to RA is catalyzed by the retinaldehyde dehydrogenases (RALDHs). We previously reported that RALDH1 mRNA was expressed in the anterior pituitary glands of adult rats and suppressed by administration of 17beta-estradiol in vivo. However, little is known about the mechanism regulating pituitary RALDH1 expression. In order to characterize the mechanism of estrogen-induced RALDH1 reduction, we examined the effect of 17beta-estradiol on the regulation of pituitary RALDH1 gene expression and protein production both in vivo and in vitro. Using quantitative real-time PCR and immunoblot analysis, we found that levels of RALDH1 gene expression and protein production markedly decreased after 1-week treatment with 17beta-estradiol in male rats. In immunohistochemical analysis, RALDH1-immunoreaction was observed in prolactin cells and folliculo-stellate cells. In 17beta-estradiol-treated rats, RALDH1-immunoreactivity was lower in prolactin cells, but not in folliculo-stellate cells. Treatment of isolated anterior pituitary cells with 17beta-estradiol (10(-14) - 10(-8) M) decreased expression of RALDH1 mRNA in a dose-dependent manner. Estradiol-induced suppression of RALDH1 expression was completely blocked by the estrogen receptor (ER) antagonist ICI 182, 780. The ERalpha-selective agonist propylpyrazole triol (10(-8) M) mimicked the effect of 17beta-estradiol on RALDH1 expression, but the ERbeta-selective agonist diarylpropionitrile (10(-8) M) did not. These results strongly suggest that RALDH1 mRNA expression is suppressed by 17beta-estradiol through ERalpha, and that estrogen regulates the expression of RALDH1 and production of RA in the anterior pituitary gland.

Non-P450 aldehyde oxidizing enzymes: the aldehyde dehydrogenase superfamily

BACKGROUND

Aldehydes are highly reactive molecules. While several non-P450 enzyme systems participate in their metabolism, one of the most important is the aldehyde dehydrogenase (ALDH) superfamily, composed of NAD(P)+-dependent enzymes that catalyze aldehyde oxidation.

OBJECTIVE

This article presents a review of what is currently known about each member of the human ALDH superfamily including the pathophysiological significance of these enzymes.

METHODS

Relevant literature involving all members of the human ALDH family was extensively reviewed, with the primary focus on recent and novel findings.

CONCLUSION

To date, 19 ALDH genes have been identified in the human genome and mutations in these genes and subsequent inborn errors in aldehyde metabolism are the molecular basis of several diseases, including Sjögren-Larsson syndrome, type II hyperprolinemia, gamma-hydroxybutyric aciduria and pyridoxine-dependent seizures. ALDH enzymes also play important roles in embryogenesis and development, neurotransmission, oxidative stress and cancer. Finally, ALDH enzymes display multiple catalytic and non-catalytic functions including ester hydrolysis, antioxidant properties, xenobiotic bioactivation and UV light absorption.

Retinoic acid modulates retinaldehyde dehydrogenase 1 gene expression through the induction of GADD153-C/EBPbeta interaction

Mammalian class I aldehyde dehydrogenase (ALDH) plays an important role in the biosynthesis of the hormone retinoic acid (RA), which modulates gene expression and cell differentiation. RA has been shown to mediate control of human ALDH1 gene expression through modulation of the retinoic acid receptor alpha (RARalpha) and the CCAAT/enhancer binding protein beta (C/EBPbeta). The positive activation of these transcription factors on the ALDH1 promoter is inhibited by RA through a decrease of C/EBPbeta binding to the ALDH1 CCAAT box response element. However, the mechanism of this effect remains unknown. Here we report that the RARalpha/retinoid X receptor beta (RXRbeta) complex binds to the mouse retinaldehyde dehydrogenase 1 (Raldh1) promoter at a non-consensus RA response element (RARE) with similar affinity to that of the consensus RARE. We found that C/EBPbeta binds to a Raldh1 CCAAT box located at -82/-58bp, adjacent to the RARE. Treatment with RA increases GADD153 and GADD153-C/EBPbeta interaction resulting in a decreased cellular availability of C/EBPbeta for binding to the Raldh1 CCAAT box. These data support a model in which high RA levels inhibit Raldh1 gene expression by sequestering C/EBPbeta through its interaction to GADD153.

Tissue distribution, ontogeny, and regulation of aldehyde dehydrogenase (Aldh) enzymes mRNA by prototypical microsomal enzyme inducers in mice

Aldehyde dehydrogenases (Aldhs) are a group of nicotinamide adenine dinucleotide phosphate-dependent enzymes that catalyze the oxidation of a wide spectrum of aldehydes to carboxylic acids. Tissue distribution and developmental changes in the expression of the messenger RNA (mRNA) of 15 Aldh enzymes were quantified in male and female mice tissues using the branched DNA signal amplification assay. Furthermore, the regulation of the mRNA expression of Aldhs by 15 typical microsomal enzyme inducers (MEIs) was studied. Aldh1a1 mRNA expression was highest in ovary; 1a2 in testis; 1a3 in placenta; 1a7 in lung; 1b1 in small intestine; 2 in liver; 3a1 in stomach; 3a2 and 3b1 expression was ubiquitous; 4a1, 6a1, 7a1, and 8a1 in liver and kidney; 9a1 in liver, kidney, and small intestine; and 18a1 in ovary and small intestine. mRNAs of different Aldh enzymes were detected at lower levels in fetuses than adult mice and gradually increased after birth to reach adult levels between 15 and 45 days of age, when the gender difference began to appear. Aromatic hydrocarbon receptor (AhR) ligands induced the liver mRNA expression of Aldh1a7, 1b1, and 3a1, constitutive androstane receptor (CAR) activators induced Aldh1a1 and 1a7, whereas pregnane X receptor (PXR) ligands and NF-E2 related factor 2 (Nrf2) activators induced Aldh1a1, 1a7, and 1b1. Peroxisome proliferator activator receptor alpha (PPAR alpha) ligands induced the mRNA expression in liver of almost all Aldhs. The Aldh organ-specific distribution may be important in elucidating their role in metabolism, elimination, and organ-specific toxicity of xenobiotics. Finally, in contrast to other phase-I metabolic enzymes such as CYP450 enzymes, Aldh mRNA expression seems to be generally insensitive to typical microsomal inducers except PPAR alpha ligands.

Altered vitamin A homeostasis and increased size and adiposity in the rdh1-null mouse

Rat RoDH performs efficiently (V(m)/K(m)) in a pathway of all-trans-retinoic acid biosynthesis in cells and recognizes the physiological form of vitamin A, i.e., retinol bound with cellular retinol binding-protein, type I. Here we report that mouse embryo (e7.5 to e18.5) and liver (e12.5 to P2M) display inversely related mRNA expression of an Rodh ortholog, rdh1, and a major retinoic acid catabolic enzyme, cyp26a1, suggesting coordinate modulation of retinoic acid homeostasis. Rdh1 inactivation by homologous recombination produces mice with decreased liver cyp26a1 mRNA and protein and increased liver and kidney retinoid stores, when fed vitamin A-restricted diets. Thus, null mice autocompensate by down-regulating cyp26a1 and sparing retinoids, indicating that rdh1 metabolizes retinoids in vivo. Surprisingly, rdh1-null mice grow longer than wild type, with increased weight and adiposity, when restricted in vitamin A. Liver, kidney, and multiple fat pads increase in weight. Some differences reflect the larger sizes of rdh1-null mice, but mesentery, femoral, and inguinal fat pads grow disproportionately larger. These data reveal an unexpected contribution of Rdh1 to size and adiposity and provide the first genetic evidence of a candidate retinol dehydrogenase affecting either vitamin A-related homeostasis physiologically or vitamin A-related gene expression or biological function in vivo.

Specificity of zebrafish retinol saturase: formation of all-trans-13,14-dihydroretinol and all-trans-7,8- dihydroretinol

Metabolism of vitamin A, all-trans-retinol, leads to the formation of 11-cis-retinaldehyde, the visual chromophore, and all-trans-retinoic acid, which is involved in the regulation of gene expression through the retinoic acid receptor. Enzymes and binding proteins involved in retinoid metabolism are highly conserved across species. We previously described a novel mammalian enzyme that saturates the 13-14 double bond of all-trans-retinol to produce all-trans-13,14-dihydroretinol, which then follows the same metabolic fate as that of all-trans-retinol. Specifically, all-trans-13,14-dihydroretinol is transiently oxidized to all-trans-13,14-dihydroretinoic acid before being oxidized further by Cyp26 enzymes. Here, we report the identification of two putative RetSat homologues in zebrafish, one of which, zebrafish RetSat A (zRetSat A), also had retinol saturase activity, whereas zebrafish RetSat B (zRetSat B) was inactive under similar conditions. Unlike mouse RetSat (mRetSat), zRetSat A had an altered bond specificity saturating either the 13-14 or 7-8 double bonds of all-trans-retinol to produce either all-trans-13,14-dihydroretinol or all-trans-7,8-dihydroretinol, respectively. zRetSat A also saturated the 13-14 or 7-8 double bonds of all-trans-3,4-didehydroretinol (vitamin A2), a second endogenous form of vitamin A in zebrafish. The dual enzymatic activity of zRetSat A displays a newly acquired specificity for the 13-14 double bond retained in higher vertebrates and also the evolutionarily preserved activity of bacterial phytoene desaturases and plant carotenoid isomerases. Expression of zRetSat A was restricted to the liver and intestine of hatchlings and adult zebrafish, whereas zRetSat B was expressed in the same tissues but at earlier developmental stages. Exogenous all-trans-retinol, all-trans-13,14-dihydroretinol, or all-trans-7,8-dihydroretinol led to the strong induction of the expression of the retinoic acid-metabolizing enzyme, Cyp26A1, arguing for an active signaling function of dihydroretinoid metabolites in zebrafish. These findings point to a conserved function but altered specificity of RetSat in vertebrates, leading to the generation of various dihydroretinoid compounds, some of which could have signaling functions.

Uncompetitive inhibition of Xenopus laevis aldehyde dehydrogenase 1A1 by divalent cations

Aldehyde dehydrogenases (ALDHs) convert aldehydes into their corresponding carboxylic acids. ALDH1A1, also known as ALDH class 1 (ALDH1) or retinaldehyde dehydrogenase (RALDH1), prefers retinal to acetaldehyde as a substrate. To investigate the effects of divalent cations on the dehydrogenase activity of Xenopus laevis ALDH1A1, the formation of acetate and retinoic acid from acetaldehyde and retinal, respectively, was investigated in the presence of Ca2+, Mg2+, Mn2+ or Zn2+. All divalent cations tested inhibited the oxidation of acetaldehyde and retinal by ALDH1A1. When acetaldehyde was used as a substrate, the 50% inhibitory concentrations (IC50) were 10, 24, 35 and 220 microM for Zn2+, Mn2+, Mg2+ and Ca2+, respectively. Kinetic studies of ALDH1A1 dehydrogenase activity in the presence or absence of each cation revealed that the inhibition mode by cations was uncompetitive against acetaldehyde, retinal, and NAD+, and that their inhibitory potencies were greater against acetaldehyde than retinal. It was concluded that the divalent cations inhibited X. laevis ALDH1A1 activity in a substrate-dependent manner by affecting a step of the dehydrogenase reaction that occurred after the formation of the ternary complex of the enzyme, substrate, and coenzyme.

Localization of retinaldehyde dehydrogenases and retinoid binding proteins to sustentacular cells, glia, Bowman's gland cells, and stroma: potential sites of retinoic acid synthesis in the postnatal rat olfactory organ

Work from our laboratory suggests that retinoic acid (RA) influences neuron development in the postnatal olfactory epithelium (OE). The studies reported here were carried out to identify and localize retinaldehyde dehydrogenase (RALDH) expression in postnatal rat OE to gain a better understanding of potential in vivo RA synthesis sites in this continuously regenerating tissue. RALDH 1, 2, and 3 mRNAs were detected in postnatal rat olfactory tissue by RT-PCR analysis, but RALDH 1 and 2 transcripts were predominant. RALDH 1 immunoreactivity was localized to sustentacular cells in the OE and to Bowman's gland cells, and GFAP(+)/p75(-) olfactory ensheathing cells (OECs) in the underlying lamina propria (LP). RALDH 2 did not colocalize with RALDH 1, but appeared to be expressed in GFAP(-)/RALDH 1(-) OECs as well as in unidentified structures in the LP. Cellular RA binding protein (CRABP II) colocalized with RALDH 1. Cellular retinol/retinaldehyde binding protein (CRBP I) was localized to RALDH 1(+) sites in the OE and LP and RALDH 2(+) sites, primarily surrounding nerve fiber bundles in the LP. Vitamin A deficiency altered RALDH 1, but not RALDH 2 protein expression. The isozymes and binding proteins exhibited random variability in levels and areas of expression both within and between animals. These findings support the hypothesis that RA is synthesized in the postnatal OE (catalyzed by RALDH 1) and underlying LP (differentially catalyzed by RALDH 1 and RALDH 2) at sites that could influence the development, maturation, targeting, and/or turnover of olfactory receptor neurons throughout the olfactory organ.

Characterization of retinaldehyde dehydrogenase 3

RALDH3 (retinal dehydrogenase 3) was characterized by kinetic and binding studies, protein engineering, homology modelling, ligand docking and electrostatic-potential calculations. The major recognition determinant of an RALDH3 substrate was shown to be an eight-carbon chain bonded to the aldehyde group whose kinetic influence (kcat/Km at pH 8.5) decreases when shortened or lengthened. Surprisingly, the b-ionone ring of all-trans-retinal is not a major recognition site. The dissociation constants (Kd) of the complexes of RALDH3 with octanal, NAD+ and NADH were determined by intrinsic tryptophan fluorescence. The similarity of the Kd values for the complexes with NAD+ and with octanal suggests a random kinetic mechanism for RALDH3, in contrast with the ordered sequential mechanism often associated with aldehyde dehydrogenase enzymes. Inhibition of RALDH3 by tri-iodothyronine binding in competition with NAD+, predicted by the modelling, was established kinetically and by immunoprecipitation. Mechanistic implications of the kinetically influential ionizations with macroscopic pKa values of 5.0 and 7.5 revealed by the pH-dependence of kcat are discussed. Analogies with data for non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase from Streptococcus mutans, together with the present modelled structure of the thioacyl RALDH3, suggest (a) that kcat characterizes deacylation of this intermediate for specific substrates and (b) the assignment of the pKa of the major ionization (approximating to 7.5) to the perturbed carboxy group of Glu280 whose conjugate base is envisaged as supplying general base catalysis to attack of a water molecule. The macroscopic pKa of the minor ionization (5.0) is considered to approximate to that of the carboxy group of Glu488.

Developmental Expression of Aldehyde Dehydrogenase in Rat: a Comparison of Liver and Lung Development

Metabolism is one of the major determinants for age-related changes in susceptibility to chemicals. Aldehydes are highly reactive molecules present in the environment that also can be produced during biotransformation of xenobiotics and endogenous metabolism. Although the lung is a major target for aldehyde toxicity, early development of aldehyde dehydrogenases (ALDHs) in lung has been poorly studied. The expression of ALDH in liver and lung across ages (postnatal day 1, 8, 22, and 60) was investigated in Wistar-Han rats. In adult, the majority of hepatic ALDH activity was found in mitochondria, while cytosolic ALDH activity was the highest contributor in lung. Total aldehyde oxidation capability in liver increases with age, but stays constant in lung. These overall developmental profiles of ALDH expression in a tissue appear to be determined by the different composition of ALDH isoforms within the tissue and their independent temporal and tissue-specific development. ALDH2 showed the most notable tissue-specific development. Hepatic ALDH2 was increased with age, while the pulmonary form did not. ALDH1 was at its maximum value at postnatal day 1 (PND1) and decreased thereafter both in liver and lung. ALDH3 increased with age in liver and lung, although ALDH3A1 was only detectible in lung. Collectively, the present study indicates that, in the case of aldehyde exposure, the in vivo responses would be tissue and age dependent.

Cloning and expression analysis of retinoic acid receptors in the zebra finch brain

The vitamin A derivative retinoic acid is produced postembryonically in discrete portions of the songbird brain, including some of the nuclei involved in song production and song learning, and its synthesis is required for the normal maturation of song behavior. To identify the brain targets for retinoic acid action, we cloned the zebra finch homologs of the alpha, beta, and gamma classes of retinoic acid receptors (RARs). In situ hybridization analysis revealed that the mRNAs for all three RARs are expressed at different levels in several brain areas, with a broader distribution than the mRNA for retinaldehyde-specific aldehyde dehydrogenase (zRalDH), a retinoic acid-synthesizing enzyme. Detectable RAR expression was found in all nuclei of the song control system, with the most marked expression occurring within the striatal song nucleus area X. These observations are consistent with a persistent action of retinoic acid in the postembryonic and adult songbird brain and provide further evidence for an involvement of retinoic acid signaling in the control of learned vocal behavior in a songbird species. They also suggest that the striatum is a major target of retinoic acid in songbirds.

Biochemical properties of purified human retinol dehydrogenase 12 (RDH12): catalytic efficiency toward retinoids and C9 aldehydes and effects of cellular retinol-binding protein type I (CRBPI) and cellular retinaldehyde-binding protein (CRALBP) on the oxidation and reduction of retinoids

Retinol dehydrogenase 12 (RDH12) is a novel member of the short-chain dehydrogenase/reductase superfamily of proteins that was recently linked to Leber's congenital amaurosis 3 (LCA). We report the first biochemical characterization of purified human RDH12 and analysis of its expression in human tissues. RDH12 exhibits approximately 2000-fold lower K(m) values for NADP(+) and NADPH than for NAD(+) and NADH and recognizes both retinoids and lipid peroxidation products (C(9) aldehydes) as substrates. The k(cat) values of RDH12 for retinaldehydes and C(9) aldehydes are similar, but the K(m) values are, in general, lower for retinoids. The enzyme exhibits the highest catalytic efficiency for all-trans-retinal (k(cat)/K(m) approximately 900 min(-)(1) microM(-)(1)), followed by 11-cis-retinal (450 min(-)(1) mM(-)(1)) and 9-cis-retinal (100 min(-)(1) mM(-)(1)). Analysis of RDH12 activity toward retinoids in the presence of cellular retinol-binding protein (CRBP) type I or cellular retinaldehyde-binding protein (CRALBP) suggests that RDH12 utilizes the unbound forms of all-trans- and 11-cis-retinoids. As a result, the widely expressed CRBPI, which binds all-trans-retinol with much higher affinity than all-trans-retinaldehyde, restricts the oxidation of all-trans-retinol by RDH12, but has little effect on the reduction of all-trans-retinaldehyde, and CRALBP inhibits the reduction of 11-cis-retinal stronger than the oxidation of 11-cis-retinol, in accord with its higher affinity for 11-cis-retinal. Together, the tissue distribution of RDH12 and its catalytic properties suggest that, in most tissues, RDH12 primarily contributes to the reduction of all-trans-retinaldehyde; however, at saturating concentrations of peroxidic aldehydes in the cells undergoing oxidative stress, for example, photoreceptors, RDH12 might also play a role in detoxification of lipid peroxidation products.

Metabolism and transactivation activity of 13,14-dihydroretinoic acid

The metabolism of vitamin A is a highly regulated process that generates essential mediators involved in the development, cellular differentiation, immunity, and vision of vertebrates. Retinol saturase converts all-trans-retinol to all-trans-13,14-dihydroretinol (Moise, A. R., Kuksa, V., Imanishi, Y., and Palczewski, K. (2004) J. Biol. Chem. 279, 50230-50242). Here we demonstrate that the enzymes involved in oxidation of retinol to retinoic acid and then to oxidized retinoic acid metabolites are also involved in the synthesis and oxidation of all-trans-13,14-dihydroretinoic acid. All-trans-13,14-dihydroretinoic acid can activate retinoic acid receptor/retinoid X receptor heterodimers but not retinoid X receptor homodimers in reporter cell assays. All-trans-13,14-dihydroretinoic acid was detected in vivo in Lrat-/- mice supplemented with retinyl palmitate. Thus, all-trans-13,14-dihydroretinoic acid is a naturally occurring retinoid and a potential ligand for nuclear receptors. This new metabolite can also be an intermediate in a retinol degradation pathway or it can serve as a precursor for the synthesis of bioactive 13,14-dihydroretinoid metabolites.

Retinoic acid synthesis by a population of NG2-positive cells in the injured spinal cord

Retinoic acid (RA) promotes growth and differentiation in many developing tissues but less is known about its influence on CNS regeneration. We investigated the possible involvement of RA in rat spinal cord injury (SCI) using the New York University (NYU) impactor to induce mild or moderate spinal cord contusion injury. Changes in RA at the lesion site were determined by measuring the activity of the enzymes for its synthesis, the retinaldehyde dehydrogenases (RALDHs). A marked increase in enzyme activity occurred by day 4 and peaked at days 8-14 following the injuries. RALDH2 was the only detectable RALDH present in the control or injured spinal cord. The cellular localization of RALDH2 was identified by immunostaining. In the noninjured spinal cord, RALDH2 was detected in oligodendroglia positive for the markers RIP and CNPase. Expression was also intense in the arachnoid membrane surrounding the spinal cord. After SCI the increase in RALDH2 was independent of the RIP- and CNPase-positive cells, which were severely depleted. Instead, RALDH2 was present in a cell type not previously identified as capable of synthesizing RA, that expressed NG2 and that was negative for markers of astrocytes, oligodendroglia, microglia, neurons, Schwann cells and immature lymphocytes. We postulate that the RALDH2- and NG2-positive cells migrate into the injured sites from the adjacent arachnoid membrane, where the RALDH2-positive cells proliferate substantially following SCI. These findings indicate that close correlations exist between RA synthesis and SCI and that RA may play a role in the secondary events that follow acute SCI.

Activation of retinoic acid receptor-dependent transcription by organochlorine pesticides

Five organochlorine pesticides, namely, chlordane, dieldrin, aldrin, endrin, and endosulfan, activate human retinoic acid receptor (RAR)-mediated gene transcription via a retinoic acid response element (RARE). Transactivation studies were performed with stable RARalpha, beta, or gamma reporter cell lines in which the RAR DNA-binding domain (DBD) was replaced by that of estrogen receptor alpha (ERalpha)? Five of the organochlorine pesticides tested activated RARbeta and RARgamma but not RARalpha; their half-maximal luciferase activity (EC(50)) was determined. Furthermore, that activity was RAR-specific and organochlorine pesticides did not activate the retinoid X receptor (RXR) pathway. However, competitive binding experiments with [(3)H]-CD367, a pan-RAR agonist, showed that only chlordane could bind RARbeta and RARgamma, albeit with low affinity. In addition, organochlorine pesticides strongly induce cytochrome P450RAI1 (P450RAI1), a key factor of retinoic acid level regulation in many tissues and whose expression and activity are strongly induced by retinoic acid. This study shows that organochlorine pesticides can activate two RAR homologues, with low-binding affinity. Although the agonistic potential of organochlorine pesticides is lower than that of (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1-propenyl] benzoic acid (TTNPB), they are able to induce RAR-mediated gene transcription as P450RAI1 and may disrupt the retinoid signaling pathway. Because these chemicals are extremely persistent and tend to accumulate in biological tissues, these results support the hypothesis that the increase in teratogenicity observed in some developing countries could be due to prolonged exposure to organochlorine pesticides ubiquitously present in the environment.

Role of human aldehyde dehydrogenases in endobiotic and xenobiotic metabolism

The human genome contains at least 17 genes that are members of the aldehyde dehydrogenase (ALDH) superfamily. These genes encode NAD(P)(+)-dependent enzymes that oxidize a wide range of aldehydes to their corresponding carboxylic acids. Aldehydes are highly reactive molecules that are intermediates or products involved in a broad spectrum of physiologic, biologic, and pharmacologic processes. Aldehydes are generated during retinoic acid biosynthesis and the metabolism of amino acids, lipids, carbohydrates, and drugs. Mutations in several ALDH genes are the molecular basis of inborn errors of metabolism and contribute to environmentally induced diseases.

Identification of all-trans-retinol:all-trans-13,14-dihydroretinol saturase

Retinoids carry out essential functions in vertebrate development and vision. Many of the retinoid processing enzymes remain to be identified at the molecular level. To expand the knowledge of retinoid biochemistry in vertebrates, we studied the enzymes involved in plant metabolism of carotenoids, a related group of compounds. We identified a family of vertebrate enzymes that share significant similarity and a putative phytoene desaturase domain with a recently described plant carotenoid isomerase (CRTISO), which isomerizes prolycopene to all-trans-lycopene. Comparison of heterologously expressed mouse and plant enzymes indicates that unlike plant CRTISO, the CRTISO-related mouse enzyme is inactive toward prolycopene. Instead, the CRTISO-related mouse enzyme is a retinol saturase carrying out the saturation of the 13-14 double bond of all-trans-retinol to produce all-trans-13,14-dihydroretinol. The product of mouse retinol saturase (RetSat) has a shifted UV absorbance maximum, lambda(max) = 290 nm, compared with the parent compound, all-trans-retinol (lambda(max) = 325 nm), and its MS analysis (m/z = 288) indicates saturation of a double bond. The product was further identified as all-trans-13,14-dihydroretinol, since its characteristics were identical to those of a synthetic standard. Mouse RetSat is membrane-associated and expressed in many tissues, with the highest levels in liver, kidney, and intestine. All-trans-13,14-dihydroretinol was also detected in several tissues of animals maintained on a normal diet. Thus, saturation of all-trans-retinol to all-trans-13,14-dihydroretinol by RetSat produces a new metabolite of yet unknown biological function.

A disorder to order transition accompanies catalysis in retinaldehyde dehydrogenase type II

Retinaldehyde dehydrogenase II (RalDH2) converts retinal to the transcriptional regulator retinoic acid in the developing embryo. The x-ray structure of the enzyme revealed an important structural difference between this protein and other aldehyde dehydrogenases of the same enzyme superfamily; a 20-amino acid span in the substrate access channel in retinaldehyde dehydrogenase II is disordered, whereas in other aldehyde dehydrogenases this region forms a well defined wall of the substrate access channel. We asked whether this disordered loop might order during the course of catalysis and provide a means for an enzyme that requires a large substrate access channel to restrict access to the catalytic machinery by smaller compounds that might potentially enter the active site and be metabolized. Our experiments, a combination of kinetic, spectroscopic, and crystallographic techniques, suggest that a disorder to order transition is linked to catalytic activity.

Regulation and biochemistry of mouse molybdo-flavoenzymes. The DBA/2 mouse is selectively deficient in the expression of aldehyde oxidase homologues 1 and 2 and represents a unique source for the purification and characterization of aldehyde oxidase

Mouse molybdo-flavoenzymes consist of xanthine oxidoreductase, aldehyde oxidase (AOX1), and two recently identified proteins, AOH1 and AOH2 (aldehyde oxidase homologues 1 and 2). Here we demonstrate that CD-1, C57BL/6, 129/Sv, and other mouse strains synthesize high levels of AOH1 in the liver and AOH2 in the skin. By contrast, the DBA/2 and CBA strains are unique, having a selective deficit in the expression of the AOH1 and AOH2 genes. DBA/2 animals synthesize trace amounts of a catalytically active AOH1 protein. However, relative to CD-1 animals, an over 2 log reduction in the steady-state levels of liver AOH1 mRNA, protein, and enzymatic activity is observed in basal conditions and following administration of testosterone. The DBA/2 mouse represents a unique opportunity to purify AOX1 and compare its enzymatic characteristics to those of the AOH1 protein. The spectroscopy and biochemistry of AOX1 are very similar to those of AOH1 except for a differential sensitivity to the non-competitive inhibitory effect of norharmane. AOX1 and AOH1 oxidize an overlapping set of aldehydes and heterocycles. For most compounds, the substrate efficiency (V(max)/K(m)) of AOX1 is superior to that of AOH1. Alkylic alcohols and acetaldehyde, the toxic metabolite of ethanol, are poor substrates of both enzymes. Consistent with this, the levels of acetaldehyde in the livers of ethanol administered CD-1 and DBA/2 mice are similar, indicating that neither enzyme is involved in the in vivo biotransformation of acetaldehyde.

Human aldehyde dehydrogenases: potential pathological, pharmacological, and toxicological impact

Aldehyde dehydrogenases catalyze the pyridine nucleotide-dependent oxidation of aldehydes to acids. Seventeen enzymes are currently viewed as belonging to the human aldehyde dehydrogenase superfamily. Summarized herein, insofar as the information is available, are the structural composition, physical properties, tissue distribution, subcellular location, substrate specificity, and cofactor preference of each member of this superfamily. Also summarized are the chromosomal locations and organization of the genes that encode these enzymes and the biological consequences when enzyme activity is lost or substantially diminished. Broadly, aldehyde dehydrogenases can be categorized as critical for normal development and/or physiological homeostasis (1). even when the organism is in a friendly environment or (2). only when the organism finds itself in a hostile environment. The primary, if not sole, evolved raison d'être of first category aldehyde dehydrogenases appears to be to catalyze the biotransformation of a single endobiotic for which they are relatively specific and of which the resultant metabolite is essential to the organism. Most of the human aldehyde dehydrogenases for which the relevant information is available fall into this category. Second category aldehyde dehydrogenases are relatively substrate nonspecific and their evolved raison d'être seems to be to protect the organism from potentially harmful xenobiotics, specifically aldehydes or xenobiotics that give rise to aldehydes, by catalyzing their detoxification. Thus, the lack of a fully functional first category aldehyde dehydrogenase results in a gross pathological phenotype in the absence of any insult, whereas the lack of a functional second category aldehyde dehydrogenase is ordinarily of no consequence with respect to gross phenotype, but is of consequence in that regard when the organism is subjected to a relevant insult.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Cyp26C1 encodes a novel retinoic acid-metabolizing enzyme expressed in the hindbrain, inner ear, first branchial arch and tooth buds during murine development

Retinoic acid (RA), an active metabolite of vitamin A, is a crucial signaling molecule involved in tissue morphogenesis during embryonic development. RA distribution and concentration is precisely regulated during embryogenesis by balanced complementary activities of RA synthesizing (RALDH) and metabolizing (CYP26) enzymes. Here, we describe the identification of a novel murine p450 cytochrome belonging to the CYP26 family, mCYP26C1. Sequence alignment show that mCYP26C1 is more closely related to mCYP26B1 than mCYP26A1. At early developmental stages (E8.0-E8.5), mCyp26C1 is expressed in prospective rhombomeres 2 and 4, in the first branchial arch and along the lateral surface mesenchyme adjacent to the rostral hindbrain. At E9.5, mCyp26C1 expression persists in rhombomere 2 and in the maxillary and mandibular components of the first branchial arch, and is strongly induced in the lateral cervical mesenchyme. By mid-gestation, mCyp26C1 is weakly expressed in the cervical mesenchyme and in the maxillary component of the first branchial arch. At E11.5, mCyp26C1 can only be seen in a narrow band in the lateral cervical mesenchyme. During late gestation, mCyp26C1 exhibits region-specific expression in the inner ear epithelium and a persistent expression in the inner dental epithelium of the developing teeth. This pattern of expression suggests that mCYP26C1 may play an important role in protecting the hindbrain, first branchial arch, otocyst and tooth buds against RA exposure during embryonic development.

13-cis-retinoic acid competitively inhibits 3 alpha-hydroxysteroid oxidation by retinol dehydrogenase RoDH-4: a mechanism for its anti-androgenic effects in sebaceous glands?

Retinol dehydrogenase-4 (RoDH-4) converts retinol and 13-cis-retinol to corresponding aldehydes in human liver and skin in the presence of NAD(+). RoDH-4 also converts 3 alpha-androstanediol and androsterone into dihydrotestosterone and androstanedione, which may stimulate sebum secretion. This oxidative 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) activity of RoDH-4 is competitively inhibited by retinol and 13-cis-retinol. Here, we further examine the substrate specificity of RoDH-4 and the inhibition of its 3 alpha-HSD activity by retinoids. Recombinant RoDH-4 oxidized 3,4-didehydroretinol-a major form of vitamin A in the skin-to its corresponding aldehyde. 13-cis-retinoic acid (isotretinoin), 3,4-didehydroretinoic acid, and 3,4-didehydroretinol, but not all-trans-retinoic acid or the synthetic retinoids acitretin and adapalene, were potent competitive inhibitors of the oxidative 3 alpha-HSD activity of RoDH-4, i.e., reduced the formation of dihydrotestosterone and androstandione in vitro. Extrapolated to the in vivo situation, this effect might explain the unique sebosuppressive effect of isotretinoin when treating acne.

Mouse retinal dehydrogenase 4 (RALDH4), molecular cloning, cellular expression, and activity in 9-cis-retinoic acid biosynthesis in intact cells

This study describes cDNA cloning and characterization of mouse RALDH4. The 2.3-kb cDNA encodes an aldehyde dehydrogenase of 487 amino acid residues, about two-orders of magnitude more active in vitro with 9-cis-retinal than with all-trans-retinal. RALDH4 recognizes as substrate 9-cis-retinal generated in transfected cells by the short-chain dehydrogenases CRAD1, CRAD3, or RDH1, to reconstitute a path of 9-cis-retinoic acid biosynthesis in situ. Northern blot analysis showed expression of RALDH4 mRNA in adult mouse liver and kidney. In situ hybridization revealed expression of RALDH4 in liver on embryo day 14.5, in adult hepatocytes, and kidney cortex. Immunohistochemistry confirmed RALDH4 expression in hepatocytes and showed that hepatocytes also express RALDH1, RALDH2, and RALDH3. Kidney expresses the RALDH4 protein primarily in the proximal and distal convoluted tubules of the cortex but not in the glomeruli or the medulla. Kidney expresses RALDH2 in the proximal convoluted tubules of the cortex but not in the distal convoluted tubules or glomeruli. Kidney expresses RALDH1 and RALDH2 in the medulla. The enzymatic characteristics of RALDH4, its expression in fetal liver, and its unique expression pattern in adult kidney compared with RALDH1, -2, and -3 suggest that it could meet specific needs for 9-cis-retinoic acid biosynthesis.

Gene structure and minimal promoter of mouse rdh1

Mouse rdh1 encodes retinol dehydrogenase type 1 (RDH1), a short-chain dehydrogenase, which recognizes as substrates all-trans-retinol, 9-cis-retinol, 5alpha-androstan-3,17-diol and 5alpha-androstan-3-ol-17-one. RDH1 is the most efficient known mouse short-chain dehydrogenase that catalyzes dehydrogenation of all-trans-retinol, and contributes to a reconstituted path of all-trans-retinoic acid biosynthesis, when coexpressed in reporter cells with any one of three retinal dehydrogenases. Rdh1 shows widespread, if not ubiquitous, mRNA expression in the mouse beginning no later than embryo day 7. Here we report genomic organization, chromosomal localization and analysis of a minimum promoter of mouse rdh1. Rdh1 consists of four exons and three introns and spans approximately 14412 bp. Rdh1 is a single copy gene that maps to chromosome 10D3 with rdh5-9, but no known disorder maps precisely to rdh1. Rdh1 has three transcription start sites in kidney and one start site in liver. The rdh1 5'-region between -424 and +43 induces transcription maximally in COS7, mouse kidney RAG, and mouse liver NMu3Li cells. This section has no TATA box, but has a CCAAT box beginning 65 bp upstream of the major transcription start site, which is required for transcription of transfected reporter constructs. An AP1 binding site at -119 also activates transfected reporter constructs, and mediates 2-O-tetradecanoylphorbol-13-acetate (TPA) induced transcription. All-trans-retinoic acid antagonizes the TPA affect; however, no RARE or RXRE was found in the proximal promoter region, consistent with indirect regulation by all-trans-retinoic acid.

The role of vitamin A in mammalian reproduction and embryonic development

Since the late 1980s, there has been an explosion of information on the molecular mechanisms and functions of vitamin A. This review focuses on the essential role of vitamin A in female reproduction and embryonic development and the metabolism of vitamin A (retinol) that results in these functions. Evidence strongly supports that in situ-generated all-trans retinoic acid (atRA) is the functional form of vitamin A in female reproduction and embryonic development. This is supported by the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency with atRA, the block in embryonic development that occurs in retinaldehyde dehydrogenase type 2 null mutant mice, and the essential roles of the retinoic acid receptors, at least in embryogenesis. Early studies of embryos from marginally vitamin A-deficient (VAD) pregnant rats revealed a collection of defects called the vitamin A-deficiency syndrome. The manipulation of all-trans retinoic acid (atRA) levels in the diet of VAD female rats undergoing a reproduction cycle has proved to be an important new tool in deciphering the points of atRA function in early embryos and has provided a means to generate large numbers of embryos at later stages of development with the vitamin A-deficiency syndrome. The essentiality of the retinoid receptors in mediating the activity of atRA is exemplified by the many compound null mutant embryos that now recapitulate both the original vitamin A-deficiency syndrome and exhibit a host of new defects, many of which can also be observed in the VAD-atRA-supported rat embryo model and in retinaldehyde dehydrogenase type 2 (RALDH2) mutant mice. A major task for the future is to elucidate the atRA-dependent pathways that are normally operational in vitamin A-sufficient animals and that are perturbed in deficiency, thus leading to the characteristic VAD phenotypes described above.

Evidence that the human gene for prostate short-chain dehydrogenase/reductase (PSDR1) encodes a novel retinal reductase (RalR1)

All-trans-retinoic acid is a metabolite of vitamin A (all-trans-retinol) that functions as an activating ligand for a family of nuclear retinoic acid receptors. The intracellular levels of retinoic acid in tissues are tightly regulated, although the mechanisms underlying the control of retinoid metabolism at the level of specific enzymes are not completely understood. In this report we present the first characterization of the retinoid substrate specificity of a novel short-chain dehydrogenase/reductase (SDR) encoded by RalR1/PSDR1, a cDNA recently isolated from the human prostate (Lin, B., White, J. T., Ferguson, C., Wang, S., Vessella, R., Bumgarner, R., True, L. D., Hood, L., and Nelson, P. S. (2001) Cancer Res. 61, 1611-1618). We demonstrate that RalR1 exhibits an oxidoreductive catalytic activity toward retinoids, but not steroids, with at least an 800-fold lower apparent K(m) values for NADP+ and NADPH versus NAD+ and NADH as cofactors. The enzyme is approximately 50-fold more efficient for the reduction of all-trans-retinal than for the oxidation of all-trans-retinol. Importantly, RalR1 reduces all-trans-retinal in the presence of a 10-fold molar excess of cellular retinol-binding protein type I, which is believed to sequester all-trans-retinal from nonspecific enzymes. As shown by immunostaining of human prostate and LNCaP cells with monoclonal anti-RalR1 antibodies, the enzyme is highly expressed in the epithelial cell layer of human prostate and localizes to the endoplasmic reticulum. The enzymatic properties and expression pattern of RalR1 in prostate epithelium suggest that it might play a role in the regulation of retinoid homeostasis in human prostate.

Recombinant class I aldehyde dehydrogenases specific for all-trans- or 9-cis-retinal

The molecular basis for the specificity of aldehyde dehydrogenases (ALDHs) for retinal, the precursor of the morphogen retinoic acid, is still poorly understood. We have expressed in Escherichia coli both retinal dehydrogenase (RALDH), a cytosolic aldehyde dehydrogenase originally isolated from rat kidney, and the highly homologous phenobarbital-induced aldehyde dehydrogenase (PB-ALDH). Oxidation of propanal was observed with both enzymes. On the other hand, recombinant RALDH efficiently catalyzed oxidation of 9-cis- and all-trans-retinal, whereas PB-ALDH was inactive with all-trans-retinal and poorly active with 9-cis-retinal. A striking difference between PB-ALDH and all other class I ALDHs is the identity of the amino acid immediately preceding the active nucleophile Cys(302) (Ile(301) instead of Cys(301)). Nevertheless, these amino acids could be exchanged in either RALDH or PB-ALDH without affecting substrate specificity. Characterization of chimeric enzymes demonstrates that distinct groups of amino acids control the differential activity of RALDH and PB-ALDH with all-trans- and 9-cis-retinal. Of 52 divergent amino acids, the first 17 are crucial for activity with all-trans-retinal, whereas the next 25 are important for catalysis of 9-cis-retinal oxidation. Recombinant enzymes with specificity for all-trans- or 9-cis-retinal were obtained, which should provide useful tools to study the relative importance of local production of all-trans- versus 9-cis-retinoic acid in development and tissue differentiation.

Distinct retinoid metabolic functions for alcohol dehydrogenase genes Adh1 and Adh4 in protection against vitamin A toxicity or deficiency revealed in double null mutant mice

The ability of class I alcohol dehydrogenase (ADH1) and class IV alcohol dehydrogenase (ADH4) to metabolize retinol to retinoic acid is supported by genetic studies in mice carrying Adh1 or Adh4 gene disruptions. To differentiate the physiological roles of ADH1 and ADH4 in retinoid metabolism we report here the generation of an Adh1/4 double null mutant mouse and its comparison to single null mutants. We demonstrate that loss of both ADH1 and ADH4 does not have additive effects, either for production of retinoic acid needed for development or for retinol turnover to minimize toxicity. During gestational vitamin A deficiency Adh4 and Adh1/4 mutants exhibit completely penetrant postnatal lethality by day 15 and day 24, respectively, while 60% of Adh1 mutants survive to adulthood similar to wild-type. Following administration of a 50-mg/kg dose of retinol to examine retinol turnover, Adh1 and Adh1/4 mutants exhibit similar 10-fold decreases in retinoic acid production, whereas Adh4 mutants have only a slight decrease. LD(50) studies indicate a large increase in acute retinol toxicity for Adh1 mutants, a small increase for Adh4 mutants, and an intermediate increase for Adh1/4 mutants. Chronic retinol supplementation during gestation resulted in 65% postnatal lethality in Adh1 mutants, whereas only approximately 5% for Adh1/4 and Adh4 mutants. These studies indicate that ADH1 provides considerable protection against vitamin A toxicity, whereas ADH4 promotes survival during vitamin A deficiency, thus demonstrating largely non-overlapping functions for these enzymes in retinoid metabolism.

Kinetic analysis of mouse retinal dehydrogenase type-2 (RALDH2) for retinal substrates

Retinal dehydrogenase (RALDH) isozymes catalyze the terminal oxidation of retinol into retinoic acid (RA) that is essential for embryogenesis and tissue differentiation. To understand the role of mouse type 2 RALDH in synthesizing the ligands (all-trans and 9-cis RA) needed to bind and activate nuclear RA receptors, we determined the detailed kinetic properties of RALDH2 for various retinal substrates. Purified recombinant RALDH2 showed a pH optimum of 9.0 for all-trans retinal oxidation. The activity of the enzyme was lower at 37 degrees C compared to 25 degrees C. The efficiency of conversion of all-trans retinal to RA was 2- and 5-fold higher than 13-cis and 9-cis retinal, respectively. The K(m) for all-trans and 13-cis retinal were similar (0.66 and 0.62 microM, respectively). However, the K(m) of RALDH2 for 9-cis retinal substrate (2.25 microM) was 3-fold higher compared to all-trans and 13-cis retinal substrates. Among several reagents tested for their ability to either inhibit or activate RALDH2, citral and para-hydroxymercuribenzoic acid (p-HMB) inhibited and MgCl(2) activated the reaction. Comparison of the kinetic properties of RALDH2 for retinal substrates and its activity towards various reagents with those of previously reported rat kidney RALDH1 and human liver aldehyde dehydrogenase-1 showed distinct differences. Since RALDH2 has low K(m) and high catalytic efficiency for all-trans retinal, it may likely be involved in the production of all-trans RA in vivo.

4-(N,N-dipropylamino)benzaldehyde inhibits the oxidation of all-trans retinal to all-trans retinoic acid by ALDH1A1, but not the differentiation of HL-60 promyelocytic leukemia cells exposed to all-trans retinal

BACKGROUND

The signal transduction pathways mediated by retinoic acid play a critical role in the regulation of cell growth and differentiation during embryogenesis and hematopoiesis as well as in a variety of tumor cell lines in culture. Following the reports that two members of the superfamily of aldehyde dehydrogenase (ALDH) enzymes, ALDH1A1 and ALDH1A2, were capable of catalyzing the oxidation of all-trans retinal to all-trans retinoic acid with submicromolar Km values, we initiated an investigation of the ability of 4-(N,N-dipropylamino)benzaldehyde (DPAB) to inhibit the oxidation of retinal by purified mouse and human ALDH1A1.

RESULTS

Our results show that DPAB potently inhibits retinal oxidation, with IC50 values of 0.11 and 0.13 microM for purified mouse and human ALDH1A1, respectively. Since the HL-60 human myeloid leukemic cell line has been used extensively to study the retinoic acid induced differentiation of HL-60 cells to granulocytes, and ALDH1A1 activity had previously been reported in HL-60 cells, we investigated the ability of DPAB to block differentiation of HL-60 promyelocytic leukemia cells exposed to retinal in culture. In HL-60 cells coincubated with 1 microM retinal and 50 microM DPAB for 144 hours, cell differentiation was inhibited only 30%. Furthermore, the NAD-dependent oxidation of propanal or retinal was less than 0.05 nmoles NADH formed/min-10(7) cells in spectrophotometric assays using HL-60 cell extracts.

CONCLUSION

Although ALDH1A1 may be the major catalytic activity for retinal oxidation in some retinoid-dependent mouse and Xenopus embryonic tissues and in adult human and mouse hematopoietic stem cells, another catalytic activity appears to synthesize the retinoic acid ligand necessary to stimulate the differentiation of HL-60 cells to end stage granulocytes.

Differential expression of retinoic acid-synthesizing (RALDH) enzymes during fetal development and organ differentiation in the mouse

Three retinaldehyde dehydrogenases (RALDH1, RALDH2 and RALDH3), which catalyze the oxidation of retinaldehyde into retinoic acid, have been shown to be differentially expressed during early embryogenesis. Here, we report their differential expression patterns throughout later mouse organogenesis. Raldh1 is prominently expressed in developing lung (notably in bronchial and tracheal epithelia), and shows stage-specific expression in stomach and intestine epithelial and mesenchymal layers. Raldh3 expression is specific to the differentiating intestinal lamina propria. Raldh2 is expressed throughout the kidney nephrogenic zone, whereas Raldh1 and Raldh3 are mostly expressed in collecting duct epithelia. Raldh3 expression is more restricted than that of Raldh1 in the urogenital tract and sex gland epithelia, whereas Raldh2 expression is mesenchymal. Raldh1 is coexpressed with Raldh2 in the early heart epicardium, and is later specifically expressed in developing heart valves. All three genes exhibit distinct expression patterns in respiratory and olfactory epithelia and/or mesenchymes, and in developing teeth. Only Raldh1 expression is seen after birth in specific brain structures. These data indicate a requirement for regulated RA synthesis in various differentiating organs.

Molecular characterization of a mouse short chain dehydrogenase/reductase active with all-trans-retinol in intact cells, mRDH1

Metabolic activation of retinol (vitamin A) via sequential actions of retinol and retinal dehydrogenases produces the active metabolite all-trans-retinoic acid. This work reports cDNA cloning, enzymatic characterization, function in a reconstituted path of all-trans-retinoic acid biosynthesis in cell culture, and mRNA expression patterns in adult tissues and embryos of a mouse retinol dehydrogenase, RDH1. RDH1 represents a new member of the short chain dehydrogenase/reductase superfamily that differs from other mouse RDH in relative activity with all-trans and cis-retinols. RDH1 has a multifunctional catalytic nature, as do other short chain dehydrogenase/reductases. In addition to retinol dehydrogenase activity, RDH1 has strong 3alpha-hydroxy and weak 17beta-hydroxy steroid dehydrogenase activities. RDH1 has widespread and intense mRNA expression in tissues of embryonic and adult mice. The mouse embryo expresses RDH1 as early as 7.0 days post-coitus, and expression is especially intense within the neural tube, gut, and neural crest at embryo day 10.5. Cells cotransfected with RDH1 and any one of three retinal dehydrogenase isozymes synthesize all-trans-retinoic acid from retinol, demonstrating that RDH1contributes to a path of all-trans-retinoic acid biosynthesis in intact cells. These characteristics are consistent with RDH1 functioning in a path of all-trans-retinoic acid biosynthesis starting early during embryogenesis.

Hindbrain patterning revisited: timing and effects of retinoic acid signalling

Retinoids play a critical role in patterning, segmentation, and neurogenesis of the posterior hindbrain and it has been proposed that they act as a posteriorising signal during hindbrain development. Until now, direct evidence that endogenous retinoid signalling acts through a gradient to specify cell fates along the anteroposterior axis has been missing. Two recent studies tested the requirement for retinoid signalling in the developing hindbrain through systematic application of a pan-retinoic acid receptor antagonist. They demonstrate a stage-dependent requirement for increasing retinoid signalling activity along the hindbrain that proceeds from anterior to posterior. Together these findings challenge the concept of a stable gradient of retinoic acid across the hindbrain and warrant a re-interpretation of the phenotypes obtained by genetic and nutritional disruption of retinoid signalling in the amniote embryo.

Analysis of mouse retinal dehydrogenase type 2 promoter and expression

The mouse RALDH2 gene spans >50 kb, has a structure similar to that of human class 1 aldehyde dehydrogenase genes, and localizes to the central region of chromosome 9 by single-strand polymorphism analysis. Expression of mouse RALDH2 was detected in testis, lung, brain, and heart (Northern blot) and in liver and kidney (RNase protection assays). Expression was not detected by RNase protection assay in testis of vitamin A-deficient rats, and all-trans-retinoic acid dosing did not increase expression in vitamin A-deficient rat testis. A 2.3-kb section of the gene 5' to the transcription start site included neither retinoic acid nor retinoid X response elements, but included TATA and CCAAT motifs and AP, AHR, CREB, ER, Ets, and SREBP sites. The promoter initiated transcription of a luciferase reporter in human embryonic kidney cells (EBNA) and mouse Leydig- (TM3) and Sertoli-derived (TM4) cell lines, but neither all-trans-retinoic acid nor 9-cis-retinoic acid affected reporter transcription. These data suggest that relatively weak RALDH2 expression in vitamin A-deficient testis reflects vastly decreased numbers of germ cells, the major site of expression.

Development of the limb neuromuscular system

Appendages, such as wings of a fly or limbs of a vertebrate, are excellent models to study the principles of patterning and morphogenesis. In the adult these structures are used for a variety of behaviors, including locomotion. Although support structures of the adult vertebrate limb are generated within the limb bud, its dynamic elements are derived from the somitic mesoderm and neural tube. Recent studies show that regional patterns set up in the mesenchyme-filled limb bud guide muscle precursors and developing motor axons to their proper location within the limb. Subsequent development of the neuromuscular system is regulated by cell surface interactions between pre-specified muscle fibers and motor axons.

Increased XRALDH2 activity has a posteriorizing effect on the central nervous system of Xenopus embryos

Retinoic acid (RA) metabolizing enzymes play important roles in RA signaling during vertebrate embryogenesis. We have previously reported on a RA degrading enzyme, XCYP26, which appears to be critical for the anteroposterior patterning of the central nervous system (EMBO J. 17 (1998) 7361). Here, we report on the sequence, expression and function of its counterpart, XRALDH2, a RA generating enzyme in Xenopus. During gastrulation and neurulation, XRALDH2 and XCYP26 show non-overlapping, complementary expression domains. Upon misexpression, XRALDH2 is found to reduce the forebrain territory and to posteriorize the molecular identity of midbrain and individual hindbrain rhombomeres in Xenopus embryos. Furthermore, ectopic XRALDH2, in combination with its substrate, all-trans-retinal (ATR), can mimic the RA phenotype to result in microcephalic embryos. Taken together, our data support the notion that XRALDH2 plays an important role in RA homeostasis by the creation of a critical RA concentration gradient along the anteroposterior axis of early embryos, which is essential for proper patterning of the central nervous system in Xenopus.

Follicle-stimulating hormone and leukemia inhibitory factor regulate Sertoli cell retinol metabolism

Sertoli cells, the somatic epithelial cells of the seminiferous tubules, provide both structural and biochemical support for developing male germ cells. The Sertoli cells are targets of retinoid action in the testis. We have found that FSH, (Bu)(2)cAMP, and leukemia inhibitory factor elicit substantial changes in the metabolism of [(3)H]retinol (vitamin A) in primary cultures of purified rat Sertoli cells. Addition of (Bu)(2)cAMP for 2 h or FSH for 6 h results in a 3-fold increase in the metabolism of [(3)H]retinol to [(3)H]retinoic acid ([(3)H]RA); the esterification of [(3)H]retinol to [(3)H]retinyl esters, especially [(3)H]retinyl palmitate, is also increased by approximately 5-fold. The addition of 1 microM all-trans-RA also elicits changes in [(3)H]retinol metabolism, but in this case the metabolism of [(3)H]retinol to [(3)H]RA is inhibited, whereas the metabolism of [(3)H]retinol to [(3)H]retinyl esters is increased by over 50-fold. Leukemia inhibitory factor increases the esterification of [(3)H]retinol by 2- to 3-fold. FSH leads to a reduction in the level of cellular retinol binding protein I transcripts, whereas RA increases the cellular retinol binding protein I messenger RNA level by about 2-fold at approximately 24 h. Levels of AHD-2 (aldehyde dehydrogenase-2) and RALDH-2 (retinaldehyde dehydrogenase-2) messenger RNAs, which encode enzymes that convert [(3)H]retinaldehyde to [(3)H]RA, are increased by about 2-fold by FSH, whereas no change in CYP26 (RA hydroxylase) expression is seen. Our results suggest that one function of FSH (and/or (Bu)(2)cAMP) in Sertoli cells is to increase the metabolism of retinol to the biologically active metabolite RA and to retinyl esters.

Cellular expression of retinal dehydrogenase types 1 and 2: effects of vitamin A status on testis mRNA

We examined expression of retinal dehydrogenase (RALDH) types 1 and 2 in liver and lung, and the effect of vitamin A status on testis expression by in situ hybridization. Liver expressed RALDH1 and RALDH2 only in stellate cells and hepatocytes, respectively. Lung expressed RALDH1 and RALDH2 throughout the epithelia of the airways, from the principal bronchi to the respiratory bronchiole. Vitamin A-sufficient rats expressed RALDH1 in spermatocytes, with less intense expression in spermatogonia and spermatids, and expressed RALDH2 in interstitial cells, spermatogonia, and spermatocytes. Neither Sertoli nor peritubular cells showed detectable RALDH1 or RALDH2 mRNA. Vitamin A deficiency produced a sevenfold increase in RALDH1 and a 70-fold decrease in RALDH2 mRNA in testis. In each case, the net change reflected extensive loss of germ cells, increased intensity of expression in residual germ cells, and expression in Sertoli and peritubular cells. Low-dose RA relatively early during vitamin A depletion supported spermatogenesis and affected expression of both RALDHs, but did not reinstate "vitamin A normal" expression patterns. These results show that: RALDH1 and RALDH2 have distinct mRNA expression patterns in multiple cell types in three vitamin A target tissues; RALDH expression occurs in cell types that express cellular retinol-binding protein and retinol dehydrogenase isozymes (except stellate cells, for which retinol dehydrogenase expression remains unknown); vitamin A deficiency and RA supplementation affects the loci and intensity of RALDH mRNAs in testis; and low-dose RA does not substitute completely for retinol. Overall, these data provide insight into the unique functions of RALDH1 and RALDH2 in retinoid metabolism.

Characterization of Xenopus cytosolic thyroid-hormone-binding protein (xCTBP) with aldehyde dehydrogenase activity

Multiple cytosolic thyroid-hormone-binding proteins (CTBPs) with varying characteristics, depending on the species and tissue, have been reported. We first purified a 59-kDa CTBP from Xenopus liver (xCTBP), and found that it is responsible for major [125I]T(3)-binding activity in Xenopus liver cytosol. Amino acid sequencing of internal peptide fragments derived from xCTBP demonstrated high identity to the corresponding sequence of mammalian aldehyde dehydrogenases 1 (ALDH1). To confirm whether or not xCTBP is identical to xALDH1, we isolated cDNAs encoding xALDH1 from an adult Xenopus hepatic cDNA library. The amino acid sequences deduced from the two isolated xALDH1 cDNAs were very similar to those of mammalian ALDH1 enzymes. The recombinant xALDH1 protein exhibited both T(3)-binding activity and ALDH activity converting retinal to retinoic acid (RA), which were similar to those of xCTBP purified from liver cytosol. The T(3)-binding activity was inhibited by NAD, while the ALDH activity was inhibited by thyroid hormones. Our results demonstrate that xCTBP is identical to ALDH1 and suggest that this protein might modulate RA synthesis and intracellular concentration of free T(3). Communications between thyroid hormone and retinoid pathways are discussed.

Genetic dissection of retinoid dehydrogenases

Biochemical studies indicate that alcohol dehydrogenase (ADH) metabolizes retinol to retinal, and that aldehyde dehydrogenase (ALDH) metabolizes retinal to retinoic acid, a molecule essential for growth and development. Summarized herein are several genetic studies supporting in vivo functions for ADH and ALDH in retinoic acid synthesis. Gene targeting was used to create knockout mice for either Adh1 or Adh4. Both knockout mice were viable and fertile without obvious defects. However, when wild-type and Adh4 knockout mice were subjected to vitamin A deficiency during gestation, the survival rate at birth was 3.3-fold lower for Adh4 knockout mice. When adult mice were examined for production of retinoic acid following retinol administration, Adh1 knockout mice exhibited 10-fold lower retinoic acid levels in liver compared with wild-type, whereas Adh4 knockout mice differed from wild-type by less than 2-fold. Thus, Adh1 plays a major role in the metabolism of a large dose of retinol to retinoic acid in adults, whereas Adh4 plays a role in maintaining sufficient retinol metabolism for development during retinol deficiency. ALDHs were examined by overexpression studies in frog embryos. Injection of mRNAs for either mouse Raldh1 or Raldh2 stimulated retinoic acid synthesis in frog embryos at the blastula stage when retinoic acid is normally undetectable. Overexpression of human ALDH2, human ALDH3, and mouse Aldh-pb did not stimulate retinoic acid production. In addition, Raldh2 knockout mice exhibit embryonic lethality with defects in retinoid-dependent tissues. Overall, these studies provide genetic evidence that Adh1, Adh4, Raldh1, and Raldh2 encode retinoid dehydrogenases involved in retinoic acid synthesis in vivo.

The retinoic acid-inactivating enzyme CYP26 is essential for establishing an uneven distribution of retinoic acid along the anterio-posterior axis within the mouse embryo

Retinoic acid (RA), a derivative of vitamin A, plays a pivotal role in vertebrate development. The level of RA may be determined by the balance between its synthesis and degradation. We have examined the role of CYP26, a P450 enzyme that may degrade RA, by generating mutant mice that lack CYP26. CYP26(-/-) mice exhibited anomalies, including caudal agenesis, similar to those induced by administration of excess RA. The concentration of endogenous RA, as revealed by marker gene activity, was markedly increased in the tailbud of the mutant animals, in which CYP26 is normally expressed. Expression of T (Brachyury) and Wnt3a in the tailbud was down-regulated in CYP26(-/-) mice, which may underlie the caudal truncation. The lack of CYP26 also resulted in homeotic transformation of vertebrae as well as in misspecification of the rostral hindbrain associated with anterior expansion of RA-positive domains. These results suggest that local degradation of RA by CYP26 is required for establishing an uneven distribution of RA along the anterio-posterior axis, which is essential for patterning the hindbrain, vertebrae, and tailbud.

Profiling the specific reactivity of the proteome with non-directed activity-based probes

BACKGROUND

The field of proteomics aims to characterize dynamics in protein function on a global level. However, several classes of proteins, in particular low abundance proteins, remain difficult to characterize using standard proteomics technologies. Recently, chemical strategies have emerged that profile classes of proteins based on activity rather than quantity, thereby greatly facilitating the analysis of low abundance constituents of the proteome.

RESULTS

In order to expand the classes of proteins susceptible to analysis by activity-based methods, we have synthesized a library of biotinylated sulfonate esters and applied its members to complex proteomes under conditions that distinguish patterns of specific protein reactivity. Individual sulfonates exhibited unique profiles of proteome reactivity that in extreme cases appeared nearly orthogonal to one another. A robustly labeled protein was identified as a class I aldehyde dehydrogenase and shown to be irreversibly inhibited by members of the sulfonate library.

CONCLUSIONS

Through screening the proteome with a non-directed library of chemical probes, diverse patterns of protein reactivity were uncovered. These probes labeled protein targets based on properties other than abundance, circumventing one of the major challenges facing contemporary proteomics research. Considering further that the probes were found to inhibit a target enzyme's catalytic activity, the methods described herein should facilitate the identification of compounds possessing both selective proteome reactivities and novel bioactivities.

Aldehyde dehydrogenase 6, a cytosolic retinaldehyde dehydrogenase prominently expressed in sensory neuroepithelia during development

We have isolated the chick and mouse homologs of human aldehyde dehydrogenase 6 (ALDH6) that encode a third cytosolic retinaldehyde-specific aldehyde dehydrogenase. In both chick and mouse embryos, strong expression is observed in the sensory neuroepithelia of the head. In situ hybridization analysis in chick shows compartmentalized expression primarily in the ventral retina, olfactory epithelium, and otic vesicle; additional sites of expression include the isthmus, Rathke's pouch, posterior spinal cord interneurons, and developing limbs. Recombinant chick ALDH6 has a K(0.5) = 0.26 microm, V(max) = 48.4 nmol/min/mg and exhibits strong positive cooperativity (H = 1.9) toward all-trans-retinaldehyde; mouse ALDH6 has similar kinetic parameters. Expression constructs can confer 1000-fold increased sensitivity to retinoic acid receptor-dependent signaling from retinol in transient transfections experiments. The localization of ALDH6 to the developing sensory neuroepithelia of the eye, nose, and ear and discreet sites within the CNS suggests a role for RA signaling during primary neurogenesis at these sites.

cDNA cloning and expression of a human aldehyde dehydrogenase (ALDH) active with 9-cis-retinal and identification of a rat ortholog, ALDH12

This report describes the isolation of a heretofore uncharacterized aldehyde dehydrogenase (ALDH) with retinal dehydrogenase activity from rat kidney and the cloning and expression of a cDNA that encodes its human ortholog, the previously unknown ALDH12. The human ALDH12 cDNA predicts a 487-residue protein with the 23 invariant amino acids, four conserved regions, cofactor binding motif (G(209)XGX(3)G), and active site cysteine residue (Cys(287)) that typify members of the ALDH superfamily. ALDH12 seems at least as efficient (V(m)/K(m)) in converting 9-cis-retinal into the retinoid X receptor ligand 9-cis-retinoic acid as two previously identified ALDHs with 9-cis-retinal dehydrogenase activity, rat retinal dehydrogenase (RALDH) 1 and RALDH2. ALDH12, however, has approximately 40-fold higher activity with 9-cis- retinal than with all-trans-retinal, whereas RALDH1 and RALDH2 have equivalent and approximately 4-fold less efficiencies for 9-cis-retinal versus all-trans-retinal, respectively. Therefore, ALDH12 is the first known ALDH to show a preference for 9-cis-retinal relative to all-trans-retinal. Evidence consistent with the possibility that ALDH12 could function in a pathway of 9-cis-retinoic acid biosynthesis in vivo includes biosynthesis of 9-cis-retinoic acid from 9-cis-retinol in cells co-transfected with cDNAs encoding ALDH12 and the 9-cis-retinol/androgen dehydrogenase, cis-retinoid/androgen dehydrogenase type 1. Intense ALDH12 mRNA expression in adult and fetal liver and kidney, two organs that reportedly have relatively high concentrations of 9-cis-retinol, reinforces this notion.

Role of aldehyde dehydrogenases in endogenous and xenobiotic metabolism

Aldehydes are highly reactive molecules that are intermediates or products involved in a broad spectrum of physiologic, biologic and pharmacologic processes. Aldehydes are generated from chemically diverse endogenous and exogenous precursors and aldehyde-mediated effects vary from homeostatic and therapeutic to cytotoxic, and genotoxic. One of the most important pathways for aldehyde metabolism is their oxidation to carboxylic acids by aldehyde dehydrogenases (ALDHs). Oxidation of the carbonyl functional group is considered a general detoxification process in that polymorphisms of several human ALDHs are associated a disease phenotypes or pathophysiologies. However, a number of ALDH-mediated oxidation form products that are known to possess significant biologic, therapeutic and/or toxic activities. These include the retinoic acid, an important element for vertebrate development, gamma-aminobutyric acid (GABA), an important neurotransmitter, and trichloroacetic acid, a potential toxicant. This review summarizes the ALDHs with an emphasis on catalytic properties and xenobiotic substrates of these enzymes.

A gene knockout corroborates the integral function of cellular retinol-binding protein in retinoid metabolism

Continually expanding evidence has moved inexorably toward establishing key functions for cellular retinol-binding protein (CRBP) in retinoid metabolism. These experimental data integrate into a model of CRBP as a chaperone that protects retinol from the cellular milieu and interacts with certain retinoid-metabolizing enzymes. Mutant mice with an inactivated CRBP gene show decreased liver retinyl ester storage, a shorter elimination half-life of liver retinoids, and predisposition to vitamin A deficiency. No morphologic phenotype was observed until vitamin A was exhausted. Although the mechanisms underlying diminished vitamin A in the CRBP-null mice have not been elucidated, the observations support the model of CRBP as a chaperone of retinoid metabolism.

Site-specific retinoic acid production in the brain of adult songbirds

The song system of songbirds, a set of brain nuclei necessary for song learning and production, has distinctive morphological and functional properties. Utilizing differential display, we searched for molecular components involved in song system regulation. We identified a cDNA (zRalDH) that encodes a class 1 aldehyde dehydrogenase. zRalDH was highly expressed in various song nuclei and synthesized retinoic acid efficiently. Brain areas expressing zRalDH generated retinoic acid. Within song nucleus HVC, only projection neurons not undergoing adult neurogenesis expressed zRalDH. Blocking zRalDH activity in the HVC of juveniles interfered with normal song development. Our results provide conclusive evidence for localized retinoic acid synthesis in an adult vertebrate brain and indicate that the retinoic acid-generating system plays a significant role in the maturation of a learned behavior.

Families of retinoid dehydrogenases regulating vitamin A function: production of visual pigment and retinoic acid

Vitamin A (retinol) and provitamin A (beta-carotene) are metabolized to specific retinoid derivatives which function in either vision or growth and development. The metabolite 11-cis-retinal functions in light absorption for vision in chordate and nonchordate animals, whereas all-trans-retinoic acid and 9-cis-retinoic acid function as ligands for nuclear retinoic acid receptors that regulate gene expression only in chordate animals. Investigation of retinoid metabolic pathways has resulted in the identification of numerous retinoid dehydrogenases that potentially contribute to metabolism of various retinoid isomers to produce active forms. These enzymes fall into three major families. Dehydrogenases catalyzing the reversible oxidation/reduction of retinol and retinal are members of either the alcohol dehydrogenase (ADH) or short-chain dehydrogenase/reductase (SDR) enzyme families, whereas dehydrogenases catalyzing the oxidation of retinal to retinoic acid are members of the aldehyde dehydrogenase (ALDH) family. Compilation of the known retinoid dehydrogenases indicates the existence of 17 nonorthologous forms: five ADHs, eight SDRs, and four ALDHs, eight of which are conserved in both mouse and human. Genetic studies indicate in vivo roles for two ADHs (ADH1 and ADH4), one SDR (RDH5), and two ALDHs (ALDH1 and RALDH2) all of which are conserved between humans and rodents. For several SDRs (RoDH1, RoDH4, CRAD1, and CRAD2) androgens rather than retinoids are the predominant substrates suggesting a function in androgen metabolism as well as retinoid metabolism.