Human aldehyde dehydrogenase. cDNA cloning and primary structure of the enzyme that catalyzes dehydrogenation of 4-aminobutyraldehyde

L-Carnitine in Drosophila: A Review

L-Carnitine is an amino acid derivative that plays a key role in the metabolism of fatty acids, including the shuttling of long-chain fatty acyl CoA to fuel mitochondrial β-oxidation. In addition, L-carnitine reduces oxidative damage and plays an essential role in the maintenance of cellular energy homeostasis. L-carnitine also plays an essential role in the control of cerebral functions, and the aberrant regulation of genes involved in carnitine biosynthesis and mitochondrial carnitine transport in Drosophila models has been linked to neurodegeneration. Drosophila models of neurodegenerative diseases provide a powerful platform to both unravel the molecular pathways that contribute to neurodegeneration and identify potential therapeutic targets. Drosophila can biosynthesize L-carnitine, and its carnitine transport system is similar to the human transport system; moreover, evidence from a defective Drosophila mutant for one of the carnitine shuttle genes supports the hypothesis of the occurrence of β-oxidation in glial cells. Hence, Drosophila models could advance the understanding of the links between L-carnitine and the development of neurodegenerative disorders. This review summarizes the current knowledge on L-carnitine in Drosophila and discusses the role of the L-carnitine pathway in fly models of neurodegeneration.

Reconstruction of the carnitine biosynthesis pathway from Neurospora crassa in the yeast Saccharomyces cerevisiae

Industrial synthesis of L-carnitine is currently performed by whole-cell biotransformation of industrial waste products, mostly D-carnitine and cronobetaine, through specific bacterial species. No comparable system has been established using eukaryotic microorganisms, even though there is a significant and growing international demand for either the pure compound or carnitine-enriched consumables. In eukaryotes, including the fungus Neurospora crassa, L-carnitine is biosynthesized through a four-step metabolic conversion of trimethyllysine to L-carnitine. In contrast, the industrial yeast, Saccharomyces cerevisiae lacks the enzymes of the eukaryotic biosynthesis pathway and is unable to synthesize carnitine. This study describes the cloning of all four of the N. crassa carnitine biosynthesis genes and the reconstruction of the entire pathway in S. cerevisiae. The engineered yeast strains were able to catalyze the synthesis of L-carnitine, which was quantified using hydrophilic interaction liquid chromatography electrospray ionization mass spectrometry (HILIC-ESI-MS) analyses, from trimethyllysine. Furthermore, the yeast threonine aldolase Gly1p was shown to effectively catalyze the second step of the pathway, fulfilling the role of a serine hydroxymethyltransferase. The analyses also identified yeast enzymes that interact with the introduced pathway, including Can1p, which was identified as the yeast transporter for trimethyllysine, and the two yeast serine hydroxymethyltransferases, Shm1p and Shm2p. Together, this study opens the possibility of using an engineered, carnitine-producing yeast in various industrial applications while providing insight into possible future strategies aimed at tailoring the production capacity of such strains.

Overview of the role of alcohol dehydrogenase and aldehyde dehydrogenase and their variants in the genesis of alcohol-related pathology

Alcohol dehydrogenase (ADH) and mitochondrial aldehyde dehydrogenase (ALDH2) are responsible for metabolizing the bulk of ethanol consumed as part of the diet and their activities contribute to the rate of ethanol elimination from the blood. They are expressed at highest levels in liver, but at lower levels in many tissues. This pathway probably evolved as a detoxification mechanism for environmental alcohols. However, with the consumption of large amounts of ethanol, the oxidation of ethanol can become a major energy source and, particularly in the liver, interferes with the metabolism of other nutrients. Polymorphic variants of the genes for these enzymes encode enzymes with altered kinetic properties. The pathophysiological effects of these variants may be mediated by accumulation of acetaldehyde; high-activity ADH variants are predicted to increase the rate of acetaldehyde generation, while the low-activity ALDH2 variant is associated with an inability to metabolize this compound. The effects of acetaldehyde may be expressed either in the cells generating it, or by delivery of acetaldehyde to various tissues by the bloodstream or even saliva. Inheritance of the high-activity ADH β2, encoded by theADH2*2gene, and the inactiveALDH2*2gene product have been conclusively associated with reduced risk of alcoholism. This association is influenced by gene–environment interactions, such as religion and national origin. The variants have also been studied for association with alcoholic liver disease, cancer, fetal alcohol syndrome, CVD, gout, asthma and clearance of xenobiotics. The strongest correlations found to date have been those between theALDH2*2allele and cancers of the oro-pharynx and oesophagus. It will be important to replicate other interesting associations between these variants and other cancers and heart disease, and to determine the biochemical mechanisms underlying the associations.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Human aldehyde dehydrogenase catalytic activity and structural interactions with coenzyme analogs

K(m) and V(max) values for 10 coenzyme analogs never previously studied with any aldehyde dehydrogenase and NADP(+) were compared with those for NAD(+) for three human aldehyde dehydrogenases (EC 1.2.1.3); the cytoplasmic E1 (the product of the aldh1 gene), the mitochondrial E2 (the product of the aldh2 gene) and the cytoplasmic E3 (the product of the aldh9 gene) isozymes. Structural information on changes in coenzyme-protein interactions were obtained via molecular dynamics (MD) studies with the E2 isozyme and quantum mechanical (QM) calculations were used to study changes in charge distribution of the pyridine ring and relative free energies of solvation of the purine ring in the analogs. E1 showed the broadest substrate specificity and was the only isozyme subject to substrate inhibition, both of which are suggested to be due to the two coenzyme conformations observed previously in the sheep crystal structure. NADP(+) selectivity is indicated to be influenced by Glu195 in E1 and E2. Substitutions in the purine ring affected K(m) but not V(max), with the changes in K(m) being dominated by the hydrophobicity of the purine ring as indicted by the QM calculations. Substitutions in the pyridine ring sometimes rendered the coenzymes inactive, with no consistent pattern observed for the three coenzymes. Structural analysis of the coenzyme analog-E2 MD simulations revealed different structural perturbations of the surrounding active site, though interactions with Asn169 and Glu399 were preserved in all cases.

Betaine aldehyde, betaine, and choline levels in rat livers during ethanol metabolism

Betaine aldehyde levels were determined in rat livers following 4 weeks of ethanol feeding, employing the Lieber-De Carli liquid diet. The results showed that the levels of betaine aldehyde are unaffected by alcohol feeding to rats. These levels in both experimental and control animals were found to be quite low, 5.5 nmol/g liver. Betaine aldehyde levels have not been determined previously in mammalian liver because of methodological difficulties. This investigation employed fast atom bombardment-mass spectroscopy to determine the levels of betaine aldehyde, betaine, and choline. The decrease in betaine levels following ethanol administration confirmed the results of other investigators. Choline levels determined during this investigation were lower than previously reported. The reason for starting this investigation was the fact that the enzyme that catalyzes betaine aldehyde dehydrogenation to betaine, which is distributed in both mitochondria and the cytoplasm, was found to also metabolize acetaldehyde with K(m) and V(max) values lower than those for betaine aldehyde. Thus, it appeared likely that the metabolism of acetaldehyde during ethanol metabolism might inhibit betaine aldehyde conversion to betaine and thereby result in decreased betaine levels (Barak et al., Alcohol 13: 395-398, 1996). The fact that betaine aldehyde levels in alcohol-fed animals were similar to those in controls demonstrates that competition between acetaldehyde and betaine aldehyde for the same enzyme does not occur. This complete lack of competition suggests that betaine aldehyde dehydrogenase in the mitochondrial matrix may totally metabolize betaine aldehyde to betaine without any involvement of cytoplasmic betaine aldehyde dehydrogenase.

Molecular and biochemical characterization of rat gamma-trimethylaminobutyraldehyde dehydrogenase and evidence for the involvement of human aldehyde dehydrogenase 9 in carnitine biosynthesis

The penultimate step in carnitine biosynthesis is mediated by gamma-trimethylaminobutyraldehyde dehydrogenase (EC 1.2.1.47), a cytosolic NAD(+)-dependent aldehyde dehydrogenase that converts gamma-trimethylaminobutyraldehyde into gamma-butyrobetaine. This enzyme was purified from rat liver, and two internal peptide fragments were sequenced by Edman degradation. The peptide sequences were used to search the Expressed Sequence Tag data base, which led to the identification of a rat cDNA containing an open reading frame of 1485 base pairs encoding a polypeptide of 494 amino acids with a calculated molecular mass of 55 kDa. Expression of the coding sequence in Escherichia coli confirmed that the cDNA encodes gamma-trimethylaminobutyraldehyde dehydrogenase. The previously identified human aldehyde dehydrogenase 9 (EC 1.2.1.19) has 92% identity with rat trimethylaminobutyraldehyde dehydrogenase and has been reported to convert substrates that resemble gamma-trimethylaminobutyraldehyde. When aldehyde dehydrogenase 9 was expressed in E. coli, it exhibited high trimethylaminobutyraldehyde dehydrogenase activity. Furthermore, comparison of the enzymatic characteristics of the heterologously expressed human and rat dehydrogenases with those of purified rat liver trimethylaminobutyraldehyde dehydrogenase revealed that the three enzymes have highly similar substrate specificities. In addition, the highest V(max)/K(m) values were obtained with gamma-trimethylaminobutyraldehyde as substrate. This indicates that human aldehyde dehydrogenase 9 is the gamma-trimethylaminobutyraldehyde dehydrogenase, which functions in carnitine biosynthesis.

Evidence for mitochondrial localization of betaine aldehyde dehydrogenase in rat liver: purification, characterization, and comparison with human cytoplasmic E3 isozyme

Betaine aldehyde dehydrogenase has been purified to homogeneity from rat liver mitochondria. The properties of betaine aldehyde dehydrogenase were similar to those of human cytoplasmic E3 isozyme in substrate specificity and kinetic constants for substrates. The primary structure of four tryptic peptides was also similar; only two substitutions, at most, per peptide were observed. Thus, betaine aldehyde dehydrogenase is not a specific enzyme, as formerly believed; activity with betaine aldehyde is a property of aldehyde dehydrogenase (EC 1.2.1.3), which has broad substrate specificity. Up to the present time the enzyme was thought to be cytoplasmic in mammals. This report establishes, for the first time, mitochondrial subcellular localization for aldehyde dehydrogenase, which dehydrogenates betaine aldehyde, and its colocalization with choline dehydrogenase. Betaine aldehyde dehydrogenation is an important function in the metabolism of choline to betaine, a major osmolyte. Betaine is also important in mammalian organisms as a major methyl group donor and nitrogen source. This is the first purification and characterization of mitochondrial betaine aldehyde dehydrogenase from any mammalian species.Key words: betaine, aldehyde, dehydrogenase, mitochondria, rat liver.

Methylglyoxal as substrate and inhibitor of human aldehyde dehydrogenase: comparison of kinetic properties among the three isozymes

Methylglyoxal was demonstrated to be a substrate for the isozymes E1, E2 and E3 of human aldehyde dehydrogenase. Pyruvate was the product from the oxidation of methylglyoxal by the three isozymes. At pH 7.4 and 25 degrees C, the major and minor components of the E3 isozyme catalyzed the reaction with Vmax of 1.1 and 0.8 mumol NADH min-1 mg-1 protein, respectively, compared to 0.067 and 0.060 mumol NADH min-1 mg-1 protein for the E1 and E2 isozymes, respectively. The E2 isozyme had a K(m) for methylglyoxal of 8.6 microM, the lowest compared to 46 microM for E1 and 586 and 552 microM for the major and minor components of the E3 isozyme, respectively. Both components of the E3 isozyme showed substrate inhibition by methylglyoxal, with Ki values of 2.0 mM for the major component and 12 mM for the minor component at pH 9.0. Substrate inhibition by methylglyoxal was not observed with the E1 and E2 isozymes. Methylglyoxal strongly inhibited the glycolaldehyde activity of the E1 and E2 isozymes. Mixed-type models of inhibition were employed as an approach to calculate the inhibition constants, 44 and 10.6 microM for E1 and E2 isozymes, respectively.

Tissue distribution of human aldehyde dehydrogenase E3 (ALDH9): comparison of enzyme activity with E3 protein and mRNA distribution

The tissue distribution of the E3 isozyme of human aldehyde dehydrogenase has been investigated by three methods: enzyme activity assay employing betaine aldehyde as substrate, Western blotting employing E3 isozyme-specific antibodies, and Northern blotting using a human liver E3 cDNA as probe. All three methods showed that E3 isozyme was universally distributed among all tissues tested. The highest levels of the E3 isozyme activity were found in liver, adrenal gland, and kidney. These same tissues also showed highest levels of the E3 protein via the Western blot. This distribution is consistent with the possible physiological role of E3 isozyme in the synthesis of the osmolyte, betaine, and the neurotransmitter, GABA. Northern blot analysis, however, differed from that of enzyme assay and the Western blot in that it showed highest mRNA levels in skeletal and heart muscles, which had low enzyme activities and E3 protein levels.

Sequencing and expression of the human ALDH8 encoding a new member of the aldehyde dehydrogenase family

The human aldehyde dehydrogenase gene (ALDH) family is characterized by two major conserved DNA sequences encoding residues which are possibly involved in the catalytic function and the maintenance of the functional conformation of the ALDH enzyme. This property is the basis for synthesizing the degenerate primers to clone several cDNAs of the ALDH isozymes. In this report, we describe the cDNA sequence and the expression of a new member of this family, ALDH8. The human ALDH8 gene was identified during the process of the screening for the human ALDH7 genomic clones. Overlapping ALDH8 cDNA clones were isolated by polymerase chain reaction (PCR) amplification of human salivary gland total RNA or lambda gt11 cDNA library. When the ALDH8 cDNA sequence was aligned with that of the ALDH7 which encodes a polypeptide chain of 468 amino acid (aa) residues, it was found that a termination codon (TGA) is placed in frame at the ALDH8 sequence corresponding to the codon GCG for the seventeenth aa position of the ALDH7. Therefore, the human ALDH8 gene is a potential nonprocessed pseudogene in the ALDH multigene family which has no other pseudogenes reported so far. Alternatively, the ALDH8 gene is a functional gene if the premature stop codon is suppressed, or if the first downstream in-frame ATG serves as the initiator codon. This longest putative open reading frame (ORF) encodes a polypeptide chain of 385 aa residues, includes the two ALDH conserved regions, and demonstrates 86% identity with the corresponding ORF region of the human ALDH7. The expression of the ALDH8 transcripts is restricted to the salivary gland among the human tissues examined.

Aldehyde dehydrogenase from adult human brain that dehydrogenates gamma-aminobutyraldehyde: purification, characterization, cloning and distribution

Enzyme purification and characterization, cDNA cloning and Northern blot analysis were the techniques utilized during this investigation to determine the identity and occurrence of the aldehyde dehydrogenase that metabolizes gamma-aminobutyraldehyde in adult human brain. The purification yielded one major protein which was active with gamma-aminobutyraldehyde. It had the physico-chemical and kinetic properties of the human liver E3 isoenzyme of aldehyde dehydrogenase (EC 1.2.1.3), and also interacted with an anti-(liver E3 isoenzyme) antibody. Tryptic peptides derived from the purified brain protein matched the amino acid sequence of the liver E3 isoenzyme. Employing liver E3 cDNA, a human cerebellar cDNA library was screened and a 2.0 kb cDNA fragment was isolated. The cerebellar cDNA yielded a derived primary structure which differed from the liver E3 amino acid sequence by a single serine-to-cysteine substitution at position 88 (position 84 in the liver sequence). Thus the gamma-amino-butyraldehyde-metabolizing enzyme from human brain can be identified as E3', a variant of the E3 isoenzyme. The catalytic properties of the brain variant were indistinguishable from those of E3, and so the functional importance of this variant is at present unknown. The distribution of this enzyme in brain was investigated by Northern blot analysis, which demonstrated the presence of E3' mRNA in all regions of the human brain. mRNA levels were variable in the different brain areas, with the highest levels in the spinal cord and the lowest in the occipital pole.

Mouse microsomal Class 3 aldehyde dehydrogenase: AHD3 cDNA sequence, inducibility by dioxin and clofibrate, and genetic mapping

We have cloned and sequenced the mouse AHD3 cDNA, which codes for the Class 3 microsomal aldehyde dehydrogenase (ALDH3m). The cDNA is 2,997 bp in length excluding the poly(A)+ tail, and has 5' and 3' non-translated regions of 113 bp and 1,429 bp, respectively. The deduced amino acid sequence consists of 484 amino acids, including the first methionine (Mr = 53,942), and contains a hydrophobic segment at the carboxyl terminus which is the putative membrane anchor. The mouse AHD3 protein was found to be: 95% similar to the rat microsomal ALDH3m protein, 65% identical to the mouse, rat and human cytosolic ALDH3c protein, and <28% similar to the rat Class 1 and Class 2 ALDH and methylmalonate-semialdehyde dehydrogenase proteins. Southern hybridization analysis of mouse cDNA probed with the full-length AHD3 cDNA revealed that the Ahd3 gene likely spans less than a total of 25 kb. The mouse Ahd3 gene is very tightly linked to the Ahd4 gene on chromosome 11. Mouse AHD3 mRNA levels are increased by dioxin in mouse Hepa-1c1c7 hepatoma wild-type (wt) cells but not in the Ah receptor nuclear translocator (ARNT)-defective (c4) mutant line, indicating that the induction process is mediated by the Ah (aromatic hydrocarbon) dioxin-binding receptor. AHD3 mRNA levels are also inducible by clofibrate in both the wt and c4 lines. AHD3 mRNA levels are not elevated in the CYP1A1 metabolism-deficient c37 mutant line or as part of the oxidative stress response found in the untreated 14CoS/14CoS mouse cell line. These data indicate that, although inducible by dioxin, the Ahd3 gene does not qualify as a member of the aromatic hydrocarbon [Ah] gene battery.

Molecular cloning of the mature NAD(+)-dependent succinic semialdehyde dehydrogenase from rat and human. cDNA isolation, evolutionary homology, and tissue expression

Three rat brain cDNA clones approximately 3500, 1465, and 1135 base pairs in length encoding succinic semialdehyde dehydrogenase (SSADH; EC 1.2.1.24) were isolated from two cDNA libraries using a polymerase chain reaction derived probe. Restriction mapping and DNA sequencing revealed that the 3.5-kilobase clone contained an 84-base pair (28 amino acid) insert in the coding region. Composite clones encoding mature SSADH predicted proteins with 488 amino acids (M(r) = 52,188) when including the insert and 460 amino acids (M(r) = 48,854) without the insert. The cDNA clones were confirmed by expression of enzyme activity in bacteria and protein sequence data obtained from sequencing purified rat brain SSADH. Two human liver SSADH cDNA clones of 1091 and 899 base pairs were also isolated. Human and rat SSADH share 83 and 91% identity in nucleotide and protein sequence, respectively. Northern blot analysis revealed two differentially expressed SSADH transcripts of approximately 2.0 and 6.0 kilobases in both rat and human tissues. Human genomic Southern blots indicate that the two SSADH transcripts are encoded by a greater than 20-kilobase single copy gene. Mammalian SSADH contains significant homology to bacterial NADP(+)-succinic semialdehyde dehydrogenase (EC 1.2.1.16) and conserved regions of general aldehyde dehydrogenases (EC 1.2.1.3), suggesting it is a member of the aldehyde dehydrogenase superfamily of proteins.

Cloning of a cDNA encoding human ALDH7, a new member of the aldehyde dehydrogenase family

Aldehyde dehydrogenases (ALDH; EC 1.2.1.3) are a family of isozymes which have been suggested to play a major role in the detoxification of aldehydes generated by alcohol metabolism and lipid peroxidation. Five non-allelic ALDH genes, encoding the ALDH1, 2, 3, 5 and 6 isozymes, have previously been identified and cloned in our laboratory. In this paper, we report the cloning and sequencing of a cDNA encoding a new human ALDH (ALDH7). Degenerate oligodeoxyribonucleotides derived from conserved regions of known ALDH cDNAs amplified a 408-bp product from human kidney total RNA by the reverse transcription-polymerase chain reaction (RT-PCR) procedures [Hsu et al., J. Biol. Chem. 266 (1992) 3030-3037]. This PCR product was subcloned, selected and used as a probe to screen a human kidney cDNA library. The full-length human kidney cDNA (ALDH7) is 2791 bp in length and contains an open reading frame encoding 468 amino acids (aa). The deduced sequence of ALDH7 is longer than that of the human stomach ALDH3 by 15 aa at the C terminus. The degree of identity between the two isozymes is 52% with a positional alignment of 453 aa. Northern blot analysis demonstrated that lung is another major tissue expressing ALDH7.

Aldehyde dehydrogenase of mice inhibited by thiocarbamate herbicides

The herbicide S-ethyl N,N-dipropylthiocarbamate (EPTC) and three of its candidate metabolites (the sulfoxide, N-depropyl and S-methyl derivatives) inhibit mitochondrial low-Km aldehyde dehydrogenase (ALDH) in liver by 56 to 82% 2 hr after these thiocarbamates are administered intraperitoneally (ip) to mice at 8 mg/kg. They also greatly elevate the acetaldehyde level (determined as the O-benzyloxime ether) in blood (up to 500 microM) and brain (up to 3 ppm) 30 min after two ip treatments, the first with the thiocarbamate at 40 mg/kg and 2 hr later with ethanol at 1000 mg/kg. EPTC at 4 mg/kg inhibits liver ALDH activity by 50% and at 8 and 18 mg/kg gives half of the maximum ethanol-dependent elevation of acetaldehyde levels in blood and brain, respectively. The in vivo effects of other thiocarbamate herbicides at 8 mg/kg on ALDH activity and 40 mg/kg on acetaldehyde levels decrease in the order of thiobencarb, pebulate, vernolate and molinate > butylate and triallate >> cycloate. The percentage inhibition of liver ALDH activity generally correlates with the elevation in blood and brain acetaldehyde under these treatment protocols. B.W. Hart and M.D. Faiman (Biochem. Pharmacol. 43 403-406, 1992) have shown that the alcohol-aversion drug disulfiram is metabolized to S-methyl N,N-diethylthiocarbamate and its sulfoxide as the penultimate and ultimate metabolites inhibiting ALDH. Thus, the thiocarbamate herbicides and their metabolites are similar to the disulfiram metabolites not only in homologous structure but also in their potency range as ALDH inhibitors in vivo. On this basis some of the thiocarbamate herbicides may sensitize agricultural workers to ethanol intoxication.