Simulation of biogenic amine metabolism in the brain

Pleiotropic Actions of Aldehyde Reductase (AKR1A)

We provide an overview of the physiological roles of aldehyde reductase (AKR1A) and also discuss the functions of aldose reductase (AKR1B) and other family members when necessary. Many types of aldehyde compounds are cytotoxic and some are even carcinogenic. Such toxic aldehydes are detoxified via the action of AKR in an NADPH-dependent manner and the resulting products may exert anti-diabetic and anti-tumorigenic activity. AKR1A is capable of reducing 3-deoxyglucosone and methylglyoxal, which are reactive intermediates that are involved in glycation, a non-enzymatic glycosylation reaction. Accordingly, AKR1A is thought to suppress the formation of advanced glycation end products (AGEs) and prevent diabetic complications. AKR1A and, in part, AKR1B are responsible for the conversion of d-glucuronate to l-gulonate which constitutes a process for ascorbate (vitamin C) synthesis in competent animals. AKR1A is also involved in the reduction of S-nitrosylated glutathione and coenzyme A and thereby suppresses the protein S-nitrosylation that occurs under conditions in which the production of nitric oxide is stimulated. As the physiological functions of AKR1A are currently not completely understood, the genetic modification of Akr1a could reveal the latent functions of AKR1A and differentiate it from other family members.

Methylmercury impairs canonical dopamine metabolism in rat undifferentiated pheochromocytoma (PC12) cells by indirect inhibition of aldehyde dehydrogenase

The environmental neurotoxicant methylmercury (MeHg) disrupts dopamine (DA) neurochemical homeostasis by stimulating DA synthesis and release. Evidence also suggests that DA metabolism is independently impaired. The present investigation was designed to characterize the DA metabolomic profile induced by MeHg, and examine potential mechanisms by which MeHg inhibits the DA metabolic enzyme aldehyde dehydrogenase (ALDH) in rat undifferentiated PC12 cells. MeHg decreases the intracellular concentration of 3,4-dihydroxyphenylacetic acid (DOPAC). This is associated with a concomitant increase in intracellular concentrations of the intermediate metabolite 3,4-dihydroxyphenylaldehyde (DOPAL) and the reduced metabolic product 3,4-dihydroxyethanol. This metabolomic profile is consistent with inhibition of ALDH, which catalyzes oxidation of DOPAL to DOPAC. MeHg does not directly impair ALDH enzymatic activity, however MeHg depletes cytosolic levels of the ALDH cofactor NAD(+), which could contribute to impaired ALDH activity following exposure to MeHg. The observation that MeHg shunts DA metabolism along an alternative metabolic pathway and leads to the accumulation of DOPAL, a reactive species associated with protein and DNA damage, as well as cell death, is of significant consequence. As a specific metabolite of DA, the observed accumulation of DOPAL provides evidence for a specific mechanism by which DA neurons may be selectively vulnerable to MeHg.

Benzo(a)pyrene induces hepatic AKR1A1 mRNA expression in tilapia fish (Oreochromis niloticus)

AKR1A1 or aldehyde reductase is a member of the aldo-keto reductases superfamily that is evolutionarily conserved among species. AKR1A1 is one of the five AKRs (AKR1A1 and 1C1-1C4) implicated in the metabolic benzo(a)pyrene (BaP) activation to reactive BaP 7,8-dione. BaP is a polycyclic aromatic hydrocarbon (PAH) widely distributed in aquatic ecosystems and its metabolic activation is necessary to produce its toxic effects. Although the presence of AKR1A1 in fish has been reported, its tissue distribution in tilapia (Oreochromis niloticus) and AKR1A1 inducibility by BaP are not known yet. Moreover, cytochrome P4501A (CYP1A) mRNA expression in fish has been used as a PAH biomarker of effect. Therefore, BaP effects on AKR1A1 and CYP1A gene expressions in tilapia, a species of commercial interest, were investigated by real-time RT-PCR. A partial AKR1A1 cDNA was identified, sequenced and compared with AKR1A1 reported sequences in the GenBank DNA database. Constitutive AKR1A1 mRNA expression was detected mainly in liver, similarly to that of CYP1A. BaP exposure resulted in statistically significant AKR1A1 and CYP1A mRNA induction in liver (20- and 120-fold, respectively) at 24 h. On the other hand, ethoxyquin (EQ) was used as control inducer for AKR1A1 mRNA. Interestingly, EQ also induced CYP1A mRNA levels in tilapia liver. Our results suggest that teleost AKR1A1, in addition to CYP1A, are inducible by BaP. The mechanism of AKR1A1 induction by BaP and its role in fish susceptibility to BaP toxic effects remains to be elucidated.

Products of oxidative stress inhibit aldehyde oxidation and reduction pathways in dopamine catabolism yielding elevated levels of a reactive intermediate

Dopamine (DA) has been implicated as an endogenous neurotoxin to explain the selective neurodegeneration as observed for Parkinson's disease (PD). In addition, oxidative stress and lipid peroxidation are hypothesized culprits in PD pathogenesis. DA undergoes catabolism by monoamine oxidase (MAO) to 3,4-dihydroxyphenylacetaldehyde (DOPAL), which is further oxidized to 3,4-dihydroxyphenylacetic acid (DOPAC) via aldehyde dehydrogenase (ALDH). As a minor and compensatory metabolic pathway, DOPAL can be reduced to 3,4-dihydroxyphenylethanol (DOPET) via cytosolic aldehyde or aldose reductase (AR). Previous studies have found DOPAL to be significantly more toxic to DA cells than DA and that the major lipid peroxidation products, that is, 4-hydroxynonenal (4HNE) and malondialdehyde (MDA), potently inhibit DOPAL oxidation via ALDH. The hypothesis of this work is that lipid peroxidation products inhibit DOPAL oxidation, yielding aberrant levels of the toxic aldehyde intermediate. To test this hypothesis, nerve growth factor-differentiated PC6-3 cells were used as a model for DA neurons. Cell viability in the presence of 4HNE and MDA (2-100 microM) was measured by MTT assay, and it was found that only 100 microM 4HNE exhibited significant cytotoxicity. Treatment of cells with varying concentrations of 4HNE and MDA resulted in reduced DOPAC production and significant elevation of DOPAL levels, suggesting inhibition of ALDH. In cells treated with 4HNE that exhibited elevated DOPAL, there was a significant increase in DOPET. However, elevated DOPET was not observed for the cells treated with MDA, suggesting MDA to be an inhibitor of AR. Using isolated cytosolic AR, it was found that MDA but not 4HNE inhibited reductase activity toward DOPAL, surprisingly. These data demonstrate that the oxidative stress products 4HNE and MDA inhibit the aldehyde biotransformation step of DA catabolism yielding elevated levels of the endogenous neurotoxin DOPAL, which may link oxidative stress to selective neurodegeneration as seen in PD.

Lipid Peroxidation Products Inhibit Dopamine Catabolism Yielding Aberrant Levels of a Reactive Intermediate

Recent work indicates that oxidative stress is a factor in Parkinson's disease (PD); however, it is unknown how this condition causes selective dopaminergic cell death. The neurotransmitter dopamine (DA) has been implicated as an endogenous neurotoxin to explain the selective neurodegeneration. DA undergoes catabolism by monoamine oxidase (MAO) to the reactive intermediate 3,4-dihydroxyphenylacetaldehyde (DOPAL), which is further oxidized to 3,4-dihydroxyphenylacetic (DOPAC) acid via mitochondrial aldehyde dehydrogenase (ALDH). Previous studies found DOPAL to be more toxic than DA, and the major lipid peroxidation products, that is, 4-hydroxynonenal (4HNE) and malondialdehyde (MDA), potently inhibit ALDH. The hypothesis of this work is that lipid peroxidation products inhibit DOPAL oxidation, yielding aberrant levels of the reactive aldehyde intermediate. Treatment of striatal synaptosomes with 2-100 microM 4HNE or 2-50 microM MDA impaired DOPAL oxidation, resulting in elevated [DOPAL]. The aberrant concentration of DOPAL yielded an increase in protein modification by the DA-derived aldehyde, evident via staining of proteins with nitroblue tetrazolium (NBT). Pretreatment of synaptosomes with an MAO inhibitor significantly decreased NBT staining. On the basis of NBT staining, the order of protein reactivity for DA and metabolites was found to be DOPAL>>DOPAC>DA. Mass spectrometric analysis of a model peptide reacted with DOPAL revealed the adduct to be a Schiff base product. In summary, these data demonstrate the sensitivity of DA catabolism to the lipid peroxidation products 4HNE and MDA even at low, physiologic levels and suggest a mechanistic link between oxidative stress and generation of aberrant levels of an endogenous and protein reactive dopaminergic toxin relevant to PD.

Inhibition of the oxidative metabolism of 3,4-dihydroxyphenylacetaldehyde, a reactive intermediate of dopamine metabolism, by 4-hydroxy-2-nonenal

Recent evidence indicates a role for oxidative stress and resulting products, e.g. 4-hydroxy-2-nonenal (4HNE) in the pathogenesis of Parkinson's disease (PD). 4HNE is a known inhibitor of mitochondrial aldehyde dehydrogenase (ALDH2), an enzyme very important to the dopamine (DA) metabolic pathway. DA undergoes monoamine oxidase-catalyzed oxidative deamination to 3,4-dihydroxyphenylacetaldehyde (DOPAL), which is metabolized primarily to 3,4-dihydroxyphenylacetic acid (DOPAC) via ALDH2. The biotransformation of DOPAL is critical as previous studies have demonstrated this DA-derived aldehyde to be a reactive electrophile and toxic to dopaminergic cells. Therefore, 4HNE produced via oxidative stress may inhibit ALDH2-mediated oxidation of the endogenous neurotoxin DOPAL. To test this hypothesis, ALDH2 in various model systems was treated with 4HNE and activity toward DOPAL measured. Incubation of human recombinant ALDH2 with 4HNE (1.5-30 microM) yielded inhibition of activity toward DOPAL. Furthermore, ALDH2 in rat brain mitochondrial lysate as well as isolated rat brain mitochondria was also sensitive to the lipid peroxidation product at low micromolar, as evident by a decrease in the rate of DOPAL to DOPAC conversion measured using HPLC. Taken together, these data indicate that 4HNE at low micromolar inhibits mitochondrial biotransformation of DOPAL to DOPAC, and generation of the lipid peroxidation product may represent a mechanism yielding aberrant levels of DOPAL, thus linking oxidative stress to the uncontrolled production of an endogenous neurotoxin relevant to PD.

Inducible gene trapping with drug-selectable markers and Cre/loxP to identify developmentally regulated genes

Gene trapping in mouse embryonic stem cells is an important genetic approach that allows simultaneous mutation of genes and generation of corresponding mutant mice. We designed a selection scheme with drug selection markers and Cre/loxP technology which allows screening of gene trap events that responded to a signaling molecule in a 96-well format. Nine hundred twenty gene trap clones were assayed, and 258 were classified as gene traps induced by in vitro differentiation. Sixty-five of the in vitro differentiation-inducible gene traps were also responsive to retinoic acid treatment. In vivo analysis revealed that 85% of the retinoic acid-inducible gene traps trapped developmentally regulated genes, consistent with the observation that genes induced by retinoic acid treatment are likely to be developmentally regulated. Our results demonstrate that the inducible gene trapping system described here can be used to enrich in vitro for traps in genes of interest. Furthermore, we demonstrate that the cre reporter is extremely sensitive and can be used to explore chromosomal regions that are not detectable with neo as a selection cassette.

Regulation of aldehyde reductase expression by STAF and CHOP

Aldehyde reductase is involved in the reductive detoxification of reactive aldehydes that can modify cellular macromolecules. To analyze the mechanism of basal regulation of aldehyde reductase expression, we cloned the murine gene and adjacent regulatory region and compared it to the human gene. The mouse enzyme exhibits substrate specificity similar to that of the human enzyme, but with a 2-fold higher catalytic efficiency. In contrast to the mouse gene, the human aldehyde reductase gene has two alternatively spliced transcripts. A fragment of 57 bp is sufficient for 25% of human promoter activity and consists of two elements. The 3' element binds transcription factors of the Sp1 family. Gel-shift assays and chromatin immunoprecipitation as well as deletion/mutation analysis reveal that selenocysteine tRNA transcription activating factor (STAF) binds to the 5' element and drives constitutive expression of both mouse and human aldehyde reductase. Aldehyde reductase thus becomes the fourth protein-encoding gene regulated by STAF. The human, but not the mouse, promoter also binds C/EBP homologous protein (CHOP), which competes with STAF for the same binding site. Transfection of the human promoter into ethoxyquin-treated mouse 3T3 cells induces a 3.5-fold increase in promoter activity and a CHOP-C/EBP band appears on gel shifts performed with the 5' probe from the human aldehyde reductase promoter. Induction is attenuated in similar transfection studies of the mouse promoter. Mutation of the CHOP-binding site in the human promoter abolishes CHOP binding and significantly reduces ethoxyquin induction, suggesting that CHOP mediates stimulated expression in response to antioxidants in the human. This subtle difference in the human promoter suggests a further evolution of the promoter toward responsiveness to exogenous stress and/or toxins.

Aldose reductase: an aldehyde scavenging enzyme in the intraneuronal metabolism of norepinephrine in human sympathetic ganglia

The neurotransmitter norepinephrine is metabolized by monoamine oxidase into an aldehyde intermediate that is further metabolized to the stable glycol derivative, 3,4-dihydroxyphenylglycol (DHPG). In this study, the possible role of aldose reductase in reducing this aldehyde intermediate in human sympathetic neurons has been examined. DHPG is formed when norepinephrine is incubated with aldose reductase in the presence of monoamine oxidase. DHPG metabolism is inhibited by the monoamine oxidase inhibitor, pargyline which prevents the deamination of norepinephrine, and by the aldose reductase inhibitor AL 1576, which inhibits DHPG formation without affecting the deamination of norepinephrine. Although similar formation of DHPG was observed with human liver aldehyde reductase, the production of DHPG was more effective with aldose reductase than aldehyde reductase. Two peaks of reductase activity corresponding to aldose reductase and aldehyde reductase were observed when sympathetic ganglia were chromatofocused. Molecular modeling studies indicate that the energy-minimized structure of 3,4-dihydroxymandelaldehyde bound to aldose reductase is similar to that of glyceraldehyde where the 2'-hydroxyl group forms hydrogen bonds with Trp111 and NADPH. These results suggest that aldose reductase may be important in metabolizing the potentially toxic aldehyde intermediate formed from norepinephrine in human sympathetic ganglia.

Purification from rat liver of a novel constitutively expressed member of the aldo-keto reductase 7 family that is widely distributed in extrahepatic tissues

Antiserum raised against human aflatoxin B(1) aldehyde reductase 1 (hAFAR1) has been used to identify a previously unrecognized rat aldo-keto reductase (AKR). This novel enzyme is designated rat aflatoxin B(1) aldehyde reductase 2 (rAFAR2) and it characteristically migrates faster during SDS/PAGE than does the archetypal ethoxyquin-inducible rAFAR protein (now called rAFAR1). Significantly, rAFAR2 is essentially unreactive with polyclonal antibodies raised against rAFAR1. Besides its distinct electrophoretic and immunochemical properties, rAFAR2 appears to be regulated differently from rAFAR1 as it is expressed in most rat tissues and does not appear to be induced by ethoxyquin. Multiple forms of rAFAR2 have been identified. Anion-exchange chromatography on Q-Sepharose, followed by adsorption chromatography on columns of Matrex Orange A and Cibacron Blue, have been employed to purify rAFAR2 from rat liver cytosol. The Q-Sepharose chromatography step resulted in the resolution of rAFAR2 into three peaks of AKR activity, two of which were purified and shown to be capable of catalysing the reduction of 2-carboxybenzaldehyde, succinic semialdehyde, 4-nitrobenzaldehyde and 9,10-phenathrenequinone. The two most highly purified rAFAR2-containing preparations eluted from the Cibacron Blue column were 91 and 98% homogeneous. Analysis of these by SDS/PAGE indicated that the least anionic (peak CBA5) comprised a polypeptide of 37.0 kDa, whereas the most anionic (peak CBA6) contained two closely migrating polypeptides of 36.8 and 37.0 kDa; by contrast, in the present study, rAFAR1 was estimated by SDS/PAGE to be composed of 38.0 kDa subunits. Final purification of the 37 kDa polypeptide in CBA5 and CBA6 was accomplished by reversed-phase HPLC. Partial proteolysis of the two preparations of the 37 kDa polypeptide with Staphylococcus aureus V8 protease yielded fragments of identical size, suggesting that they represent the product of a single gene. Furthermore, the peptide maps from CBA5 and CBA6 differed substantially from that yielded by rAFAR1, indicating that they are genetically distinct from the inducible reductase. A peptide generated by CNBr digestion of the 37 kDa polypeptide from CBA6 was shown by Edman degradation to share 88% sequence identity with residues Tyr(168)-Leu(183) of rAFAR1. This provides evidence that the rat protein identified by its cross-reactivity with anti-hAFAR1 serum is an additional member of the AKR7 family.

Cloning and developmental expression of mouse aldehyde reductase (AKR1A4)

The aldo-keto reductase superfamily catalyzes the reduction of a broad range of aldehydes and ketones to their corresponding alcohols. Here we report the cloning of the mouse aldehyde reductase cDNA and its embryonic pattern of expression. From stages E7.5 to E13.5 the gene encoding for this enzyme is expressed at high levels in several tissues, including the neural ectoderm, gut endoderm, somites, branchial arches, otic vesicles, limb buds, and tail bud. In adult mice aldehyde reductase was expressed in all tissues examined.

Purification from rat liver of a novel constitutively expressed member of the aldo-keto reductase 7 family that is widely distributed in extrahepatic tissues

Antiserum raised against human aflatoxin B(1) aldehyde reductase 1 (hAFAR1) has been used to identify a previously unrecognized rat aldo-keto reductase (AKR). This novel enzyme is designated rat aflatoxin B(1) aldehyde reductase 2 (rAFAR2) and it characteristically migrates faster during SDS/PAGE than does the archetypal ethoxyquin-inducible rAFAR protein (now called rAFAR1). Significantly, rAFAR2 is essentially unreactive with polyclonal antibodies raised against rAFAR1. Besides its distinct electrophoretic and immunochemical properties, rAFAR2 appears to be regulated differently from rAFAR1 as it is expressed in most rat tissues and does not appear to be induced by ethoxyquin. Multiple forms of rAFAR2 have been identified. Anion-exchange chromatography on Q-Sepharose, followed by adsorption chromatography on columns of Matrex Orange A and Cibacron Blue, have been employed to purify rAFAR2 from rat liver cytosol. The Q-Sepharose chromatography step resulted in the resolution of rAFAR2 into three peaks of AKR activity, two of which were purified and shown to be capable of catalysing the reduction of 2-carboxybenzaldehyde, succinic semialdehyde, 4-nitrobenzaldehyde and 9,10-phenathrenequinone. The two most highly purified rAFAR2-containing preparations eluted from the Cibacron Blue column were 91 and 98% homogeneous. Analysis of these by SDS/PAGE indicated that the least anionic (peak CBA5) comprised a polypeptide of 37.0 kDa, whereas the most anionic (peak CBA6) contained two closely migrating polypeptides of 36.8 and 37.0 kDa; by contrast, in the present study, rAFAR1 was estimated by SDS/PAGE to be composed of 38.0 kDa subunits. Final purification of the 37 kDa polypeptide in CBA5 and CBA6 was accomplished by reversed-phase HPLC. Partial proteolysis of the two preparations of the 37 kDa polypeptide with Staphylococcus aureus V8 protease yielded fragments of identical size, suggesting that they represent the product of a single gene. Furthermore, the peptide maps from CBA5 and CBA6 differed substantially from that yielded by rAFAR1, indicating that they are genetically distinct from the inducible reductase. A peptide generated by CNBr digestion of the 37 kDa polypeptide from CBA6 was shown by Edman degradation to share 88% sequence identity with residues Tyr(168)-Leu(183) of rAFAR1. This provides evidence that the rat protein identified by its cross-reactivity with anti-hAFAR1 serum is an additional member of the AKR7 family.

Characterization of the human aldehyde reductase gene and promoter

Aldehyde reductase (EC 1.1.1.2; AKR1A1) is involved in the reduction of biogenic and xenobiotic aldehydes and is present in virtually every tissue. To study the regulation of its expression, the human aldehyde reductase gene and promoter were cloned and characterized. The protein coding region consists of eight exons, with two additional upstream exons, separated by a large intron of 9.4 kb, that code for the 5' untranslated region of the mRNA. Two mRNA transcripts that encode the same protein and that originate from alternative splicing were identified. The shorter transcript is the major form as shown by Northern blots and reverse transcription-PCR experiments. Northern blots of multiple tissues indicate that aldehyde reductase mRNA is present in all tissues examined and is most abundant in kidney, liver, and thyroid, which is consistent with the tissue enzyme distribution. The two mRNA transcripts do not exhibit differential tissue distribution. A construct containing a promoter region insert in a pGL3 vector drives transcription of a luciferase reporter gene and is 290-fold more active than a control vector without insert in transfected HepG2 cells. The activity of the full promoter construct is comparable to that of a pGL3 vector containing the SV40 promoter with an enhancer. The promoter does not contain a TATA box, but contains multiple GC-rich islands and exhibits bidirectional activity in transfection studies. The major active promoter element was localized by nested deletions and mutations to a DNA element (TGCAAT, -59 to -54) that presumptively binds the transcription factor CHOP [CAAT enhancer binding protein (C/EBP) homologous protein]. Comparison of the aldehyde reductase gene structure to all other characterized human genes of the aldo-keto reductase superfamily (aldose reductase, bile acid binder, and type I and type II 3alpha-hydroxysteroid dehydrogenases) indicates that it is more distantly related to these genes than they are among themselves.

Quantitation of 3,4-dihydroxyphenylacetaldehyde and 3, 4-dihydroxyphenylglycolaldehyde, the monoamine oxidase metabolites of dopamine and noradrenaline, in human tissues by microcolumn high-performance liquid chromatography

We recently described the chemical synthesis of 3, 4-dihydroxyphenylacetaldehyde and 3,4-dihydroxyphenylglycolaldehyde, the monamine oxidase metabolites of dopamine and noradrenaline, respectively. We demonstrated the neurotoxicity of these compounds. Catecholamine nerve cells which synthesize these aldehydes die in degenerative brain diseases, such as Parkinson's and Alzheimer's. Here we describe a sensitive method for separating these catecholaldehydes from catecholamines and their other oxidative and methylated metabolites by microcolumn high-performance liquid chromatography with electrochemical detection. We then quantitate catecholamines and their major metabolites in human brain, plasma, and urine. The method can be used to determine the role of these catecholaldehydes in human disease.

Molecular determinants of steroid recognition and catalysis in aldo-keto reductases. Lessons from 3alpha-hydroxysteroid dehydrogenase

Hydroxysteroid Dehydrogenases (HSDs) regulate the occupancy of steroid hormone receptors by converting active steroid hormones into their cognate inactive metabolites. HSDs belong to either the Short-chain Dehydrogenase/Reductases (SDRs) or the Aldo-Keto Reductases (AKRs). The AKRs include virtually all mammalian 3alpha-HSDs, Type 5 17beta-HSD, ovarian 20alpha-HSDs as well as the steroid 5beta-reductases. Selective inhibitors of 3alpha-HSD isoforms could control occupancy of the androgen and GABA(A) receptors, while broader based AKR inhibitors targeting 3alpha-HSD, 20alpha-HSD and prostaglandin F2alpha synthase could maintain pregnancy. We have determined three X-ray crystal structures of rat liver 3alpha-HSD, a representative AKR. These structures are of the apoenzyme (E), the binary-complex (E.NADP-), and the ternary complex (E.NADP+.testosterone). These structures are being used with site-directed mutagenesis to define the molecular determinants of steroid recognition and catalysis as a first step in rational inhibitor design. A conserved catalytic tetrad (Tyr55, Lys84, His117 and Asp50) participates in a 'proton-relay' in which Tyr55 acts as general acid/base catalyst. Its bifunctionality relies on contributions from His117 and Lys84 which alter the pKb and pKa, respectively of this residue. Point mutation of the tetrad results in different enzymatic activities. H117E mutants display 5beta-reductase activity while Y55F and Y55S mutants retain quinone reductase activity. Our results suggest that different transition states are involved in these reaction mechanisms. The ternary complex structure shows that the mature steroid binding pocket is comprised of ten residues recruited from five loops, and that there is significant movement of a C-terminal loop on binding ligand. Mutagenesis of pocket tryptophans shows that steroid substrates and classes of nonsteroidal inhibitors exhibit different binding modes which may reflect ligand-induced loop movement. Exploitation of these findings using steroidal and nonsteroidal mechanism based inactivators may lead to selective and broad based AKR inhibitors.

Comparative anatomy of the aldo-keto reductase superfamily

The aldo-keto reductases metabolize a wide range of substrates and are potential drug targets. This protein superfamily includes aldose reductases, aldehyde reductases, hydroxysteroid dehydrogenases and dihydrodiol dehydrogenases. By combining multiple sequence alignments with known three-dimensional structures and the results of site-directed mutagenesis studies, we have developed a structure/function analysis of this superfamily. Our studies suggest that the (alpha/beta)8-barrel fold provides a common scaffold for an NAD(P)(H)-dependent catalytic activity, with substrate specificity determined by variation of loops on the C-terminal side of the barrel. All the aldo-keto reductases are dependent on nicotinamide cofactors for catalysis and retain a similar cofactor binding site, even among proteins with less than 30% amino acid sequence identity. Likewise, the aldo-keto reductase active site is highly conserved. However, our alignments indicate that variation ofa single residue in the active site may alter the reaction mechanism from carbonyl oxidoreduction to carbon-carbon double-bond reduction, as in the 3-oxo-5beta-steroid 4-dehydrogenases (Delta4-3-ketosteroid 5beta-reductases) of the superfamily. Comparison of the proposed substrate binding pocket suggests residues 54 and 118, near the active site, as possible discriminators between sugar and steroid substrates. In addition, sequence alignment and subsequent homology modelling of mouse liver 17beta-hydroxysteroid dehydrogenase and rat ovary 20alpha-hydroxysteroid dehydrogenase indicate that three loops on the C-terminal side of the barrel play potential roles in determining the positional and stereo-specificity of the hydroxysteroid dehydrogenases. Finally, we propose that the aldo-keto reductase superfamily may represent an example of divergent evolution from an ancestral multifunctional oxidoreductase and an example of convergent evolution to the same active-site constellation as the short-chain dehydrogenase/reductase superfamily.

A new nomenclature for the aldo-keto reductase superfamily

The aldo-keto reductases (AKRs) represent a growing oxidoreductase superfamily. Forty proteins have been identified and characterized as AKRs, and an additional fourteen genes may encode proteins related to the superfamily. Found in eukaryotes and prokaryotes, the AKRs metabolize a wide range of substrates, including aliphatic aldehydes, monosaccharides, steroids, prostaglandins, and xenobiotics. This broad substrate specificity has caused problems in naming these proteins. Enzymes capable of these reactions have been referred to as aldehyde reductase (ALR1), aldose reductase (ALR2), and carbonyl reductase (ALR3); however, ALR3 is not a member of the AKR superfamily. Also, some AKRs have multiple names based upon substrate specificity. For example, human 3alpha-hydroxysteroid dehydrogenase (3apha-HSD) type I is also known as dihydrodiol dehydrogenase 4 and chlordecone reductase. To address these issues, we propose a new nomenclature system for the AKR superfamily based on amino acid sequence identities. Cluster analysis of the AKRs shows seven distinct families at the 40% amino acid identity level. The largest family (AKR1) contains the aldose reductases, aldehyde reductases, and HSDs. Other families include the prokaryotic AKRs, the plant chalcone reductases, the Shaker channels, and the ethoxyquin-inducible aflatoxin B1 aldehyde reductase. At the level of 60% amino acid identity, subfamilies are discernible. For example, the AKR1 family includes five subfamilies: (A) aldehyde reductases (mammalian); (B) aldose reductases; (C) HSDs; (D) delta4-3-ketosteroid-5beta-reductases; and (E) aldehyde reductases (plant). This cluster analysis forms the basis for our nomenclature system. Recommendations for naming an aldo-keto reductase include the root symbol "AKR," an Arabic number designating the family, a letter indicating the subfamily when multiple subfamilies exist, and an Arabic numeral representing the unique protein sequence. For example, human aldehyde reductase would be assigned as AKR1A1. Our nomenclature is both systematic and expandable, thereby allowing assignment of consistent designations for newly identified members of the superfamily.

Is monoamine oxidase activity in the outer mitochondrial membrane influenced by the mitochondrial respiratory state?

Monoamine oxidase activity was measured in isolated rat liver mitochondria using the radiochemical assay with [14C]tyramine as substrate. With toluene as the extracting solvent the apparent activity in the resting state (State 4) was much higher than in the active state (State 3) in agreement with Smith and Reid (Smith, G.S. and Reid, R.A. (1978) Biochem. J. 176, 1011-1014). However, with ethyl acetate or diethyl ether as extracting solvents, the activity in both states was almost identical and several times higher than that measured with toluene. p-Hydroxyphenylacetaldehyde, p-hydroxyphenylacetalcohol and p-hydroxyphenylacetic acid were identified as final reaction products, the latter one being hardly extractable with toluene. It is concluded that monoamine oxidase activity is not influenced by the respiratory state of mitochondria and that differences found by Smith and Reid are due to different extractability of secondary reaction products. NADPH-dependent aldehyde reductase was tentatively identified in rat liver mitochondria, its specific activity amounting to about one fourth of that in the cytosol.

Effect of rapid eye movement sleep deprivation on rat brain monoamine oxidases

Monoamine oxidase, monoamine oxidase-A, and monoamine oxidase-B activities were compared in free moving, rapid eye movement sleep-deprived, recovered, and control rat brains. The activities were estimated in the whole brain, cerebrum, cerebellum, whole brainstem, medulla, pons, and midbrain. The flowerpot method was used for continuing deprivation for one, two, or four days. Monoamine oxidase activity decreased significantly in the cerebrum and the cerebellum of the sleep-deprived rats, whereas monoamine oxidase-A and monoamine oxidase-B were differentially affected. Medullary MAO-A was the first to be affected, showing an increase after just one day of rapid eye movement sleep deprivation, while longer deprivation decreased its activity. The activity of monoamine oxidase-B was not significantly affected in any brain areas of the deprived rats until after two days of rapid eye movement sleep deprivation. All the altered enzyme activities returned to control levels after recovery. Control experiments suggest that the decrease was primarily caused by the rapid eye movement sleep deprivation and was not due to nonspecific effects. These findings are consistent with past studies and may help to explain earlier observations. The results support the involvement of aminergic mechanisms in rapid eye movement sleep. The plausible reasons for the changes in the activities of monoamine oxidases, after rapid eye movement sleep deprivation, are discussed.

High levels of aldehyde dehydrogenase transcripts in the undifferentiated chick retina

A cDNA clone corresponding to chicken aldehyde dehydrogenase (ALDH) mRNA was isolated from a library representing the polyadenylated RNAs expressed in the retina of day 3.5 chick embryos. The profile of ALDH RNA expression was examined in different tissues as well as at different stages of development in the chick embryo. A notable feature of this analysis was the high level of ALDH transcripts found in the undifferentiated cells of the retina. A 20-fold decrease in ALDH RNA levels was observed upon retinal differentiation, in contrast to the kidney, liver and gut where tissue maturation was accompanied by an increase in ALDH mRNA levels. The observations reported here suggest an important role for the ALDH enzyme in retinal development. One possibility is that retinal, the aldehyde form of vitamin A, serves as a substrate for ALDH in the developing retina, resulting in the formation of retinoic acid which has been implicated in various differentiation processes.

Lens aldo-keto reductase of Camelus dromedarius: purification and properties

Aldo-keto reductase has been purified 13,000-fold from the lens of the camel (Camelus dromedarius) to a specific activity of 85 U/mg protein. The enzyme is a monomeric protein, exhibiting a Mr = 40,000 upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. Camel lens aldo-keto reductase shows a broad substrate specificity, which is strictly dependent on NADPH, and is insensitive to inhibition by Sorbinil and valproate. Aldoses with a carbon chain with more than four residues, as well as glucuronate, are not reduced by the enzyme. On the basis of substrate specificity and sensitivity to inhibition, camel lens aldo-keto reductase appears to be distinct from the so far described aldose, aldehyde and carbonyl reductases.

Detection of 3,4-dihydroxyphenylglycolaldehyde in human brain by high-performance liquid chromatography

The monoamine oxidase A metabolite of noradrenaline, 3,4-dihydroxyphenylglycolaldehyde, is the precursor of 3,4-dihydroxymandelic acid, and 3,4-dihydroxyphenylglycol, metabolites of noradrenaline. Owing to difficulties in purifying this aldehyde, it has not been previously characterized or identified in biological sources. This paper describes an enzymatic synthesis, purification, and characterization of 3,4-dihydroxyphenylglycolaldehyde. The aldehyde metabolite is identified in postmortem human brain using high-performance liquid chromatography and electrochemical detection. We estimate the concentration in human hippocampus to be 0.164 +/- 0.05 nmol/g. The importance of this aldehyde metabolite of noradrenaline is discussed.

Human class II (pi) alcohol dehydrogenase has a redox-specific function in norepinephrine metabolism

Studies of the function of human alcohol dehydrogenase (ADH) have revealed substrates that are virtually unique for class II ADH (pi ADH). It catalyzes the formation of the intermediary glycols of norepinephrine metabolism, 3,4-dihydroxyphenylglycol and 4-hydroxy-3-methoxyphenylglycol, from the corresponding aldehydes 3,4-dihydroxymandelaldehyde and 4-hydroxy-3-methoxymandelaldehyde with Km values of 55 and 120 microM and kcat/Km ratios of 14,000 and 17,000 mM-1 X min-1; these are from 60- to 210-fold higher than those obtained with class I ADH isozymes. The catalytic preference of class II ADH also extends to benzaldehydes. The kcat/Km values for the reduction of benzaldehyde, 3,4-dihydroxybenzaldehyde and 4-hydroxy-3-methoxybenzaldehyde by pi ADH are from 9- to 29-fold higher than those for a class I isozyme, beta 1 gamma 2 ADH. Furthermore, the norepinephrine aldehydes are potent inhibitors of alcohol (ethanol) oxidation by pi ADH. The high catalytic activity of pi ADH-catalyzed reduction of the aldehydes in combination with a possible regulatory function of the aldehydes in the oxidative direction leads to essentially "unidirectional" catalysis by pi ADH. These features and the presence of pi ADH in human liver imply a physiological role for pi ADH in the degradation of circulating epinephrine and norepinephrine.

Age-related changes in the activities of the amine metabolizing enzymes of rat eye

Eye MAO-A, MAO-B, semicarbazide-sensitive amine oxidase (SSAO) and aldehyde reductase (AR) activities were measured in young and old rats. When enzyme activity is expressed as nmol (mg protein)-1 min-1, a significant decrease (18-23%) of SSAO activity in the eye of old rats was found, whereas there was no significant difference in MAO-A and MAO-B activities. A significant increase of AR activity with D-xylose (67%), DL-glyceraldehyde (64%), D-glucuronate (43%) and D-glucose (21%) was found in the eye of old rats. These results suggest that changes in the activities of the amine metabolizing enzymes of rat eye with age might have consequences for their function in senescence; particularly, the increase of AR activity might be involved in cataract formation.

Identification of pig brain aldehyde reductases with the high-Km aldehyde reductase, the low-Km aldehyde reductase and aldose reductase, carbonyl reductase, and succinic semialdehyde reductase

Four NADPH-dependent aldehyde reductases (ALRs) isolated from pig brain have been characterized with respect to substrate specificity, inhibition by drugs, and immunological criteria. The major enzyme, ALR1, is identical in these respects with the high-Km aldehyde reductase, glucuronate reductase, and tissue-specific, e.g., pig kidney aldehyde reductase. A second enzyme, ALR2, is identical with the low-Km aldehyde reductase and aldose reductase. The third enzyme, ALR3, is carbonyl reductase and has several features in common with prostaglandin-9-ketoreductase and xenobiotic ketoreductase. The fourth enzyme, unlike the other three which are monomeric, is a dimeric succinic semialdehyde reductase. All four of these enzymes are capable of reducing aldehydes derived from the biogenic amines. However, from a consideration of their substrate specificities and the relevant Km and Vmax values, it is likely that it is ALR2 which plays a primary role in biogenic aldehyde metabolism. Both ALR1 and ALR2 may be involved in the reduction of isocorticosteroids. Despite its capacity to reduce ketones, ALR3 is primarily an aldehyde reductase, but clues as to its physiological role in brain cannot be discerned from its substrate specificity. The capacity of succinic semialdehyde reductase to reduce succinic semialdehyde better than any other substrate shows that this reductase is aptly named and suggests that its primary role is the maintenance in brain of physiological levels of gamma-hydroxybutyrate.

Kinetic studies with the low-Km aldehyde reductase from ox brain

Initial-rate studies of the low-Km aldehyde reductase-catalysed reduction of pyridine-3-aldehyde by NADPH gave families of parallel double-reciprocal plots, consistent with a double-displacement mechanism being obeyed. Studies on the variation of the initial velocity with the concentration of a mixture of the two substrates were also consistent with a double-displacement mechanism. In contrast, the initial-rate data indicated that a sequential mechanism was followed when NADH was used as the coenzyme. Product-inhibition studies, however, indicated that a compulsory-order mechanism was followed in which NADPH bound before pyridine-3-aldehyde with a ternary complex being formed and the release of pyrid-3-ylcarbinol before NADP+. The apparently parallel double-reciprocal plots obtained in the initial-rate studies with NADPH and pyridine-3-aldehyde were thus attributed to the apparent dissociation constant for the binary complex between the enzyme and coenzyme being finite but very low.

Purification and properties of low-Km aldehyde reductase from ox brain

A low-Km aldehyde reductase (alcohol:NADP+ oxidoreductase, EC 1.1.1.2), which may be identical with aldose reductase (alditol:NADP+ 1-oxidoreductase, EC 1.1.1.21), has been purified from ox brain to homogeneity. It was shown to be a monomer with Mr values of 31 000 and 35 100 being obtained by gel filtration and polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate, respectively. The enzyme catalyses the NADPH-dependent reduction of a number of aromatic and sugar aldehydes. The activity of the enzyme with 133 microM NADH was about one-third of that with 120 microM NADPH. Activity with both these coenzymes was optimum at pH 6.2 and was inhibited by increasing the ionic strength with KCl, NaCl or NaNO3. In contrast, the activity was stimulated by sodium phosphate. The activity with NADH as the coenzyme was more sensitive to stimulation by phosphate and to inhibition by increasing ionic strength than that determined with NADPH.

Purification and properties of an NADPH-linked aldehyde reductase from rat kidney

Rat kidney was shown to contain two NADPH-linked aldehyde reductases (alcohol:NADP+) oxidoreductase, EC 1.1.1.2) with different substrate affinities. The high-Km aldehyde reductase, which was purified to apparent homogeneity, had a molecular weight of 32 000 as determined by Sephadex G-100 gel filtration, and of 37 000 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The purified enzyme reduced various aliphatic aldehydes of different carbon-chain lengths besides many chemicals containing aldehyde groups. The Km values for n-hexadecanal and n-octadecanal were 8 microM and 4 microM, respectively. Bovine serum albumin (1.8 mM) stimulated the reduction of n-hexadecanal and n-octadecanal, and increased the Vmax values by about 15-fold without changing the Km values. The kidney enzyme was not distinguishable from the brain and liver high-Km aldehyde reductases in mobility on polyacrylamide gel electrophoresis, immunological properties, peptide maps or substrate specificity.

Age-related changes in aldehyde dehydrogenase activity of rat brain, liver, and heart

Aldehyde dehydrogenase (ALDH) activity was measured in brains, livers, and hearts of 23-26-month-old and 3-month-old rats. A significant increase of ALDH activity was found in whole brain of old rats with both acetaldehyde (39%) and propionylaldehyde (15%) used as substrates. In different brain areas of old rats, with acetaldehyde used as substrate, a significant increase of ALDH activity was found in striatum (30-50%) and cerebral cortex (37%). However, no significant difference in ALDH activity was found in livers and hearts of young and old rats. Preliminary experiments showed a significant increase of aldehyde reductase activity (52%) with p-nitrobenzaldehyde used as substrate in whole brain of old rats compared with young rats. The present work indicates that an increase of ALDH activity in brain of old rats may be an adaptive phenomenon.

Isolation and characterization of multiforms of aldehyde reductase in chicken kidney

Three multiforms of NADPH-dependent aldehyde reductase have been purified to homogeneity from chicken kidney. The enzymes were monomeric proteins with similar molecular weights around 39 000 but with different pI values. Two of them exhibited, almost the same heat stability, a broad optimal pH of 6-6.5 and preference for NADPH as a cofactor. They were high-Km aldehyde reductases which reduced various aldehydes, hexonates, alpha-diketones, xylose, glucuronolactone and ethyl acetoacetate, and which were inhibited to similar degrees by heavy metal ions, organic acids, p-chloromercuribenzoate, indomethacin and phenobarbital. The third aldehyde reductase was distinct from the other two enzymes in its heat lability, sharp acidic pH optimum of 6.0, and dual cofactor specificity. The enzyme showed low Km values for the above-mentioned substrates except hexonates and 4-carboxybenzaldehyde, and it specifically reduced lauryl aldehyde, acetoin and ketosteroids. This enzyme was less sensitive to the above-mentioned inhibitors than the high-Km aldehyde reductases, but the enzyme was moderately inhibited by chlorpromazine and was activated about 1.5 times by the addition of sulfate ions. It was demonstrated using antibodies against one of the high-Km forms and the low-Km form of aldehyde reductase that the two high-Km forms were immunologically identical with each other but not with the low-Km form, and that the high-Km and low-Km forms were widely distributed in various tissues of chicken.

A monoamine oxidase-B inhibitor, MD 780236, metabolized essentially by the A form of the enzyme in the rat

In-vivo studies on the metabolism of [14C]MD 780236 a short-acting selective type B MAO inhibitor in the rat showed the acid to be the major metabolite in plasma and urine, whereas it was minor in brain, where the alcohol was the major metabolite. Pretreatment with SKF 525-A did not modify the metabolite profile in brain, but benserazide decreased the alcohol. Pretreatment with (-)-selegiline had no effect, but clorgyline or clorgyline with (-)-selegiline significantly decreased the alcohol and increased the primary amine metabolite in brain. In-vivo results suggest that MAO-A is the enzyme responsible for the metabolism of MD 780236. This was confirmed by in-vitro studies. Rat brain homogenates extensively metabolized the drug, with the aldehyde being the major metabolite formed (28% of the total radioactivity in the incubation mixture after 60 min incubation). The acid (12%) was more important than the alcohol (4%) in-vitro. The addition of all metabolites originating from possible MAO activity gave 46% when the incubation was carried out at pH 7.4 and 82% at pH 8.8. The presence of NADPH or NAD+ did not alter the relative amounts of metabolites formed. Total metabolites originating from MAO activity in the presence of (-)-selegiline accounted for 40% of total radioactivity, whereas in the presence of clorgyline they accounted for 8% and in the presence of both clorgyline and (-)-selegiline they were reduced to 3%, compared with 45% in controls. As a further proof of the importance of MAO-A in the metabolism of MD 780236, rats were pretreated with clorgyline 1 h before the drug and MAO-B inhibition measured at different times ex-vivo in brain and liver. The short-lasting phase of inhibition of MAO-B disappeared after pretreatment with clorgyline, and inhibition at 24 h was as high as that at 1 h. These results demonstrate the importance of the A form of MAO for the metabolism of MD 780236.

Effects of aldehyde dehydrogenase inhibitors on enzymes involved in the metabolism of biogenic aldehydes in rat liver and brain

The effects of the aldehyde dehydrogenase inhibitors disulfiram, coprine and cyanamide on enzymes involved in the metabolism of biogenic aldehydes in rat liver and brain were studied. Both liver and brain aldehyde dehydrogenase activities were significantly decreased in rats pretreated with these drugs. In the liver, the low-Km aldehyde dehydrogenase activity was markedly decreased by all three drugs after 2 and 24 hr whereas only cyanamide inhibited the high-Km enzymes. The brain ALDH-activity with a low acetaldehyde concentration was significantly decreased by coprine and cyanamide at both times tested, whereas disulfiram caused no change after 2 hr but an inhibition of 38% after 24 hr. The brain ALDH-activity with a high acetaldehyde concentration was significantly decreased by coprine and cyanamide but not by disulfiram. The activity of the substrate specific enzyme succinate semialdehyde dehydrogenase in brain was slightly but significantly decreased in rats pretreated with cyanamide but not in rats pretreated with disulfiram or coprine. None of the drugs caused any changes in the activities of aldehyde reductase and monoamine oxidase in brains in vivo. The activity of monoamine oxidase in liver was significantly decreased by coprine after 24 hr. In contrast to the effects obtained in vivo, disulfiram was found to be an inhibitor in vitro of brain succinate semialdehyde dehydrogenase and liver monoamine oxidase. Aldehyde reductase was slightly inhibited by both disulfiram and 1-aminocyclopropanol in vitro.

Estimation of the cytoplasmic catecholamine concentrations in pheochromocytoma cells

Pheochromocytoma cells contain amine oxidase (flavin-containing), and convert dopamine and norepinephrine to deaminated metabolites. Dihydroxyphenylacetic acid is the major dopamine metabolite produced by the cells, whereas dihydroxyphenylglycol is the predominant metabolite of norepinephrine. Cells incubated under control conditions produce deaminated dopamine metabolites at a rate of about 30 pmol/min per mg protein, and dihydroxyphenylglycol at a rate of approx. 10 pmol/min per mg protein. Activation of tyrosine 3-monooxygenase increases the formation of dihydroxyphenylacetic acid, but does not greatly affect the production of dihydroxyphenylglycol. Inhibition of aromatic-L-amino-acid decarboxylase decreases the production of dihydroxyphenylacetic acid, but does not alter the production of dihydroxyphenylglycol. These results are consistent with the idea that newly synthesized dopamine represents the major source of cytoplasmic dopamine, whereas cytoplasmic norepinephrine is derived largely from catecholamine stores in secretory vesicles. The concentrations of dopamine and of norepinephrine in the cytoplasm of pheochromocytoma cells were estimated by measuring the substrate dependence of amine oxidase activity in extracts of these cells. By this method, the cytoplasmic concentrations of dopamine and of norepinephrine were estimated to be in the range of 0.5 to 1 microM. Incubation of the cells with extracellular norepinephrine or with reserpine results in an increase in the production of dihydroxyphenylglycol, and in inhibition of tyrosine 3-monoxygenase activity. Both of these effects are presumably mediated by a rise in the cytoplasmic norepinephrine concentration. Analysis of the relationship between norepinephrine metabolism and tyrosine 3-monooxygenase activity indicates that the apparent Ki of this enzyme for norepinephrine in intact cells is 10-15-times the basal cytoplasmic concentration of norepinephrine, or approx. 10 microM.

Biogenic aldehyde metabolism in rat brain: subcellular distribution of aldose reductase and valproate-sensitive aldehyde reductase

Reductase activity towards two aldose substrates has been examined in subcellular fractions prepared from rat brain. The reduction of glucuronate, which is sensitive to inhibition by the anticonvulsant drug sodium valproate, corresponds to the major high-Km aldehyde reductase in brain. Xylose reduction that is insensitive to valproate inhibition has characteristics consistent with the activity of aldose reductase (EC 1.1.1.21). Both enzymes are predominantly localized in the cytosolic fraction. The significance of the location of these two reductases is discussed in relation to the compartmentation of catecholamine metabolism in brain.

Kinetic studies of the reduction of succinic semialdehyde by rat-brain aldehyde reductase

Initial rate studies have been used to investigate the kinetic mechanism followed by the purified high-Km (AR1) form of rat brain aldehyde reductase at pH 7.0. The effects of varying the aldehyde and NADPH concentrations, together with the inhibition given by the products of the reaction, are consistent with the reduction of succinic semialdehyde and p-nitrobenzaldehyde following an ordered reaction mechanism involving the formation of an intermediate ternary complex and in which NADPH is the first substrate to bind to the enzyme. Both these aldehyde substrates inhibit the enzyme at higher concentrations. This inhibition, which is uncompetitive with respect to NADPH, suggests that many previous studies on the specificity of this enzyme, that have been based on the activity determined at a single arbitrary concentration of each substrate, may have given erroneous results.

Subcellular localization of aldehyde reductase activities in ox brain

The distribution of the two principal isoenzymes of aldehyde reductase (EC 1.1.1.2) has been studied in ox brain. The more active of these, which has been termed the high-Km enzyme, has been shown to be located in the cytosol and the less abundant low-Km form has a similar localization. p-Nitrobenzaldehyde, which has been used as a substrate in previous studies, caused the reduction of NADH in the presence of the mitochondrial fraction, but mixed substrate experiments with 1,3-dinitrobenzene and the effects of pH on the activity indicate that this is due to the presence of a nitro reductase activity which has been recently described (Köchli, Wermuth & von Wartburg (1980) Biochim. Biophys. Acta 616, 133-142] rather than to the low-Km aldehyde reductase activity. Fractionation of the mitochondria indicated this activity to be largely confined to the mitochondrial inner membrane.

Purification of the high-Km aldehyde reductase from rat brain and liver and from ox brain

A procedure is described that yields an apparently homogeneous preparation of the high-Km aldehyde reductase from rat brain. This procedure is also applicable to the purification of this enzyme from rat liver and ox brain. In the latter case, however, the purified preparation could be resolved into two protein bands, both of which had enzyme activity, by polyacrylamide-gel electrophoresis. Since a sample of the ox brain enzyme from an earlier step in the purification procedure only showed the presence of a single band of activity after electrophoresis, this apparent multiplicity probably results from modification of the enzyme, possibly by oxidation, during the final step of the purification. A number of properties of the rat brain enzyme were determined and these were compared with those of the enzyme from rat liver. The two preparations were similar in their stabilities, behaviour during purification, kinetic properties, electrophoretic mobilities and amino acid compositions. Antibodies to the rat liver enzyme cross-reacted with that from brain and the inhibition of both these preparations by the antiserum was similar, further supporting the view that the enzymes from these two sources were closely similar if not identical.

Biogenic aldehyde metabolism in rat brain. Differential sensitivity of aldehyde reductase isoenzymes to sodium valproate

The effects of inhibitors of aldehyde reductase (alcohol:NADP+ oxidoreductase, EC 1.1.1.2) on the formation of 3-methoxy-4-hydroxyphenethylene glycol from normetanephrine have been studied in rat brain homogenates. The reaction pathway was shown to be unaffected by several inhibitors of the major (high Km) form of aldehyde reductase such as sodium valproate. Two isoenzymes of aldehyde reductase have been separated and characterized from rat brain. The minor (low Km) isoenzyme is shown to be relatively insensitive to sodium valproate and exhibits a similar inhibitor-sensitivity profile to that obtained for methoxyhydroxyphenethylene glycol formation. The low Km isoenzyme is therefore implicated in catecholamine metabolism. The metabolism of succinic semialdehyde and xylose by rat brain cytosol has also been examined. Aldose metabolism may also be attributed to the action of the low Km reductase, but the existence of a separate succinic semialdehyde reductase is postulated. The possible roles of aldehyde reductases in brain metabolism and the relationship between these enzymes and aldose reductase (alditol:NADP+ 1-oxidoreductase, EC 1.1.1.21) are discussed.

Brain concentrations of biogenic amine metabolites in acutely treated and ethanol-dependent rats

1 Mass fragmentography was used to measure whole brain concentrations of some of the major metabolites of tyramine, octopamine, dopamine and noradrenaline in acutely treated and in ethanol-dependent rats. 2 Treatments with ethanol, either acutely or chronically, failed to alter significantly brain concentration of p-hydroxphenylacetic and p-hydroxymandelic acid (metabolites derived from tyramine and octopamine respectively). The effect on catecholamine metabolites was marked and therefore suggests that ethanol is selective in its effect on central metabolism of biogenic amines. 3 Acute ethanol treatment significantly increased brain concentration of homovanillic acid (HVA), 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxy-4-hydroxyphenylglycol (MHPG). Vanilmandelic acid (VMA) was not affected. All four metabolites (HVA, DOPAC, MHPG and VMA) were increased in the brains of rats rendered dependent on ethanol while still intoxicated (blood ethanol levels above 200 mg/dl). In ethanol-dependent rats undergoing ethanol withdrawal syndrome (no ethanol present in blood), the brain concentrations of HVA and DOPAC were normal while those of MHPG and VMA continued to be elevated. 4 From the decline in the concentrations of HVA and DOPAC after 50 mg pargyline/kg in control rats and rats acutely treated with ethanol, it was concluded that ethanol has no effect on the transport of phenolic acids across the blood brain barrier. 5 No reversal in the metabolism of catecholamines from an oxidative to a reductive pathway, analogous to that produced by ethanol in the periphery, could be established in the brain. 6 The increase in catecholamine metabolite concentrations after ethanol treatment, either acute or chronic, were interpreted as manifestations of increases catecholamine turnover.