Human brain glyceraldehyde-3-phosphate dehydrogenase, succinic semialdehyde dehydrogenase and aldehyde dehydrogenase isozymes: substrate specificity and sensitivity to disulfiram

Activation of ALDH1A1 by omeprazole reduces cell oxidative stress damage

Under physiological conditions, cells produce low basal levels of reactive oxygen species (ROS); however, in pathologic conditions ROS production increases dramatically, generating high concentrations of toxic unsaturated aldehydes. Aldehyde dehydrogenases (ALDHs) are responsible for detoxification of these aldehydes protecting the cell. Due to the physiological relevance of these enzymes, it is important to design strategies to modulate their activity. It was previously reported that omeprazole activation of ALDH1A1 protected Escherichia coli cells overexpressing this enzyme, from oxidative stress generated by H2 O2 . In this work, omeprazole cell protection potential was evaluated in eukaryotic cells. AS-30D cell or hepatocyte suspensions were subjected to a treatment with omeprazole and exposure to light (that is required to activate omeprazole in the active site of ALDH) and then exposed to H2 O2 . Cells showed viability similar to control cells, total activity of ALDH was preserved, while cell levels of lipid aldehydes and oxidative stress markers were maintained low. Cell protection by omeprazole was avoided by inhibition of ALDHs with disulfiram, revealing the key role of these enzymes in the protection. Additionally, omeprazole also preserved ALDH2 (mitochondrial isoform) activity, diminishing lipid aldehyde levels and oxidative stress in this organelle, protecting mitochondrial respiration and transmembrane potential formation capacity, from the stress generated by H2 O2 . These results highlight the important role of ALDHs as part of the antioxidant system of the cell, since if the activity of these enzymes decreases under stress conditions, the viability of the cell is compromised.

Interaction of Ethanol and Oral ANS-6637, a Selective ALDH2 Inhibitor in Males: A Randomized, Double-Blind, Placebo-Controlled, Single-Ascending Dose Cohort Study

BACKGROUND

ANS-6637, an orally bioavailable selective and reversible aldehyde dehydrogenase-2 (ALDH2) inhibitor, is under development for drug and alcohol use disorders. During the elimination of alcohol, ALDH2 metabolizes acetaldehyde to acetate; inhibiting this enzyme can lead to aversive reactions due to the accumulation of acetaldehyde. Thus, understanding the safety and tolerability of ANS-6637 in combination with alcohol is essential.

TRIAL DESIGN AND METHODS

Forty eight healthy males participated in a randomized, double-blind, placebo-controlled, single-ascending dose cohort study of oral ANS-6637. Eligible participants were randomized to ANS-6637 (n = 36) or placebo (n = 12) in a 3:1 fashion in each of 6 dose cohorts (8 per cohort; ANS-6637 dose levels were 25, 50, 100, 200, 400, and 600 mg). Two hours after receiving study drug, participants drank up to 5 standard drinks, 1 every 30 minutes. Safety assessments, pharmacodynamic measures, and pharmacokinetic blood samples were obtained.

RESULTS

Flushing was the most common adverse event (AE) associated with ANS-6637 (24 of 36 participants) compared with placebo (3 of 12). Statistically significant, but modest, increases in heart rate (HR) occurred (+10.5 bpm after 2 drinks; +16.9 to + 20.5 bpm after 3^rd through 5^th drink). No participant met HR or systolic blood pressure criteria for stopping ethanol administration. There were no clinically significant QTc interval prolongations. Individuals receiving ANS-6637 reported lower ratings of liking, alcohol effects, and feeling drunk.

CONCLUSIONS

A single oral dose of ANS-6637 with up to 5 standards drinks over 2.5 hours was generally well tolerated in healthy males. The most common pharmacological response was flushing and an increase in HR, which are known effects of acetaldehyde accumulation and consistent with inhibition of ALDH2 with oral ANS-6637 in combination with alcohol. The results of this alcohol interaction study support further testing of ANS-6637 in individuals who consume alcohol heavily.

Role of Aldehyde Dehydrogenases in Physiopathological Processes

Many different diseases are associated with oxidative stress. One of the main consequences of oxidative stress at the cellular level is lipid peroxidation, from which toxic aldehydes may be generated. Below their toxicity thresholds, some aldehydes are involved in signaling processes, while others are intermediaries in the metabolism of lipids, amino acids, neurotransmitters, and carbohydrates. Some aldehydes ubiquitously distributed in the environment, such as acrolein or formaldehyde, are extremely toxic to the cell. On the other hand, aldehyde dehydrogenases (ALDHs) are able to detoxify a wide variety of aldehydes to their corresponding carboxylic acids, thus helping to protect from oxidative stress. ALDHs are located in different subcellular compartments such as cytosol, mitochondria, nucleus, and endoplasmic reticulum. The aim of this review is to analyze, and highlight, the role of different ALDH isoforms in the detoxification of aldehydes generated in processes that involve high levels of oxidative stress. The ALDH physiological relevance becomes evident by the observation that their expression and activity are enhanced in different pathologies that involve oxidative stress such as neurodegenerative disorders, cardiopathies, atherosclerosis, and cancer as well as inflammatory processes. Furthermore, ALDH mutations bring about several disorders in the cell. Thus, understanding the mechanisms by which these enzymes participate in diverse cellular processes may lead to better contend with the damage caused by toxic aldehydes in different pathologies by designing modulators and/or protocols to modify their activity or expression.

Inhibition of the Aldehyde Dehydrogenase 1/2 Family by Psoralen and Coumarin Derivatives

Aldehyde dehydrogenase 2 (ALDH2), one of 19 ALDH superfamily members, catalyzes the NAD⁺-dependent oxidation of aldehydes to their respective carboxylic acids. In this study, we further characterized the inhibition of four psoralen and coumarin derivatives toward ALDH2 and compared them to the ALDH2 inhibitor daidzin for selectivity against five ALDH1/2 isoenzymes. Compound 2 (K_i = 19 nM) binds within the aldehyde-binding site of the free enzyme species of ALDH2. Thirty-three structural analogs were examined to develop a stronger SAR profile. Seven compounds maintained or improved upon the selectivity toward one of the five ALDH1/2 isoenzymes, including compound 36, a selective inhibitor for ALDH2 (K_i = 2.4 μM), and compound 32, which was 10-fold selective for ALDH1A1 (K_i = 1.2 μM) versus ALDH1A2. Further medicinal chemistry on the compounds' basic scaffold could enhance the potency and selectivity for ALDH1A1 or ALDH2 and generate chemical probes to examine the unique and overlapping functions of the ALDH1/2 isoenzymes.

Discovery of a series of aromatic lactones as ALDH1/2-directed inhibitors

In humans, the aldehyde dehydrogenase superfamily consists of 19 isoenzymes which mostly catalyze the NAD(P)(+)-dependent oxidation of aldehydes. Many of these isoenzymes have overlapping substrate specificities and therefore their potential physiological functions may overlap. Thus the development of new isoenzyme-selective probes would be able to better delineate the function of a single isoenzyme and its individual contribution to the metabolism of a particular substrate. This specific study was designed to find a novel modulator of ALDH2, a mitochondrial ALDH isoenzyme most well-known for its role in acetaldehyde oxidation. 53 compounds were initially identified to modulate the activity of ALDH2 by a high-throughput esterase screen from a library of 63,000 compounds. Of these initial 53 compounds, 12 were found to also modulate the oxidation of propionaldehyde by ALDH2. Single concentration measurements at 10μM compound were performed using ALDH1A1, ALDH1A2, ALDH1A3, ALDH2, ALDH1B1, ALDH3A1, ALDH4A1, and/or ALDH5A1 to determine the selectivity of these 12 compounds toward ALDH2. Four of the twelve compounds shared an aromatic lactone structure and were found to be potent inhibitors of the ALDH1/2 isoenzymes, but have no inhibitory effect on ALDH3A1, ALDH4A1 or ALDH5A1. Two of the aromatic lactones show selectivity within the ALDH1/2 class, and one appears to be selective for ALDH2 compared to all other isoenzymes tested.

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.

Neurotoxicity and metabolism of the catecholamine-derived 3,4-dihydroxyphenylacetaldehyde and 3,4-dihydroxyphenylglycolaldehyde: the role of aldehyde dehydrogenase

Aldehydes are highly reactive molecules formed during the biotransformation of numerous endogenous and exogenous compounds, including biogenic amines. 3,4-Dihydroxyphenylacetaldehyde is the aldehyde metabolite of dopamine, and 3,4-dihydroxyphenylglycolaldehyde is the aldehyde metabolite of both norepinephrine and epinephrine. There is an increasing body of evidence suggesting that these compounds are neurotoxic, and it has been recently hypothesized that neurodegenerative disorders may be associated with increased levels of these biogenic aldehydes. Aldehyde dehydrogenases are a group of NAD(P)+ -dependent enzymes that catalyze the oxidation of aldehydes, such as those derived from catecholamines, to their corresponding carboxylic acids. To date, 19 aldehyde dehydrogenase genes have been identified in the human genome. 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, most of which are characterized by neurological abnormalities. Several pharmaceutical agents and environmental toxins are also known to disrupt or inhibit aldehyde dehydrogenase function. It is, therefore, possible to speculate that reduced detoxification of 3,4-dihydroxyphenylacetaldehyde and 3,4-dihydroxyphenylglycolaldehyde from impaired or deficient aldehyde dehydrogenase function may be a contributing factor in the suggested neurotoxicity of these compounds. This article presents a comprehensive review of what is currently known of both the neurotoxicity and respective metabolism pathways of 3,4-dihydroxyphenylacetaldehyde and 3,4-dihydroxyphenylglycolaldehyde with an emphasis on the role that aldehyde dehydrogenase enzymes play in the detoxification of these two aldehydes.

Inhibition of aldehyde detoxification in CNS mitochondria by fungicides

Among the several converging factors leading to Parkinson's disease, epidemiological studies indicate a correlation between Parkinson's disease (PD) with living in a rural area and/or exposure to agricultural pesticides. In this present study, we examined the potential of multiple agricultural pesticides for their ability to inhibit the function of whole, respiring rat brain mitochondria using the oxidation of the neurotoxic lipid-aldehyde trans-4-hydroxy-2-nonenal (HNE) as a biomarker for mitochondrial aldehyde dehydrogenase (ALDH) activity in situ. We chose an arbitrary cutoff concentration of 10 microM of each pesticide. Our data demonstrate that only four of the eighteen compounds tested inhibited oxidation of HNE to trans-4-hydroxy-2-nonenoic acid (HNEAcid). These compounds included rotenone, maneb, mancozeb, and benomyl. Surprisingly, maneb, mancozeb, and benomyl did not inhibit mitochondrial respiration but inhibited the activity of purified rat ALDH2 and rat ALDH5A, enzymes found in brain mitochondria that oxidize HNE and aldehydes derived from neurotransmitters. Our data demonstrate that mitochondrial ALDHs are sensitive targets of pesticide inactivation and that pesticides such as maneb and benomyl can decrease the detoxification of lipid peroxidation derived aldehydes such as HNE and, likely, aldehydes derived from neurotransmitters.

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.

Enantioselective metabolism of trans-4-hydroxy-2-nonenal by brain mitochondria

Trans-4-hydroxy-2-nonenal (HNE) is a product of lipid peroxidation with many cellular effects. HNE possesses a stereogenic center at the C4 carbon that influences the metabolism and alkylation targets of HNE. We tested the hypothesis that rat brain mitochondria metabolize HNE in an enantioselective manner after exposure to racemic HNE. The study of HNE chirality, however, is hindered by the lack of facile methods to chromatographically resolve (R)-HNE and (S)-HNE. We used a chiral hydrazine, (S)-carbidopa, as a derivatization reagent to form diastereomers with (R)-HNE and (S)-HNE that were separated by reverse-phase HPLC. After exposure to racemic HNE, rat brain mitochondria metabolized HNE enantioselectively with a higher rate of (R)-HNE metabolism. By using the purified enantiomers of HNE, we found that this enantioselective metabolism of HNE was the result of higher rates of enzymatic oxidation of (R)-HNE by aldehyde dehydrogenases compared to (S)-HNE. Conjugation of HNE to glutathione was a minor metabolic pathway and was not enantioselective. These studies demonstrate that the chirality of HNE affects its mitochondrial metabolism and potentially other processes in the central nervous system.

Mitochondrial oxidation of 4-hydroxy-2-nonenal in rat cerebral cortex

4-hydroxy-trans-2-nonenal (HNE) is a neurotoxic product of lipid peroxidation whose levels are elevated in multiple neurodegenerative diseases and CNS trauma. The detoxification of HNE may take the route of glutathione conjugation to the C3 carbon and the oxidation or reduction of the C1 aldehyde. In this work, we examined whether the oxidation of HNE to its corresponding carboxylic acid, 4-hydroxy-trans-2-nonenoate (HNEAcid) was detoxifying event, if it occurred in rat cerebral cortex, and in which subcellular compartments. Our results show that HNEAcid did not form protein adducts and was non-toxic to Neuro 2A cells. HNEAcid formation occurred in rat cerebral cortex slices following exposure to HNE in a time-dependent and dose-dependent fashion. Homogenate studies indicated that HNEAcid formation was NAD+ dependent. Subcellular fractionation demonstrated that mitochondria had the highest specific activity for HNEAcid formation with a KM of 21 micro m HNE. These data indicate that oxidation of HNE to its corresponding acid is a major detoxification pathway of HNE in the CNS and that mitochondria play a role in this process.

Inhibition of succinic semialdehyde dehydrogenase activity by alkenal products of lipid peroxidation

Lipid peroxidation causes the generation of the neurotoxic aldehydes acrolein and 4-hydroxy-trans-2-nonenal (HNE). These products are elevated in neurodegenerative diseases and acute CNS trauma. Previous studies demonstrate that mitochondrial class 2 aldehyde dehydrogenase (ALDH2) is susceptible to inactivation by these alkenals. In the liver and brain another mitochondrial aldehyde dehydrogenase, succinic semialdehyde dehydrogenase (SSADH/ALDH5A1), is present. In this study, we tested the hypothesis that aldehyde products of lipid peroxidation inhibit SSADH activity using the endogenous substrate, succinic semialdehyde (SSA, 50 microM). Acrolein potently inhibited SSADH activity (IC(50)=15 microM) in rat brain mitochondrial preparations. This inhibition was of an irreversible and noncompetitive nature. HNE inhibited activity with an IC(50) of 110 microM. Trans-2-hexenal (HEX) and crotonaldehyde (100 microM each) did not inhibit activity. These data suggest that acrolein and HNE disrupt SSA metabolism and may have subsequent effects on CNS neurochemistry.

Expression and activities of aldo-keto oxidoreductases in Alzheimer disease

A reactive intermediate generated by lipid peroxidation, 4-hydroxy-2-nonenal (HNE), has received considerable attention as a potential effector of oxidative damage and Abeta peptide-mediated neurotoxicity in Alzheimer disease (AD). However, little is known about aldo-keto oxidoreductases, a group of enzymes that constitute a major detoxifying pathway for HNE and related reactive aldehydes in human brain. We have determined the regional, cellular, and class distribution in human brain of the 4 major aldo-keto oxidoreductases that detoxify HNE: aldehyde dehydrogenase (ALDH): aldose reductase; aldehyde reductase: and alcohol dehydrogenase (ADH). Of these 4 enzymes, only ALDH and aldose reductase were expressed in cerebral cortex. hippocampus, basal ganglia, and midbrain: all 4 enzymes were present in cerebellum. In cerebrum and hippocampus, aldose reductase was localized to pyramidal neurons and mitochondrial class 2 ALDH was localized to glia and senile plaques. ALDH, but not aldose reductase, activity was significantly increased in temporal cortex from patients with AD compared to age-matched controls. These results suggest that in brain regions involved in AD, neurons and glia utilize different mechanisms to detoxify HNE, and that increased ALDH activity is a protective response of cerebral cortex to AD.