Human brain aldehyde dehydrogenase: activity with DOPAL and isozyme distribution

Xenobiotic transport and metabolism in the human brain

Organisms have metabolic pathways responsible for eliminating endogenous and exogenous toxicants. Generally, we associate the liver par excellence as the organ in charge of detoxifying the body; however, this process occurs in all tissues, including the brain. Due to the presence of the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB), the Central Nervous System (CNS) is considered a partially isolated organ, but similar to other organs, the CNS possess xenobiotic transporters and metabolic pathways associated with the elimination of xenobiotic agents. In this review, we describe the different systems related to the detoxification of xenobiotics in the CNS, providing examples in which their association with neurodegenerative processes is suspected. The CNS detoxifying systems include carrier-mediated, active efflux and receptor-mediated transport, and detoxifying systems that include phase I and phase II enzymes, as well as those enzymes in charge of neutralizing compounds such as electrophilic agents, reactive oxygen species (ROS), and free radicals, which are products of the bioactivation of xenobiotics. Moreover, we discuss the differential expression of these systems in different regions of the CNS, showing the different detoxifying needs and the composition of each region in terms of the cell type, neurotransmitter content, and the accumulation of xenobiotics and/or reactive compounds.

Alpha-Synuclein in Alcohol Use Disorder, Connections with Parkinson's Disease and Potential Therapeutic Role of 5' Untranslated Region-Directed Small Molecules

Alpha-synuclein (α-Syn) is a 140-amino acid (aa) protein encoded by the Synuclein alpha SNCA gene. It is the synaptic protein associated with Parkinson's disease (PD) and is the most highly expressed protein in the Lewy bodies associated with PD and other alpha synucleopathies, including Lewy body dementia (LBD) and multiple system atrophy (MSA). Iron deposits are present in the core of Lewy bodies, and there are reports suggesting that divalent metal ions including Cu2+ and Fe2+ enhance the aggregation of α-Syn. Differential expression of α-Syn is associated with alcohol use disorder (AUD), and specific genetic variants contribute to the risk for alcoholism, including alcohol craving. Spliced variants of α-Syn, leading to the expression of several shorter forms which are more prone to aggregation, are associated with both PD and AUD, and common transcript variants may be able to predict at-risk populations for some movement disorders or subtypes of PD, including secondary Parkinsonism. Both PD and AUD are associated with liver and brain iron dyshomeostasis. Research over the past decade has shown that α-Syn has iron import functions with an ability to oxidize the Fe3+ form of iron to Fe2+ to facilitate its entry into cells. Our prior research has identified an iron-responsive element (IRE) in the 5' untranslated region (5'UTR) of α-Syn mRNA, and we have used the α-Syn 5'UTR to screen for small molecules that modulate its expression in the H4 neuronal cell line. These screens have led us to identify several interesting small molecules capable of both decreasing and increasing α-Syn expression and that may have the potential, together with the recently described mesenchymal stem cell therapies, to normalize α-Syn expression in different regions of the alcoholic and PD brain.

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 mitochondrial aldehyde dehydrogenase by nitric oxide-mediated S-nitrosylation

Mitochondrial aldehyde dehydrogenase (ALDH2) is responsible for the metabolism of acetaldehyde and other toxic lipid aldehydes. Despite many reports about the inhibition of ALDH2 by toxic chemicals, it is unknown whether nitric oxide (NO) can alter the ALDH2 activity in intact cells or in vivo animals. The aim of this study was to investigate the effects of NO on ALDH2 activity in H4IIE-C3 rat hepatoma cells. NO donors such as S-nitrosoglutathione (GSNO), S-nitroso-N-acetylpenicillamine, and 3-morpholinosydnonimine significantly increased the nitrite concentration while they inhibited the ALDH2 activity. Addition of GSH-ethylester (GSH-EE) completely blocked the GSNO-mediated ALDH2 inhibition and increased nitrite concentration. To directly demonstrate the NO-mediated S-nitrosylation and inactivation, ALDH2 was immunopurified from control or GSNO-treated cells and subjected to immunoblot analysis. The anti-nitrosocysteine antibody recognized the immunopurified ALDH2 only from the GSNO-treated samples. All these results indicate that S-nitrosylation of ALDH2 in intact cells leads to reversible inhibition of ALDH2 activity.

Changes in behavioural contingencies produce a loss of tolerance to amphetamine hypophagia in rats despite continued feeding tests while drugged

Rats administered amphetamine prior to access to milk in bottles develop tolerance to the hypophagic effect of the drug by learning to suppress stereotyped behaviours that interfere with feeding. When tolerant rats are later allowed to drink milk from a bottle in an unintoxicated state, tolerance is lost, even when drug exposure is held constant by administration of the drug after the test. In the present experiment, we show that tolerance can also be lost in the face of continued administration of amphetamine prior to milk tests, as a result of changes in the contingencies of reinforcement that govern the suppression of stereotypy. Rats were injected with 2 mg/kg amphetamine and given access to milk in bottles for 16 trials. Tolerance to the hypophagic effect was confirmed by dose-response tests in which milk was available in bottles. The rats were then injected with 2 mg/kg amphetamine prior to intraoral milk infusions for 21 trials. This method of feeding did not require the suppression of stereotypy to obtain milk. Subsequent dose-response tests in which milk was again presented in bottles revealed that tolerance was lost, even though intoxicated feedings were never interrupted. These results demonstrate that the contingencies of reinforcement governing the suppression of stereotypy determine whether tolerance is retained or lost.

Reduced aldehyde dehydrogenase levels in the brain of patients with Down syndrome

Aldehyde dehydrogenase (ALDH) is a key enzyme in fructose, acetaldehyde and oxalate metabolism and represents a major detoxification system for reactive carbonyls and aldehydes. In the brain, ALDH exerts a major function in the metabolism of biogenic aldehydes, norepinephrine, dopamine and diamines and gamma-aminobutyric acid. Subtractive hybridization studies in Down Syndrome (DS) fetal brain showed that mRNA for ALDH are downregulated. Here we studied the protein levels in the brain of adult patients. The proteins from five brain regions of 9 aged patients with DS and 9 controls were analyzed by two-dimensional (2-D) gel electrophoresis and identified by matrix-assisted laser desorption ionization mass spectrometry. ALDH levels were reduced in the brain regions of at least half of the patients with Down Syndrome, as compared to controls. The decreased ALDH levels in the DS brain may result in accumulation of aldehydes which can lead to the formation of plaques and tangles reflecting abnormally cross-linked, insoluble and modified proteins, found in aged DS brain. Furthermore, we constructed a 2-Dmap including approximately 120 identified human brain proteins.

Distribution and kinetics of ethanol metabolism in rat brain

It was found that the accumulation of acetaldehyde produced from 50 mM ethanol in rat brain homogenates takes place in all major brain regions. The velocity varied between 3.5 to 7.1 nmol/mg of protein/hr. The rate increased in the following order: brain hemispheres, striatum, brainstem, hypothalamus, and cerebellum. Significant regional differences in this process were found: in the initial period of incubation (5 min), acetaldehyde accumulation was maximal in the brain hemispheres; but, in the 30- to 60-min period, it became significantly higher in the cerebellum. Inhibition of this process by the catalase inhibitor, 3-amino-1,2,4-triazole (8 mM), was minimal in the brainstem (27%) and maximal (57%) in the cerebellum, despite nearly complete inhibition of catalase. This would indicate that processes other than catalase activity must contribute to acetaldehyde accumulation.

Redox cycling of MPP+: evidence for a new mechanism involving hydride transfer with xanthine oxidase, aldehyde dehydrogenase, and lipoamide dehydrogenase

MPP+ is redox active in the presence of cytochrome P450 reductase and induces the formation of O2.- and HO(.). In this study, we report the redox cycling capability of MPP+ with additional enzymes and with UV photolysis detected through ESR techniques. The treatment of MPP+ with UV light resulted in the production of HO. trapped as a spin adduct. Two of the enzymes examined in this study, xanthine oxidase and aldehyde dehydrogenase, produced O2.- in the presence of substrate. However, when MPP+ was added to the incubations, the radical trapped by DMPO was HO(.). This indicates that MPP+ redox cycles in the presence of these two enzymes or UV light, which produces HO.. Our data also suggest that MPP+ is reduced by lipoamide dehydrogenase. MPP+ stimulated the oxidation of reduced nicotinamide adenine dinucleotide (NADH) by the enzyme at concentrations between 2 mM and 8 mM of MPP+. Higher concentrations of MPP+ inhibited lipoamide dehydrogenase. MPP+ appears to be redox active with a number of redox enzymes. The mechanism involved may be hydride transfer from the enzymes to MPP+, rather than a direct single-electron reduction.

Ethanol intake: effect on liver and brain mitochondrial function and acetaldehyde oxidation

The effect of a chronic ethanol consumption by forcing rats to drink a 20% v/v ethanol solution as sole drinking fluid, for 3 months, was evaluated on: liver and brain mitochondrial function, the capacity of isolated mitochondria to oxidize acetaldehyde, as well as on the low Km mitochondrial AlDH activity, in rats. The O2 uptake by liver and brain mitochondria in the presence of glutamate + malate, succinate or ascorbate + TMPD, was measured polarographically with a Clark electrode. Acetaldehyde oxidation was measured by the disappearance rate in presence of the intact or disrupted mitochondria (AlDH activity) by gas chromatography. Results indicate that an ethanol intake of 11 g/kg b.wt. per day produce a significant reduction of the liver mitochondrial respiration tested with all the substrates used, including acetaldehyde. In contrast, the activity of AlDH in disrupted mitochondria remained unchanged. These results are in accord with the idea that a progressive deterioration of liver mitochondrial function appears with the increase in amount of ethanol consumed, and that alterations of acetaldehyde oxidation by intact mitochondria can be detected before an alteration of the AlDH activity. Concerning the brain, this ethanol consumption regimen did not affect the brain mitochondrial respiration tested with glutamate + malate, succinate or ascorbate + TMPD, but it induces an increase in acetaldehyde oxidation rate by intact brain mitochondria. The imposed increase in the cerebral aldehyde oxidizing capacity could reflect a principal biochemical mechanism underlying neural adaptation to ethanol.

Histochemical study of aldehyde dehydrogenase in the rat CNS

A quantitative histochemical method was developed to determine aldehyde dehydrogenase (EC 1.2.1.3; ALDH) activity in the CNS. The distribution of ALDH activity in all rat brain and spinal cord regions is described. Among the CNS neuron structures, high enzyme activity was found in receptor and effector neurons, whereas low activity was noted in perikarya of the majority of intermediate neurons, including all aminergic neurons. A positive correlation was demonstrated between the distribution of ALDH activity among rat CNS microregions (our own data) and the density of dopaminergic terminals, dopamine content, and monoamine oxidase activity (literature data) among the same microregions. They may reflect a spatial linkage between ALDH and the predicted sites of natural aldehyde production. Lower enzyme activity was found in phylogenetically younger brain structures. It may explain the differential resistance of CNS structures to ethanol (acetaldehyde). Among the barrier CNS structures, moderate ALDH activity was found in capillaries and surrounding astrocytes and high activity was noted in ependimocytes covering the brain cavities and those of the vascular plexus. This provides realization of the function of ALDH as a brain metabolic barrier for aldehydes.