Effects of induction of rat liver cytosolic aldehyde dehydrogenase on the oxidation of biogenic aldehydes

From aggression to autism: new perspectives on the behavioral sequelae of monoamine oxidase deficiency

The two monoamine oxidase (MAO) enzymes, A and B, catalyze the metabolism of monoamine neurotransmitters, such as serotonin, norepinephrine, and dopamine. The phenotypic outcomes of MAO congenital deficiency have been studied in humans and animal models, to explore the role of these enzymes in behavioral regulation. The clinical condition caused by MAOA deficiency, Brunner syndrome, was first described as a disorder characterized by overt antisocial and aggressive conduct. Building on this discovery, subsequent studies were focused on the characterization of the role of MAOA in the neurobiology of antisocial conduct. MAO A knockout mice were found to display high levels of intermale aggression; however, further analyses of these mutants unveiled additional behavioral abnormalities mimicking the core symptoms of autism-spectrum disorder. These findings were strikingly confirmed in newly reported cases of Brunner syndrome. The role of MAOB in behavioral regulation remains less well-understood, even though Maob-deficient mice have been found to exhibit greater behavioral disinhibition and risk-taking responses, supporting previous clinical studies showing associations between low MAO B activity and impulsivity. Furthermore, lack of MAOB was found to exacerbate the severity of psychopathological deficits induced by concurrent MAOA deficiency. Here, we summarize how the convergence of clinical reports and behavioral phenotyping in mutant mice has helped frame a complex picture of psychopathological features in MAO-deficient individuals, which encompass a broad spectrum of neurodevelopmental problems. This emerging knowledge poses novel conceptual challenges towards the identification of the endophenotypes shared by autism-spectrum disorder, antisocial behavior and impulse-control problems, as well as their monoaminergic underpinnings.

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

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

Involvement of enzymes other than CYPs in the oxidative metabolism of xenobiotics

Although the majority of oxidative metabolic reactions are mediated by the CYP superfamily of enzymes, non-CYP-mediated oxidative reactions can play an important role in the metabolism of xenobiotics. The (major) oxidative enzymes, other than CYPs, involved in the metabolism of drugs and other xenobiotics are: the flavin-containing monooxygenases, the molybdenum hydroxylases (aldehyde oxidase and xanthine oxidase), the prostaglandin H synthase, the lipoxygenases, the amine oxidases (monoamine, polyamine, diamine and semicarbazide-sensitive amine oxidases) and the alcohol and aldehyde dehydrogenases. In a similar manner to CYPs, these oxidative enzymes can also produce therapeutically active metabolites and reactive/toxic metabolites, modulate the efficacy of therapeutically active drugs or contribute to detoxification. Many of them have been shown to be important in endobiotic metabolism, and, consequently, interactions between drugs and endogenous compounds might occur when they are involved in drug metabolism. In general, most non-CYP oxidative enzymes appear to be noninducible or much less inducible than the CYP system, although some of them may be as inducible as some CYPs. Some of these oxidative enzymes exhibit polymorphic expression, as do some CYPs. It is possible that the contribution of non-CYP oxidative enzymes to the overall metabolism of xenobiotics is underestimated, as most investigations of drug metabolism in discovery and lead optimisation are performed using in vitro test systems optimised for CYP activity.

Monoamine metabolism and behavioral responses to ethanol in mitochondrial aldehyde dehydrogenase knockout mice

BACKGROUND

It is widely accepted that, in addition to removing acetaldehyde produced during the metabolism of ethanol, mitochondrial aldehyde dehydrogenase (ALDH2) functions in the pathway by which aldehyde metabolites of the monoamines dopamine (DA) and serotonin (5-HT) are converted to their acidic metabolites. Moreover, studies of ALDH2 inhibitors used for treating alcoholism suggest that their antidipsotropic effects may be related to inhibition of monoamine metabolism. Therefore, we examined the hypothesis that altered brain monoamine metabolism is related to the influence of ALDH2 on behavioral responses to ethanol.

METHODS

Mice were generated with a gene-trap mutation of the ALDH2 gene. ALDH2 mRNA was absent in ALDH2-/- mice. Western blot analysis of liver mitochondria confirmed the absence of ALDH2 protein in the ALDH2-/- mice. Wild-type and ALDH2-deficient mice were tested for the effects of different doses of ethanol on locomotor activity, ataxia, and a 2-bottle ethanol-water preference test.

RESULTS

Wild-type and ALDH2+/- mice preferred ethanol to water. However, ALDH2-/- mice drank significantly less ethanol than wild-type or ALDH2+/- mice. Locomotor activity and ataxia were significantly more affected by ethanol in ALDH2-/- mice than in wild-type or ALDH2+/- mice. There was no effect of genotype on levels of 5-HT, DA, or their precursors or metabolites in several brain regions, as measured by HPLCec.

CONCLUSIONS

The results indicate that: (1) the effect of the mutant genotype on behavioral responses to ethanol is unrelated to altered brain monoamine metabolism and (2) ALDH2 is not required for the metabolism of brain monoamines in vivo.

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

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

Evaluation of aldehyde dehydrogenase 1 promoter polymorphisms identified in human populations

BACKGROUND

Cytosolic aldehyde dehydrogenase, or ALDH1A1, functions in ethanol detoxification, metabolism of neurotransmitters, and synthesis of retinoic acid. Because the promoter region of a gene can influence gene expression, the ALDH1A1 promoter regions were studied to identify polymorphism, to assess their functional significance, and to determine whether they were associated with a risk for developing alcoholism.

METHODS

Sequence analysis was performed in the promoter region by using Asian, Caucasian, and African American subjects. The resulting polymorphisms were assessed for frequency in Asian, Caucasian, Jewish, and African American populations and tested for associations with alcohol dependence in Asian and African American populations of alcoholics and controls. The functional significance of each polymorphism was determined through in vitro expression analysis by using HeLa and HepG2 cells.

RESULTS

Two polymorphisms, a 17 base pair (bp) deletion (-416/-432) and a 3 bp insertion (-524), were discovered in the ALDH1A1 promoter region: ALDH1A1*2 and ALDH1A1*3, respectively. ALDH1A1*2 was observed at frequencies of 0.035, 0.023, 0.023, and 0.012 in the Asian, Caucasian, Jewish, and African American populations, respectively. ALDH1A1*3 was observed only in the African American population, at a frequency of 0.029. By using HeLa and HepG2 cells for in vitro expression, the activity of the luciferase reporter gene was significantly decreased after transient transfection of ALDH1A1*3-luciferase compared with the wild-type construct ALDH1A1*1-luciferase. In an African American population, a trend for higher frequencies of the ALDH1A1*2 and ALDH1A1*3 alleles was observed in a population of alcoholics (p = 0.03 and f = 0.12, respectively) compared with the control population.

CONCLUSIONS

ALDH1A1*2 and ALDH1A1*3 may influence ALDH1A1 gene expression. Both ALDH1A1*2 and ALDH1A1*3 produce a trend in an African American population that may be indicative of an association with alcoholism; however, more samples are required to validate this observation. The underlying mechanisms contributing to these trends are still unknown.

Endocrine factors modulate the phenobarbital-mediated induction of cytochromes P450 and phase II enzymes in a similar strain-dependent manner

Phenobarbital (PB)-mediated induction of five forms of cytochrome P450 (CYP2B1, CYP2B2, CYP3A1, CYP2A1, and CYP2C6) and epoxide hydrolase is highly suppressed, at the transcriptional level, in Wistar Furth (WF) relative to Fischer 344 (F344) female rats. Either hypophysectomy or thyroid hormone depletion by methimazole largely reverses the suppression in WF animals. Here we show that this strain-dependent polymorphism and unusual endocrine regulation extend to PB induction of phase II enzymes UGT2B1 uridine diphosphate-glucuronosyl transferase (UDPGT), PB-inducible aldehyde dehydrogenase (ALDH), and glutathione transferases Ya1 and Ya2 (GSTYa1 and GSTYa2). UDPGT, ALDH, GSTYa1, and GSTYa2 had mRNA levels induced by PB in a similar strain-dependent manner (F344 > WF). The extent to which mRNA induction was favored in female F344 relative to female WF was gene dependent (UDPGT 5 x; ALDH 15 x; GSTYa1 2 x; GSTYa2 3-5 x). Again, thyroid suppression by methimazole treatment selectively enhanced mRNA induced levels in female WF animals to remove much of the strain difference. Since thyroid hormone action is linked to fatty acid (FA) homeostasis, we tested the possibility that FAs participated in this endocrine polymorphism by using three isocaloric diets: low fat (LFD), polyunsaturated fatty acid (PUFAD), or saturated fatty acid (SFAD). The LFD suppressed PB-induction of CYP mRNA and protein in WF but not F344 rats. This had no parallel in phase II mRNA induction, possibly indicating that FA and thyroid hormone effects are uncoupled. We conclude that the PB-response mechanism for induction of multiple P450 and phase II genes share a pathway that has as a common feature the linkage between chemical stimulation and thyroid hormone suppression that is seen in female WF relative to female F344 rats but not in male rats.

A review of alcohol clearance in humans

The level of blood or brain alcohol is considered to influence alcohol ingestion by causing subjective perceptions or neural activations that are reinforcing or rewarding. Alcohol-dependent people may try to maintain some desired tissue level, drinking to replace the millimolar levels that were cleared from the blood by metabolism. The biomedical literature describes many approaches to understanding the role of blood alcohol levels in human physiology and behavior, and this review examines some of the published results. They include the general kinetics of intake and removal of beverage alcohol as well as the characteristics of many different catalysts that can interact with alcohol. Because ingested alcohol creates blood levels that are a 1000-fold greater than those normally experienced during abstinence, ethanol may impose itself as an alternate substrate for the many oxidoreductases that act physiologically on other endogenous alcohols. Many enzymes that can act on millimolar ethanol have been isolated, and their structural genes are sequenced. Unfortunately, the genetic sequence does not indicate the physiological material upon which the translated gene product may act. In a sense, the set of enzymes with catalytic sites occupied by millimolar ethanol during alcohol drinking might constructively be regarded as "orphan gene products" whose physiological role remains to be clarified. This review is designed to indicate some of what is known, what is not known, and what needs to be known to improve the interpretations regarding adaptations to beverage alcohol and the ability of millimolar levels of alcohol to diminish dysphoria. The dysphoria may be influenced by ethanol, by ethanol metabolites, or by altered metabolism of currently unspecified endogenous substrates. A major challenge is to evaluate the multiple alternative variables within a context that stimulates curiosity and encourages quantitative tests of the relative contribution of each variable to the overall physiology of an individual.

High levels of a retinoic acid-generating dehydrogenase in the meso-telencephalic dopamine system

Retinoic acid is synthesized from retinaldehyde by several different dehydrogenases, which are arranged in conserved spatial and developmentally regulated patterns. Here we show for the mouse that a class-1 aldehyde dehydrogenase, characterized by oxidation and disulfiram sensitivity, is found in the brain at high levels only in the basal forebrain. It is present in axons and terminals of a subpopulation of dopaminergic neurons of the mesostriatal and mesolimbic system, forming a retinoic acid-generating projection from the ventral tegmentum to the corpus striatum and the shell of the nucleus accumbens. In the striatum the projection is heaviest to dorsal and rostral regions, declining gradually toward ventral. The enzyme is expressed early in development, shortly after appearance of tyrosine hydroxylase. Other dopaminergic neurons in the brain, as well as the chromaffin cells of the adrenal medulla, do not contain this dehydrogenase. The presence of this enzyme may be a factor in the long-term success of transplants of dopaminergic cells to the corpus striatum in Parkinson disease, and it may play a role in parkinsonism and catatonia due to disulfiram (Antabuse) neurotoxicity.

Mitochondria as the primary site of acetaldehyde metabolism in beef and pig liver slices

Aldehyde dehydrogenase (ALDH) is the major enzyme involved in the oxidation of acetaldehyde. It has been shown that the liver enzyme is located in both cytosol and mitochondria. It has not been established where the subcellular oxidation of acetaldehyde occurs in species other than rat. Using slices isolated from beef and pig livers and selectively inhibiting the mitochondria enzyme with cyanamide or the cytosolic enzyme with disulfiram, it was possible to address this question. It was found that with both beef and pig liver slices 60% of the oxidation was catalyzed by the mitochondrial ALDH and 20% by the higher Km cytosolic enzyme. The remainder of the metabolism was the result of non-ALDH involvement. Furthermore, any decrease in the level of the low Km mitochondrial aldehyde dehydrogenase activity resulted in a decreased rate of acetaldehyde oxidation showing that its activity governed the rate of acetaldehyde oxidation. These were the same conclusions previously reached using rat liver tissue slices. Thus, it appears that for all mammalian tissue, mitochondria is the primary location of acetaldehyde oxidation.