Subcellular localization of aldehyde dehydrogenase isozymes in human liver

Aldehyde dehydrogenase 2 deficiency promotes skeletal muscle atrophy in aged mice

Aldehyde dehydrogenase 2 (ALDH2) detoxifies acetaldehyde produced from ethanol. A missense single nucleotide polymorphism (SNP) rs671 in ALDH2 exhibits a dominant-negative form of the ALDH2 protein. Nearly 40% of people in East Asia carry an inactive ALDH2*2 mutation. Previous studies reported that ALDH2*2 is associated with increased risk of several diseases. In this study, we examined the effect of ALDH2 deficiency on age-related muscle atrophy and its underlying mechanisms. We found that ALDH2 deficiency promotes age-related loss of muscle fiber cross-sectional areas, especially in oxidative fibers. Furthermore, ALDH2 deficiency exacerbated age-related accumulation of 4-hydroxy-2-nonenal (4-HNE), a marker of oxidative stress in the gastrocnemius muscle. Similarly, mitochondrial reactive oxygen species (ROS) production increased in aged ALDH2-knockout mice, indicating that ALDH2 deficiency induced mitochondrial dysfunction. In summary, ALDH2 deficiency promotes age-related muscle loss, especially in oxidative fibers, which may be associated with an increased accumulation of oxidative stress via mitochondrial dysfunction.

Acetaldehyde dehydrogenase 2 deficiency increases mitochondrial reactive oxygen species emission and induces mitochondrial protease Omi/HtrA2 in skeletal muscle

Acetaldehyde dehydrogenase 2 (ALDH2) is an enzyme involved in redox homeostasis as well as the detoxification process in alcohol metabolism. Nearly 8% of the world’s population have an inactivating mutation in the ALDH2 gene. However, the expression patterns and specific functions of ALDH2 in skeletal muscles are still unclear. Herein, we report that ALDH2 is expressed in skeletal muscle and is localized to the mitochondrial fraction. Oxidative muscles had a higher amount of ALDH2 protein than glycolytic muscles. We next comprehensively investigated whether ALDH2 knockout in mice induces mitochondrial adaptations in gastrocnemius muscle (for example, content, enzymatic activity, respiratory function, supercomplex formation, and functional networking). We found that ALDH2 deficiency resulted in partial mitochondrial dysfunction in gastrocnemius muscle because it increased mitochondrial reactive oxygen species (ROS) emission (2′,7′-dichlorofluorescein and MitoSOX oxidation rate during respiration) and the frequency of regional mitochondrial depolarization. Moreover, we determined whether ALDH2 deficiency and the related mitochondrial dysfunction trigger mitochondrial stress and quality control responses in gastrocnemius muscle (for example, mitophagy markers, dynamics, and the unfolded protein response). We found that ALDH2 deficiency upregulated the mitochondrial serine protease Omi/HtrA2 (a marker of the activation of a branch of the mitochondrial unfolded protein response). In summary, ALDH2 deficiency leads to greater mitochondrial ROS production, but homeostasis can be maintained via an appropriate stress response.

Role of mitochondrial aldehyde dehydrogenase in nitrate tolerance

Glyceryl trinitrate (GTN) is used in the treatment of angina pectoris and cardiac failure, but the rapid onset of GTN tolerance limits its clinical utility. Research suggests that a principal cause of tolerance is inhibition of an enzyme responsible for the production of physiologically active concentrations of NO from GTN. This enzyme has not conclusively been identified. However, the mitochondrial aldehyde dehydrogenase (ALDH2) is inhibited in GTN-tolerant tissues and produces NO2- from GTN, which is proposed to be converted to NO within mitochondria. To investigate the role of this enzyme in GTN tolerance, cumulative GTN concentration-response curves were obtained for both GTN-tolerant and -nontolerant rat aortic rings treated with the ALDH inhibitor cyanamide or the ALDH substrate propionaldehyde. Tolerance to GTN was induced using both in vivo and in vitro protocols. The in vivo protocol resulted in almost complete inhibition of ALDH2 activity and GTN biotransformation in hepatic mitochondria, indicating that long-term GTN exposure results in inactivation of the enzyme. Treatment with cyanamide or propionaldehyde caused a dose-dependent increase in the EC50 value for GTN-induced relaxation of similar magnitude in both tolerant and nontolerant aorta, suggesting that although cyanamide and propionaldehyde inhibit GTN-induced vasodilation, these inhibitors do not affect the enzyme or system involved in tolerance development to GTN. Treatment with cyanamide or propionaldehyde did not significantly inhibit 1,1-diethyl-2-hydroxy-2-nitrosohydrazine-mediated vasodilation in tolerant or nontolerant aorta, indicating that these ALDH inhibitors do not affect the downstream effectors of NO-induced vasodilation. Immunoblot analysis indicated that the majority of vascular ALDH2 is present in the cytoplasm, suggesting that mitochondrial biotransformation of GTN by ALDH2 plays a minor role in the overall vascular biotransformation of GTN by this enzyme.

Genotyping of the aldehyde dehydrogenase 2 (ALDH2) gene using the polymerase chain reaction: evidence for single point mutation in the ALDH2 gene of ALDH2-deficiency

About half of all Japanese lack the activity of aldehyde dehydrogenase 2 (ALDH2), and suffer a flush after alcohol intake due to the marked elevation of blood acetaldehyde concentration. The cause of ALDH2 deficiency is thought to be a single point mutation in codon 487 of the ALDH2 gene. However, this mutant ALDH2 gene has not yet been cloned and sequenced. We amplified and cloned the exon 12 of the ALDH2 gene using polymerase chain reaction (PCR), and revealed that normal GAA coding glutamic acid is replaced for AAA coding lysine in codon 487 of the mutant ALDH2 gene. Based on this finding, we performed the genotyping of the ALDH2 gene using PCR and allele-specific oligonucleotide probes. The genotypes of 13 subjects with ALDH2-active phenotype were all homozygous for the normal ALDH2 gene (ALDH2(1)), while in 9 subjects with ALDH2-deficient phenotype 2 subjects were homozygous for the mutant ALDH2 gene (ALDH2(2)) and the other 7 subjects were heterozygous for both genes, indicating that the mutant ALDH2 gene is dominant. In 20 normal control subjects, the prevalence of ALDH2(1)/ALDH2(1), ALDH2(1)/ALDH2(2) and ALDH2(2)/ALDH2(2) was 45%, 45% and 10% respectively. On the other hand, in 36 alcoholic liver disease patients, the prevalence of the genotypes was 83%, 17% and 0%. These results confirmed the previous observation that the incidence of ALDH2 deficiency is much lower in alcoholic liver disease patients than in the general population, and suggested that most of the ALDH2 deficient patients with alcoholic liver disease are heterozygous for the normal and mutant ALDH2 genes.

Acetaldehyde metabolism in different aldehyde dehydrogenase-2 genotypes

In order to clarify the relationships between acetaldehyde (Ac-CHO) metabolism and low Km (mitochondrial) aldehyde dehydrogenase (ALDH2) genotypes, hepatic ALDH2 activity was determined and serial changes of blood Ac-CHO levels after ethanol administration were analyzed in the individuals homozygous for the normal ALDH2 genes, heterozygous for the normal and mutant ALDH2 genes, and homozygous for the mutant ALDH2 genes. Genomic DNA was extracted from white blood cells and genotyping of ALDH2 was performed using the polymerase chain reaction technique and slot blot hybridization with synthesized oligonucleotide probes specific to the normal and mutant ALDH2 genes. ALDH2 activity was not detectable in the liver in two cases of the mutant homozygote. In four out of eight cases of the heterozygote, hepatic ALDH2 activity was measurable, although the activity was lower compared with that in the normal homozygote. Blood ethanol levels after alcohol administration were not different among the three different ALDH2 genotypes. Blood Ac-CHO levels after drinking of alcohol were significantly higher in the heterozygotes and the mutant homozygotes than in the normal homozygotes. The levels after a moderate amount of ethanol (0.8 g/kg of body weight) in a case of the mutant homozygote were not different from those of the heterozygotes. However, the levels after a small amount of ethanol (0.1 g/kg of body weight) were significantly higher in the mutant homozygotes than in the heterozygotes. These results indicate that hepatic ALDH2 activity is lacking completely, and metabolism of Ac-CHO in the liver is severely impaired in the homozygotes of the mutant ALDH2 genes.(ABSTRACT TRUNCATED AT 250 WORDS)