Aldehyde dehydrogenases, aldehyde oxidase and xanthine oxidase from baboon tissues: phenotypic variability and subcellular distribution in liver and brain

The mammalian aldehyde oxidase gene family

Aldehyde oxidases (EC 1.2.3.1) are a small group of structurally conserved cytosolic proteins represented in both the animal and plant kingdoms. In vertebrates, aldehyde oxidases constitute the small sub-family of molybdo-flavoenzymes, along with the evolutionarily and structurally related protein, xanthine oxidoreductase. These enzymes require a molybdo-pterin cofactor (molybdenum cofactor, MoCo) and flavin adenine dinucleotide for their catalytic activity. Aldehyde oxidases have broad substrate specificity and catalyse the hydroxylation of N-heterocycles and the oxidation of aldehydes to the corresponding acid. In humans, a single aldehyde oxidase gene (AOX1) and two pseudogenes clustering on a short stretch of chromosome 2q are known. In other mammals, a variable number of structurally conserved aldehyde oxidase genes has been described. Four genes (Aox1, Aox3, Aox4 and Aox3l1), coding for an equivalent number of catalytically active enzymes, are present in the mouse and rat genomes. Although human AOX1 and its homologous proteins are best known as drug metabolising enzymes, the physiological substrate(s) and function(s) are as yet unknown. The present paper provides an update of the available information on the evolutionary history, tissue- and cell-specific distribution and function of mammalian aldehyde oxidases.

Advantages and limitations of nonhuman primates as animal models in genetic research on complex diseases

The genetic similarity between humans and nonhuman primates makes nonhuman primates uniquely suited as models for genetic research on complex physiological and behavioral phenotypes. By comparison with human subjects, nonhuman primates, like other animal models, have several advantages for these types of studies: 1) constant environmental conditions can be maintained over long periods of time, greatly increasing the power to detect genetic effects; 2) different environmental conditions can be imposed sequentially on individuals to characterize genotype-environment interactions; 3) complex pedigrees that are much more powerful for genetic analysis than typically available human pedigrees can be generated; 4) genetic hypotheses can be tested prospectively by selective matings; and 5) essential invasive and terminal experiments can be conducted. Limitations of genetic research with nonhuman primates include cost and availability. However, the ability to manipulate both genetic and environmental factors in captive primate populations indicates the promise of genetic research with these important animal models for illuminating complex disease processes. The utility of nonhuman primates for biomedical research on human health problems is illustrated by examples concerning the use of baboons in studies of osteoporosis, alcohol metabolism, and lipoproteins.

Rapid conversion to high xanthine oxidase activity in viable Kupffer cells during hypoxia

It has been widely postulated that the central mechanism of hepatic reperfusion injury involves the conversion, during ischemia, of the enzyme xanthine dehydrogenase (XDH) to its free radical-producing form, xanthine oxidase (XOD). However, this theory has been questioned because (a) XDH to XOD conversion in whole liver occurs very slowly; (b) the cellular distribution of XDH/XOD is unclear; and (c) the direct demonstration of XDH to XOD conversion in viable cells is lacking. In this paper, we address all three issues by measuring XDH to XOD conversion and cell viability in purified populations of hepatic endothelial cells (EC), Kupffer cells (KC), and hepatocytes (HEP). Although XDH/XOD activity on a cellular basis was greater in hepatocytes (0.92 +/- 0.12 mU/10(6) cells) than ECs (0.03 +/- 0.01) or KCs (0.12 +/- 0.04), XDH + XOD specific activity was similar in all three cell types (HEP 1.85 +/- 0.10 U/g protein; EC 1.69 +/- 0.54; KC 2.30 +/- 0.22). Over 150 min of warm (37 degrees C) or 24 h of cold (4 degrees C) hypoxia, percent XOD activity increased slowly in ECs, from 21 +/- 2% (basal) to 39 +/- 3% (warm) and 49 +/- 5% (cold) and in HEPs (29 +/- 2% to 38 +/- 3% and 49 +/- 2%), but converted significantly faster in KCs (28 +/- 3% to 91 +/- 7% and 94 +/- 4%). The dramatic changes in Kupffer cell XOD during cold hypoxia occurred despite only minor changes in cell viability. When hypoxic KCs were reoxygenated after 16 h of cold hypoxia, there was a marked increase in cell death that was significantly blocked by allopurinol. These data suggest that significant conversion to the free radical-producing state occurs within viable KCs, and that Kupffer cell XOD may play an important role in mediating reperfusion injury in the liver.

Aldehyde dehydrogenases and their role in carcinogenesis

Aldehydes are highly reactive molecules that may have a variety of effects on biological systems. They can be generated from a virtually limitless number of endogenous and exogenous sources. Although some aldehyde-mediated effects such as vision are beneficial, many effects are deleterious, including cytotoxicity, mutagenicity, and carcinogenicity. A variety of enzymes have evolved to metabolize aldehydes to less reactive forms. Among the most effective pathways for aldehyde metabolism is their oxidation to carboxylic acids by aldehyde dehydrogenases (ALDHs). ALDHs are a family of NADP-dependent enzymes with common structural and functional features that catalyze the oxidation of a broad spectrum of aliphatic and aromatic aldehydes. Based on primary sequence analysis, three major classes of mammalian ALDHs--1, 2, and 3--have been identified. Classes 1 and 3 contain both constitutively expressed and inducible cytosolic forms. Class 2 consists of constitutive mitochondrial enzymes. Each class appears to oxidize a variety of substrates that may be derived either from endogenous sources such as amino acid, biogenic amine, or lipid metabolism or from exogenous sources, including aldehydes derived from xenobiotic metabolism. Changes in ALDH activity have been observed during experimental liver and urinary bladder carcinogenesis and in a number of human tumors, including some liver, colon, and mammary cancers. Changes in ALDH define at least one population of preneoplastic cells having a high probability of progressing to overt neoplasms. The most common change is the appearance of class 3 ALDH dehydrogenase activity in tumors arising in tissues that normally do not express this form. The changes in enzyme activity occur early in tumorigenesis and are the result of permanent changes in ALDH gene expression. This review discusses several aspects of ALDH expression during carcinogenesis. A brief introduction examines the variety of sources of aldehydes. This is followed by a discussion of the mammalian ALDHs. Because the ALDHs are a relatively understudied family of enzymes, this section presents what is currently known about the general structural and functional properties of the enzymes and the interrelationships of the various forms. The remainder of the review discusses various aspects of the ALDHs in relation to tumorigenesis. The expression of ALDH during experimental carcinogenesis and what is known about the molecular mechanisms underlying those changes are discussed. This is followed by an extended discussion of the potential roles for ALDH in tumorigenesis. The role of ALDH in the metabolism of cyclophosphamidelike chemotherapeutic agents is described. This work suggests that modulation of ALDH activity may an important determinant of the effectiveness of certain chemotherapeutic agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of mouse liver aldehyde dehydrogenases that catalyze the oxidation of retinaldehyde to retinoic acid

NAD(P)-linked aldehyde dehydrogenases catalyze the oxidation of a wide variety of aldehydes. Thirteen of these enzymes have been identified in mouse tissues; eleven are found in the liver. Some are substrate-nonspecific; others are relatively substrate-specific. The present investigation sought to determine which of these enzymes are operative in catalyzing the oxidation of retinaldehyde to retinoic acid, a metabolite of vitamin A that promotes the differentiation of epithelial and other cells. Spectrophotometric and HPLC assays were used for this purpose. Enzyme-catalyzed oxidation of retinaldehyde (25 microM) was restricted to the cytosol (105,000 g supernatant fraction) and occurred at a rate of 211 nmol/min/g liver; oxidation of acetaldehyde (4 mM) by this fraction proceeds about ten times faster. At least 90% of this activity was NAD dependent. Of the approximately 10% that was apparently NAD independent, two-thirds was inhibited by 1 mM pyridoxal, a known inhibitor of aldehyde oxidase. Of the six cytosolic aldehyde dehydrogenases, only two, viz. AHD-2 and AHD-7, catalyzed the oxidation of retinaldehyde to retinoic acid. An additional NAD-dependent enzyme, viz. xanthine oxidase (dehydrogenase form), also catalyzed the reaction. Catalysis by AHD-2 accounted for more than 90% of the total NAD-dependent activity. Km values were 0.7, 0.6 and 0.9 microM, respectively, for the AHD-2-, AHD-7- and xanthine oxidase (dehydrogenase form)-catalyzed reaction. AHD-4, an aldehyde dehydrogenase found in the cytosol of mouse stomach epithelium and cornea, did not catalyze the reaction.

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.

Genetics of alcohol dehydrogenase and aldehyde dehydrogenase from Monodelphis domestica cornea: further evidence for identity of corneal aldehyde dehydrogenase with a major soluble protein

A didelphid marsupial, the gray short-tailed opossum (Monodelphis domestica), was used as a model species to study the biochemical genetics of alcohol dehydrogenases (ADHs) and aldehyde dehydrogenase (ALDH) in corneal tissue. Isoelectric point variants of corneal ALDH (designated ALDH3) and a major soluble protein in corneal extracts were observed among eight families of animals used in studying the genetics of these proteins. Both phenotypes exhibited identical patterns following PAGE-IEF and were inherited in a normal Mendelian fashion, with two alleles at a single locus (ALDH3) showing codominant expression. The data provided evidence for genetic identity of corneal ALDH with this major soluble protein, and supported biochemical evidence, recently reported for purified bovine corneal ALDH, that this enzyme constitutes a major portion of soluble corneal protein (Abedinia et al. 1990). Isoelectric point variants for corneal ADH were also observed, with patterns for the two major forms (ADH3 and ADH4) and one minor form (ADH5) being consistent with the presence of two ADH subunits (designated gamma and delta), and variant phenotypes existing for the gamma subunit. The genetics of this enzyme was studied in the eight families, and the results were consistent with codominant expression of two alleles at a single locus (designated ADH3). It is relevant that a major detoxification function has been proposed for corneal ADH and ALDH, in the oxidoreduction of peroxidic aldehydes induced by available oxygen and UV-B light (Holmes & VandeBerg, 1986a). In addition, a direct role for corneal ALDH as a UV-B photoreceptor in this anterior eye tissue has also been proposed (Abedinia et al. 1990).

Purification and properties of mouse stomach aldehyde dehydrogenase. Evidence for a role in the oxidation of peroxidic and aromatic aldehydes

The major isozyme of aldehyde dehydrogenase in mouse stomach, AHD-4, has been purified to homogeneity and characterized with a range of aldehyde substrates at pH 7.4. The enzyme was a dimer with a subunit size of 65 kDa. Using V/Km values as an indication of substrate efficacy, aromatic aldehydes were the preferred substrates. The enzyme used either NAD+ or NADP+ as cofactor, but showed a preference for NAD+. AHD-4 showed 'high-Km' properties with respect to acetaldehyde, but differed from the 'high-Km' liver mitochondrial enzyme (AHD-1), in that it was not a semialdehyde dehydrogenase. The enzyme was significantly active towards the peroxidic aldehyde, 4-hydroxynonenal, and may play a role in vivo in the detoxification of aromatic aldehydes and the aldehyde products of lipid peroxidation.

Subcellular localization of aldehyde dehydrogenase isozymes in human liver

The subcellular distribution of aldehyde dehydrogenase (ALDH) isozymes in human liver was studied by isoelectric focusing and biochemical procedures in biopsied liver specimens obtained during surgical procedures. Four types of ALDH isozymes (ALDH I, II, III and IV) were identified in human liver by isoelectric focusing. In 6 of the 13 livers examined, ALDH I was not detected, indicating that about half of the Japanese people may be classified as the unusual type. ALDH I, which exhibits a low Km with respect to acetaldehyde (Ac-CHO), was located mainly in the mitochondrial and cytosolic fractions. ALDH II (high Km for Ac-CHO) was found to be localized mainly in the microsomal and cytosolic fractions. ALDH III and IV (very high Km for Ac-CHO) were localized in all fractions, except for ALDH III in the microsomal fraction. Biochemical analysis indicates that low Km ALDH activity was localized in the mitochondrial and cytosolic fractions, while high Km and whole ALDH activities were detected in all 3 fractions. More than 80% of the low Km, high Km and whole ALDH activity was found in the cytosolic fraction. These distribution patterns were quite different from those in rats. These results indicate that the results obtained in animal experiments cannot be directly applied to humans and that the main site of Ac-CHO oxidation in the human liver is in the cytosol.

Purification and characterization of beef and pig liver aldehyde dehydrogenases

Beef liver cytosolic, mitochondrial, and pig liver mitochondrial aldehyde dehydrogenases (ALDH) had been purified to homogeneity. The two mitochondrial enzymes as with other mammalian mitochondrial enzymes had properties very similar to that of the corresponding human enzyme. These include immunological as well as basic kinetic properties such as low Km for aldehyde, activation by Mg2+ ions, and lack of inhibition by disulfiram. A major difference between these two enzymes and the human mitochondrial enzyme was that they contained an N-terminal-blocked amino acid. Cytosolic ALDHs from human and horse liver have been shown to possess an N-acetyl serine as the N-terminal residue; beef cytosolic ALDH was also found to be blocked. Tissue preparations and subcellular fractions from beef or pig liver could be used to study acetaldehyde oxidation. This is the subject of the accompanying paper (Cao Q-N, Tu G-C, Weiner H, Alcohol Clin Exp Res 12:xxx-xxx, 1988).

Human stomach alcohol and aldehyde dehydrogenases (ALDH): a genetic model proposed for ALDH III isozymes

Isozyme phenotypes of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) from human gastroendoscopic as well as surgical gastric biopsies were determined by starch gel electrophoresis and agarose isoelectric focusing. gamma gamma ADH isozymes were expressed predominantly in the mucosal layer of the stomach, whereas beta beta isozymes were in the muscular layer. In the 56 gastroendoscopic mucosal biopsies examined, the homozygous ADH3 1-1 phenotype was found in 75% of the samples, and the heterozygous ADH3 2-1 phenotype in 25%. Accordingly, the gene frequencies of the alleles ADH1/3 and ADH2/3 were calculated to be 0.88 and 0.12, respectively. Using a modified agarose isoelectric focusing procedure, gastric ALDH I, ALDH II, and up to five ALDH III forms could be clearly resolved. The ALDH III isozymes accounted for more than 80% of the total ALDH activities in gastric mucosa and exhibited Km values in the millimolar range for propionaldehyde at pH 9.0. Forty-five percent of the 55 gastroendoscopic biopsies studied lacked ALDH I isozyme. The complex gastric ALDH III isozyme phenotypes seen in these biopsies fall into three patterns. They can be interpreted by a genetic hypothesis, based on a dimeric molecule, in which there are two separate genes, ALDH3a and ALDH3b, with the ALDH3b locus exhibiting polymorphism. The homozygous phenotypes ALDH3b 1-1 and ALDH3b 2-2 were found to be 4 and 76%, respectively, and the heterozygous ALDH3b 2-1 phenotype 20%, of the total. Therefore, the allele frequencies for ALDH1/3b and ALDH2/3b were calculated to be 0.14 and 0.86, respectively. Several lines of biochemical evidence consistent with this genetic model are discussed.

Ocular NAD-dependent alcohol dehydrogenase and aldehyde dehydrogenase in the baboon

Isoelectric focusing (IEF) techniques and spectrophotometric analyses were used to examine the distribution and properties of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) isozymes in ocular tissue of olive and yellow baboons. Cornea extracts exhibited very high specific activities of the 'stomach-specific' ADH and ALDH isozymes (designated ADH-3 and ALDH-III respectively), and were devoid of the major liver and kidney isozymes. Lens extracts exhibited lower activities of ADH-3 and ALDH-III, and also showed significant activity of ALDH-II (the major liver cytosolic isozyme) and a group of 'lens-specific' ALDHs of low isoelectric point. Extracts of baboon retina also exhibited ADH-3 and ALDH-III activities, together with activities of the major liver cytosolic (ALDH-II) and mitochondrial (ALDH-I) isozymes of ALDH; and ADH-5 (or chi-ADH) activity. Evidence was obtained for individual variation of ALDH-III activity in the lens. An electrophoretic variant for ADH-3 indicated genetic identity of the major stomach and ocular ADH isozyme. The catalytic properties of the high specific activity corneal ADH and ALDH isozymes indicated a role in the detoxification of lipid peroxidation by-products.