Pterin and folate salvage. Plants and Escherichia coli lack capacity to reduce oxidized pterins

Dihydropterins are intermediates of folate synthesis and products of folate breakdown that are readily oxidized to their aromatic forms. In trypanosomatid parasites, reduction of such oxidized pterins is crucial for pterin and folate salvage. We therefore sought evidence for this reaction in plants. Three lines of evidence indicated its absence. First, when pterin-6-aldehyde or 6-hydroxymethylpterin was supplied to Arabidopsis (Arabidopsis thaliana), pea (Pisum sativum), or tomato (Lycopersicon esculentum) tissues, no reduction of the pterin ring was seen after 15 h, although reduction and oxidation of the side chain of pterin-6-aldehyde were readily detected. Second, no label was incorporated into folates when 6-[(3)H]hydroxymethylpterin was fed to cultured Arabidopsis plantlets for 7 d, whereas [(3)H]folate synthesis from p-[(3)H]aminobenzoate was extensive. Third, no NAD(P)H-dependent pterin ring reduction was found in tissue extracts. Genetic evidence showed a similar situation in Escherichia coli: a GTP cyclohydrolase I (folE) mutant, deficient in pterin synthesis, was rescued by dihydropterins but not by the corresponding oxidized forms. Expression of a trypanosomatid pterin reductase (PTR1) enabled rescue of the mutant by oxidized pterins, establishing that E. coli can take up oxidized pterins but cannot reduce them. Similarly, a GTP cyclohydrolase I (fol2) mutant of yeast (Saccharomyces cerevisiae) was rescued by dihydropterins but not by most oxidized pterins, 6-hydroxymethylpterin being an exception. These results show that the capacity to reduce oxidized pterins is not ubiquitous in folate-synthesizing organisms. If it is lacking, folate precursors or breakdown products that become oxidized will permanently exit the metabolically active pterin pool.     

Purification and properties of aldehyde reductases from human placenta

Aldehyde reductases (alcohol: NADP⁺-oxidoreductases, EC 1.1.1.2) I and II from human placenta have been purified to homogeneity. Aldehyde reductase I, molecular weight about 74 000, is a dimer of two nonidentical subunits of molecular weigths of about 32 500 and 39 000, whereas aldehyde erductase II is a monomer of about 32 500. Aldehyde reductase I can be dissociated into subunits under high ionic concentrations. The isoelectric pH for aldehyde reductases I and II are 5.76 and 5.20, respectively. Amino acid compositions of the two enzymes are significantly different. Placenta aldehyde reductase I can utilize glucose with a lower affinity, whereas aldehyde reductase II is not capable to reducing aldo-sugars. Similarly, aldehyde reductase I does not catalyse the reduction of glucuronate while aldehyde reductase II has a high affinity for glucuronate. Both enzymes, however, exhibit strong affinity towards various other aldehydes such as glyceraldehyde, propionaldehyde, and pyridine-3-aldehyde. The pH optima for aldehyde reductases I and II are 6.0 and 7.0, respectively. Aldehyde reductaase I can use both NADH and NADPH as cofactors, whereas aldehyde reductase II activity is dependent on NADPH only. Both enzymes are susceptible to inhibition by sulfhydryl group reagents, aldose reductase inhibitors, lithium sulfate, and sodium chloride to varying degrees.     

Evidence for folate-salvage reactions in plants

Folates in vivo undergo oxidative cleavage, giving pterin and p -aminobenzoylglutamate ( p ABAGlu) moieties. These breakdown products are excreted in animals, but their fate is unclear in microorganisms and unknown in plants. As indirect evidence from this and previous studies strongly suggests that plants can have high folate-breakdown rates (approximately 10% per day), salvage of the cleavage products seems likely. Four sets of observations support this possibility. First, cleavage products do not normally accumulate: pools of p ABAGlu (including its polyglutamyl forms) are equivalent to, at most, 4–14% of typical total folate pools in Arabidopsis thaliana , Lycopersicon esculentum and Pisum sativum tissues. Pools of the pterin oxidation end-product pterin-6-carboxylate are, likewise, fairly small (3–37%) relative to total folate pools. Second, little p ABAGlu built up in A. thaliana plantlets when net folate breakdown was induced by blocking folate synthesis with sulfanilamide. Third, A. thaliana and L. esculentum tissues readily converted supplied breakdown products to folate synthesis precursors: p ABAGlu was hydrolysed to p -aminobenzoate and glutamate, and dihydropterin-6-aldehyde was reduced to 6-hydroxymethyldihydropterin. Fourth, both these reactions were detected in vitro ; the reduction used NADPH as cofactor. An alternative salvage route for p ABAGlu, direct reincorporation into dihydrofolate via the action of dihydropteroate synthase, appears implausible from the properties of this enzyme. We conclude that plants are excellent organisms in which to explore the biochemistry of folate salvage.     

NADPH-dependent reductases involved in the detoxification of reactive carbonyls in plants

Reactive carbonyls, especially α,β-unsaturated carbonyls produced through lipid peroxidation, damage biomolecules such as proteins and nucleotides; elimination of these carbonyls is therefore essential for maintaining cellular homeostasis. In this study, we focused on an NADPH-dependent detoxification of reactive carbonyls in plants and explored the enzyme system involved in this detoxification process. Using acrolein (CH(2) = CHCHO) as a model α,β-unsaturated carbonyl, we purified a predominant NADPH-dependent acrolein-reducing enzyme from cucumber leaves, and we identified the enzyme as an alkenal/one oxidoreductase (AOR) catalyzing reduction of an α,β-unsaturated bond. Cloning of cDNA encoding AORs revealed that cucumber contains two distinct AORs, chloroplastic AOR and cytosolic AOR. Homologs of cucumber AORs were found among various plant species, including Arabidopsis, and we confirmed that a homolog of Arabidopsis (At1g23740) also had AOR activity. Phylogenetic analysis showed that these AORs belong to a novel class of AORs. They preferentially reduced α,β-unsaturated ketones rather than α,β-unsaturated aldehydes. Furthermore, we selected candidates of other classes of enzymes involved in NADPH-dependent reduction of carbonyls based on the bioinformatic information, and we found that an aldo-keto reductase (At2g37770) and aldehyde reductases (At1g54870 and At3g04000) were implicated in the reduction of an aldehyde group of saturated aldehydes and methylglyoxal as well as α,β-unsaturated aldehydes in chloroplasts. These results suggest that different classes of NADPH-dependent reductases cooperatively contribute to the detoxification of reactive carbonyls.     

Distribution and immunological characterization of microbial aldehyde reductases

The distribution of microbial aldo-keto reductases was examined and their immunochemical characterization was performed. p-Nitrobenzaldehyde, pyridine-3-aldehyde and ethyl 4-chloro-3-oxobutanoate reductase activities were found to be widely distributed in a variety of microorganisms. In immunodiffusion studies, most yeasts belonging to the genera Sporobolomyces, Sporidiobolus and Rhodotorula formed precipitin bands with anti-Sporobolomyces salmonicolor aldehyde reductase serum. Furthermore, the results of immunotitration experiments suggested that Sporobolomyces salmonicolor AKU 4429 contains other enzyme(s) which can reduce p-nitrobenzaldehyde, pyridine-3-aldehyde and/or ethyl 4-chloro-3-oxobutanoate, and which are inactivated by anti-Sporobolomyces salmonicolor aldehyde reductase serum.     

Resolution and partial characterization of two aldehyde reductases of mammalian liver.

Investigation of NADP-dependent aldehyde reductase activity in mouse liver led to the finding that two distinct reductases are separable by DE52 ion exchange chromatography. Aldehyde reductase I (AR I) appears in the effluent, while aldehyde reductase II (AR II) is eluted with a salt gradient. By several procedures AR II was purified over 1100-fold from liver supernatant fraction, but AR I could be pruified only 107-fold because of its instability. The two enzymes are different in regard to pH optimum, substrate specificity, response to inhibitors, and reactivity with antibody to AR II. While both enzymes utilize aromatic aldehydes well, only AR II ACTS ON D-glucuronate, indicating that it is the aldyhyde reductase recently reported to be identical to NADP-L-gulonate dehydrogenase. The presence of two NADP-linked aldehyde reductases in liver has apparently not heretofore been reported.     

Functional expression of two Arabidopsis aldehyde oxidases in the yeast Pichia pastoris

To investigate the biochemical and enzymatic properties of two aldehyde oxidase (AO) isoforms of Arabidopsis thaliana, we expressed AAO1 and AAO2 cDNAs in a heterologous yeast (Pichia pastoris) system and successfully obtained the proteins in active forms. The expressed AAO1 and AAO2 proteins gave activity bands with the same mobilities on native gel electrophoresis and exhibited the same substrate preferences on zymograms with 8 aldehydes as those of AOalpha and AOgamma in Arabidopsis seedlings, respectively. Furthermore, anti-AAO1 and anti-AAO2 antibodies, which specifically recognize the seedling AOalpha and AOgamma, respectively, reacted with the AAO1 and AAO2 proteins produced in P. pastoris, respectively. These results indicate that these AO proteins are accurately produced in the yeast system, as in Arabidopsis seedlings. Using AO preparations from P. pastoris, the enzymatic properties of Arabidopsis AOalpha and AOgamma were investigated. AOalpha showed a relatively wide substrate specificity for 7 aldehydes tested, with high affinity to benzaldehyde and indole-3-aldehyde, while AOgamma could most efficiently oxidize naphthaldehyde. AOalpha was strongly inhibited by iodoacetate and KCN, while AOgamma was inhibited not only by iodoacetate and KCN but also by 2-mercaptethanol, dithiothreitol, menadion, and estradiol. AOalpha and AOgamma showed the highest activity at around 65 and 50 degrees C, respectively, and exhibited pH dependence around pH 8.0. These results indicate that the two AO isoforms in Arabidopsis seedlings have different enzymatic properties and may have different physiological roles in vivo.     

Purification and characterization of aldose reductase and aldehyde reductase from human kidney.

Aldose reductase and aldehyde reductases have been purified to homogeneity from human kidney and have molecular weights of 32,000 and 40,000 and isoelectric pH 5.8 and 5.3, respectively. Aldose reductase, beside catalyzing the reduction of various aldehydes, reduces aldo-sugars, whereas aldehyde reductase, does not reduce aldo-sugars. Aldose reductase activity is expressed with either NADH or NADPH as cofactor, whereas aldehyde reductase utilizes only NADPH. Both enzymes are inhibited to varying degrees by aldose reductase inhibitors. Antibodies against bovine lens aldose reductase precipitated aldose reductase but not aldehyde reductase. The sequence of addition of the substrates to aldehyde reductase is ordered and to aldose reductase is random, whereas for both the enzymes the release of product is ordered with NADP released last.     

拟南芥醛还原酶编码基因At3g04000表达模式分析及过量表达转基因植株获得

植物体内的α,β-不饱和活性醛类化合物对植物细胞具有毒害作用,清除这些α,β-不饱和活性醛类化合物对于植物细胞维持正常的生命活动至关重要。前人研究报道通过体外酶活测定和异源瞬时表达鉴定拟南芥 At3g04000基因编码的蛋白为 NADPH 依赖的叶绿体醛还原酶(Arabidopsis NADPH-dependent chloroplastic aldehyde reductases, AtChlADRs),推测其在清除叶绿体中长链(≥5)α,β-不饱和醛类物质中具有重要的功能。该研究主要构建了拟南芥 At3g04000基因的表达模式分析载体 ProAt3g04000：GUS、亚细胞定位分析载体At3g04000-EGFP 和过量表达载体 At3g04000-OE,并获得了转基因拟南芥,并通过实时定量 PCR 分析了At3g04000基因在拟南芥不同组织中的转录水平。结果表明：拟南芥 At3g04000基因在幼苗中的转录水平最高,在莲座叶、茎生叶、花序和角果中均有较高的转录水平;而在根部和茎秆中的转录水平较低。通过对ProAt3g04000：GUS 转基因植株的 GUS 染色分析可知,At3g04000基因在子叶、莲座叶和萼片的维管组织和保卫细胞中均有较强的表达,在根的维管组织中有较弱的表达。通过共聚焦显微镜对 At3g04000-EGFP 转基因植株的观察和分析发现,At3g04000不是定位于叶绿体中,而是定位在细胞质和细胞核中。该研究结果为深入研究拟南芥醛还原酶编码基因 At3g04000的功能奠定了基础。     

Cloning and expression of succinic semialdehyde reductase from human brain. Identity with aflatoxin B1 aldehyde reductase.

The neuromodulator gamma-hydroxybutyrate is synthesized in vivo from gamma-aminobutyrate by transamination to succinic semialdehyde and subsequent reduction of the aldehyde group. In human brain, succinic semialdehyde reductase is thought to be responsible for the conversion of succinic semialdehyde to gamma-hydroxybutyrate. In the present work, we cloned the cDNA coding for succinic semialdehyde reductase and expressed it in Escherichia coli. A data bank search indicated that the enzyme is identical with aflatoxin B1-aldehyde reductase, an enzyme implicated in the detoxification of xenobiotic carbonyl compounds. Structurally, succinic semialdehyde reductase thus belongs to the aldo-keto reductase superfamily. The recombinant protein was indistinguishable from native human brain succinic semialdehyde reductase by SDS/PAGE. In addition to succinic semialdehyde, it readily catalyzed the reduction 9,10-phenanthrene quinone, phenylglyoxal and 4-nitrobenzaldehyde, typical substrates of aflatoxin B1 aldehyde reductase. The results suggest multiple functions of succinic semialdehyde reductase/aflatoxin B1 aldehyde reductase in the biosynthesis of gamma-hydroxybutyrate and the detoxification of xenobiotic carbonyl compounds, respectively.     

Aldose reductase-catalyzed reduction of aldehyde phospholipids

Oxidation of unsaturated phospholipids results in the generation of aldehyde side chains that remain esterified to the phospholipid backbone. Such "core" aldehydes elicit immune responses and promote inflammation. However, the biochemical mechanisms by which phospholipid aldehydes are metabolized or detoxified are not well understood. In the studies reported here, we examined whether aldose reductase (AR), which reduces hydrophobic aldehydes, metabolizes phospholipid aldehydes. Incubation with AR led to the reduction of 5-oxovaleroyl, 7-oxo-5-heptenoyl, 5-hydroxy-6-oxo-caproyl, and 5-hydroxy-8-oxo-6-octenoyl phospholipids generated upon oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PAPC). The enzyme also catalyzed the reduction of phospholipid aldehydes generated from the oxidation of 1-alkyl, and 1-alkenyl analogs of PAPC, and 1-palmitoyl-2-arachidonoyl phosphatidic acid or phosphoglycerol. Aldose reductase catalyzed the reduction of chemically synthesized 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphatidylcholine (POVPC) with a K(m) of 10 mum. Addition of POVPC to the culture medium led to incorporation and reduction of the aldehyde in COS-7 and THP-1 cells. Reduction of POVPC in these cells was prevented by the AR inhibitors sorbinil and tolrestat and was increased in COS-7 cells overexpressing AR. Together, these observations suggest that AR may be a significant participant in the metabolism of several structurally diverse phospholipid aldehydes. This metabolism may be a critical regulator of the pro-inflammatory and immunogenic effects of oxidized phospholipids.     

Purification and properties of aldose reductase and aldehyde reductase II from human erythrocyte

Aldose reductase (EC 1.1.1.21) and aldehyde reductase II (L-hexonate dehydrogenase, EC 1.1.1.2) have been purified to homogeneity from human erythrocytes by using ion-exchange chromatography, chromatofocusing, affinity chromatography, and Sephadex gel filtration. Both enzymes are monomeric, Mr 32,500, by the criteria of the Sephadex gel filtration and polyacrylamide slab gel electrophoresis under denaturing conditions. The isoelectric pH's for aldose reductase and aldehyde reductase II were determined to be 5.47 and 5.06, respectively. Substrate specificity studies showed that aldose reductase, besides catalyzing the reduction of various aldehydes such as propionaldehyde, pyridine-3-aldehyde and glyceraldehyde, utilizes aldo-sugars such as glucose and galactose. Aldehyde reductase II, however, did not use aldo-sugars as substrate. Aldose reductase activity is expressed with either NADH or NADPH as cofactors, whereas aldehyde reductase II can utilize only NADPH. The pH optima for aldose reductase and aldehyde reductase II are 6.2 and 7.0, respectively. Both enzymes are susceptible to the inhibition by p-hydroxymercuribenzoate and N-ethylmaleimide. They are also inhibited to varying degrees by aldose reductase inhibitors such as sorbinil, alrestatin, quercetrin, tetramethylene glutaric acid, and sodium phenobarbital. The presence of 0.4 M lithium sulfate in the assay mixture is essential for the full expression of aldose reductase activity whereas it completely inhibits aldehyde reductase II. Amino acid compositions and immunological studies further show that erythrocyte aldose reductase is similar to human and bovine lens aldose reductase, and that aldehyde reductase II is similar to human liver and brain aldehyde reductase II.     

Reduced triphosphopyridine-liked aldehyde reductase. II. Species and tissue distribution

A 30,000 MW, barbiturate sensitive TPNH-linked aldehyde reductase which reduces aromatic aldehydes (3-pyridinecarboxaldehyde, 4-nitrobenzaldehyde, 4-cyanobenzaldehyde), d-glyceraldehyde, d-glucuronate and (+)-camphorquinone was found in liver, kidney, brain and heart tissue from a variety of animals. In livers, rabbit kidney, rabbit heart, and bovine brain a high molecular weight reductase was also found, which was less sensitive to barbiturate inhibition and had higher reactivity for cyclohexanone and d-ribose. The low molecular weight, TPNH-linked aldehyde reductases are probably homologous and should be classified under the systematic name alcohol:NADP oxidoreductase (EC 1.1.1.2). Since the substrate specificity of aldehyde reductase overlaps several previously described TPNH-linked reductases from the same tissues, reexamination of the properties of these enzymes for reclassification as EC 1.1.1.2 is necessary.     

Primary structure of three mannosyl-glycoasparagines and nine sialyl-oligosaccharides isolated from the urine of two patients with Gaucher''s disease (infantile form)

Initial-rate measurements were made of the reduction of pyridine-3-aldehyde and p-carboxybenzaldehyde by NADPH catalyzed by pig liver aldehyde reductase I. The initial velocity analysis and product inhibition data suggest that aldehyde reductase I obeys a compulsory-order mechanism with pyridine-3-aldehyde as substrate but follows a partially random-order pathway with p-carboxybenzaldehyde. The partially random-order pathway would be operative only at high concentrations of p-carboxybenzaldehyde. In both cases, aldehydes and the corresponding alcohol substrates inhibit the enzyme at high concentration. Abortive ternary complexes are shown to be formed with pyridine-3-aldehyde and with p-carboxybenzaldehyde. Dissociation of the coenzyme from the abortive ternary complex seems only to be observed with p-carboxybenzaldehyde. This study suggests overall that an enzyme kinetic mechanism may be different, depending on whether specific interactions can occur between certain amino acid residue(s) of the protein active site and substrates. Finally, the mechanism of the inhibition of pyridine-3-aldehyde reduction by diacid derivatives is discussed.     

Localization, isolation and properties of three NADPH-dependent aldehyde reducing enzymes from dog kidney

Three kinds of NADPH-dependent aldehyde reducing enzymes were present in the dog kidney. Aldose reductase was located in the inner medulla region and aldehyde reductase in all regions of the renal cortex, outer medulla and inner medulla. In addition, a new reductase designated tentatively as high-Km aldose reductase, which was converted into an aldose reductase-like enzyme, was present in the inner medulla region of the kidney. Aldose reductase, aldehyde reductase and high-Km aldose reductase were purified to homogeneity from each region of the dog kidney. The molecular weight of aldose reductase was estimated to be 38,500 by SDS-polyacrylamide gel electrophoresis and the isoelectric point was found to be 5.7 by chromatofocusing. Aldose reductase had activity for aldo-sugars such as D-xylose, D-glucose and D-galactose as substrates and utilized both NADPH and NADH as coenzymes. Sulfate ions resulted in over 2-fold activation of aldose reductase. All aldehyde reductases from the three regions had the same properties. The molecular weights and isoelectric points of aldehyde reductases were 40,000 and 6.1, respectively. The aldehyde reductases were inactive for D-hexose, utilized only NADPH as coenzyme and were not affected by sulfate ions. High-Km aldose reductase had a molecular weight of 38,500 and an isoelectric point of 5.4. It had activity for aldo-sugars, but showed much higher Km and lower kcat/Km values than aldose reductase. Sulfate ions inhibited high-Km aldose reductase. It was converted into an aldose reductase-like enzyme by incubation in phosphate buffer at pH 7.0. The three kinds of enzymes were strongly inhibited by the known aldose reductase inhibitors. However, aldehyde reductase and high-Km aldose reductase were, in general, less susceptible than aldose reductase.     

Kinetics and mechanism of action of aldehyde reductase from pig kidney.

An improved procedure for purifying aldehyde reductase is described. Utilization of Blue Dextran--Sepharose 4B and elimination of hydroxyapatite chromatography greatly improves the yield and ease of purification. Starting with 340 g of kidney tissue (two pig kidneys) approx. 50 mg of purified reductase may be routinely and reproducibly obtained. The purified reductase was used to establish the kinetic reaction mechanism of the enzyme. Initial-velocity analysis and product-inhibition data revealed that pig kidney aldehyde reductase follows an Ordered Bi Bi reaction mechanism in which NADPH binds first before D-glyceraldehyde. The limiting Michaelis constants for D-glyceraldehyde and NADPH were 4.8 +/- 0.7 mM and 9.1 +/- 2.1 micrometer respectively. The mechanism is similar to that of another monomeric oxidoreductase, octopine dehydrogenase, towards which aldehyde reductase exhibits several similarities, but differs from that of other aldehyde reductases. Phenobarbital is a potent inhibitor of aldehyde reductase, inhibiting both substrate and cofactor non-competitively (Ki = 80.4 +/- 10.5 micrometer and 66.9 +/- 1.6 micrometer respectively). Barbiturate inhibition seems to be a common property of NADPH-dependent aldehyde reductases.     

Purification and properties of an NADPH-linked aldehyde reductase from rat kidney

Rat kidney was shown to contain two NADPH-linked aldehyde reductases (alcohol:NADP+) oxidoreductase, EC 1.1.1.2) with different substrate affinities. The high-Km aldehyde reductase, which was purified to apparent homogeneity, had a molecular weight of 32 000 as determined by Sephadex G-100 gel filtration, and of 37 000 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The purified enzyme reduced various aliphatic aldehydes of different carbon-chain lengths besides many chemicals containing aldehyde groups. The Km values for n-hexadecanal and n-octadecanal were 8 microM and 4 microM, respectively. Bovine serum albumin (1.8 mM) stimulated the reduction of n-hexadecanal and n-octadecanal, and increased the Vmax values by about 15-fold without changing the Km values. The kidney enzyme was not distinguishable from the brain and liver high-Km aldehyde reductases in mobility on polyacrylamide gel electrophoresis, immunological properties, peptide maps or substrate specificity.     

A functional arginine residue in NADPH-dependent aldehyde reductase from pig kidney.

Pig kidney aldehyde reductase is inactivated by 2,3-butanedione, phenylglyoxal, methylglyoxal, and 1,2-cyclohexanedione. 2,3-Butanedione caused the most rapid loss in enzyme activity, the rate of loss being proportional to the concentration of 2,3-butanedione. Neither D-glyceraldehyde nor pyridine 3-aldehyde, both substrates for this broadly specific enzyme, protected the enzyme from inactivation but 1 mM NADPH or NADP completely prevented the loss of activity by 2,3-butanedione suggesting the involvement of arginine in the binding of cofactor. Nicotinamide mononucleotide (NMN) (reduced form) offered no protection to inactivation whereas ADP-ribose phosphate gave complete protection indicating that it is the latter portion of NADPH which interacts with the essential arginine. Both NMN and ADP-ribose phosphate are competitive inhibitors of aldehyde reductase with respect to NADPH. Butanedione-modified aldehyde reductase could still bind to a blue dextran-Sepharose 4B column suggesting that the modified arginine did not bind NADPH. This was confirmed by fluorescence spectra which showed that chemically modified aldehyde reductase caused the same blue shift of NADPH fluorescence as did native aldehyde reductase. Of additional interest was the quenching of NADPH fluorescence by aldehyde reductase which, with one exception, is in contrast to the fluorescence behavior of all other oxidoreductases.     

Phylogeny and evolution of aldehyde dehydrogenase-homologous folate enzymes

Folate coenzymes function as one-carbon group carriers in intracellular metabolic pathways. Folate-dependent reactions are compartmentalized within the cell and are catalyzed by two distinct groups of enzymes, cytosolic and mitochondrial. Some folate enzymes are present in both compartments and are likely the products of gene duplications. A well-characterized cytosolic folate enzyme, FDH (10-formyltetrahydro-folate dehydrogenase, ALDH1L1), contains a domain with significant sequence similarity to aldehyde dehydrogenases. This domain enables FDH to catalyze the NADP(+)-dependent conversion of short-chain aldehydes to corresponding acids in vitro. The aldehyde dehydrogenase-like reaction is the final step in the overall FDH mechanism, by which a tetrahydrofolate-bound formyl group is oxidized to CO(2) in an NADP(+)-dependent fashion. We have recently cloned and characterized another folate enzyme containing an ALDH domain, a mitochondrial FDH. Here the biological roles of the two enzymes, a comparison of the respective genes, and some potential evolutionary implications are discussed. The phylogenic analysis suggests that the vertebrate ALDH1L2 gene arose from a duplication event of the ALDH1L1 gene prior to the emergence of osseous fish >500 millions years ago.