Ethanol formation in adh0 mutants reveals the existence of a novel acetaldehyde-reducing activity in Saccharomyces cerevisiae.

A strain of Saccharomyces cerevisiae has been constructed which is deficient in the four alcohol dehydrogenase (ADH) isozymes known at present. This strain (adh0), being irreversibly mutated in the genes ADH1, ADH3, and ADH4 and carrying a point mutation in the gene ADH2 coding for the glucose-repressible isozyme ADHII, still produces up to one third of the theoretical maximum yield of ethanol in a homofermentative conversion of glucose to ethanol. Analysis of the glucose metabolism of adh0 cells shows that the lack of all known ADH isozymes results in the formation of glycerol as a major fermentation product, accompanied by a significant production of acetaldehyde and acetate. Treatment of glucose-growing adh0 cells with the respiratory-chain inhibitor antimycin A leads to an immediate cessation of ethanol production, demonstrating that ethanol production in adh0 cells is dependent on mitochondrial electron transport. Reduction of acetaldehyde to ethanol in isolated mitochondria could also be demonstrated. This reduction is apparently linked to the oxidation of acetaldehyde to acetate. Preliminary data suggest that this novel type of ethanol formation in S. cerevisiae is associated with the inner mitochondrial membrane.     

Aldehyde dehydrogenase is essential for both adult and larval ethanol resistance in Drosophila melanogaster

The enzyme aldehyde dehydrogenase (ALDH) is essential for ethanol metabolism in mammals, converting the highly toxic intermediate acetaldehyde to acetate. The role of ALDH in Drosophila has been debated, with some authors arguing that, at least in larvae, acetaldehyde detoxification is carried out mainly by alcohol dehydrogenase (ADH), the enzyme responsible for converting ethanol to acetaldehyde. Here, we report the creation and characterization of four null mutants of Aldh, the putative structural locus for ALDH. Aldh null larvae and adults are poisoned by ethanol concentrations easily tolerated by wild-types; their ethanol sensitivity is in fact comparable to that of Adh nulls. The results refute the view that ALDH plays only a minor role in ethanol detoxification in larvae, and suggest that Aldh and Adh may be equally important players in the evolution of ethanol resistance in fruit-breeding Drosophila.     

Effects of ADH2 overexpression in Saccharomyces bayanus during alcoholic fermentation

The effect of overexpression of the gene ADH2 on metabolic and biological activity in Saccharomyces bayanus V5 during alcoholic fermentation has been evaluated. This gene is known to encode alcohol dehydrogenase II (ADH II). During the biological aging of sherry wines, where yeasts have to grow on ethanol owing to the absence of glucose, this isoenzyme plays a prominent role by converting the ethanol into acetaldehyde and producing NADH in the process. Overexpression of the gene ADH2 during alcoholic fermentation has no effect on the proteomic profile or the net production of some metabolites associated with glycolysis and alcoholic fermentation such as ethanol, acetaldehyde, and glycerol. However, it affects indirectly glucose and ammonium uptakes, cell growth, and intracellular redox potential, which lead to an altered metabolome. The increased contents in acetoin, acetic acid, and L-proline present in the fermentation medium under these conditions can be ascribed to detoxification by removal of excess acetaldehyde and the need to restore and maintain the intracellular redox potential balance.     

Distinct retinoid metabolic functions for alcohol dehydrogenase genes Adh1 and Adh4 in protection against vitamin A toxicity or deficiency revealed in double null mutant mice

The ability of class I alcohol dehydrogenase (ADH1) and class IV alcohol dehydrogenase (ADH4) to metabolize retinol to retinoic acid is supported by genetic studies in mice carrying Adh1 or Adh4 gene disruptions. To differentiate the physiological roles of ADH1 and ADH4 in retinoid metabolism we report here the generation of an Adh1/4 double null mutant mouse and its comparison to single null mutants. We demonstrate that loss of both ADH1 and ADH4 does not have additive effects, either for production of retinoic acid needed for development or for retinol turnover to minimize toxicity. During gestational vitamin A deficiency Adh4 and Adh1/4 mutants exhibit completely penetrant postnatal lethality by day 15 and day 24, respectively, while 60% of Adh1 mutants survive to adulthood similar to wild-type. Following administration of a 50-mg/kg dose of retinol to examine retinol turnover, Adh1 and Adh1/4 mutants exhibit similar 10-fold decreases in retinoic acid production, whereas Adh4 mutants have only a slight decrease. LD(50) studies indicate a large increase in acute retinol toxicity for Adh1 mutants, a small increase for Adh4 mutants, and an intermediate increase for Adh1/4 mutants. Chronic retinol supplementation during gestation resulted in 65% postnatal lethality in Adh1 mutants, whereas only approximately 5% for Adh1/4 and Adh4 mutants. These studies indicate that ADH1 provides considerable protection against vitamin A toxicity, whereas ADH4 promotes survival during vitamin A deficiency, thus demonstrating largely non-overlapping functions for these enzymes in retinoid metabolism.     

Characterization of alcohol dehydrogenase 3 of the thermotolerant methylotrophic yeast Hansenula polymorpha

In this study, we identified and characterized mitochondrial alcohol dehydrogenase 3 from the thermotolerant methylotrophic yeast Hansenula polymorpha (HpADH3). The amino acid sequence of HpADH3 shares over 70% of its identity with the alcohol dehydrogenases of other yeasts and exhibits the highest similarity of 91% with the alcohol dehydrogenase 1 of H. polymorpha. However, unlike the cytosolic HpADH1, HpADH3 appears to be a mitochondrial enzyme, as a mitochondrial targeting extension exists at its N terminus. The recombinant HpADH3 overexpressed in Escherichia coli showed similar catalytic efficiencies for ethanol oxidation and acetaldehyde reduction. The HpADH3 displayed substrate specificities with clear preferences for medium chain length primary alcohols and acetaldehyde for an oxidation reaction and a reduction reaction, respectively. Although the H. polymorpha ADH3 gene was induced by ethanol in the culture medium, both an ADH isozyme pattern analysis and an ADH activity assay indicated that HpADH3 is not the major ADH in H. polymorpha DL-1. Moreover, HpADH3 deletion did not affect the cell growth on different carbon sources. However, when the HpADH3 mutant was complemented by an HpADH3 expression cassette fused to a strong constitutive promoter, the resulting strain produced a significantly increased amount of ethanol compared to the wild-type strain in a glucose medium. In contrast, in a xylose medium, the ethanol production was dramatically reduced in an HpADH3 overproduction strain compared to that in the wild-type strain. Taken together, our results suggest that the expression of HpADH3 would be an ideal engineering target to develop H. polymorpha as a substrate specific bioethanol production strain.     

ADH enzyme activity and Adh gene expression in Drosophila melanogaster lines differentially selected for increased alcohol tolerance

In Drosophila melanogaster, alcohol dehydrogenase (ADH) activity is essential for ethanol tolerance, but its role may not be restricted to alcohol metabolism alone. Here we describe ADH activity and Adh expression level upon selection for increased alcohol tolerance in different life-stages of D. melanogaster lines with two distinct Adh genotypes: Adh(FF) and Adh(SS). We demonstrate a positive within genotype response for increased alcohol tolerance. Life-stage dependent selection was observed in larvae only. A slight constitutive increase in adult ADH activity for all selection regimes and genotypes was observed, that was not paralleled by Adh expression. Larval Adh expression showed a constitutive increase, that was not reflected in ADH activity. Upon exposure to environmental ethanol, sex, selection regime life stage and genotype appear to have differential effects. Increased ADH activity accompanies increased ethanol tolerance in D. melanogaster but this increase is not paralleled by expression of the Adh gene.     

Reduction of furan derivatives by overexpressing NADH-dependent Adh1 improves ethanol fermentation using xylose as sole carbon source with Saccharomyces cerevisiae harboring XR–XDH pathway

Several alcohol dehydrogenase (ADH)-related genes have been identified as enzymes for reducing levels of toxic compounds, such as, furfural and/or 5-hydroxymethylfurfural (5-HMF), in hydrolysates of pretreated lignocelluloses. To date, overexpression of these ADH genes in yeast cells have aided ethanol production from glucose or glucose/xylose mixture in the presence of furfural or 5-HMF. However, the effects of these ADH isozymes on ethanol production from xylose as a sole carbon source remain uncertain. We showed that overexpression of mutant NADH-dependent ADH1 derived from TMB3000 strain in the recombinant Saccharomyces cerevisiae, into which xylose reductase (XR) and xylitol dehydrogenase (XDH) pathway of Pichia stipitis has been introduced, improved ethanol production from xylose as a sole carbon source in the presence of 5-HMF. Enhanced furan-reducing activity is able to regenerate NAD⁺ to relieve redox imbalance, resulting in increased ethanol yield arising from decreased xylitol accumulation. In addition, we found that overexpression of wild-type ADH1 prevented the more severe inhibitory effects of furfural in xylose fermentation as well as overexpression of TMB3000-derived mutant. After 120 h of fermentation, the recombinant strains overexpressing wild-type and mutant ADH1 completely consumed 50 g/L xylose in the presence of 40 mM furfural and most efficiently produced ethanol (15.70 g/L and 15.24 g/L) when compared with any other test conditions. This is the first report describing the improvement of ethanol production from xylose as the sole carbon source in the presence of furan derivatives with xylose-utilizing recombinant yeast strains via the overexpression of ADH-related genes.     

Synthesis of Drosophila melanogaster alcohol dehydrogenase in yeast

Expression systems for the heterologous expression of Drosophila melanogaster alcohol dehydrogenase (ADH) in Saccharomyces cerevisiae have been designed, analyzed and compared. Four different yeast/Escherichia coli shuttle vectors were constructed and used to transform four different yeast strains. Expression was detectable in ADH- yeast strains, from either a constitutive promoter, yeast ADH1 promoter (ADCp), or a regulated promoter, yeast GALp. The highest amount of D. melanogaster ADH was obtained from a multicopy plasmid with the D. melanogaster Adh gene expressed constitutively under the control of yeast ADCp promoter. The D. melanogaster enzyme was produced in cell extracts, as assessed by Coomassie blue staining and Western blotting after polyacrylamide-gel electrophoresis and it was fully active and able to complement the yeast ADH deficiency. Results show that D. melanogaster ADH subunits synthesized in yeast are able to assemble into functional dimeric forms. The synthesized D. melanogaster ADH represents up to 3.5% of the total extracted yeast protein.     

Coproduction of acetaldehyde and hydrogen during glucose fermentation by Escherichia coli

Escherichia coli K-12 strain MG1655 was engineered to coproduce acetaldehyde and hydrogen during glucose fermentation by the use of exogenous acetyl-coenzyme A (acetyl-CoA) reductase (for the conversion of acetyl-CoA to acetaldehyde) and the native formate hydrogen lyase. A putative acetaldehyde dehydrogenase/acetyl-CoA reductase from Salmonella enterica (SeEutE) was cloned, produced at high levels, and purified by nickel affinity chromatography. In vitro assays showed that this enzyme had both acetaldehyde dehydrogenase activity (68.07 ± 1.63 μmol min(-1) mg(-1)) and the desired acetyl-CoA reductase activity (49.23 ± 2.88 μmol min(-1) mg(-1)). The eutE gene was engineered into an E. coli mutant lacking native glucose fermentation pathways (ΔadhE, ΔackA-pta, ΔldhA, and ΔfrdC). The engineered strain (ZH88) produced 4.91 ± 0.29 mM acetaldehyde while consuming 11.05 mM glucose but also produced 6.44 ± 0.26 mM ethanol. Studies showed that ethanol was produced by an unknown alcohol dehydrogenase(s) that converted the acetaldehyde produced by SeEutE to ethanol. Allyl alcohol was used to select for mutants with reduced alcohol dehydrogenase activity. Three allyl alcohol-resistant mutants were isolated; all produced more acetaldehyde and less ethanol than ZH88. It was also found that modifying the growth medium by adding 1 g of yeast extract/liter and lowering the pH to 6.0 further increased the coproduction of acetaldehyde and hydrogen. Under optimal conditions, strain ZH136 converted glucose to acetaldehyde and hydrogen in a 1:1 ratio with a specific acetaldehyde production rate of 0.68 ± 0.20 g h(-1) g(-1) dry cell weight and at 86% of the maximum theoretical yield. This specific production rate is the highest reported thus far and is promising for industrial application. The possibility of a more efficient "no-distill" ethanol fermentation procedure based on the coproduction of acetaldehyde and hydrogen is discussed.     

Molecular analysis of alcohol dehydrogenase electromorphs in wild type and transformed Drosophila melanogaster

The protein expressed by the alcohol dehydrogenase locus (Adh) in D. melanogaster comprises a small group of electromorphs. We are able to study the expression of these electromorphs by electrophoretic separation and subsequent probing of blots of the separated polypeptides with antiserum for alcohol dehydrogenase (ADH). In the present study we have utilized this technique to study and compare the ADH electromorphs in wild type D. melanogaster with D. melanogaster transformants which carry an Adh gene from D. grimshawi, D. hawaiiensis or D. affinidisjuncta and produced functional ADH (10, 19). We have determined that polypeptides are produced by the donor loci in the transformed flies and further show that although the molecular weight of the expressed polypeptides is similar to D. melanogaster electromorphs, the isoelectric points are not similar. Thus this methodology offers the potential to study naturally occurring ADH electromorphs and null alleles independent of enzymatic activity assays.     

Effects of differential alcohol dehydrogenase activity on the developmental toxicity of ethanol in Drosophila melanogaster

The role of alcohol dehydrogenase (ADH) activity in ethanol toxicity was investigated in Drosophila melanogaster. Flies from three congenic Adh strains (high, medium, and low ADH activity) were allowed to deposit eggs on medium containing 0, 4, or 8% ethanol. The resulting larvae were allowed to complete their development in the medium, and emerging flies were examined for defects. Flies with high ADH activity had malformation incidences of 0.8, 2.4, and 5.2% at 0, 4, and 8% ethanol, respectively. The comparable incidences for the low ADH strain were 1.0, 4.1, and 8.4%, while those for the medium ADH strain were intermediate in value. These results indicate that ethanol teratogenesis may be inversely related to ADH activity. When larvae were treated with ethanol for different lengths of time during development, the incidence of defects in flies from the high ADH strain was 3.9% when exposure started at the first instar and 3.09% when exposure started at the third instar. Results of the same exposures for the intermediate ADH strain were 5.2 and 3.4%, respectively, while those for the low ADH strain were 6.9 and 5.5%, respectively. Thus, length of ethanol exposure was directly related to the increased incidence of malformations in all tested Drosophila strains. For all tested strains, defect incidences appeared to be dose-related as well, regardless of length of exposure. ADH in Drosophila has a dual function and thus can catalyze oxidation of both ethanol and its toxic metabolite, acetaldehyde. This suggests that ethanol is the proximate teratogen in Drosophila.     

The catabolism of amino acids to long chain and complex alcohols in Saccharomyces cerevisiae

The catabolism of phenylalanine to 2-phenylethanol and of tryptophan to tryptophol were studied by (13)C NMR spectroscopy and gas chromatography-mass spectrometry. Phenylalanine and tryptophan are first deaminated (to 3-phenylpyruvate and 3-indolepyruvate, respectively) and then decarboxylated. This decarboxylation can be effected by any of Pdc1p, Pdc5p, Pdc6p, or Ydr380wp; Ydl080cp has no role in the catabolism of either amino acid. We also report that in leucine catabolism Ydr380wp is the minor decarboxylase. Hence, all amino acid catabolic pathways studied to date use a subtly different spectrum of decarboxylases from the five-membered family that comprises Pdc1p, Pdc5p, Pdc6p, Ydl080cp, and Ydr380wp. Using strains containing all possible combinations of mutations affecting the seven AAD genes (putative aryl alcohol dehydrogenases), five ADH genes, and SFA1, showed that the final step of amino acid catabolism (conversion of an aldehyde to a long chain or complex alcohol) can be accomplished by any one of the ethanol dehydrogenases (Adh1p, Adh2p, Adh3p, Adh4p, Adh5p) or by Sfa1p (formaldehyde dehydrogenase.)     

Correct developmental expression of a cloned alcohol dehydrogenase gene transduced into the Drosophila germ line

We have used P-element-mediated transformation to introduce a cloned Drosophila alcohol dehydrogenase (Adh) gene into the germ line of ADH null flies. Six independent transformants expressing ADH were identified by their acquired resistance to ethanol. Each transformant carries a single copy of the cloned Adh gene in a different chromosomal location. Four of the six transformant lines exhibit normal Adh expression by the following criteria: quantitative levels of ADH enzyme activity in larvae and adults; qualitative tissue specificity; the size of stable Adh mRNA; and the characteristic developmental switch in utilization of two different Adh promoters. The remaining two transformants express ADH enzyme activity with the correct tissue specificity, but at a lower level than wild type. These results demonstrate that an 11.8 kb chromosomal fragment containing the Adh gene includes the cis-acting sequences necessary for its correct developmental expression, and that a variety of chromosomal sites permit proper Adh gene function.     

Retinol/ethanol drug interaction during acute alcohol intoxication in mice involves inhibition of retinol metabolism to retinoic acid by alcohol dehydrogenase

Substantial evidence indicates that one consequence of alcohol intoxication is a reduction in retinoic acid (RA) levels. Studies on the mechanism have shown that chronic ethanol consumption induces P450 enzymes that increase RA degradation, thus accounting for much but not all of the observed decrease in RA. A reduction in RA synthesis may also be involved as ethanol competitively inhibits retinol oxidation catalyzed by alcohol dehydrogenase (ADH) in vitro. This may be important during acute ethanol intoxication and may contribute to adverse retinol/ethanol drug interactions. Here we have examined mice for the effect of either acute ethanol intoxication or Adh1 gene disruption on RA synthesis and degradation. RA produced following a dose of retinol (50 mg/kg) was reduced 87% by pretreatment with an intoxicating dose of ethanol (3.5 g/kg). RA produced in Adh1-null mutant mice following a 50-mg/kg dose of retinol was reduced 82% relative to wild-type mice, thus similar to wild-type mice pretreated with ethanol. Reduced RA production was associated with increased retinol levels in both ethanol-treated wild-type mice and Adh1-null mutant mice, indicating reduced clearance of the retinol dose. RA degradation following a dose of RA (10 mg/kg) was increased only 42% by ethanol pretreatment (3.5 g/kg) and only 26% in Adh1-null mutant mice relative to wild-type mice. These findings demonstrate that the reduced RA levels observed during acute retinol/ethanol drug interaction are due primarily to a decrease in ADH-catalyzed RA synthesis and secondarily to an increase in RA degradation.     

Acetaldehyde coenzyme A dehydrogenase of Escherichia coli.

Mutants of Escherichia coli (adh) in which alcohol dehydrogenase is derepressed under aerobic conditions were also found to overproduce acetaldehyde coenzyme a dehydrogenase. However, acetaldehyde coenzyme A dehydrogenase was induced by ethanol or acetaldehyde and subject to strong catabolite repression, whereas alcohol dehydrogenase was little affected by these conditions. Mutants no longer able to use ethanol as carbon source were isolated from an adh strain. Some of these mutants were revertants at the adh locus and no longer produced either alcohol dehydrogenase or acetaldehyde coenzyme A dehydrogenase. Others, designated acd, were found to lack only acetaldehyde coenzyme A dehydrogenase. The acd mutation was located at min 62 of the E. coli genetic map, the gene order being thyA-lysA-acd-serA-fda. Isolation of Tn10 insertions cotransducible with acd greatly simplified the mapping procedure.     

Characterization of alcohol dehydrogenase 1 of the thermotolerant methylotrophic yeast Hansenula polymorpha

The thermotolerant methylotrophic yeast Hansenula polymorpha has recently been gaining interest as a promising host for bioethanol production due to its ability to ferment xylose, glucose, and cellobiose at elevated temperatures up to 48 °C. In this study, we identified and characterized alcohol dehydrogenase 1 of H. polymorpha (HpADH1). HpADH1 seems to be a cytoplasmic protein since no N-terminal mitochondrial targeting extension was detected. Compared to the ADHs of other yeasts, recombinant HpADH1 overexpressed in Escherichia coli exhibited much higher catalytic efficiency for ethanol oxidation along with similar levels of acetaldehyde reduction. HpADH1 showed broad substrate specificity for alcohol oxidation but had an apparent preference for medium chain length alcohols. Both ADH isozyme pattern analysis and ADH activity assay indicated that ADH1 is the major ADH in H. polymorpha DL-1. Moreover, an HpADH1-deleted mutant strain produced less ethanol in glucose or glycerol media compared to wild-type. Interestingly, when the ADH1 mutant was complemented with an HpADH1 expression cassette, the resulting strain produced significantly increased amounts of ethanol compared to wild-type, up to 36.7 g l⁻¹. Taken together, our results suggest that optimization of ADH1 expression would be an ideal method for developing H. polymorpha into an efficient bioethanol production strain.     

Selective recruitment of <Emphasis Type="Italic">Adh</Emphasis> genes for distinct enzymatic functions in <Emphasis Type="Italic">Petunia hybrida</Emphasis>

Alcohol dehydrogenase (ADH) activity in plants is generally associated with glycolytic fermentation, which facilitates cell survival during episodes of low-oxygen stress in water-logged roots as well as chronically hypoxic regions surrounding the vascular core. Work with tobacco and potato has implicated ADH activity in additional metabolic roles, including aerobic fermentation, acetaldehyde detoxification and carbon reutilization. Here a combination of approaches has been used to examine tissue-specific patterns of Adh gene expression in order to provide insight into the potential roles of alcohol dehydrogenases, using Petunia hybrida, a solanaceous species with well-characterized genetics. A reporter-gene study, relying on the promoters of Adh1 and Adh2 to drive expression of the gene for a green fluorescent protein derivative, mgfp5, revealed unexpectedly complex patterns of GFP fluorescence in floral tissues, particularly the stigma, style and nectary. Results of GC-MS analysis suggest the association of ADH with production of aromatic compounds in the nectary. Overall the results demonstrate selective recruitment of Adh gene family members in tissues and organs associated with diverse ADH functions.     

A distinct type of alcohol dehydrogenase, adh4+, complements ethanol fermentation in an adh1-deficient strain of Schizosaccharomyces pombe

In the fission yeast Schizosaccharomyces pombe, only one alcohol dehydrogenase gene, adh1(+), has been identified. To elucidate the influence of adh1(+) on ethanol fermentation, we constructed the adh1 null strain (delta adh1). The delta adh1 cells still produced ethanol and grew fermentatively as the wild-type cells. Both DNA microarray and RT-PCR analysis demonstrated that this ethanol production is caused by the enhanced expression of a Saccharomyces cerevisiae ADH4-like gene product (SPAC5H10.06C named adh4(+)). Since the strain lacking both adh1 and adh4 genes (delta adh1 delta adh4) showed non-fermentative retarded growth, only these two ADHs produce ethanol for fermentative growth. This is the first observation that a S. cerevisiae ADH4-like alcohol dehydrogenase functions in yeast ethanol fermentation.