cDNA Sequence and Catalytic Properties of a Chick Embryo Alcohol Dehydrogenase That Oxidizes Retinol and 3beta ,5alpha -Hydroxysteroids*

(Received for publication, October 16, 1996, and in revised form, December 17, 1996)

Natalia Y. Kedishvili Dagger §, Wendy H. Gough Dagger , Ellen A. G. Chernoff , Thomas D. Hurley Dagger , Carol L. Stone Dagger , Kenneth D. Bowman Dagger , Kirill M. Popov Dagger , William F. Bosron Dagger and Ting-Kai Li par

From the Dagger  Department of Biochemistry and Molecular Biology and the par  Department of Medicine, School of Medicine, Indiana University and the  Department of Biology, School of Science, Purdue University, Indianapolis, Indiana 46202-5122

ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
FOOTNOTES
REFERENCES


ABSTRACT

This study was undertaken to identify the cytosolic 40-kDa zinc-containing alcohol dehydrogenases that oxidize all-trans-retinol and steroid alcohols in fetal tissues. Degenerate oligonucleotide primers were used to amplify by polymerase chain reaction 500-base pair fragments of alcohol dehydrogenase cDNAs from chick embryo limb buds and heart. cDNA fragments that encode an unknown putative alcohol dehydrogenase as well as the class III alcohol dehydrogenase were identified. The new cDNA hybridized with two messages of ~2 and 3 kilobase pairs in the adult chicken liver but not in the adult heart, muscle, testis, or brain. The corresponding complete cDNA clones with a total length of 1390 base pairs were isolated from a chicken liver lambda gt11 cDNA library. The open reading frame encoded a 375-amino acid polypeptide that exhibited 67 and 68% sequence identity with chicken class I and III alcohol dehydrogenases, respectively, and had lower identity with mammalian class II (55-58%) and IV (62%) isozymes. Expression of the new cDNA in Escherichia coli yielded an active alcohol dehydrogenase (ADH-F) with subunit molecular mass of ~40 kDa. The specific activity of the recombinant enzyme, calculated from active site titration of NADH binding, was 3.4 min-1 for ethanol at pH 7.4 and 25 °C. ADH-F was stereospecific for the 3beta ,5alpha - versus 3beta ,5beta -hydroxysteroids. The Km value for ethanol at pH 7.4 was 17 mM compared with 56 µM for all-trans-retinol and 31 µM for epiandrosterone. Antiserum against ADH-F recognized corresponding protein in the chicken liver homogenate. We suggest that ADH-F represents a new class of alcohol dehydrogenase, class VII, based on its primary structure and catalytic properties.


INTRODUCTION

Cytosolic zinc-containing alcohol dehydrogenases (ADH)1 with 40-kDa subunits are capable of oxidizing a variety of primary, secondary, and aliphatic alcohols and a limited number of cyclic alcohols (1). Six classes of dimeric ADH isozymes have been identified in mammals (1). Except for class I, all other classes of ADH have high Km values for ethanol and oxidize medium-chain and long-chain alcohols most effectively.

Potential physiological substrates for ADH isozymes include retinoid and steroid alcohols. Human class IV is the most efficient retinol dehydrogenase, followed by class II and the class I alpha alpha ADH (2). Class IV and II ADH are not active with steroid alcohols, whereas class I isozymes oxidize both retinoid and steroid substrates with relatively low catalytic efficiency. Class I isozymes2 exhibit stereospecificity toward alcohol substrates. For example, horse SS and human gamma gamma ADH oxidize 3beta -hydroxysteroids but not 3alpha -hydroxysteroids (3). ADH isozymes vary in their tissue distribution; class IV ADH is expressed in the epithelial tissues of mammals and is present in the human stomach mucosa and esophagus (1), whereas class II pi pi is found in fetal and adult liver. Class I isozymes beta 1beta 1, beta 2beta 2, beta 3beta 3, gamma 1gamma 1, gamma 2gamma 2, alpha alpha , and their heterodimers, as well as class II pi pi , are the predominant forms responsible for ethanol oxidation in the human adult liver. Class I isozymes are also expressed to a lesser extent in certain adult and fetal tissues, such as kidney, skin, gastrointestinal tract, and lung.

The physiological significance of the cytosolic ADHs for steroid and retinoid metabolism is not clear. The retinoid and steroid hormones play a major role in fetal development and are detected in the embryonal tissues during the early developmental stages. Since ADH isozymes appear at different stages of embryogenesis, it is important to determine which isozymes are present in the embryo during the stages when retinoid and steroid hormones are synthesized. The chick embryo is used as a model to study the effects of various hormones on gene expression during development. In this study, we analyzed the mRNA isolated from the chick fetal heart and limb buds at stage 21 for the presence of messages encoding cytosolic ADH.


EXPERIMENTAL PROCEDURES

PCR Amplification of ADH Isozymes

Degenerate oligonucleotide primers were synthesized based on the peptide sequences E(D/E)(I/V)EVAP and FGLGGVG, which are conserved in all animal alcohol dehydrogenases (4). The first region corresponds to amino acids 24-30 (sense primer) and the second region to amino acids 198-204 (antisense primer) of the human class I beta 1 ADH. Four oligonucleotides were synthesized for the sense orientation: 1), GA(G/A)GA(T/C)GTIGA(G/ A)GTIGCICC; 2), GA(G/A)GA(T/C)AT(A/C/T)GA(G/A)GTIGCICC; 3), GA(G/A)GA(G/A)GTIGA(G/A)GTIGCICC; and 4), GA(G/A)GA(G/A)AT(A/C/T)GA(G/A)GTIGCICC. Two oligonucleotides were synthesized for the antisense orientation: 1), TT(T/C)GGICTIGGIGGIGTIGG; and 2), TT(T/C)GGITT(A/G)GGIGGIGTIGG. Inosines were incorporated in all positions that required a degeneracy of 4. Limb buds and hearts were dissected from 30 chick stage-21 embryos (3 days old). Total RNA was isolated from the pooled limb buds and from the pooled hearts (RNAzol, Cinna/Biotecx Laboratories, Inc., Houston, TX). The total RNA from each pool was reverse transcribed and used for PCR amplification of ADH isozymes. PCR was performed with various combinations of 4 sense and 2 antisense primers for 30 cycles with annealing at 50 °C (1 min), extension at 72 °C (2 min), and denaturing at 94 °C (1 min). Several combinations of primers produced a ~500-bp product. Each ~500-bp band was isolated, reamplified, and subcloned into M13mp19RF (Life Technologies, Inc.) vector for sequencing (U. S. Biochemical Corp.).

Northern Blot Analysis and Screening of the cDNA Library

The liver, kidney, lung, heart, brain, skeletal muscle, and bladder tissues were dissected from a 7-week-old chicken and frozen immediately in liquid nitrogen. Total RNA was isolated from each tissue with RNAzol according to the manufacturer's protocol. Twenty micrograms of each RNA preparation were loaded onto a formaldehyde-agarose gel and separated by electrophoresis. After transfer to the Nytran filter (Shleicher & Schuell), the separated mRNAs were hybridized with the [alpha -32P]dATP-labeled ~500-bp PCR product in 50% formamide, 5 × Denhardt's solution, 5 × saline/sodium/phosphate/EDTA, 0.1 mg/ml salmon sperm DNA, and 0.1% SDS at 42 °C overnight. After hybridization, the filter was washed several times in 2 × SSC, 0.1% SDS at room temperature, and the final wash was performed in 0.1 × SSC, 0.1% SDS at 65 °C for 30 min.

A chicken liver lambda gt11 cDNA library (Clontech) was screened with the radiolabeled ~500-bp PCR product. The hybridization and washing conditions were the same as those described for the Northern blot analysis. Positive plaques were purified through three more rounds of screening. The purified lambda  phage was cleaved with EcoRI restriction endonuclease, and the cDNA insert was isolated and subcloned into M13mp19RF digested with EcoRI. Sense and antisense single-stranded M13 DNA were prepared, and each was sequenced at least three times.

Expression in Escherichia coli

The coding region of the new ADH cDNA was amplified by PCR with the following primers: CTCA<UNL>GGATCC</UNL>ATGGCCACTTCTGGAAAAGTT for the sense strand and TGG<UNL>GAATTC</UNL>TCAGAAGAGCATCACGGTGC for the antisense strand. The sense and antisense primers contained recognition sequences for the restriction endonucleases BamHI and EcoRI (underlined in the nucleotide sequence above), respectively. The amplified coding region of the new cDNA was subcloned into the expression vector pGEX-2T (Pharmacia Biotech Inc.). The final construct encoded a 375-amino acid polypeptide fused with glutathione S-transferase (GST). The expression of the fusion protein in the E. coli TG-1 cells was performed as described for human stomach sigma -ADH (5). Cells were harvested by centrifugation and suspended in phosphate-buffered saline with Tween 80 (138 mM NaCl, 2.7 mM KCl, 1.2 mM KH2PO4, 8.1 mM Na2HPO4, pH 7.5, and 0.05% (w/v) Tween 80 (PBST)) containing 0.1% beta -mercaptoethanol, 10 µM ZnSO4, and the protease inhibitors phenylmethylsulfonyl fluoride (50 µg/ml) and benzamidine (5 mM). The cells were homogenized using a French press, and the insoluble fraction was separated by centrifugation. The fusion protein was purified by glutathione-agarose affinity chromatography. The alcohol dehydrogenase activity of the recombinant protein was determined in a standard assay containing 4.7 mM cinnamyl alcohol, 2.5 mM NAD+ in 0.1 M sodium phosphate, pH 7.4, at 25 °C. The GST domain was separated from chick ADH by cleavage with human thrombin (Sigma). The efficiency of the cleavage was monitored by the appearance of separate 40- (ADH) and 26-kDa (GST) protein bands in SDS-polyacrylamide gel electrophoresis. ADH was purified from GST by chromatography over S Sepharose and eluted with a NaCl gradient in 10 mM sodium phosphate buffer, pH 6.5, 10% glycerol, 2 mM dithiothreitol. Chick ADH eluted at 100 mM NaCl. GST did not bind to the resin under these conditions. Glycerol and dithiothreitol were found to stabilize enzyme activity. Therefore, purified ADH was stored in 10 mM sodium phosphate, pH 7.4, 50% glycerol, and 2 mM dithiothreitol at -20 °C. The concentration of glycerol was reduced to 10% before each experiment. Glycerol never exceeded 0.5% in the assay mixture, and this concentration did not alter ADH-F activity measurements. The concentration of ADH active sites was determined by observing fluorescence (excitation wavelength at 328 nm and emission at 425 nm) while titrating enzyme (1-2 mg/ml) with NADH in the presence of 99 mM isobutyramide in 10 mM sodium phosphate at pH 7.4. The concentration of NADH binding sites was evaluated from the intersection point of the linear regression of the fluorescence titration above and below NADH saturation (6). The specific activity of the chick ADH-F was calculated based on the concentration of NADH binding sites. Total protein concentration was determined by a dye-binding assay (Bio-Rad) using bovine serum albumin as a standard.

The kinetic constants for retinol were determined by monitoring the production of all-trans-retinal at 400 nm (epsilon  = 29.5 mM-1 cm-1) (7). The retinol stock solution was prepared in acetone, and aqueous retinol solutions were prepared by dissolving the calculated amount of retinol stock solution in 0.1 M sodium phosphate, pH 7.5, and 0.02% Tween 80. The addition of 0.02% Tween 80 did not inhibit the enzyme activity. The concentration of retinol in aqueous solution was determined by measuring the absorbance at 328 nm (epsilon  = 39.5 mM-1 cm-1), and solutions were used immediately.

All kinetic studies were performed in 0.1 M sodium phosphate, pH 7.4, at 25 °C with 2.4 mM NAD+ or 0.2 mM NADH. The kinetic constants for alcohols other than retinol were obtained by monitoring the production of NADH at 340 nm (epsilon  = 6.22 mM-1 cm-1). Reaction mixtures with steroid substrates contained 0.02% Tween 80. Steroid stock solutions were prepared in methanol, and concentration of methanol in the assay mixtures was kept constant at 0.3 M. Chick ADH-F was neither active toward nor inhibited by methanol up to 3 M at pH 7.5. The Vmax and Km values for alcohol substrates (at 2.4 mM NAD+) were calculated from a fit of the kinetic data to the Michaelis-Menten equation, V = VmaxA/(Km + A), where A is the concentration of the varied substrate. The kcat (min-1) was obtained by dividing Vmax by the concentration of active sites assuming a subunit Mr of 40,000. The apparent Km values for NAD+ and NADH were determined with 1 mM cinnamyl alcohol and 100 µM cinnamyl aldehyde, respectively. The inhibition constant for 4-methylpyrazole was determined with butanol as a substrate by varying both butanol (68-200 µM) and 4-methylpyrazole (75-350 µM) concentrations. The Ki of 4-methylpyrazole was calculated from a fit of the kinetic data to the equation for competitive inhibition, V = VmaxB/(KB (1 + I/Kis) + B), where B and I are butanol and 4-methylpyrazole concentrations, respectively (8). The Ki value for NADH was determined by varying NAD+ (15-60 µM) at 1 mM cinnamyl alcohol, using 0-10 µM NADH as the inhibitor.

The rabbit antiserum was raised against recombinant ADH-F. A 1:5,000 dilution of this antiserum detected 10 ng of purified ADH-F. Frozen chicken liver was homogenized in 10 mM Tris-HCl, pH 7.4, plus 5 mM benzamidine and 1 mM dithiothreitol. The homogenate was centrifuged at 10,000 × g for 15 min, and the supernatant was concentrated twice. Glycerol was added to 50% concentration, and the liver extract was stored at -20 °C. The proteins in the chicken liver homogenate were separated by isoelectic focusing using 3-10 pH gradient isoelectic focusing agarose plates (FMC Bioproducts, Inc.). After focusing, the separated proteins were transferred to nitrocellulose membrane, blocked with 3% bovine serum albumin in PBST, and incubated with a 1:5,000 dilution of antiserum. The binding of anti-ADH-F antibodies was visualized with 125I-protein A.

The amino acid substitutions occurring in chick ADH-F were model-built into the human beta 1-structure using the molecular graphics program QUANTA (Molecular Simulations, Inc.). Following substitution of all amino acid side chains in the dimer, the model structure was subjected to 100 cycles of energy minimization using X-PLOR 3.1 with the x-ray energy term omitted (9). The position for the epiandrosterone molecule in the human beta 1-structure was found by manually adjusting its position to minimize close contacts between the enzyme active site and the substrate molecule.

Sequences of human class I beta , alpha , and gamma  ADH; class I ADHs from the alligator, cod, frog, horse E and S, mouse, ostrich, quail, rabbit, and rat; class II ADHs from human and rat; class III ADHs from human, horse, mouse, and rat; class IV ADHs from human and mouse; human ADH6; and class VI from deer mouse were aligned with ADH-F by a progressive alignment method according to Feng and Doolittle (10). Sequences of ADHs were obtained from the GenBankTM.


RESULTS

The pool of ~500-bp PCR products obtained with ADH-specific primers from the limb buds and heart mRNA of chick embryos at stage 21 was subcloned in M13 vector, and 48 individual clones were sequenced. Two of the clones from heart mRNA were found to have a novel sequence with a high resemblance to ADH sequences, and 6 of the clones from limb bud mRNA encoded a fragment that exhibited 87% protein sequence identity with human chi -ADH (11, 12). The rest of the clones contained cDNA sequences that were not related to ADH. Since human class III ADH is not active with retinol, we did not pursue the cloning and characterization of this chi -ADH-like chick isozyme further.

The deduced protein sequence of the two novel identical PCR clones had a high resemblance to ADH sequences but was different from that of the chi -ADH-like chick ADH and the chick class I ADH (13). A Northern blot analysis of adult chicken tissues demonstrated that this partial PCR product from embryonal heart hybridized with two messages of approximately 2 and 3 kilobase pairs in adult chicken liver (Fig. 1). Other tissues (brain, testis, skeletal muscle, and heart) did not show a detectable hybridization signal after 24 h of exposure. A chicken liver lambda gt11 cDNA library was used to isolate a full-length cDNA. Three independent clones hybridizing with the partial cDNA were isolated. Two clones encoded a complete cDNA, and one lacked the N terminus. The total composite cDNA sequence was 1408 bp long with the ATG starting codon at nucleotide 74 and the TGA stop codon at nucleotide 1202 (Fig. 2). The open reading frame encoded a 375-amino acid mature polypeptide with predicted Mr of 40,016. 


Fig. 1. Northern blot analysis of adult chicken tissues. 20 µg of total RNA from brain (lane 1), testis (lane 2), muscle (lane 3), liver (lane 4), and heart (lane 5) were separated by size in the formaldehyde-agarose gel and transferred to Nytran membrane. The membrane was hybridized with the 500-bp PCR product encoding new chick ADH-F (panel A). The numbers on the right refer to sizes of the mRNA ladder (kilobases). Panel B shows the amount of ribosomal RNA present in each lane.
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Fig. 2. Nucleotide sequence and deduced protein sequence of chick ADH-F. Numbers on the right correspond to nucleotide sequence, and numbers on the left correspond to amino acid sequence. The peptide regions that were used to design degenerate oligonucleotides are underlined. The starting Met is present at nucleotide 74 (MET). The amino acid sequence is numbered from the Ala following the initiating Met codon in accordance with numbering of other ADH isozymes. The termination codon is indicated with an asterisk. The residues discussed in the text are shown in reversed color (white on black background). The insertion of N at amino acid 56 is shown in italic.
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The relationships of this presumed new ADH (ADH-F) with other ADH isozymes were analyzed by progressive alignment (10). Table I shows percentage identity of the new chick enzyme (ADH-F) with other ADH classes (11) as well as the range of percentage identity of the isozymes from different species within the same ADH class. The identity of the new ADH was highest with class I isozymes (69%). Class II and VI ADH were the least similar (about 60% identity) (Table I).

Table I.

Intraclass isozyme variability and percentage amino acid identity of ADH-F with other ADH classes

Percentage identity calculated by progressive alignment method according to Feng and Doolittle (10). The first two lines of this table have been published in Ref. 11.
ADH class Ia IIb IIIc IVd Ve VIf

ADH-F 61 -69 54 -62 62 -64 62 -63  63  57
I 69 -83 53 -61 62 -66 66 -71 59 -67 55 -60
II 66 -72 55 -63 51 -58 54 -60 47 -51
III 93 -95 60 -61  58  55
IV 87 -89  60a 56 -57
V 100b  67a
VI 100c

a Includes isozymes from human (alpha , beta 1, and gamma 1), horse (S and E), rat, mouse, rabbit, chicken, ostrich, alligator, and frog.
b Includes isozymes from human, rat, and ostrich.
c Includes isozymes from human, horse, rat, and mouse.
d Includes isozymes from human, rat, and mouse.
e A single representative of the class, human ADH6, is known.
f Class VI is represented by the deer mouse isozyme.

To characterize the catalytic properties of the new isozyme, the ADH-F cDNA was expressed in E. coli as fusion protein with GST (14). The recombinant enzyme separated from GST by thrombin cleavage had an apparent subunit Mr of 40,000 on SDS-polyacrylamide gel electrophoresis. 1 to 2 units of activity were obtained from a 1-liter culture, which corresponded to 12-24 mg of active enzyme. The specific activity of the ADH after thrombin cleavage was the same as that for the ADH-GST fusion protein. Specific activity was determined utilizing fluorescence active site titration by directly measuring the concentration of NADH binding sites. The Km value of the new ADH for ethanol was relatively high, 17 mM, and the kcat value was 3.4 min-1 (Table II). The Km values were several orders of magnitude lower for long-chain and large hydrophobic alcohols than for ethanol (Table II). The Km for cinnamyl alcohol was 8.4 µM, and the kcat/Km value was 580 min-1 mM-1. The apparent Km values for NAD+ and NADH were 5.4 and 5.3 µM, respectively (Table II). Inhibition of NAD+ reduction by NADH with cinnamyl alcohol held constant at 1 mM was consistent with competitive inhibition. The Ki for NADH was 4.0 ± 0.5 µM. Inhibition of butanol oxidation by 4-methylpyrazole also fitted best the competitive inhibition model. The Ki of 4-methylpyrazole was 300 ± 50 µM. These data are consistent with results for horse liver ADH (15, 16) and an Ordered Bi Bi mechanism.

Table II.

Km values of chick ADH-F for alcohol substrates and cofactors compared with other ADH isozymes

Kinetic constants for human class IV ADH are from Kedishvili et al. (5), and kinetic constants of class I and II ADH for retinol oxidation are from Yang et al. (2). Remaining kinetic data were reported as follows from the sources listed in the footnotes. All kinetic data were obtained in 0.1 M sodium phosphate at pH 7.4 and 25 °C. Km values for alcohols were determined with saturating 2.4 mM NAD+, for NAD+ with saturating 1 mM cinnamyl alcohol, and for NADH with saturating 100 µM cinnamyl aldehyde.
Substrate/cofactor Km
ADH-F Class I gamma gamma Class II pi pi Class IV sigma sigma

mM
Ethanol 17  ± 1.1 0.33a 34b 28
Butanol 0.087  ± 0.008 0.040a 0.15  ± 0.02c 0.79
(S)-(+)-2-Butanol 0.63  ± 0.10 1.9a 24  ± 3c 120
(R)-(-)-2-Butanol 0.14  ± 0.02 1.2a 49  ± 10c -
Cyclohexanol 0.017  ± 0.002 0.063a NSd NAd
all-trans-Retinol 0.056  ± 0.005 0.29 0.014 0.009
trans-2-Hexen-1-ol 0.0081  ± 0.0004 - 0.025  ± 0.003c 0.019  ± 0.002c
NAD+ 0.0054  ± 0.0005 0.0079e 0.014b 0.21
NADH 0.0053  ± 0.0007 0.007e 0.016b 0.22

a Stone et al. (24).
b Bosron et al. (23).
c Data obtained in this study.
d NS, not saturable at the maximum solubility; NA, not active with 240 mM cyclohexanol at pH 7.5.
e Bosron et al. (30).

The secondary alcohol (R)-(-)-2-butanol had a kcat/Km value seven times greater than (S)-(+)-2-butanol (Table III). The catalytic efficiencies with all-trans-retinol (Table III) and 3beta -hydroxy-5alpha -steroids (Table IV) were about 102 times higher than with ethanol, due primarily to the much lower Km values (Tables II and IV). The catalytic efficiency with the steroid alcohols epiandrosterone, dehydroepiandrosterone, and dihydrotestosterone was similar to that with retinol (Tables III and IV). No activity was observed with steroid alcohols having a hydrogen in the 5beta configuration (3beta -hydroxy-5beta -androstan-17-one or 5beta -pregnenolone) (Table IV). Stereospecificity of the new ADH for 3beta -hydroxy-5alpha -steroids was confirmed in the reverse reaction. ADH-F was active in reducing the steroid aldehyde 5alpha -androstan-17beta -ol-3-one with NADH, but it was not active with 5beta -androstan-17beta -ol-3-one (Table IV).

Table III.

kcat/Km values of chick ADH-F for alcohol substrates compared with other ADH isozymes

Kinetic constants for human class IV ADH are from Kedishvili et al. (5), and kinetic constants of class I and II ADH for retinol oxidation are from Yang et al. (2). Remaining kinetic data were reported as follows from the sources listed in the footnotes. All kinetic data were obtained in 0.1 M sodium phosphate at pH 7.4 and 25 °C. Km values for alcohols were determined with saturating 2.4 mM NAD+, for NAD+ with saturating 1 mM cinnamyl alcohol, and for NADH with saturating 100 µM cinnamyl aldehyde.
Substrate kcat/Km
ADH-F Class I gamma gamma Class II pi pi Class IV sigma sigma

min-1mM-1
Ethanol 0.20  ± 0.01 150a 0.6b 65
Butanol 50  ± 7 1,800a 140  ± 13c 2,600
(S)-(+)-2-Butanol 3.6  ± 0.4 8.0a 0.086  ± 0.006c 1.2
(R)-(-)-2-Butanol 24  ± 2 16a 0.064  ± 0.008c -
Cyclohexanol 209  ± 15 600a NSd NAd
all-trans-Retinol 26  ± 1 19 650 2,600
trans-2-Hexen-1-ol 1,009  ± 25 - 5,400  ± 470c 64,000  ± 3,000c

a Stone et al. (24).
b Bosron et al. (23).
c Data obtained in this study.
d NS, not saturable at the maximum solubility; NA, not active with 240 mM cyclohexanol at pH 7.4.

Table IV.

Substrate specificity of chick ADH-F for steroid alcohols and aldehydes at pH 7.4 

Kinetic constants were determined in 0.1 M sodium phosphate, pH 7.4, 0.02% Tween 80, and either 2.4 mM NAD+ or 0.2 mM NADH at 25 °C.
Substrate Km Kcat/Km

mM min-1 mM-1
5alpha -Androstan-3beta -ol-17-onea (epiandrosterone) 0.031  ± 0.004 17  ± 2
5beta -Androstan-3beta -ol-17-onea - Not active
5alpha -Androstan-17beta -ol-3-oneb (dihydrotestosterone) 0.079  ± 0.009 15.0  ± 0.3
5beta -Androstan-17beta -ol-3-oneb - Not active
5alpha -Androsten-3beta -ol-17-onea (dehydroepiandrosterone) 0.022  ± 0.003 23.0  ± 2.3
5beta -Pregnan-3beta -ol-20-onea (epipregnanolone) - Not active

a 2.4 mM NAD+.
b 0.2 mM NADH.

The protein corresponding to the wild-type ADH-F was detected in the chicken liver homogenate with the rabbit antiserum raised against recombinant ADH-F (Fig. 3). This antiserum cross-reacted with 100 ng of human class I, II, and IV but not class III ADH proteins (not shown), all of which have similar subunit molecular weights and cannot be separated by SDS-polyacrylamide gel electrophoresis. Thus, isoelectric focusing was employed to separate the 80-kDa dimers of chick ADH isozymes according to their isoelectric points. Recombinant ADH-F appeared as a smear of multiple protein bands with pI values ranging from 7.1 to 8.0 (Fig. 3, lane 1). All bands exhibited activity with 100 mM ethanol, 5 mM trans-2-hexen-1-ol, and 100 µM 3beta -hydroxysteroid alcohols. Protein bands binding anti-ADH-F antibodies also appeared in the chicken liver homogenate separated by isoelectric focusing in the same range of pI values as the recombinant ADH-F (Fig. 3, lanes 2 and 3).


Fig. 3. Western blot analysis of ADH-F in chicken liver homogenate. Lane 1, 100 ng of recombinant ADH-F. Lanes 2 and 3, 1 and 10 µg of liver homogenate, respectively. Numbers on the right indicate isoelectric points of the isoelectic focusing standard proteins.
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DISCUSSION

This study was undertaken to identify the isoforms of cytosolic 40-kDa (subunit) ADH in chick fetal tissues that oxidize the retinoid and steroid alcohols. The only ADH-related PCR product amplified from limb bud mRNA was identified as chick class III ADH based on the 87% protein sequence identity with human class III chi -ADH (12). Because human class III ADH is not active with retinol, we were not interested in characterizing the corresponding chick enzyme. However, PCR amplification of embryonal heart mRNA yielded a novel ADH-related cDNA. This new cDNA encoded an active enzyme named ADH-F.

Northern blot analysis of tissues from a 7-week-old chicken showed that among seven tissues analyzed, only the liver contained an mRNA hybridizing with the partial PCR product from the fetal heart. The adult heart mRNA did not contain a message that hybridized with this PCR product. A change in the tissue-specific expression of ADH gene between fetal and adult organism was also observed for the class I ADH mRNA in rat (17) and may reflect different metabolic needs of the tissues at different stages of development. The complete cDNA isolated from the chicken liver cDNA library encoded an ADH (ADH-F) with the polypeptide size similar to those of other ADH isozymes (375 amino acids without the starting methionine) (Fig. 2). ADH-F contained 13 residues (Fig. 2) that are conserved in the 47 members of the zinc-containing ADH family excluding zeta -crystallin (4). Sequence alignment with other animal ADHs indicated a single amino acid insertion after position 55. Therefore, the numbers of the conserved amino acid residues after Gly-55 were shifted by one position in chick ADH-F (Fig. 2) when compared with class I ADH (18). The conserved glycines and the valine of the substrate binding domain were present at positions 67, 72, 78, 87, and 81 (Fig. 2). The four glycines of the coenzyme binding domain were in positions 193, 202, 205, and 237. Conserved ligands to the catalytic zinc, Cys-46 and His-68, were also present. The new ADH-F sequence had Asp-224, which has been suggested to determine the specificity for NAD+ versus NADP+ as a coenzyme, and Thr-48, which is thought to form a hydrogen bond with the alcohol hydroxyl group bound to the catalytic zinc (19). Cysteines 98, 101, 104, and 112, which are responsible for binding the noncatalytic zinc (20), were conserved in the new ADH. However, the sequence of this ADH had less than 68% identity with any of the known ADH isozymes (Table I); hence, we conclude that ADH-F belongs to a separate class in the family of ADHs, class VII.

The new ADH gene encoded an active enzyme when produced as a recombinant protein in E. coli. Antiserum against this recombinant ADH-F recognized protein bands in the chicken liver homogenate with the same range of isoelectric points as the multiple ADH-F forms (Fig. 3). The slightly more basic pI of the recombinant protein is consistent with the lack of N-acetylation in E. coli-expressed proteins (21).

The functional and kinetic properties of the new recombinant ADH-F were compared with those of other ADHs. The yield of ADH-F protein was high (up to 14 mg/L of E. coli culture), but the specific activity with saturating ethanol at pH 7.4 was relatively low (0.08 unit/mg), a value that is similar to that of the human class I beta 1-ADH (0.1 unit/mg) (22). The Km value for ethanol (17 mM) was close to that of human stomach class IV ADH (29 mM) (5) and human liver class II ADH (34 mM) (23) (Table II). NADH inhibited NAD+ reduction competitively, and 4-methylpyrazole was a competitive inhibitor of butanol. These inhibition results are consistent with the Ordered Bi Bi mechanism suggested for other ADHs. ADH-F sensitivity to 4-methylpyrazole inhibition was similar to that of the human class IV ADH (Ki = 350 µM) (5). The Ki value (300 µM) was greater than that of class I and less than that of class II ADH.

The catalytic efficiency of ADH-F toward the secondary alcohol (R)-(-)-2-butanol was seven times that of (S)-(+)-2-butanol. This specificity appeared to be similar to that of the human alpha alpha isozyme, where its catalytic efficiency was about four times higher with (R)-(-)-2-butanol (136 min-1 mM-1) than with (S)-(+)-2-butanol (37.6 min-1 mM-1) (24). It has been suggested that the specificity of ADHs toward secondary alcohols is affected by amino acids at positions 48 and 93. Chick ADH-F has Thr-48 as in alpha alpha ADH and a unique Pro at the position homologous to residue 93 (94 in ADH-F). Modeling of the amino acid substitutions present in chick ADH shows that Pro can easily be accommodated at position 93 (Fig. 4) and that the region of the active site occupied by the secondary alcohol more closely resembles alpha alpha , with Ala at position 93, than beta 1beta 1, with Phe at position 93. Thus, it is not surprising that this new ADH isozyme possesses a stereospecificity for small secondary alcohols that is more similar to alpha alpha than beta 1beta 1 (or horse ADH). The catalytic efficiency (kcat/Km) of ADH-F was higher for oxidation of large alcohols. For example, ADH-F was 5 × 103 times more efficient with trans-2-hexen-1-ol than with ethanol.


Fig. 4. Binding of epiandrosterone to chick ADH-F. A, a stereo diagram of the model-built structure of chick ADH-F with bound epiandrosterone (bold lines). The position for epiandrosterone was found by minimizing the close contacts between the enzyme active site and epiandrosterone. All contact distances are greater than 2.6 Å, except for the distance between the 3beta -hydroxyl group and the catalytic zinc atom, which was modeled at 2.3 Å. B, aligned active site structures of human beta 1beta 1 ADH (thin lines) and chick ADH-F (thick lines). The modeled position of epiandrosterone is shown with heavy lines. The unfavorable van der Waals contact (a distance of 2.0 Å) generated between Phe-93 (F93) in human beta 1beta 1 ADH and epiandrosterone is shown as a dashed line.
[View Larger Version of this Image (30K GIF file)]


Chick ADH-F was similar to human class I gamma gamma isozyme in that it oxidized both retinoid and steroid alcohols (Tables III and IV). It was different from the other two retinol-oxidizing ADH isozymes, class IV sigma sigma and class II pi pi , which were not active with steroids (5, 25). ADH-F oxidized epiandrosterone about 88 times more slowly than horse SS ADH (0.51 min-1 versus 44 min-1) (26). The catalytic properties of ADH-F suggest that it may function as a steroid/retinoid dehydrogenase in chick. However, the physiological significance of chick ADH-F for steroid and retinoid metabolism will be clarified once the tissue-specific expression pattern during development and the amount of the active enzyme in tissues are determined.

In general, no single amino acid difference appears to be responsible for the unique kinetic properties of the new chick ADH. The ability to oxidize large hydrophobic alcohols, such as retinol and 3beta ,5alpha -hydroxysteroids, appears to be the result of several amino acid substitutions near the active site zinc atom and at the entrance to the alcohol binding pocket. With current knowledge, the ability of ADH isozymes to oxidize 3beta -hydroxysteroids has been limited to those that possess a Ser at position 48 (human gamma gamma and horse SS ADH). ADH-F has a Thr at position 48. Its ability to bind steroids productively may be due to substitutions in the vicinity of position 93. In sterol-oxidizing horse SS and human gamma gamma , which have a Ser at position 48, there is also a Phe at position 93, which is usually preceded by a Pro-Leu sequence. The sequence Leu-Phe-Pro in chick ADH-F, instead of Pro-Leu-Phe as in most class I isozymes, may account for the difference in steroid alcohol specificity. In addition, there are unusual substitutions at positions 318 and 319, where Leu and Ala substitute for Ile and Phe, respectively. Thus, the "floor" of the alcohol binding pocket appears to be more open in ADH-F compared with class I beta 1beta 1, and this could make the site more accessible to large substrates (Fig. 4). The enzyme appears to be sensitive to the configuration at the 5-position of steroid alcohol, since the 5beta -hydroxysteroid alcohols are inactive (Table IV). Molecular modeling suggests that the stereospecificity at the 5alpha -position of the sterol may be due to the presence of the extra methyl group of Thr-48 and the rearranged floor of the substrate binding pocket in the ADH-F compared with beta 1beta 1 (Fig. 4).

The productive association of large hydrophobic substrates leading to efficient oxidation is usually associated with rearrangements near the entrance to the alcohol binding pocket. It was shown that both the horse SS and the human class IV sigma sigma isozyme efficiently oxidize sterols and retinol, respectively, due to alterations in the loop at the entrance to the alcohol binding site comprising residues 112-119 (5, 27). Both of these isozymes possess single amino acid deletions that appear to widen the mouth of the substrate binding site, permitting easier access for these large substrates. Although chick ADH-F isozyme does not possess such a deletion, the presence of His and Trp in positions 115 and 142, respectively, may affect the conformation of this loop. A neutral or acidic residue at position 115 helps to correctly position this loop by hydrogen bonding with the peptide nitrogen of residue 118 in most class I ADH crystal structures. The His at position 115 will not perform the same function to anchor this loop structure in place, and a conformational change in the structure of this loop could create a more open substrate binding site. The substitution of Asp-115 by Trp in the cod ADH crystal structure appears to be the primary reason for the conformation of this loop to adopt an alpha -helical structure (28). It is not clear whether the insertion of one amino acid in the region between positions 55 and 60 will also affect the structure at the entrance to the substrate binding site. Mutagenesis in class III chi -ADH, which also has an insertion in this region, strongly implicates a role for Asp-57 in binding of the substrate hydroxymethyl glutathione (29). Our modeling of chick ADH would suggest that Phe-57 could form favorable van der Waals contacts with the hydrophobic face of hydroxysteroids.

Another interesting substitution occurs at position 173. In most ADH isozymes the catalytic zinc ligand Cys-174 is surrounded by two glycines. These glycines may provide the necessary flexible linkage between the catalytic and coenzyme binding domains to allow the large conformational change observed upon coenzyme binding. The presence of Ala at this position may impair the ability of this isozyme to undergo rapid conformational shifts in its structure and may explain, at least in part, the relatively low turnover rate of this isozyme.

Thus, ADH-F appears to be unique in terms of its structure-function relationships. This enzyme has low specific activity; it is active with 3beta ,5alpha -hydroxysteroids but not with 3beta ,5beta -hydroxysteroids; it is active toward steroid substrates in the absence of Ser-48; and it is active toward retinol in the absence of deletion in the loop between amino acids 115 and 120. Several amino acid substitutions discussed above suggest an explanation for some of its properties. X-ray structure determination of the enzyme will provide more complete insight into the structural basis of its substrate specificity.


FOOTNOTES

*   This work was supported by National Institute on Alcohol Abuse and Alcoholism Grants K08 AA00221-01 (to N. Y. K.), R37-AA02342 (to T. K. L.), and R37-AA07117 (to W. F. B.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) U73654[GenBank].


§   To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Dr., MS 405, Indianapolis, IN 46202-5122. Tel.: 317-278-0296; Fax: 317-274-4686.
1   The abbreviations used are: ADH, alcohol dehydrogenase; ADH-F, the new ADH in fetal chick described in this study; bp, base pairs; GST, glutathione S-transferase; PCR, polymerase chain reaction.
2   Human class I alcohol dehydrogenases include the alpha alpha , beta beta , and gamma gamma isozymes; the human class II isozyme is pi pi , the human class III isozyme is chi chi , and the human gastric class IV isozyme is sigma sigma . Polymorphic variants of class I isozymes are designated by subscripts (e.g. beta 2).

REFERENCES

  1. Edenberg, H. J., and Bosron, W. F. (1997) Comprehensive Toxicology, in press
  2. Yang, Z.-N., Davis, G. J., Hurley, T. D., Stone, C. L., Li, T.-K., and Bosron, W. F. (1994) Alcohol. Clin. Exp. Res. 18, 587-591 [Medline] [Order article via Infotrieve]
  3. Waller, G., and Theorell, H. (1965) Arch. Biochem. Biophys. 111, 671-684 [Medline] [Order article via Infotrieve]
  4. Sun, H.-W., and Plapp, B. V. (1992) J. Mol. Evol. 34, 522-535 [Medline] [Order article via Infotrieve]
  5. Kedishvili, N. Y., Bosron, W. F., Stone, C. L., Hurley, T. D., Peggs, C. F., Thomasson, H. R., Popov, K. M., Carr, L. G., Edenberg, H. J., and Li, T.-K. (1995) J. Biol. Chem. 270, 3625-3630 [Abstract/Free Full Text]
  6. Yonetani, T., and Theorell, H. (1962) Arch. Biochem. Biophys. 99, 433-446 [Medline] [Order article via Infotrieve]
  7. Julia, P., Farrés, J., and Parés, X. (1986) Exp. Eye Res. 42, 305-314 [Medline] [Order article via Infotrieve]
  8. Cleland, W. W. (1979) Methods Enzymol. 63A, 103-138 [Medline] [Order article via Infotrieve]
  9. Brünger, A. T. (1988) J. Mol. Biol. 203, 803-816 [Medline] [Order article via Infotrieve]
  10. Feng, D. F., and Doolittle, R. F. (1990) Methods Enzymol. 183, 375-389 [Medline] [Order article via Infotrieve]
  11. Kedishvili, N. Y., Stone, C. L., Popov, K. M., and Chernoff, E. A. G. (1997) in Enzymology and Molecular Biology of Carbonyl Metabolism (Weiner, H., Lindahl, R., Crabb, D. W., and Flynn, T. G., eds), Vol. 6, pp. 321-329, Plenum Press, New York
  12. Kaiser, R., Holmquist, B., Hempel, J., Vallee, B. L., and Jörnvall, H. (1988) Biochemistry 27, 1132-1140 [Medline] [Order article via Infotrieve]
  13. Estonius, M., Karlsson, C., Fox, E. A., Höög, J. O., Holmquist, B., Vallee, B. L., Davidson, W. S., and Jörnvall, H. (1990) Eur. J. Biochem. 194, 593-602 [Abstract]
  14. Guan, K. L., and Dixon, T. E. (1991) Anal. Biochem. 192, 262-267 [Medline] [Order article via Infotrieve]
  15. Wratten, C. C., and Cleland, W. W. (1963) Biochemistry 2, 935-941
  16. Dworschack, R. T., and Plapp, B. V. (1977) Biochemistry 16, 111-116 [Medline] [Order article via Infotrieve]
  17. Tietjen, T. G., Mjaatvedt, C. H., and Yang, V. W. (1994) J. Histochem. Cytochem. 42, 745-753 [Abstract/Free Full Text]
  18. Hempel, J., Buhler, R., Kaiser, R., Holmquist, B., De Zalenski, C., von Wartburg, J. P., Vallee, B. L., and Jörnvall, H. (1984) Eur. J. Biochem. 145, 437-455 [Abstract]
  19. Fan, F., Lorenzen, J. A., and Plapp, B. V. (1991) Biochemistry 30, 6397-6401 [Medline] [Order article via Infotrieve]
  20. Eklund, H., Nordström, B., Zeppezauer, M., Söderlund, G., Ohlsson, I., Boiwe, T., Söderberg, B.-O., Tapia, O., Brändén, C.-I., and Åkeson, Å. (1976) J. Mol. Biol. 102, 27-59 [Medline] [Order article via Infotrieve]
  21. Höög, J.-O., Weis, M., Zeppezauer, M., Jörnvall, H., and von Bahr-Lindström, H. (1987) Biosci. Rep. 7, 969-973 [Medline] [Order article via Infotrieve]
  22. Hurley, T. D., Edenberg, H. J., and Bosron, W. F. (1990) J. Biol. Chem. 265, 16366-16372 [Abstract/Free Full Text]
  23. Bosron, W. F., Li, T.-K., Dafeldecker, W. P., and Vallee, B. L. (1979) Biochemistry 18, 1101-1105 [Medline] [Order article via Infotrieve]
  24. Stone, C. L., Li, T.-K., and Bosron, W. F. (1989) J. Biol. Chem. 264, 11112-11116 [Abstract/Free Full Text]
  25. McEvily, A. J., Holmquist, B., Auld, D. S., and Vallee, B. L. (1988) Biochemistry 27, 4284-4288 [Medline] [Order article via Infotrieve]
  26. Cronholm, T., Larsén, C., Sjövall, J., Theorell, H., and Åkeson, Å. (1975) Acta Chem. Scand. Ser. B Org. Chem. Biochem. 29, 571-576 [Medline] [Order article via Infotrieve]
  27. Park, D.-H., and Plapp, B. V. (1991) J. Biol. Chem. 266, 13296-13302 [Abstract/Free Full Text]
  28. Ramaswamy, S., El Ahmed, M., Danielsson, O., Jörnvall, H., and Eklund, H. (1996) Protein Sci. 5, 663-671 [Abstract/Free Full Text]
  29. Estonius, M., Höög, J.-O., Danielsson, O., and Jörnvall, H. (1994) Biochemistry 33, 15080-15085 [Medline] [Order article via Infotrieve]
  30. Bosron, W. F., Magnes, L. J., and Li, T.-K. (1983) Biochemistry 22, 1852-1857 [Medline] [Order article via Infotrieve]

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