Fundamental differences in bioaffinity of amino acid dehydrogenases for N6- and S6-linked immobilized cofactors using kinetic-based enzyme-capture strategies

Five different immobilized NAD+ derivatives were employed to compare the behavior of four amino acid dehydrogenases chromatographed using kinetic-based enzyme capture strategies (KBECS): S6-, N6-, N1-, 8'-azo-, and pyrophosphate-linked immobilized NAD+. The amino acid dehydrogenases were NAD+-dependent phenylalanine (EC 1.4.1.20), alanine (EC 1.4.1.1), and leucine (EC 1.4.1.9) dehydrogenases from various microbial species and NAD(P)+-dependent glutamate dehydrogenase from bovine liver (GDH; EC 1.4.1.3). KBECS for bovine heart L-lactate dehydrogenase (EC 1.1.1.27) and yeast alcohol dehydrogenase (EC 1.1.1.1) were also applied to assist in a preliminary assessment of the immobilized cofactor derivatives. Results confirm that the majority of the enzymes studied retained affinity for NAD+ immobilized through an N6 linkage, as opposed to an N1 linkage, replacement of the nitrogen with sulfur to produce an S6 linkage, or attachment of the cofactor through the C8 position or the pyrophosphate group of the cofactor. The one exception to this was the dual-cofactor-specific GDH from bovine liver, which showed no affinity for N6-linked NAD+ but was biospecifically adsorbed to S6-linked NAD+ derivatives in the presence of its soluble KBEC ligand. The molecular basis for this is discussed together with the implications for future development and application of KBECS.

An efficient and selective enzymatic oxidation system for the synthesis of enantiomerically pure D-tert-leucine

[reaction: see text] d-tert-Leucine was prepared with an enantiomeric excess of >99% by an enzyme-catalyzed oxidative resolution of the racemic mixture of dl-tert-leucine with use of leucine dehydrogenase. The l-amino acid was oxidized completely due to coupling of the primary reaction with a highly efficient irreversible NAD(+)-regenerating step by NADH oxidase.

The hyperinsulinemic amino acid clamp increases whole-body protein synthesis in young subjects

We propose that hyperinsulinemia stimulates protein synthesis when postabsorptive plasma amino acid (AA) concentrations are maintained. During a euglycemic hyperinsulinemic clamp, many AA, notably the branched-chain amino acids (BCAA), decline markedly. Therefore, we tested whether individual plasma AA could be maintained within the range of postabsorptive concentrations to assess the effects of insulin, infused at 40 mU/m(2) x min on whole-body protein and glucose metabolism, using [1-(13)C]-leucine and [3-(3)H]-glucose methodology. Validation studies of background [(13)C] enrichment and breath (13)CO(2) recovery factors were performed in a subset of 6 subjects. In 10 healthy, young men, infusion rates of an AA solution were based on fluorometric determinations of total BCAA every 5 minutes. All 21 plasma AA remained in the target range; 15, including the BCAA, alanine, and glycine were within 13% of baseline, and only 6 (Thr, His, Arg, Asn, Cit, Tyr) varied more (18% to 42%). Notably, both leucine flux and nonoxidative leucine R(d) (protein synthesis) increased with insulin (2.36 +/- 0.06 to 2.81 +/- 0.10 and 1.79 +/- 0.05 to 2.18 +/- 0.10 micromol/kg fat-free mass (FFM) x min, respectively; P <.0005) while leucine oxidation only tended to increase (P =.05) and endogenous leucine R(a) (protein breakdown) decreased by 18% (2.36 +/- 0.06 to 1.94 +/- 0.09 micromol/kg FFM x min; P <.0005), resulting in a marked elevation of net protein synthesis (-0.57 +/- 0.02 to 0.24 +/- 0.02 micromol/kg FFM x min; P <.0000001). Thus, in vivo protein anabolism was induced when maintaining postabsorptive plasma amino acid concentrations during hyperinsulinemia through a suppression of whole-body protein breakdown, no significant change in oxidation and an elevation of synthesis compared with postabsorptive conditions.

Crystallization and preliminary X-ray analysis of substrate complexes of leucine dehydrogenase from Thermoactinomyces intermedius

Leucine dehydrogenase is an octameric enzyme which belongs to the superfamily of amino-acid dehydrogenases and catalyses the reversible oxidative deamination of leucine to 2-ketoisocaproate, with the corresponding reduction of the cofactor NAD(+). Catalysis by this enzyme is thought to involve a large-scale motion of the enzyme's two domains between an 'open' and 'closed' form, with the latter representing a conformation of the enzyme in which the partners involved in the hydride-transfer reaction are appropriately positioned for catalysis. Whilst a structure for the open form of the enzyme has been determined, the nature of the closed form has yet to be observed. In order to trap a closed form, crystals of the complexes of leucine dehydrogenase from Thermoactinomyces intermedius with 2-ketoisocaproate and with 2-ketoisocaproate and NAD(+) have been obtained by the hanging-drop vapour-diffusion method using PEG 4000 as a precipitant. The crystals of the binary complex with 2-ketoisocaproate belong to space group P2(1)2(1)2(1), with approximate unit-cell parameters a = 106, b = 118, c = 320 A and an octamer in the asymmetric unit, corresponding to a V(M) of 3.1 A(3) Da(-1). The crystals of the non-productive ternary complex belong to space group P6(1) or P6(5), with approximate unit-cell parameters a = b = 117, c = 502 A and an octamer in the asymmetric unit, corresponding to a V(M) of 3.0 A(3) Da(-1). These crystals diffract X-rays on a synchrotron-radiation source to at least 2.8 and 3.3 A resolution, respectively, and are suitable for a full structure determination.

Fragmentary form of thermostable leucine dehydrogenase of Bacillus stearothermophilus: its construction and reconstitution of active fragmentary enzyme

X-ray crystallographic studies revealed that various amino acid dehydrogenases fold into two domains in each subunit, a substrate-binding domain and an NAD(P)(+)-binding domain (Baker, P. J., Turnbull, A. P., Sedelnikova, S. E., Stillman, T. J., and Rice, D. W. (1995) Structure 3, 693-705). To elucidate the function and folding process of these two domains, we have genetically constructed a fragmentary form of thermostable leucine dehydrogenase of Bacillus stearothermophilus consisting of an N-terminal polypeptide fragment corresponding to the substrate-binding domain including an N-terminus, and a C-terminal fragment corresponding to the NAD(+)-binding domain. The two peptide fragments were expressed in separate host cells and purified. When both fragments were mixed, the leucine dehydrogenase activity with a specific activity of 1.4% of that of the wild-type enzyme appeared. This suggests that both peptide fragments mutually recognize each other, associate and fold correctly to be catalytically active, although the activity is low. However, the fragmentary form of enzyme produced catalyzed the oxidative deamination of l-leucine, l-isoleucine, and l-valine with broad substrate specificity compared to that of the wild-type enzyme. The fragmentary enzyme retained more than 75% of the initial activity after heating at 50 degrees C for 60 min. The fragmentary enzyme was more stable on heating than separate peptide fragments. These results suggest that the two domains of leucine dehydrogenase probably fold independently, and the two peptide fragments interact and associate with each other to form a functional active site.

Construction and physiological studies on a stable bioengineered strain of shengjimycin

Shengjimycin is a group of 4"-acylated spiramycins with 4"-isovalerylspiramycin as the major component, produced by recombinant S. spiramyceticus F21 harboring a 4"-O-acyltransferase gene from S. mycarofaciens 1748. A stable bioengineered strain of Streptomyces spiramyceticus WSJ-1 was constructed by integrating the 4"-O-acyltransferase gene (ist) by homologous recombination into the chromosome of the spiramycin-producing strain S. spiramyceticus F21. In this construction, a Streptomyces/E. coli shuttle plasmid pKC1139 (AmR) was used as the vector with the tsr gene used as selection marker for homologous recombination. The constructed strain, S. spiramyceticus WSJ-1,was genetically stable in production titer and proportion of components of shengjimycin as well as in maintaining the tsr selective marker when grown without selection. Southern hybridization confirmed the integrated status of the ist gene in the host genome. The production and the proportion of major component of 4"-isovalerylspiramycin of S. spiramyceticus WSJ-1 was also improved comparing with the strain harboring an autonomous plasmid -S. spiramyceticus F21/pIJ680(311) as shown by HPLC analysis. Physiological studies indicated that increase of the VDH ( valine dehydrogenase ) and LDH ( leucine dehydrogenase ) activities of WSJ-1 may be involved in this improvement.

Investigation of mathematical methods for efficient optimisation of aqueous two-phase extraction

Mathematical strategies were applied to optimise the extraction of recombinant leucine dehydrogenase from E. coli homogenates and endoglucanase 1 from culture filtrates of Trichoderma reesei in polyethylene glycol-phosphate systems. The goal was to test mathematical tools which could facilitate the optimisation procedure in aqueous two-phase systems. A modified simplex approach, the method of steepest ascent and a genetic algorithm were successfully applied and compared. The methods differ in the height of the optimum found, the number of experiments and the time required. The genetic algorithm proved to be an optimisation procedure which can be used well in aqueous two-phase systems. The simplex procedure has to be further improved.

coli[pIET98]

A method for the production of recombinant L-leucine dehydrogenase from Bacillus cereus in pilot scale is described employing the temperature induced runaway replication vector pIET98 and the Escherichia coli host strain BL21. Fed-batch cultivation using a semi-synthetic high-cell densitiy medium was adjusted in 5-L scale to yield a constant growth rate of 0,17 h(-1) and a final cell concentration of 27 g dry weight/L by exponentially increasing the nutrient supply. Runaway replication and thus, LeuDH expression was induced during the feeding phase by increasing the cultivation temperature to 41 degrees C yielding a specific enzyme activity of 110 U/mg, which corresponds to 30% of the soluble cell protein. The cultivation was terminated when the dissolved oxygen content fell below 10% saturation. The final volume activity was 600,000 U/L cultivation. No change in growth, cell density, or expression activity was observed scaling up the cultivation volume to 200 L. Thus, 120,000,000 units L-leucine dehydrogenase were obtained from one cultivation. The purification of L-leucine dehydrogenase to homogeneity was carried out by heat denaturation, liquid-liquid extraction, gel filtration, and anion-exchange chromatography to give pure enzyme in 65% yield. The integrity of the recombinant enzyme was tested measuring the molecular weight and determining the N-terminal amino acid sequence.

Investigating expression systems for the stable large-scale production of recombinant L-leucine-dehydrogenase from Bacillus cereus in Escherichia coli

The established Escherichia coli expression vectors ptrc99a, pKK223-3, pPLlambda, pAsk75, pRA95, and pRA96, which differ in copy number, mode of induction, selection marker, and use of par sequences for stabilization, were investigated for the stable expression of recombinant L-leucine dehydrogenase from Bacillus cereus with a view to large-scale production. Best results were achieved with pIET98, a runaway-replication system derived from pRA96. Expression of L-leucine dehydrogenase was controlled by its constitutive B. cereus promoter and depended on host strain, cultivation temperature, induction time, and media composition. After cell cultivation at 30 degrees C and shifting to 41 degrees C to induce plasmid replication, E. coli BL21[pIET98] yielded 200 U LeuDH/mg protein, which corresponds to >50% of the soluble cell protein. Continuous cultivation in a semisynthetic high-cell-density medium verified structural and segregational stability over 100 generations in the absence of a selection pressure.

Synthesis of probes for the active site of leucine dehydrogenase

A series of novel 3-substituted 2-oxobutanoic acids were prepared and incubated with leucine dehydrogenase giving in one case both a kinetic resolution at C-3 and reductive amination of the ketone. This is the first example of an amino acid dehydrogenase catalysed kinetic resolution and reductive amination.

Insights into the mechanism of domain closure and substrate specificity of glutamate dehydrogenase from Clostridium symbiosum

Comparisons of the structures of glutamate dehydrogenase (GluDH) and leucine dehydrogenase (LeuDH) have suggested that two substitutions, deep within the amino acid binding pockets of these homologous enzymes, from hydrophilic residues to hydrophobic ones are critical components of their differential substrate specificity. When one of these residues, K89, which hydrogen-bonds to the gamma-carboxyl group of the substrate l-glutamate in GluDH, was altered by site-directed mutagenesis to a leucine residue, the mutant enzyme showed increased substrate activity for methionine and norleucine but negligible activity with either glutamate or leucine. In order to understand the molecular basis of this shift in specificity we have determined the crystal structure of the K89L mutant of GluDH from Clostridium symbiosum. Analysis of the structure suggests that further subtle differences in the binding pocket prevent the mutant from using a branched hydrophobic substrate but permit the straight-chain amino acids to be used as substrates. The three-dimensional crystal structure of the GluDH from C. symbiosum has been previously determined in two distinct forms in the presence and absence of its substrate glutamate. A comparison of these two structures has revealed that the enzyme can adopt different conformations by flexing about the cleft between its two domains, providing a motion which is critical for orienting the partners involved in the hydride transfer reaction. It has previously been proposed that this conformational change is triggered by substrate binding. However, analysis of the K89L mutant shows that it adopts an almost identical conformation with that of the wild-type enzyme in the presence of substrate. Comparison of the mutant structure with both the wild-type open and closed forms has enabled us to separate conformational changes associated with substrate binding and domain motion and suggests that the domain closure may well be a property of the wild-type enzyme even in the absence of substrate.

NADH-dependent inhibition of branched-chain fatty acid synthesis in Bacillus subtilis

Addition of NADH to crude but not to pure branched-chain alpha-keto acid decarboxylase decreased the CO2 production from alpha-keto-beta-methylvalerate (KMV) suggesting the presence of an NADH dependent inhibitor in the crude enzyme from Bacillus subtilis. This NADH-dependent decarboxylase inhibitor was purified to homogeneity by a fast protein liquid chromatography system. The purified inhibitor was identical with leucine dehydrogenase as to N-terminal amino acid squence (35 residues) and molecular weight, and catalyzed the oxidative deamination of three branched chain amino acids (BCAAs), valine, leucine, and isoleucine. The decarboxylase inhibitor was therefore identified as leucine dehydrogenase. A decreased substrate availability caused by leucine dehydrogenase thus reasonably accounted for the NADH dependent inhibition of the decarboxylation. In turn, the observation that leucine dehydrogenase competes with the decarboxylase for branched-chain alpha-keto acid (BCKA) suggested an involvement of this enzyme in the branched chain fatty acid (BCFA) biosynthesis. This view was supported by the observation that addition of NAD to crude fatty acid synthetase increased the incorporation of isoleucine into BCFAs. Pyridoxal-5'-phosphate and alpha-ketoglutarate, cofactors for BCAA transaminase, modulated BCFA biosynthesis from isoleucine in vitro, suggesting also the involvement of transaminase reaction in BCFA biosynthesis.

Determinants of substrate specificity in the superfamily of amino acid dehydrogenases

The subunit of the enzyme glutamate dehydrogenase comprises two domains separated by a cleft harboring the active site. One domain is responsible for dinucleotide binding and the other carries the majority of residues which bind the substrate. During the catalytic cycle a large movement between the two domains occurs, closing the cleft and bringing the C4 of the nicotinamide ring and the Calpha of the substrate into the correct positioning for hydride transfer. In the active site, two residues, K89 and S380, make interactions with the gamma-carboxyl group of the glutamate substrate. In leucine dehydrogenase, an enzyme belonging to the same superfamily, the equivalent residues are L40 and V294, which create a more hydrophobic specificity pocket and provide an explanation for their differential substrate specificity. In an attempt to change the substrate specificity of glutamate dehydrogenase toward that of leucine dehydrogenase, a double mutant, K89L,S380V, of glutamate dehydrogenase has been constructed. Far from having a high specificity for leucine, this mutant appears to be devoid of any catalytic activity over a wide range of substrates tested. Determination of the three-dimensional structure of the mutant enzyme has shown that the loss of function is related to a disordering of residues linking the enzyme's two domains, probably arising from a steric clash between the valine side chain, introduced at position 380 in the mutant, and a conserved threonine residue, T193. In leucine dehydrogenase the steric clash between the equivalent valine and threonine side chains (V294, T134) does not occur owing to shifts of the main chain to which these side chains are attached. Thus, the differential substrate specificity seen in the amino acid dehydrogenase superfamily arises from both the introduction of simple point mutations and the fine tuning of the active site pocket defined by small but significant main chain rearrangements.

Analysis of the quaternary structure, substrate specificity, and catalytic mechanism of valine dehydrogenase

The solution of the three-dimensional structure of Bacillus sphaericus leucine dehydrogenase has enabled us to undertake a homology-based modeling exercise on the sequence differences between the families of leucine (LeuDH) and valine (ValDH) dehydrogenases. This analysis indicates that the secondary structure elements in the core of the two domains of a single subunit of these enzymes are conserved, as are residues directly implicated in the recognition of the nucleotide cofactor and in catalysis. Comparison of the sequences indicates that the residues in the pocket accommodating the side chain of the amino acid substrate are conserved between these two enzymes, suggesting that the small differences in specificity arise from minor changes in molecular structure, possibly associated with shifts of the main chain rather than mutation of residues in the pocket itself. While B. sphaericus LeuDH is an octamer, both Streptomyces cinnamonensis and Streptomyces coelicolor ValDHs are dimers. The differences in quaternary structure can be understood in terms of the deletion in the latter of a C-terminal loop, which forms important interactions around the four-fold axis in LeuDH.

Kinetic assay of serum and urine for urea with use of urease and leucine dehydrogenase

We describe a new kinetic assay for determining urea in serum or urine with use of urease (EC 3.5.1.5) and leucine dehydrogenase (EC 1.4.1.9). The latter enzyme is suitable for the kinetic assay of NH4+ because its Km value for NH4+ at pH 8.75 is large (approximately 500 mmol/L). Interference from endogenous NH4+ in serum or urine is obviated by subtraction of the assayed endogenous NH4+ value in a sample blank. For serum, within-assay CVs (n = 10) were 0.39-0.58%; day-to-day CVs (n = 10) were 1.56-2.30%. In urine, within-assay CVs (n = 10) were 0.86-1.15%. Analytical recovery of urea (0.893-71.4 mmol/L) added to patients' sera (urea 6.14 mmol/L) was 99.2-105.2%. The calibration curve for serum was linear through zero for urea concentrations up to 142.9 mmol/L and for urine up to 714.3 mmol/L. No influences of added ammonium ion, bilirubin, hemoglobin, ascorbic acid, or Intralipid were observed. The regression equations for this method (y) and conventional methods (x = Determiner-LUN for serum assays, Serotec UUR-R for urine) were: y = 1.016x - 0.12 mmol/L (r = 0.999, S(y/x) = 0.34 mmol/L, n = 100) for sera, and y = 1.070x - 12.6 mmol/L (r = 0.998, S(y/x) = 7.41 mmol/L, n = 100) for urine.

Construction of a new leucine dehydrogenase with preferred specificity for NADP+ by site-directed mutagenesis of the strictly NAD+-specific enzyme

On the basis of sequence comparison between NAD+-dependent leucine dehydrogenase (LeuDH) from Thermoactinomyces intermedius and NADP+-dependent dehydrogenases, a set of amino acid residues that are supposed to determine the coenzyme specificity of LeuDH were assigned. Systematic replacement of these amino acids by others was done with the aim to switch its natural coenzyme specificity to a new one preferring NADP+. Single D203A, double D203A-I204R and triple D203A-I204R-D210R mutation enzymes were constructed. The wild-type LeuDH is inactive with NADP+. However, D203A single mutant exhibited dual specificity for NAD+ and NADP+ with essentially identical k(cat)/Km values for both coenzymes, but the values were three orders of magnitude lower than that of the wild-type enzyme. Introduction of positive charge at 204 together with the removal of the negative charge at 203 in the double mutant D203A-I204R provided the enzyme with significantly high affinity for NADP+. The best k(cat)/Km value for NADP+ was shown for the triple mutant D203A-I204R-D210R: more than 2% of the k(cat)/Km value of the wild-type enzyme. Thus, we succeeded in constructing a mutant LeuDH with a new coenzyme specificity preferring NADP+ which is highly active (specific activity, 19 micromol/mg/min).

Cloning, sequencing and overexpression of the leucine dehydrogenase gene from Bacillus cereus

The L-leucine dehydrogenase gene from Bacillus cereus (DSM 626) was cloned from a partial genomic library and sequenced. The open reading frame has 1101 bp and codes for a protein of 39.9 kDa. The deduced amino acid sequence of the LeuDH from B. cereus shares 70-80% identity with LeuDH's from the thermophilic strains B. stearothermophilus and Thermoactinomyces intermedius. The active protein was overexpressed in Escherichia coli to yield approximately 30% of the total soluble protein.

Spectrophometric assay for measuring branched-chain amino acid concentrations: application for measuring the sensitivity of protein metabolism to insulin

Plasma amino acid concentrations fall during insulin infusion. Amino acid concentrations can be maintained using an infusion of amino acids if their plasma concentration can be determined within a few minutes. We developed a spectrophometric assay which determines the total concentration of all three branched-chain amino acids in plasma within 1 min. The enzyme leucine dehydrogenase oxidatively deaminates leucine, isoleucine, and valine, with stoichiometric reduction of NAD that is measured using a spectrophotometer. The assay was developed in both a kinetic and end-point format. For the kinetic assay the buffer conditions were formulated to obtain equivalent rates with all three amino acids so that it could be used in samples containing unknown mixtures. For the end-point assay additional enzyme was added so that an end-point could be reached within 1 min. The application of the kinetic assay for "clamping" the branched-chain amino acids during hyperinsulinemic euglycemic clamps in humans is demonstrated.

Postcolumn co-immobilized leucine dehydrogenase-NADH oxidase reactor for the determination of branched-chain amino acids by high-performance liquid chromatography with chemiluminescence detection

A liquid chromatographic system with a co-immobilized leucine dehydrogenase-NADH oxidase reactor is described for the determination of branched-chain amino acids such as I-leucine, I-isoleucine and I-valine. The enzymes were simultaneously immobilized on tresylate-containing poly(vinyl alcohol) beads. The separation was achieved by means of an ODS column with elution with phosphate buffer (pH 7.5). The hydrogen peroxide produced was detected chemiluminometrically via a luminol-hexacyanoferrate(III) reaction. The system gave a linear response from 0.3 to 300 mu M for each amino acid and the detection limit was 0.1 mu M.

Gene cloning, purification, and characterization of thermostable and halophilic leucine dehydrogenase from a halophilic thermophile, Bacillus licheniformis TSN9

A halophilic and thermophilic isolate from the sand of Tottori Dune was found to produce a thermostable and halophilic leucine dehydrogenase (EC 1.4.1.9). It was identified to be a new strain of Bacillus licheniformis. The enzyme gene was cloned into Escherichia coli JM109 with a vector plasmid pUC18. The enzyme was purified to homogeneity from the clone cell extract by ion-exchange column chromatography with a yield of 31%. The enzyme was found to be composed of eight subunits identical in relative molecular mass (43,000). The amino acid sequence of the enzyme, deduced from the nucleotide sequence of the gene, showed an identity of 84.6% with that of the B. stearothermophilus enzyme [Nagata S, Tanizawa K, Esaki N, Sakamoto Y, Oshima T, Tanaka H, Soda K (1988) Biochemistry 27:9056-9062], although both enzymes were similar to each other in various enzymological properties such as thermostability, substrate and coenzyme specificities, and stereospecificity for hydrogen transfer from the C-4 of NADH. However, they were markedly distinct from each other in halophilicity; the B. licheniformis enzyme was much more stable than the other in the presence of high concentrations of salts.

Complementation of defective leucine decarboxylation in fibroblasts from a maple syrup urine disease patient by retrovirus-mediated gene transfer

Maple syrup urine disease (MSUD) is a genetic disease caused by a deficiency of branched-chain keto acid dehydrogenase, a mitochondrial multienzyme complex responsible for the decarboxylation of leucine, isoleucine and valine. The complex consists of three subunits (E1, E2, and E3) and mutations in any subunit result in MSUD. No satisfactory treatment for MSUD is currently available. Here we report the successful use of retroviral gene transfer to restore leucine decarboxylation activity in fibroblasts derived from a MSUD patient containing a mutation in the E2 subunit. A full-length human E2 cDNA was inserted into a retroviral vector (MFG) and a stable CRIP producer line was generated. The amphotropic virus was then used to transduce mutant human fibroblasts. In untransduced mutant cells, 1-14C leucine decarboxylation activity was less than 2% that of the wild-type cells. Decarboxylation of 1-14C leucine in transduced mutant cells was restored to 93% of the wild-type level. Correct targeting of the expressed wild-type E2 protein to mitochondria was demonstrated by comparing the immunofluorescent pattern of E2 and a mitochondrial marker protein. Stable expression of enzyme activity has been obtained for at least 7 weeks. In contrast to most previous gene therapy attempts, which replace a single enzyme defect, the present results demonstrate complementation of a phenotype resulting from a gene defect whose product is a part of a multienzyme complex. Based on these results, studies can now be undertaken to investigate the feasibility of gene therapy to correct MSUD.

Alteration in relative activities of phenylalanine dehydrogenase towards different substrates by site-directed mutagenesis

Glycine-124 and leucine-307 of phenylalanine dehydrogenase from Bacillus sphaericus were altered by site-specific mutagenesis to the corresponding residues in leucine dehydrogenase: alanine and valine, respectively. These two residues have previously been implicated from molecular modelling as important in determining the substrate discrimination of the two enzymes. Single and double mutants displayed lower activities towards L-phenylalanine and enhanced activity towards almost all aliphatic amino acid substrates tested compared to the wild-type, thus confirming the predictions made from molecular modelling.

A role for quaternary structure in the substrate specificity of leucine dehydrogenase

BACKGROUND

Glutamate, phenylalanine and leucine dehydrogenases catalyze the NAD(P)(+)-linked oxidative deamination of L-amino acids to the corresponding 2-oxoacids, and sequence homology between these enzymes clearly indicates the existence of an enzyme superfamily related by divergent evolution. We have undertaken structural studies on a number of members of this family in order to investigate the molecular basis of their differential amino acid specificity.

RESULTS

We have solved the X-ray structure of the leucine dehydrogenase from Bacillus sphaericus to a resolution of 2.2 A. Each subunit of this octameric enzyme contains 364 amino acids and folds into two domains, separated by a deep cleft. The nicotinamide ring of the NAD+ cofactor binds deep in this cleft, which is thought to close during the hydride transfer step of the catalytic cycle.

CONCLUSIONS

Comparison of the structure of leucine dehydrogenase with a hexameric glutamate dehydrogenase has shown that these two enzymes share a related fold and possess a similar catalytic chemistry. A mechanism for the basis of the differential amino acid specificity between these enzymes involves point mutations in the amino acid side-chain specificity pocket and subtle changes in the shape of this pocket caused by the differences in quaternary structure.

Construction and characterization of chimeric enzyme consisting of an amino-terminal domain of phenylalanine dehydrogenase and a carboxy-terminal domain of leucine dehydrogenase

Phenylalanine dehydrogenase of Thermoactinomyces intermedius acts preferentially on L-phenylalanine and L-tyrosine, whereas leucine dehydrogenase of Bacillus stearothermophilus acts almost exclusively on L-leucine and some other branched-chain L-amino acids. The two enzymes share a sequence similarity (47%). Aiming at elucidation of the mechanism of substrate recognition by the two amino acid dehydrogenases, we have genetically constructed a chimeric enzyme consisting of an N-terminal domain of phenylalanine dehydrogenase containing the substrate-binding region and a C-terminal domain of leucine dehydrogenase containing the NAD(+)-binding region. The chimeric enzyme purified to homogeneity acted on phenylalanine with a specific activity of 6% of that of the parental phenylalanine dehydrogenase and showed a broad substrate specificity in the oxidative deamination, like phenylalanine dehydrogenase. However, it acted much more effectively than phenylalanine dehydrogenase on isoleucine and valine. Its Km values for L-phenylalanine and L-leucine were similar to those of phenylalanine dehydrogenase. The substrate specificity of the chimeric enzyme in the reductive amination was an admixture of those of the two parent enzymes. These results suggest that the two domains of phenylalanine dehydrogenase and leucine dehydrogenase probably can fold independently. Accordingly, their chimera forms a new active enzyme which consists of their N- and C-terminal domains containing the substrate- and coenzyme-binding regions, respectively. However, the two domains of chimeric enzyme interact and communicate with each other to form a new active site and consistently show the new substrate specificity.

Role of the conserved glycyl residues located at the active site of leucine dehydrogenase from Bacillus stearothermophilus

A tetrapeptide sequence, Gly-Gly-(Gly/Ala)-Lys, containing a catalytically important lysyl residue, is highly conserved in NAD(P)+-dependent amino acid dehydrogenases. To elucidate functional roles of the glycyl residues in this conserved sequence Gly-77, Gly-78, and Gly-79 of the recombinant leucine dehydrogenase from Bacillus stearothermophilus have been individually replaced with Ala by site-directed mutagenesis. All of the mutant enzymes had Michaelis constants for alpha-keto-iso-caproate and ammonia several times larger than the wild-type enzyme while retaining considerable catalytic activities. However, inhibition constants for a substrate analog without an alpha-carbonyl group were unchanged by the mutations. On the other hand, the rate of inactivation by pyridoxal 5'-phosphate and the microenvironment of aromatic residues, in particular of the sole tryptophanyl residue (Trp-46) located in the vicinity of the active site, were affected by the mutations of the glycyl residues. All of these results suggest that the conserved glycyl residues are important for fine-tuning of the position and/or orientation of the epsilon-amino group of Lys-80 at the active site to function efficiently as a general-base catalyst. Furthermore, the Gly-77 and Gly-78 mutant enzymes had markedly decreased thermal stabilities, showing that these two glycyl residues are also critical for the conformational stability of this thermostable enzyme.

Cloning, sequencing, and expression of Rhodococcus L-phenylalanine dehydrogenase. Sequence comparisons to amino-acid dehydrogenases

L-Phenylalanine dehydrogenase catalyzes the NAD(+)-dependent, reversible, oxidative deamination of L-phenylalanine to form ammonia, phenyl pyruvate, and NADH. The enzyme has been purified to homogeneity from Rhodococcus sp. M4, and a partial amino acid sequence was obtained. A cosmid library of Rhodococcus sp. M4 genomic DNA was prepared and used to isolate a 2.5-kilobase PstI fragment that contained the pdh gene. The open reading frame of 1068 nucleotides encodes a polypeptide of 356 amino acids, portions of which match the amino acid sequence determined for the purified enzyme. Expression of the Rhodococcus pdh gene in Escherichia coli, which does not contain a phenylalanine dehydrogenase activity, yields a soluble enzyme exhibiting phenylalanine dehydrogenase activity. Both the enzyme purified from Rhodococcus and the enzyme expressed in E. coli are post-translationally modified by removal of the amino-terminal methionine. The overall amino acid sequence is homologous to previously reported sequences of leucine and phenylalanine dehydrogenases as well as several glutamate dehydrogenases. The amino-terminal portion of the enzyme contains residues involved in L-amino acid binding and catalysis, while the carboxyl-terminal portion contains the presumptive dinucleotide-binding domain. A detailed sequence comparison of Rhodococcus phenylalanine dehydrogenase with leucine, phenylalanine, and glutamate dehydrogenases suggests residues involved in general amino acid binding and others that provide for amino acid discrimination.

The purification, characterization, cloning and sequencing of the gene for a halostable and thermostable leucine dehydrogenase from Thermoactinomyces intermedius

Leucine dehydrogenase has been purified to homogeneity from a moderate thermophilic actinomycete, Thermoactinomyces intermedius IFO 14230. The enzyme can be stored without loss of its activity at a low temperature (e.g., 4 degrees C) for over two years. The enzyme was more thermostable at higher concentrations of salts such as NaCl and KCl. It retained about 90% of activity on incubation at 70 degrees C for at least 40 min in the presence of 3 M NaCl. The Michaelis constants for NAD, L-leucine, NADH, 2-oxoisocaproate and ammonia were determined to be 0.36, 2.0, 0.042, 0.63 and 118 mM, respectively, from initial-velocity analyses. The enzyme showed pro-S stereospecificity for hydrogen transfer of NADH in the reductive amination. The enzyme gene was cloned into Escherichia coli and its complete DNA sequence was determined. The leucine dehydrogenase gene (leudh) consists of a 1098-bp open reading frame and encodes 366 amino acid residues corresponding to a subunit (M(r) 40586) of the octameric enzyme. The amino acid sequence of the enzyme showed 80.7% similarity with that of the Bacillus stearothermophilus enzyme. The enzyme was overproduced in E. coli JM 109 having a recombinant plasmid, pULDH2, which was constructed from pUC18 and the leudh gene. The enzyme was purified from the cell extract to homogeneity in one day, with 78% recovery.

Involvement of conserved lysine 68 of Bacillus stearothermophilus leucine dehydrogenase in substrate binding

Lysine 68 of Bacillus stearothermophilus leucine dehydrogenase is highly conserved in the corresponding regions of NAD(P)+-dependent amino acid dehydrogenase sequences. To elucidate its functional role, the lysyl residue of the recombinant enzyme has been replaced with alanine or arginine by site-directed mutagenesis. Either mutation resulted in nearly complete loss of activity in the oxidative deamination, whereas only the mutation to alanine led to a marked increase in Michaelis constants for both amino and keto acid substrates. On the other hand, an ionizable group in the wild-type enzyme with a pKa value of 10.1-10.7, which must be protonated for binding of substrate and competitive inhibitor with an alpha-carboxyl group, was unobservable in both mutant enzymes. These results altogether led to the conclusion that Lys-68 is located at the active site of the enzyme and involved in binding of the alpha-carboxyl group of substrate through an ionic interaction. In addition, the alanine mutant enzyme that is almost inactive in the deamination but significantly active in the amination was greatly stimulated by exogenously added ammonia, suggesting that proper binding of the substrate alpha-carboxyl group at Lys-68 is essential for catalysis.

Crystallization and quaternary structure analysis of the NAD(+)-dependent leucine dehydrogenase from Bacillus sphaericus

The NAD(+)-dependent leucine dehydrogenase from Bacillus sphaericus has been crystallized by the hanging drop method of vapour diffusion, using ammonium sulphate as the precipitant. The crystals belong to the tetragonal system and are in space group I4, with unit cell dimensions of a = b = 138.4 A and c = 121.8 A. Considerations of the values of Vm, the space group symmetry and an analysis of a self-rotation function calculated on a preliminary data set collected to 3 A resolution show that the asymmetric unit contains a dimer with the twofold axis perpendicular to the crystallographic four fold, indicating that the quaternary structure of this enzyme is octameric. Leucine dehydrogenase belongs to a superfamily of amino acid dehydrogenases which display considerable differences in amino acid specificity and elucidation of its three-dimensional structure should enable the molecular basis of this differential specificity to be examined in detail.

Amperometric flow-injection analysis of creatinine based on immobilized creatinine deiminase, leucine dehydrogenase and L-amino acid oxidase

Leucine dehydrogenase/L-amino acid oxidase was proposed as an enzymatic conversion system for ammonia and its application to amperometric assay of creatinine was investigated. Ammonia formed by creatinine deiminase catalyzed hydrolysis of creatinine was converted to L-leucine by leucine dehydrogenase, and the oxidation of L-leucine by L-amino acid oxidase was detected with an oxygen electrode. Two approaches were proposed to overcome the problem of endogenous ammonia and L-amino acids. The first was using glutamate dehydrogenase prereactor to remove endogenous ammonia; endogenous L-amino acids were corrected by a separate run. In the second approach, endogenous ammonia and L-amino acids were simultaneously compensated with a two-channel system. It resulted in double peak recording that the flow was split and rejoined between the two ends of creatinine deiminase reactor and a delay coil and a reference column were properly set at one of the two-channels. One gave the sum response of all responsible compounds, the other that of endogenous interferences except creatinine. Both approaches were applied to creatinine assay in urine and the results showed a good agreement with those obtained from the Jaffe method.

Evidence for lysine 80 as general base catalyst of leucine dehydrogenase

To elucidate the functional role of the lysyl residue highly conserved in NAD(P)(+)-dependent amino acid dehydrogenases, Lys-80 of leucine dehydrogenase from Bacillus stearothermophilus has been mutated into Ala, Arg, or Gln. All of the mutant enzymes had markedly reduced activities in the oxidative deamination, whereas the Michaelis constants for substrate and coenzyme did not change significantly upon the mutation, except for a 10-30-fold increase in Km values for alpha-keto-iso-caproate in the Ala and Gln mutants. The pH profiles of kinetic parameters of the mutants considerably differed from those of the wild type, in which two ionizable groups with pKa values of 8.9 and 10.7 must be unprotonated for catalysis and protonated for substrate binding, respectively. Combined with the analyses of solvent isotope effect and inhibition by substrate analogs, these results unequivocally show that the epsilon-amino group of Lys-80 participates in catalysis as a general base, assisting the nucleophilic attack of a water molecule to the substrate alpha-carbon atom. Furthermore, the Ala mutant was markedly stimulated by primary amines depending on the pKa and molecular volume, suggesting that in the Ala mutant the added amines can partially replace the general base function of Lys-80 in the wild type enzyme.

Evolution of substrate diversity in the superfamily of amino acid dehydrogenases. Prospects for rational chiral synthesis

We have analysed the sequence homology between glutamate, leucine and phenylalanine dehydrogenases in the light of the solution of the structure of the glutamate dehydrogenase from Clostridium symbiosum. This analysis indicates that the elements of secondary structure comprising the core of the two domains in glutamate dehydrogenase are conserved in the other two enzymes. There is a striking conservation of the residues responsible for the recognition of the nicotinamide ring of the nucleotide cofactor and the backbone of the amino acid substrates. Furthermore, residues involved in a major conformational rearrangement on amino acid binding are preserved, as are those implicated in the catalytic chemistry. In contrast, the pattern of insertions/deletions between these enzymes is consistent with possible differences in quaternary structure. Differential substrate specificity between these enzymes is achieved by critical substitutions at the base of the binding pocket, which accommodates the side-chain of the amino acid substrate. This provides insights into the mutations necessary to produce new catalysts for the chiral synthesis of novel amino acids.

Neonatal screening for maple syrup urine disease by an enzyme-mediated colorimetric method

A microplate-based, enzyme-mediated, colorimetric method using L-leucine dehydrogenase (EC 1.4.1.9) has been developed for the determination of the combined branched-chain amino acids in plasma and blood-spot specimens. The test exhibits acceptable precision and fits into a new concept according to which the different parameters of neonatal screening programs for metabolic disorders, such as phenylalanine (phenylketonuria), galactose/galactose-1-phosphate (galactosemia) and branched-chain amino acids (maple syrup urine disease) can be measured in the same blood-spot eluate by use of different specific NAD(H)-dependent enzymes.

Site-directed mutagenesis of a hexapeptide segment involved in substrate recognition of phenylalanine dehydrogenase from Thermoactinomyces intermedius

Phenylalanine dehydrogenase from Thermoactinomyces intermedius and leucine dehydrogenase from Bacillus stearothermophilus show a 59% sequence similarity in their substrate-binding domains, although their substrate specificities are different. We prepared a phenylalanine dehydrogenase mutant enzyme whose inherent hexapeptide segment (124F-V-H-A-A-129R) in the substrate-binding domain was replaced by the corresponding part of leucine dehydrogenase (M-D-I-I-Y-Q) in order to investigate the mechanism of substrate recognition by phenylalanine dehydrogenase. The catalytic efficiencies (kcat/Km) of the mutant enzyme with aliphatic amino acids and aliphatic keto acids as substrates were 0.5 to 2% of those of the wild-type enzyme. In contrast, the efficiencies for L-phenylalanine and phenylpyruvate decreased to 0.008 and 0.035% of those of the wild-type enzyme, respectively. These results suggest that the hexapeptide segment plays an important role in the substrate recognition by phenylalanine dehydrogenase.

Sequence, transcriptional, and functional analyses of the valine (branched-chain amino acid) dehydrogenase gene of Streptomyces coelicolor

The gene encoding the valine (branched-chain amino acid) dehydrogenase (Vdh) from Streptomyces coelicolor has been characterized as follows. The vdh gene was identified by hybridization to a specific oligodeoxynucleotide that was synthesized on the basis of the N-terminal amino acid sequence of purified Vdh. Nucleotide sequence analysis predicts that the vdh gene contains a 364-amino-acid open reading frame that should produce a 38,305-M(r) protein. The deduced amino acid sequence of the Vdh protein is significantly similar to those of several other amino acid dehydrogenases, especially the leucine and phenylalanine dehydrogenases from Bacillus spp. The vdh gene is apparently transcribed from a single major transcriptional start point, separated by only 8 bp from the 5' end of a divergent transcript and located 63 bp upstream from the vdh translational start point. Mutants with a disrupted vdh gene have no detectable Vdh activity and have lost the ability to grow on valine, leucine, or isoleucine as the sole nitrogen source. This vdh mutation does not significantly affect growth or actinorhodin production in a minimal medium, yet the addition of 0.2% L-valine to the medium provokes approximately 32 and 80% increases in actinorhodin production in vdh+ and vdh strains, respectively.

Alterations in mitochondrial branched-chain amino acid metabolism in brain in acute hyperammonemic states

Production of 14CO2 and [14C]branched-chain keto acids (BCKA) was determined from [U-14C]branched-chain amino acids along with the activities of branched-chain amino acid transaminase (BCAA-T) and branched-chain keto acid dehydrogenase (BCKA-DH) in mitochondria isolated from the cerebral cortex of normal and hyperammonemic rats. Results indicated that the production of CO2, but not of keto acids, was suppressed while the activities of BCAA-T and BCKA-DH were not adversely affected in the mitochondria of hyperammonemic rats. Suppression in the oxidation of BCAA in hyperammonemic states was found to be due to increased efflux of BCKA from mitochondria.

Leucine dehydrogenase from Bacillus stearothermophilus: identification of active-site lysine by modification with pyridoxal phosphate

We have constructed an efficient expression plasmid for the leucine dehydrogenase gene previously cloned from Bacillus stearothermophilus. The recombinant enzyme was overproduced in Escherichia coli cells to a level of more than 30% of the total soluble protein upon induction with isopropyl beta-D-thiogalactopyranoside. The enzyme could be readily purified to homogeneity by heat treatment and a single step of ion-exchange chromatography. The purified enzyme was inactivated in a time-dependent manner upon incubation with pyridoxal 5'-phosphate (PLP) followed by reduction with sodium borohydride. The inactivation was completely prevented in the copresence of L-leucine and NAD+. Concomitantly with the inactivation, several molecules of PLP were incorporated into each subunit of the hexameric enzyme. Sequence analysis of the fluorescent peptides isolated from a proteolytic digest of the modified protein revealed that Lys80, Lys91, Lys206, and Lys265 were labeled. Among these residues, Lys80 was predominantly labeled and, in the presence of L-leucine and NAD+, was specifically protected from the labeling. Furthermore, a linear relationship of about 1:1 was observed between the extent of inactivation and the amount of PLP incorporated into Lys80. A slightly active mutant enzyme, in which Lys80 is replaced by Ala, was not inactivated at all by incubation with PLP, showing that the inactivation is correlated with the labeling of only Lys80. Lys80is conserved in the corresponding regions of all the amino acid dehydrogenase sequences reported to date. These results suggest that Lys80 is located at the active site and plays an important role in the catalytic function of leucine dehydrogenase.

Determination of (S)- and (R)-2-oxo-3-methylvaleric acid in plasma of patients with maple syrup urine disease

An enzymatic method for the separate measurement of both chiral 2-oxo-3-methylvaleric acid (OMV) compounds, (S)- and (R)-OMV, by NADH-dependent enantioselective amination using leucine dehydrogenase in the presence of a NADH regenerating system is described. This method allows the quantitative determination of all branched-chain 2-oxo acids, simultaneously. In plasma samples from classical maple syrup urine disease patients under therapy the average (R)-OMV/(S)-OMV ratio was 0.35 and great differences in the transamination equilibria of the diastereomeric branched-chain amino acids L-isoleucine and L-alloisoleucine were demonstrated.

A large-scale preparative electrophoretic method for the purification of pyridine nucleotide-linked dehydrogenases

A large-scale preparative polyacrylamide gel electrophoresis (PAGE) method that uses a 1.5- or a 2.0-cm-thick slab gel has been developed for the purification of NAD-dependent dehydrogenases. With the 2.0-cm-thick gel, a maximum volume (up to about 160 ml) of enzyme sample was applied to a gel plate, resulting in the application of a large amount of protein and enzyme. After the electrophoretic run, the enzyme band on the gel was detected by activity staining and recovered from the gel by extraction with a fairly loose-fitting glass-Teflon homogenizer. NAD-dependent alanine dehydrogenase, leucine dehydrogenase, and glycerol dehydrogenase were purified in high yields (more than 80%) by the preparative PAGE method. The method can be carried out using a simple slab gel apparatus, which is modified from the conventional analytical apparatus for the purpose of preparative PAGE under conditions used for routine analytical runs. Thus, the method may be suitable for use in purifying NAD(P)-dependent dehydrogenases and many other enzymes after conventional chromatography such as dye-ligand affinity chromatography or ion-exchange chromatography.

Identification and analysis of the genes coding for the putative pyruvate dehydrogenase enzyme complex in Acholeplasma laidlawii

A monospecific antibody recognizing two membrane proteins in Acholeplasma laidlawii identified a plasmid clone from a genomic library. The nucleotide sequence of the 4.6-kbp insert contained four sequential genes coding for proteins of 39 kDa (E1 alpha, N terminus not cloned), 36 kDa (E1 beta), 57 kDa (E2), and 36 kDa (E3; C terminus not cloned). The N termini of the cloned E2, E1 beta, and native A. laidlawii E2 proteins were verified by amino acid sequencing. Computer-aided searches showed that the translated DNA sequences were homologous to the four subenzymes of the pyruvate dehydrogenase complexes from gram-positive bacteria and humans. The plasmid-encoded 57-kDa (E2) protein was recognized by antibodies against the E2 subenzymes of the pyruvate and oxoglutarate dehydrogenase complexes from Bacillus subtilis. A substantial fraction of the E2 protein as well as part of the pyruvate dehydrogenase enzymatic activity was associated with the cytoplasmic membrane in A. laidlawii. In vivo complementation with three different Escherichia coli pyruvate dehydrogenase-defective mutants showed that the four plasmid-encoded proteins were able to restore pyruvate dehydrogenase enzyme activity in E. coli. Since A. laidlawii lacks oxoglutarate dehydrogenase and most likely branched-chain dehydrogenase enzyme complex activities, these results strongly suggest that the sequenced genes code for the pyruvate dehydrogenase complex.

Conversion of ammonia or urea into essential amino acids, L-leucine, L-valine, and L-isoleucine using artificial cells containing an immobilized multienzyme system and dextran-NAD. L-lactic dehydrogenase for coenzyme recycling

A multienzyme system consisting of leucine dehydrogenase (EC 1.4.1.9), L-lactic dehydrogenase (EC 1.1.1.27), urease (EC 3.5.1.5), and dextran-NAD+ was microencapsulated within artificial cells. This system could convert ammonia and urea into essential amino acids, L-leucine, L-valine, and L-isoleucine. L-lactate acted as a cosubstrate for the regeneration of dextran-NADH. Greater concentrations of L-lactate favored the higher conversion ratios. The effects of ammonium salts and urea on reaction rate were also studied. The relative reaction rates in ammonium salts solutions were 44.6-78.8% of those in urea solutions. More than 90% of the original activity was retained when artificial cells were kept at 4 degrees C for 6 wk.

Leucine dehydrogenase from Corynebacterium pseudodiphtheriticum: purification and characterization

Leucine dehydrogenase [EC 1.4.1.9] was purified to homogeneity from Corynebacterium pseudodiphtheriticum ICR 2210. The enzyme consisted of a single polypeptide with a molecular weight of about 34,000. Stepwise Edman degradation provided the N-terminal sequence of the first 24 amino acids, and carboxypeptidase Y digestion provided the C-terminal sequence of the last 2 amino acids. Although the enzyme catalyzed the reversible deamination of various branched-chain L-amino acids, L-valine was the best substrate for oxidative deamination at pH 10.9 and the saturated concentration. The enzyme, however, had higher reactivity for L-leucine, and the kcat/Km value for L-leucine was higher than that for L-valine. The enzyme required NAD+ as a natural coenzyme. The NAD+ analogs 3-acetylpyridine-NAD+ and deamino-NAD+ were much better coenzymes than NAD+. The enzyme activity was significantly reduced by sulfhydryl reagents and pyridoxal 5'-phosphate. D-Enantiomers of the substrate amino acids competitively inhibited the oxidation of L-valine.

Comparison of alanine production after L-leucine and AMP deamination in an enzymatic model and in muscle specimens

The amination of 2-oxoglutarate to glutamate by the deamination of leucine followed by the transamination of pyruvate to alanine in skeletal muscle is generally accepted. However, alanine formation following AMP deamination by AMP deaminase is still questionable even though it is theoretically possible. For this reason, we investigated in an enzymatic model the dependence of alanine yield both on the increasing concentration of AMP and leucine as amino group donors as well as on AMP deaminase and leucine dehydrogenase augmentation. Up to a concentration of 375 microM neither of the amino group donors produced a difference in the glutamate nor alanine yield. At a concentration of 500 microM ammonia formation was less, but alanine production was higher when leucine was present as a starting material. However, in muscle samples obtained from trained or untrained rats we did not find an increase in alanine yield when AMP was added to the muscle sample, even though NH3 production was significantly higher in samples with than in those without AMP. This discrepancy might be further elucidated by hindquarter perfusion experiments, in which alanine release would be determined after AMP deamination enhanced by electrostimulation of these muscle groups.

Enzymatic in situ determination of stereospecificity of NAD-dependent dehydrogenases

Amino acid racemases inherently catalyze the exchange of alpha-hydrogen of amino acids with deuterium during racemization in 2H2O. When the reactions catalyzed by alanine racemase (EC 5.1.1.1) and L-alanine dehydrogenase (EC 1.4.1.1), which is pro-R specific for the C-4 hydrogen transfer of NADH, are coupled in 2H2O, [4R-2H]NADH is exclusively produced. Similarly, [4S-2H]NADH is made in 2H2O with amino-acid racemase with low substrate specificity (EC 5.1.1.10) and L-leucine dehydrogenase (EC 1.4.1.9), which is pro-S specific. We have established a simple procedure for the in situ analysis of stereospecificity of C-4 hydrogen transfer of NADH by an NAD-dependent dehydrogenase by combination with either of the above two couples of enzymes in the same reaction mixture. When the C-4 hydrogen of NAD+ is fully retained after sufficient incubation, the stereospecificity of hydrogen transfer by a dehydrogenase is the same as that of alanine dehydrogenase or leucine dehydrogenase. However, when the C-4 hydrogen of NAD+ is exchanged with deuterium, the enzyme to be examined shows the different stereospecificity from alanine dehydrogenase or leucine dehydrogenase. Thus, we can readily determine the stereospecificity by 1H NMR measurement without isolation of the coenzymes and products.

Overproduction of thermostable leucine dehydrogenase of Bacillus stearothermophilus and its one-step purification from recombinant cells of Escherichia coli

We have cloned the leucine dehydrogenase (EC 1.4.1.9) gene from a thermophile, Bacillus stearothermophilus, into Escherichia coli MV1184 with a vector plasmid, pUC119. The cloned cells produced a large amount of the thermostable enzyme, which corresponds to about 60% of the total soluble protein. The enzyme was purified to more than 95% homogeneity by only one step, heat treatment of the cell-extracts, with an average yield of 75 mg/g of wet cells (obtained from 100 ml of the culture).