Purification and properties of the flavoenzyme D-lactate dehydrogenase from Megasphaera elsdenii

Lactate dehydrogenase D is a general dehydrogenase for D-2-hydroxyacids and is associated with D-lactic acidosis

Mammalian lactate dehydrogenase D (LDHD) catalyzes the oxidation of D-lactate to pyruvate. LDHD mutations identified in patients with D-lactic acidosis lead to deficient LDHD activity. Here, we perform a systematic biochemical study of mouse LDHD (mLDHD) and determine the crystal structures of mLDHD in FAD-bound form and in complexes with FAD, Mn2+ and a series of substrates or products. We demonstrate that mLDHD is an Mn2+-dependent general dehydrogenase which exhibits catalytic activity for D-lactate and other D-2-hydroxyacids containing hydrophobic moieties, but no activity for their L-isomers or D-2-hydroxyacids containing hydrophilic moieties. The substrate-binding site contains a positively charged pocket to bind the common glycolate moiety and a hydrophobic pocket with some elasticity to bind the varied hydrophobic moieties of substrates. The structural and biochemical data together reveal the molecular basis for the substrate specificity and catalytic mechanism of LDHD, and the functional roles of mutations in the pathogenesis of D-lactic acidosis.

The Pseudomonas aeruginosa PAO1 metallo flavoprotein d-2-hydroxyglutarate dehydrogenase requires Zn2+ for substrate orientation and activation

Pseudomonas aeruginosa PAO1 d-2-hydroxyglutarate (D2HG) dehydrogenase (PaD2HGDH) oxidizes D2HG to 2-ketoglutarate during the vital l-serine biosynthesis and is a potential therapeutic target against P. aeruginosa. PaD2HGDH, which oxidizes d-malate as an alternative substrate, has been demonstrated to be a metallo flavoprotein that requires Zn2+ for activity. However, the role of Zn2+ in the enzyme has not been elucidated, making it difficult to rationalize why nature employs both a redox center and a metal ion for catalysis in PaD2HGDH and other metallo flavoenzymes. In this study, recombinant His-tagged PaD2HGDH was purified to high levels in the presence of Zn2+ or Co2+ to investigate the metal's role in catalysis. We found that the flavin reduction step was reversible and partially rate limiting for the enzyme's turnover at pH 7.4 with either D2HG or d-malate with similar rate constants for both substrates, irrespective of whether Zn2+ or Co2+ was bound to the enzyme. The steady-state pL profiles of the kcat and kcat/Km values with d-malate demonstrate that Zn2+ mediates the activation of water coordinated to the metal. Our data are consistent with a dual role for the metal, which orients the hydroxy acid substrate in the enzyme's active site and rapidly deprotonates the substrate to yield an alkoxide species for hydride transfer to the flavin. Thus, we propose a catalytic mechanism for PaD2HGDH oxidation that establishes Zn2+ as a cofactor required for substrate orientation and activation during enzymatic turnover.

Uncovering Zn2+ as a cofactor of FAD-dependent Pseudomonas aeruginosa PAO1 d-2-hydroxyglutarate dehydrogenase

Pseudomonas aeruginosa couples the oxidation of d-2-hydroxyglutarate (D2HG) to l-serine biosynthesis for survival, using d-2-hydroxyglutarate dehydrogenase from P. aeruginosa (PaD2HGDH). Knockout of PaD2HGDH impedes P. aeruginosa growth, making PaD2HGDH a potential target for therapeutics. Previous studies showed that the enzyme's activity increased with Zn2+, Co2+, or Mn2+ but did not establish the enzyme's metal composition and whether the metal is an activator or a required cofactor for the enzyme, which we addressed in this study. Comparable to the human enzyme, PaD2HGDH showed only 15% flavin reduction with D2HG or d-malate. Upon purifying PaD2HGDH with 1 mM Zn2+, the Zn2+:protein stoichiometry was 2:1, yielding an enzyme with ∼40 s-1kcat for d-malate. Treatment with 1 mM EDTA decreased the Zn2+:protein ratio to 1:1 without changing the kinetic parameters with d-malate. We observed complete enzyme inactivation for the metalloapoenzyme with 100 mM EDTA treatment, suggesting that Zn2+ is essential for PaD2HGDH activity. The presence of Zn2+ increased the flavin N3 atom pKa value to 11.9, decreased the flavin ε450 at pH 7.4 from 13.5 to 11.8 mM-1 cm-1, and yielded a charged transfer complex with a broad absorbance band >550 nm, consistent with a Zn2+-hydrate species altering the electronic properties of the enzyme-bound FAD. The exogenous addition of Zn2+, Co2+, Cd2+, Mn2+, or Ni2+ to the metalloapoenzyme reactivated the enzyme in a sigmoidal pattern, consistent with an induced fit rapid-rearrangement mechanism. Collectively, our data demonstrate that PaD2HGDH is a Zn2+-dependent metallo flavoprotein, which requires Zn2+ as an essential cofactor for enzyme activity.

Human d-lactate dehydrogenase deficiency by LDHD mutation in a patient with neurological manifestations and mitochondrial complex IV deficiency

BACKGROUND

d-lactate, one of the isomers of lactate, exists in a low concentration in healthy individuals and it can be oxidized to pyruvate catalyzed by d-lactate dehydrogenase. Excessive amount of d-lactate causes d-lactate acidosis associated with neurological manifestations.

METHODS AND RESULTS

We report here a patient with developmental delay, cerebellar ataxia, and transient hepatomegaly. Enzyme analysis in the patient's skin fibroblast showed decreased mitochondrial complex IV activity. Using whole exome sequencing, we identified compound heterozygous variants in the LDHD gene, which encodes the d-lactate dehydrogenase, consisting of a splice site variant c.469+1dupG and a missense variant c.752C>T, p.(Thr251Met) which are pathogenic and likely pathogenic respectively according to the American College of Medical Genetics and Genomics (ACMG) classification. The serum d-lactate level was subsequently detected to be elevated (0.61 mmol/L, reference value: 0-0.25 mmol/L).

CONCLUSION

This is the third report on LDHD mutations associated with d-lactate elevation and was first reported to have decreased mitochondrial complex IV activity. The study provides more information on this rare metabolic condition but the association of LDHD deficiency with the clinical presentations requires further investigations.

Identification of human D lactate dehydrogenase deficiency

Phenotypic and biochemical categorization of humans with detrimental variants can provide valuable information on gene function. We illustrate this with the identification of two different homozygous variants resulting in enzymatic loss-of-function in LDHD, encoding lactate dehydrogenase D, in two unrelated patients with elevated D-lactate urinary excretion and plasma concentrations. We establish the role of LDHD by demonstrating that LDHD loss-of-function in zebrafish results in increased concentrations of D-lactate. D-lactate levels are rescued by wildtype LDHD but not by patients’ variant LDHD, confirming these variants’ loss-of-function effect. This work provides the first in vivo evidence that LDHD is responsible for human D-lactate metabolism. This broadens the differential diagnosis of D-lactic acidosis, an increasingly recognized complication of short bowel syndrome with unpredictable onset and severity. With the expanding incidence of intestinal resection for disease or obesity, the elucidation of this metabolic pathway may have relevance for those patients with D-lactic acidosis.

D-lactic acidosis typically occurs in the context of short bowel syndrome; excess D-lactate is produced by intestinal bacteria. Here, the authors identify two point mutations in the human lactate dehydrogenase D (LDHD) gene that cause enzymatic loss of function and are associated with elevated plasma D-lactate.

New insights into the operative network of FaEO, an enone oxidoreductase from Fragaria x ananassa Duch

The 2-methylene-furan-3-one reductase or Fragaria x ananassa Enone Oxidoreductase (FaEO) catalyses the last reductive step in the biosynthesis of 4-hydroxy-2,5-dimethyl-3(2H)-furanone, a major component in the characteristic flavour of strawberries. In the present work, we describe the association between FaEO and the vacuolar membrane of strawberry fruits. Even if FaEO lacks epitopes for stable or transient membrane-interactions, it contains a calmodulin-binding region, suggesting that in vivo FaEO may be associated with the membrane via a peripheral protein complex with calmodulin. Moreover, we also found that FaEO occurs in dimeric form in vivo and, as frequently observed for calmodulin-regulated proteins, it may be expressed in different isoforms by alternative gene splicing. Further mass spectrometry analysis confirmed that the isolated FaEO consists in the already known isoform and that it is the most characteristic during ripening. Finally, a characterization by absorption spectroscopy showed that FaEO has specific flavoprotein features. The relevance of these findings and their possible physiological implications are discussed.

Interaction between NADH and electron-transferring flavoprotein from Megasphaera elsdenii

Electron-transferring flavoprotein (ETF) from the anaerobic bacterium Megasphaera elsdenii is a heterodimer containing two FAD cofactors. Isolated ETF contains only one FAD molecule, FAD-1, because the other, FAD-2, is lost during purification. FAD-2 is recovered by adding FAD to the isolated ETF. The two FAD molecules in holoETF were characterized using NADH. Spectrophotometric titration of isolated ETF with NADH showed a two-electron reduction of FAD-1 according to a monophasic profile indicating that FAD-1 receives electrons from NADH without involvement of FAD-2. When holoETF was titrated with NADH, FAD-2 was reduced to an anionic semiquinone and then was fully reduced before the reduction of FAD-1. The midpoint potential values at pH 7 were +81, -136 and -279 mV for the reduction of oxidized FAD-2 to semiquinone, semiquinone to the fully reduced FAD-2 and the two-electron reduction of FAD-1, respectively. Both FAD-1 and FAD-2 in holoETF were reduced by excess NADH very rapidly. The reduction of FAD-2 was slowed by replacement of FAD-1 with 8-cyano-FAD indicating that FAD-2 receives electrons from FAD-1 but not from NADH directly. The present results suggest that FAD-2 is the counterpart of the FAD in human ETF, which contains one FAD and one AMP.

An unconventional hexacoordinated flavohemoglobin from Mycobacterium tuberculosis

Being an obligate aerobe, Mycobacterium tuberculosis faces a number of energetic challenges when it encounters hypoxia and environmental stress during intracellular infection. Consequently, it has evolved innovative strategies to cope with these unfavorable conditions. Here, we report a novel flavohemoglobin (MtbFHb) from M. tuberculosis that exhibits unique features within its heme and reductase domains distinct from conventional FHbs, including the absence of the characteristic hydrogen bonding interactions within the proximal heme pocket and mutations in the FAD and NADH binding regions of the reductase domain. In contrast to conventional FHbs, it has a hexacoordinate low-spin heme with a proximal histidine ligand lacking imidazolate character and a distal heme pocket with a relatively low electrostatic potential. Additionally, MtbFHb carries a new FAD binding site in its reductase domain similar to that of D-lactate dehydrogenase (D-LDH). When overexpressed in Escherichia coli or Mycobacterium smegmatis, MtbFHb remained associated with the cell membrane and exhibited D-lactate:phenazine methosulfate reductase activity and oxidized D-lactate into pyruvate by converting the heme iron from Fe(3+) to Fe(2+) in a FAD-dependent manner, indicating electron transfer from D-lactate to the heme via FAD cofactor. Under oxidative stress, MtbFHb-expressing cells exhibited growth advantage with reduced levels of lipid peroxidation. Given the fact that D-lactate is a byproduct of lipid peroxidation and that M. tuberculosis lacks the gene encoding D-LDH, we propose that the novel D-lactate metabolizing activity of MtbFHb uniquely equips M. tuberculosis to balance the stress level by protecting the cell membrane from oxidative damage via cycling between the Fe(3+)/Fe(2+) redox states.

Constitutive NADPH-dependent electron transferase activity of the Nox4 dehydrogenase domain

NADPH oxidase 4 (Nox4) is constitutively active, while Nox2 requires the cytosolic regulatory subunits p47(phox) and p67(phox) and activated Rac with activation by phorbol 12-myristate 13-acetate (PMA). This study was undertaken to identify the domain on Nox4 that confers constitutive activity. Lysates from Nox4-expressing cells exhibited constitutive NADPH- but not NADH-dependent hydrogen peroxide production with a K(m) for NADPH of 55 +/- 10 microM. The concentration of Nox4 in cell lysates was estimated using Western blotting and allowed calculation of a turnover of approximately 200 mol of H(2)O(2) min(-1) (mol of Nox4)(-1). A chimeric protein (Nox2/4) consisting of the Nox2 transmembrane (TM) domain and the Nox4 dehydrogenase (DH) domain showed H(2)O(2) production in the absence of cytosolic regulatory subunits. In contrast, chimera Nox4/2, consisting of the Nox4 TM and Nox2 DH domains, exhibited PMA-dependent activation that required coexpression of regulatory subunits. Nox DH domains from several Nox isoforms were purified and evaluated for their electron transferase activities. Nox1 DH, Nox2 DH, and Nox5 DH domains exhibited barely detectable activities toward artificial electron acceptors, while the Nox4 DH domain exhibited significant rates of reduction of cytochrome c (160 min(-1), largely superoxide dismutase-independent), ferricyanide (470 min(-1)), and other electron acceptors (artificial dyes and cytochrome b(5)). Rates were similar to those observed for H(2)O(2) production by the Nox4 holoenzyme in cell lysates. The activity required added FAD and was seen with NADPH but not NADH. These results indicate that the Nox4 DH domain exists in an intrinsically activated state and that electron transfer from NADPH to FAD is likely to be rate-limiting in the NADPH-dependent reduction of oxygen by holo-Nox4.

Identification of putative mammalian D-lactate dehydrogenase enzymes

Mammalian L-isomer dehydrogenases represent an expansive and well characterized class of metabolic enzymes. Surprisingly, little is known regarding their evolutionarily distinct counterparts, D-isomer dehydrogenases, since few mammalian D-isomer 2-hydroxy acid enzymes have been isolated. Here we present the identification and initial characterization of putative human and murine D-lactate dehydrogenases (DLD) that can interact with the muscle-specific cysteine-rich protein CRP3/MLP. Sequence analysis reveals that the human and mouse transcripts encode novel proteins that display strong similarities to the yeast D-lactate dehydrogenase proteins DLD1, AIP2, and YEL071W. Expression analysis of the mammalian proteins indicates widespread distribution with transcripts present in striated muscle tissues and a variety of other tissue types. Immunofluorescence subcellular localization of the mouse DLD protein indicates that it resides within mitochondria, a feature shared by many dehydrogenases. The identification of the human and mouse DLD clones provides new insight regarding the activity of D-isomer-specific enzymes in mammalian cells.

The Membrane-bound L- and D-lactate dehydrogenase activities in mitochondria from Euglena gracilis

The activity of the pyridine nucleotide-independent lactate dehydrogenase (iLDH) was characterized in mitochondria isolated from the protist Euglena gracilis. The dissociation constants for L- and D-lactate were similar, but the V(max) was higher with the d isomer. A ping-pong kinetic mechanism was displayed with 2,4-dichlorophenol-indolphenol (DCPIP), or coenzyme Q(1), reacting as the second substrate with the modified, reduced enzyme. Oxamate was a competitive inhibitor against both L- and D-lactate. Oxalate exerted a mixed-type inhibition regarding L- or D-lactate and also against DCPIP. The rate of L-lactate uptake was partially inhibited by mersalyl and lower than the rate of dehydrogenation, which was mersalyl-insensitive. These data suggested that the active site of L-iLDH was orientated toward the intermembrane space. The following observations indicated the existence of two stereo-specific iLDH enzymes in the inner membrane of Euglena mitochondria: a greater affinity of the D-iLDH for both inhibitors, D-iLDH thermo-stability at 70 degrees C and denaturation of L-iLDH, opposite signs in the enthalpy change for the association reaction of the isomers to the enzyme, differential solubilization of both activities with detergents, and different molecular mass.

Redox potentials and their pH dependence of D-amino-acid oxidase of Rhodotorula gracilis and Trigonopsis variabilis

The redox potentials and pH characteristics of D-amino-acid oxidase (EC 1.4.3.3; DAAO) from the yeast Rhodotorula gracilis and Trigonopsis variabilis were measured in the pH range 6.5-8.5 at 15 degrees C. In the free enzyme form, the anionic red semiquinone is quantitatively formed in both DAAOs, indicating that a two single-electron transfer mechanism is active. The semiquinone species is also thermodynamically stable, as indicated by the large separation of the single-electron transfer potentials. The first electron potential is pH-independent, while the second electron transfer is pH-dependent exhibiting a approximately -60 mV/pH unit slope, consistent with a one-electron/one-proton transfer. In the presence of the substrate analogue benzoate, the two-electron transfer is the thermodynamically favoured process for both DAAOs, with only a quantitative difference in the stabilization of the anionic semiquinone. Clearly binding of the substrate (or substrate analogue) modulates the redox properties of the two enzymes. In both cases, in the presence and absence of benzoate, the slope of Em vs. pH (-30 mV/pH unit) corresponds to an overall two-electron/one-proton transfer in the reduction to yield the anionic reduced flavin. This behaviour is similar to that reported for DAAO from pig kidney. The differences in potentials and the stability of the semiquinone intermediate measured for the three DAAOs probably stem from different isoalloxazine environments. In the case of R. gracilis DAAO, the low stability of the semiquinone form in the DAAO-benzoate complex can be explained by the shift in position of the side chain of Arg285 following substrate analogue binding.

The Archaeoglobus fulgidus D-lactate dehydrogenase is a Zn(2+) flavoprotein

Archaeoglobus fulgidus, a hyperthermophilic, archaeal sulfate reducer, is one of the few organisms that can utilize D-lactate as a sole source for both carbon and electrons. The A. fulgidus open reading frame, AF0394, which is predicted to encode a D-(-)-lactate dehydrogenase (Dld), was cloned, and its product was expressed in Escherichia coli as a fusion with the maltose binding protein (MBP). The 90-kDa MBP-Dld fusion protein was more efficiently expressed in E. coli when coexpressed with the E. coli dnaY gene, encoding the arginyl tRNA for the codons AGA and AGG. When cleaved from the fusion protein by treatment with factor Xa, the recombinant Dld (rDld) has an apparent molecular mass of 50 kDa, similar to that of the native A. fulgidus Dld enzyme. Both the purified MBP-Dld fusion protein and its rDld cleavage fragment have lactate dehydrogenase activities specific for D-lactate, are stable at 80 degrees C, and retain activity after exposure to oxygen. The flavin cofactor FAD, which binds rDld apoprotein with a 1:1 stoichiometry, is essential for activity.

Effect of aniline on ethanol oxidation and carbohydrate metabolism in isolated hepatocytes

The addition of aniline to isolated hepatocytes derived from fasted rats and incubated with ethanol, caused a 30-60% decrease in the rate of ethanol oxidation. The degree of inhibition was dependent on aniline concentration, 5 mM causing near-maximal inhibition. Aniline reduced the activity of alcohol dehydrogenase in a noncompetitive manner, but had no effect on aldehyde dehydrogenase activity nor on reducing-equivalent transfer between the cytoplasm and mitochondria. The inhibition of alcohol dehydrogenase by aniline was associated with a decrease in the inhibitory effects of ethanol on glycolysis. Aniline, added to hepatocytes in the presence or absence of ethanol, inhibited gluconeogenesis from lactate and pyruvate, but not from sorbitol or fructose.

Presence of lactate dehydrogenase and lactate racemase in Megasphaera elsdenii grown on glucose or lactate

Activity of D-lactate dehydrogenase (D-LDH) was shown not only in cell extracts from Megasphaera elsdenii grown on DL-lactate, but also in cell extracts from glucose-grown cells, although glucose-grown cells contained approximately half as much D-LDH as DL-lactate-grown cells. This indicates that the D-LDH of M. elsdenii is a constitutive enzyme. However, lactate racemase (LR) activity was present in DL-lactate-grown cells, but was not detected in glucose-grown cells, suggesting that LR is induced by lactate. Acetate, propionate, and butyrate were produced similarly from both D- and L-lactate, indicating that LR can be induced by both D- and L-lactate. These results suggest that the primary reason for the inability of M. elsdenii to produce propionate from glucose is that cells fermenting glucose do not synthesize LR, which is induced by lactate.

Purification and properties of L-mandelate dehydrogenase and comparison with other membrane-bound dehydrogenases from Acinetobacter calcoaceticus

L-Mandelate dehydrogenase was purified from Acinetobacter calcoaceticus by Triton X-100 extraction from a 'wall + membrane' fraction, ion-exchange chromatography on DEAE-Sephacel, (NH4)2SO4 fractionation and gel filtration followed by further ion-exchange chromatography. The purified enzyme was partially characterized with respect to its subunit Mr (44,000), pH optimum (7.5), pI value (4.2), substrate specificity and susceptibility to various potential inhibitors including thiol-blocking reagents. FMN was identified as the non-covalently bound cofactor. The properties of L-mandelate dehydrogenase are compared with those of D-mandelate dehydrogenase, D-lactate dehydrogenase and L-lactate dehydrogenase from A. calcoaceticus.

Purification of 2-hydroxyglutaryl-CoA dehydratase from Acidaminococcus fermentans. An iron-sulfur protein

1. The (R)-2-hydroxyglutaryl-CoA dehydratase system from Acidaminococcus fermentans was separated by chromatography of cell-free extracts on Q-Sepharose into two components, an activator and the actual dehydratase. The latter enzyme was further purified to homogeneity by chromatography on blue-Sepharose. It is an iron-sulfur protein (Mr 210,000) consisting of two different polypeptides (alpha, Mr 55,000, and beta, Mr 42,000) in an alpha 2 beta 2 structure with probably two [4Fe-4S] centers. After activation this purified enzyme catalysed the dehydration of (R)-2-hydroxyglutarate only in the presence of acetyl-CoA and glutaconate CoA-transferase, demonstrating that the thiol ester and not the free acid is the substrate of the dehydration. The result led to a modification of the hydroxyglutarate pathway of glutamate fermentation. 2. The activation of the dehydratase by the flow-through from Q-Sepharose concentrated by ultrafiltration required NADH, MgCl2, ATP and strict anaerobic conditions. This fraction was designated as Ao. Later when the concentration was performed by chromatography on phenyl-Sepharose, an NADH-independent form of the activator, designated as A*, was obtained. This enzyme, which required only ATP for activation of the dehydratase, was purified further by affinity chromatography on ATP-agarose. It contains neither iron nor inorganic sulfur. A*, as well as the activated dehydratase, were irreversibly inactivated by exposure to air within less than 15 min. The activated dehydratase but not A* was also inactivated by 1 mM hydroxylamine or by 0.1 mM 2,4-dinitrophenol. 3. The (R)-2-hydroxyglutaryl-CoA dehydratase system is closely related the that of (R)-lactoyl-CoA dehydratase from Clostridium propionicum as described by R. D. Kuchta and R. H. Abeles [(1985) J. Biol. Chem. 260, 13,181-13,189].

Redox properties of electron-transferring flavoprotein from Megasphaera elsdenii

Electron-transferring flavoprotein (ETF) from the anaerobic bacterium Megasphaera elsdenii catalyzes electron transfer from NADH or D-lactate dehydrogenase to butyryl-CoA dehydrogenase. As a basis for understanding the interactions of ETF with its substrates, we report here on the redox properties of ETF alone. ETF exhibited reversible, two-electron transfer during electrochemical reduction in the presence of mediator dyes. The midpoint redox potentials of the FAD cofactor were -0.185 V at pH 5.5, -0.259 V at pH 7.1 and -0.269 +/- 0.013 V at pH 8.4, all versus the standard hydrogen electrode In the presence of the indicator dye 1-deazariboflavin, the Nernst slopes were 0.029 V and 0.026 V at pH 5.5 and pH 7.1, respectively, compared with an expected value of 0.028 V at 10 degrees C. At pH 8.4, in the presence of 2-hydroxy-1,4-naphthoquinone or phenosafranine, the Nernst slope varied from 0.021 V to 0.041 V. In the experiments at pH 8.4, equilibration was very slow in the reductive direction and a difference of as much as 30 mV was observed between reductive and oxidative midpoints. ETF exhibited no thermodynamic stabilization of the radical form of the FAD cofactor during electrochemical reduction at pH 5.5, 7.1 or 8.4. However, up to 93% of kinetically stable, anionic radical was produced by dithionite titration at pH 8.5. Molar absorptivities of ETF radical were 17,000 M-1 X cm-1 at 365 nm and 5100 M-1 X cm-1 at 450 nm. The four ETF preparations used here contained less than 7% 6-OH-FAD. However, two of the preparations contained significant amounts (up to 30%) of flavin which stabilized radical and reduced at potentials 0.2 V more positive than those required for reduction of the major form of ETF. This is referred to as the B form of ETF. The proportion of ETF-FAD in the B form was increased by incubation with free FAD or by a cycle of reduction and reoxidation. These treatments caused marked changes in the absorption spectrum of oxidized ETF and decreases of 20-25% in ETF units/A450.

Absolute stereochemistry of flavins in enzyme-catalyzed reactions

The 8-demethyl-8-hydroxy-5-deaza-5-carba analogues of FMN and FAD have been synthesized. Several apoproteins of flavoenzymes were successfully reconstituted with these analogues. This and further tests established that these analogues could serve as general probes for flavin stereospecificity in enzyme-catalyzed reactions. The method used by us involved stereoselective introduction of label on one enzyme combined with transfer to and analysis on a second enzyme. Using as a reference glutathione reductase from human erythrocytes for which the absolute stereochemistry of catalysis is known from X-ray studies [Pai, E. F., & Schulz, G. E. (1983) J. Biol. Chem. 258, 1752-1758], we were able to determine the absolute stereospecificities of other flavoenzymes. We found that glutathione reductase (NADPH), general acyl-CoA dehydrogenase (acyl-CoA), mercuric reductase (NADPH), thioredoxin reductase (NADPH), p-hydroxybenzoate hydroxylase (NADPH), melilotate hydroxylase (NADH), anthranilate hydroxylase (NADPH), and glucose oxidase (glucose) all use the re face of the flavin ring when interacting with the substrates given in parentheses.

Isolation and properties of an H2O-forming NADH oxidase from Streptococcus faecalis

An H2O-forming NADH oxidase from Streptococcus faecalis, recently described [Hoskins, D. D., Whiteley, H. R. and Mackler, B. (1962) J. Biol. Chem. 237, 2647-2651], has been isolated as a uniform protein with specific activity 690 U/mg in a total yield of 50% by a two-step affinity chromatography procedure. The enzyme is metal-free and has a molecular mass of about 51 000 Da and probably consists of a single polypeptide chain. As shown by fluorimetric titration, the prosthetic group is 1 mol FAD/mol protein. The affinity behaviour of the enzyme gives evidence for the existence of a dinucleotide-binding domain capable of binding NADH or FAD. The enzyme is specific for NADH (Km = 4.1 X 10(-5) M), NADPH is not oxidized. O2 is the preferred electron acceptor, in addition FAD and, very slowly, one-electron acceptors are reduced. It is not clear whether the reduction of FAD proceeds through the dinucleotide-binding site or by exchange of the prosthetic group. The stoichiometry of the reaction with O2 corresponds to the consumption of 2 mol NADH/mol O2, and only H2O is formed (2 NADH + 2 H+ + O2----2 NAD+ + 2 H2O). Neither H2O2 nor O2.- is detected as intermediate and H2O2 cannot replace O2 as an oxidant. The enzyme can, mainly in its reduced state, be inhibited by -SH reagents. Spectral data give no evidence for the existence of radical intermediates during reduction. The enzyme can obviously accept more than two electrons/mol. On the basis of these data two possible reaction mechanisms are discussed. A proposal for the biological purpose of the reaction is made.

Composition of the coenzyme F420-dependent formate dehydrogenase from Methanobacterium formicicum

The coenzyme F420-dependent formate dehydrogenase from Methanobacterium formicicum was purified to electrophoretic homogeneity by anoxic procedures which included the addition of azide, flavin adenine dinucleotide (FAD), glycerol, and 2-mercaptoethanol to all buffer solutions to stabilize activity. The enzyme contains, in approximate molar ratios, 1 FAD molecule and 1 molybdenum, 2 zinc, 21 to 24 iron, and 25 to 29 inorganic sulfur atoms. Denaturation of the enzyme released a molybdopterin cofactor. The enzyme has a molecular weight of 177,000 and consists of one each of two different subunits, giving the composition alpha 1 beta 1. The molecular weight of the alpha-subunit is 85,000, and that of the beta-subunit is 53,000. The UV-visible spectrum is typical of nonheme iron-sulfur flavoprotein. Reduction of the enzyme facilitated dissociation of FAD, and the FAD-depleted enzyme was unable to reduce coenzyme F420. Preincubation of the FAD-depleted enzyme with FAD restored coenzyme F420-dependent activity.

Membrane-bound lactate dehydrogenases and mandelate dehydrogenases of Acinetobacter calcoaceticus. Purification and properties

Procedures were developed for the optimal solubilization of D-lactate dehydrogenase, D-mandelate dehydrogenase, L-lactate dehydrogenase and L-mandelate dehydrogenase from wall + membrane fractions of Acinetobacter calcoaceticus. D-Lactate dehydrogenase and D-mandelate dehydrogenase were co-eluted on gel filtration, as were L-lactate dehydrogenase and L-mandelate dehydrogenase. All four enzymes could be separated by ion-exchange chromatography. D-Lactate dehydrogenase and D-mandelate dehydrogenase were purified by cholate extraction, (NH4)2SO4 fractionation, gel filtration, ion-exchange chromatography and chromatofocusing. The properties of D-lactate dehydrogenase and D-mandelate dehydrogenase were similar in several respects: they had relative molecular masses of 62 800 and 59 700 respectively, pI values of 5.8 and 5.5, considerable sensitivity to p-chloromercuribenzoate, little or no inhibition by chelating agents, and similar responses to pH. Both enzymes appeared to contain non-covalently bound FAD as cofactor.

Membrane-bound lactate dehydrogenases and mandelate dehydrogenases of Acinetobacter calcoaceticus. Location and regulation of expression

Acinetobacter calcoaceticus possesses an L(+)-lactate dehydrogenase and a D(-)-lactate dehydrogenase. Results of experiments in which enzyme activities were measured after growth of bacteria in different media indicated that the two enzymes were co-ordinately induced by either enantiomer of lactate but not by pyruvate, and repressed by succinate or L-glutamate. The two lactate dehydrogenases have very similar properties to L(+)-mandelate dehydrogenase and D(-)-mandelate dehydrogenase. All four enzymes are NAD(P)-independent and were found to be integral components of the cytoplasmic membrane. The enzymes could be solubilized in active form by detergents; Triton X-100 or Lubrol PX were particularly effective D(-)-Lactate dehydrogenase and D(-)-mandelate dehydrogenase could be selectively solubilized by the ionic detergents cholate, deoxycholate and sodium dodecyl sulphate.