Localization and biosynthesis of NADH-cytochrome b5 reductase, an integral membrane protein, in rat liver cells. II. Evidence that a single enzyme accounts for the activity in its various subcellular locations

NADH-cytochrome b5 reductases of rat liver microsomes, mitochondria, and heavy and light Golgi fractions (GF3 and GF 1+2) were compared by antibody inhibition and competition experiments, by peptide mapping, and by CNBr fragment analysis. The water-soluble portion of the microsomal enzyme, released by lysosomal digestion and purified by a published procedure, was used to raise antibodies in rabbits. Contaminant antimicrosome antibodies were removed from immune sera by immunoadsorption onto the purified antigen, and the F(ab')2 fragments of the pure antireductase antibody thus obtained were found to inhibit the NADH-cytochrome c reductase activity equally well in the four membrane fractions investigated, with similar dose-response relationships. Moreover, the purified water-soluble fragment of microsomal reductase, which by itself is very inefficient in reducing cytochrome c, competed for antibody binding with the membrane-bound enzymes, and therefore prevented the inhibition of their activity not only in microsomes but also in the other fractions. The reductases isolated from detergent-solubilized microsomes, mitochondria, GF3, and GF1+2 by immunoadsorption had identical mobilities in SDS polyacrylamide gels. The corresponding bands were eluted from gels, fragmented with pepsin or CNBr treatment, and the two families of peptides thus obtained were analyzed by two-dimensional mapping and SDS polyacrylamide gel electrophoresis, respectively. Both analyses failed to reveal differences among reductases of the four fractions. These findings support the hypothesis that NADH-cytochrome b5 reductase in its various subcellular locations is molecularly identical.     

Purification and immunological studies of glutathione reductase from rat liver. Evidence for an antigenic determinant at the nucleotide-binding domain of the enzyme

Glutathione reductase has been purified to homogeneity by a method which is an improvement of an earlier procedure (Carlberg, I. and Mannervik, B. (1975) J. Biol. Chem. 250, 5475-5480). The new steps in the purification scheme include affinity chromatography on 2',5' ADP-Sepharose 4B. Antibodies to glutathione reductase from rat liver were raised in rabbits and used for analysis of the enzyme by quantitative 'rocket' immunoelectrophoresis. Glutathione reductase from human erythrocytes, porcine erythrocytes, and calf-liver gave precipitin lines showing partial identity with the rat liver enzyme in Ouchterlony double diffusion experiments. Enzyme from spinach, yeast (Saccharomyces cerevisiae), and the photosynthetic bacterium Rhodospirillum rubrum did not give precipitates with the antibodies to the enzyme from rat liver. Titration of glutathione reductase from the different sources with antibodies confirmed the cross-reactivity of the mammalian enzymes; the human enzyme giving the strongest heterologous reaction. No reaction was observed with the enzyme from spinach, yeast, and Rhodospirillum rubrum. NADPH, NADP+, and 2',5' ADP were found to inhibit the interaction between antibodies and glutathione reductase from rat liver and human erythrocytes. NADH, glutathione, or glutathione disulfide did not protect the enzyme from reacting with the antibodies. It is concluded that glutathione reductase has an antigenic binding site for the antibodies at the pyridine nucleotide-binding site of the enzyme molecule.     

Purification and characterization of chlordecone reductase from human liver

The toxic organochlorine pesticide, chlordecone (Kepone), is excreted in human bile primarily as a stable, reduced monoalcohol metabolite. This bioreduction is catalyzed by a hepatic cytosolic enzyme activity termed chlordecone reductase. We purified this enzyme from human liver and found that chlordecone reductase resembles the family of xenobiotic metabolizing enzymes referred to as the aldo-keto reductases based on its biochemical characteristics, including its ability to catalyze the reduction of a carbonyl-containing substrate. However, analyses of liver cytosolic samples on immunoblots developed with anti-chlordecone reductase antibodies revealed that immunoreactive proteins were present only in those mammalian species that convert chlordecone to chlordecone alcohol in vivo (man, gerbil, and rabbit) and not in those species unable to reduce chlordecone (rat, mouse, and hamster). Hence, chlordecone reductase is unique among aldo-keto reductases in being species-specific. Quantitative immunoblot analyses of seven human liver specimens disclosed two immunoreactive proteins whose total concentration varied over a 6-fold range. Moreover, the amount of immunoreactive protein was directly proportional to chlordecone reductase activity in each sample. We conclude that chlordecone reductase is a unique aldo-keto reductase of potential clinical importance whose expression varies markedly among individuals.     

Purification and immunological studies of glutathione reductase from rat liver evidence for an antigenic determinant at the nucleotide-binding domain of the enzyme

Glutathione reductase has been purified to homogeneity by a method which is an improvement of an earlier procedure (Carlberg, I. and Mannervik, B. (1975) J. Biol. Chem. 250, 5475–5480). The new steps in the purification scheme include affinity chromatography on 2′,5′ ADP-Sepharose 4B. Antibodies to glutathione reductase from rat liver were raised in rabbits and used for analysis of the enzyme by quantitative ‘rocket’ immunoelectrophoresis. Glutathione reductase from human erythrocytes, porcine erythrocytes, and calf-liver gave precipitin lines showing partial identity with the rat liver enzyme in Ouchterlony double diffusion experiments. Enzyme from spinach, yeast (Saccharomyces cerevisiae), and the photosynthetic bacterium Rhodospirillum rubrum did not give precipitates with the antibodies to the enzyme from rat liver. Titration of glutathione reductase from the different sources with antibodies confirmed the cross-reactivity of the mammalian enzymes; the human enzyme giving the strongest heterologous reaction. No reaction was observed with the enzyme from spinach, yeast, and Rhodospirillum rubrum. NADPH, NADP⁺, and 2′,5′ ADP were found to inhibit the interaction between antibodies and glutathione reductase from rat liver and human erythrocytes. NADH, glutathione, or glutathione disulfide did not protect the enzyme from reacting with the antibodies. It is concluded that glutathione reductase has an antigenic binding site for the antibodies at the pyridine nucleotide-binding site of the enzyme molecule.     

Production and Characterization of Monoclonal Antibodies to Bovine Brain Succinic Semialdehyde Reductase

Abstract: Monoclonal antibodies against bovine brain succinic semialdehyde reductase were produced and characterized. A total of nine monoclonal antibodies recognizing different epitopes of the enzyme were obtained, of which two inhibited the enzyme activity and three stained cytosol of rat spinal cord neurons as observed by indirect immunofluorescence microscopy. When unfractionated total proteins of bovine brain homogenate were separated by gel electrophoresis and immunoblotted, the antibodies specifically recognized a single protein band of 34 kDa, which comigrates with purified bovine succinic semialdehyde reductase. Using the antisuccinic semialdehyde reductase antibodies as probes, we investigated the cross-reactivities of brain succinic semialdehyde reductases from some mammalian and an avian species. The immunoreactive bands on western blots appeared to be the same in molecular mass—34 kDa—in all animal species tested, including humans. The result indicates that brain succinic semialdehyde reductase is distinct from other aldehyde reductases and that mammalian brains contain only one succinic semialdehyde reductase. Moreover, the enzymes among the species are immunologically very similar, although some properties of the enzymes reported previously were different from one another.     

Immunochemical characterization of NADPH-cytochrome P-450 reductase from Jerusalem artichoke and other higher plants.

Polyclonal antibodies were prepared against NADPH-cytochrome P-450 reductase purified from Jerusalem artichoke. These antibodies inhibited efficiently the NADPH-cytochrome c reductase activity of the purified enzyme, as well as of Jerusalem artichoke microsomes. Likewise, microsomal NADPH-dependent cytochrome P-450 mono-oxygenases (cinnamate and laurate hydroxylases) were efficiently inhibited. The antibodies were only slightly inhibitory toward microsomal NADH-cytochrome c reductase activity, but lowered NADH-dependent cytochrome P-450 mono-oxygenase activities. The Jerusalem artichoke NADPH-cytochrome P-450 reductase is characterized by its high Mr (82,000) as compared with the enzyme from animals (76,000-78,000). Western blot analysis revealed cross-reactivity of the Jerusalem artichoke reductase antibodies with microsomes from plants belonging to different families (monocotyledons and dicotyledons). All of the proteins recognized by the antibodies had an Mr of approx. 82,000. No cross-reaction was observed with microsomes from rat liver or Locusta migratoria midgut. The cross-reactivity generally paralleled well the inhibition of reductase activity: the enzyme from most higher plants tested was inhibited by the antibodies; whereas Gingko biloba, Euglena gracilis, yeast, rat liver and insect midgut activities were insensitive to the antibodies. These results point to structural differences, particularly at the active site, between the reductases from higher plants and the enzymes from phylogenetically distant plants and from animals.     

Pyrimidine reducing enzymes of rat liver.

Dihydrouracil dehydrogenase (NADP+) (EC 1.3.1.2) was partially purified from the cytosol fraction of rat liver and fractionated by disc gel electrophoresis. A major and minor band were visualized by staining for enzyme activity. The substrate specificity of these bands was investigated. It was found that both bands were two to three times more active with dihydrothymine as substrate than with dihydrouracil in the presence of NADP+ and the optimum pH of 7.4. Mitochondrial fractions containing most of the NADH-dependent uracil reductase of rat liver cells were fractionated by centrifugation in sucrose density gradients. Two procedures involving linear or discontinuous gradients were used. By both, good separation of NADH- and NADPH- dependent reductases was achieved. Marker enzyme studies supported the view that the NADH-dependent enzyme is located principally in mitochondria whereas the NADPH-dependent enzyme is mainly in plasma and endoplasmic reticulum membranes. For the NADH-dependent reductase the apparent Km for thymine at pH 7.4 was 1.39 times that found for uracil whereas for the NADPH-dependent enzyme the apparent Km values were similar for the two substrates at this pH. Dihydrouracil was the principal product isolated by paper chromatography from the reaction mixture containing a partially purified fraction of mitochondria, uracil and NADH at pH 7.4. This fraction also catalyzed the formation of radioactive carbon dioxide from [2-14C]uracil. The proportion of CO2 formed by the mitochondria was about 10% of that formed by the original homogenate.     

NADPH-cytochrome c (P-450) reductase has the activity of NADPH-linked aquacobalamin reductase in rat liver microsomes.

To elucidate the mammalian system for synthesis of cobalamin coenzymes, microsomal NADPH-linked aquacobalamin reductase was purified and characterized. The enzyme was purified about 534-fold over rat liver microsomal fraction in a yield of about 32%. The purified enzyme was homogeneous in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and had a monomeric molecular weight of 79,000. The purified aquacobalamin reductase showed a high specific activity (about 55 mumol/min per mg protein) of NADPH-cytochrome c (P-450) reductase. About 33% of the NADPH-cytochrome c reductase activity found in the microsomal fraction was recovered in the final purified preparation. The activity ratio of NADPH-cytochrome c reductase/NADPH-linked aquacobalamin reductase was about 5.0 through the purification steps, indicating that the rat liver microsomal NADPH-linked aquacobalamin reductase is the NADPH-cytochrome c reductase.     

Regulation of HMG-CoA reductase: identification of the site phosphorylated by the AMP-activated protein kinase in vitro and in intact rat liver.

The intact, 100 kd microsomal enzyme and the 53 kd catalytic fragment of rat HMG-CoA reductase are both phosphorylated and inactivated by the AMP-activated protein kinase. Using the catalytic fragment, we have purified and sequenced peptides containing the single site of phosphorylation. Comparison with the amino acid sequence predicted from the cDNAs encoding other mammalian HMG-CoA reductases identifies this site as a serine residue close to the C-terminus (Ser872 in the human enzyme). Phosphopeptide mapping of native, 100 kd microsomal HMG-CoA reductase confirms that this C-terminal serine is the only major site phosphorylated in the intact enzyme by the AMP-activated protein kinase. The catalytic fragment of HMG-CoA reductase was also isolated from rat liver in the presence of protein phosphatase inhibitors under conditions where the enzyme is largely in the inactive form. HPLC, mass spectrometry and sequencing of the peptide containing Ser872 demonstrated that this site is highly phosphorylated in intact liver under these conditions. We have also identified by amino acid sequencing the N-terminus of the catalytic fragment, which corresponds to residue 423 of the human enzyme.     

Purification by affinity chromatography of 2,4-dienoyl-CoA reductases from bovine liver and Escherichia coli

1. Dye-ligand chromatography using immobilized Cibacron blue F3GA (blue Sepharose CL-6B) and Procion red HE3B (Matrex gel red A) as matrices and general ligand chromatography employing immobilized 2',5'-ADP (2',5'-ADP-Sepharose 4B) and immobilized 3',5'-ADP (3',5'-ADP-Agarose) were employed for purification of NADPH-dependent 2-enoyl-CoA reductase and 2,4-dienoyl-CoA reductase from bovine liver (formerly called 4-enoyl-CoA reductase [Kunau, W. H. and Dommes, P. (1978) Eur. J. Biochem. 91, 533-544], as well as 2,4-dienoyl-CoA reductase from Escherichia coli. 2. The NADPH-dependent 2-enoyl-CoA reductase from bovine liver mitochondria was separated from 2,4-dienoyl-CoA reductase by dye-ligand chromatography (Matrex gel red A/KCl gradient) as well as by general ligand affinity chromatography (2',5'-ADP-Sepharose 4B/NADP gradient). The enzyme was obtained in a highly purified form. 3. The NADPH-dependent 2,4-dienoyl-CoA reductase from bovine liver mitochondria was purified to homogeneity using blue Sepharose CL-6B, Matrex gel red A, and 2',5'-ADP-Sepharose 4B chromatography. 4. The bacterial 2,4-dienoyl-CoA reductase was completely purified by ion-exchange chromatography on DEAE-cellulose followed by a single affinity chromatography step employing 2',5'-ADP-Sepharose 4B and biospecific elution from the column with a substrate, trans,trans-2,4-decadienoyl-CoA. 5. The application of dye-ligand and general ligand affinity chromatography for purification of NADPH-dependent 2,4-dienoyl-CoA reductases taking part in the beta-oxidation of unsaturated fatty acids is discussed. It is concluded that making use of coenzyme specificity for binding and substrate specificity for elution is essential for obtaining homogeneous enzyme preparations.     

NADH- and NADPH-linked aquacobalamin reductases occur in both mitochondrial and microsomal membranes of rat liver

To elucidate the mechanism for the synthesis of the coenzyme forms of cobalamin in mammals, rat liver aquacobalamin reductase was partially characterized. Rat liver contained both NADH- and NADPH-linked aquacobalamin reductases. The NADH-linked enzyme was distributed in the mitochondria (approx. 40%) and microsomes (60%), identical to the distribution of the NADPH-linked enzyme. The two mitochondrial NADH- and NADPH-linked enzymes were located inside of the outer membrane.     

3-hydroxy-3-methylglutaryl coenzyme A reductase of Sulfolobus solfataricus: DNA sequence, phylogeny, expression in Escherichia coli of the hmgA gene, and purification and kinetic characterization of the gene product.

The gene (hmgA) for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (EC 1.1.1.34) from the thermophilic archaeon Sulfolobus solfataricus P2 was cloned and sequenced. S. solfataricus HMG-CoA reductase exhibited a high degree of sequence identity (47%) to the HMG-CoA reductase of the halophilic archaeon Haloferax volcanii. Phylogenetic analyses of HMG-CoA reductase protein sequences suggested that the two archaeal genes are distant homologs of eukaryotic genes. The only known bacterial HMG-CoA reductase, a strictly biodegradative enzyme from Pseudomonas mevalonii, is highly diverged from archaeal and eukaryotic HMG-CoA reductases. The S. solfataricus hmgA gene encodes a true biosynthetic HMG-CoA reductase. Expression of hmgA in Escherichia coli generated a protein that both converted HMG-CoA to mevalonate and cross-reacted with antibodies raised against rat liver HMG-CoA reductase. S. solfataricus HMG-CoA reductase was purified in 40% yield to a specific activity of 17.5 microU per mg at 50 degrees C by a sequence of steps that included heat treatment, ion-exchange chromatography, hydrophobic interaction chromatography, and affinity chromatography. The final product was homogeneous, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The substrate was (S)- not (R)-HMG-CoA; the reductant was NADPH not NADH. The Km values for HMG-CoA (17 microM) and NADPH (23 microM) were similar in magnitude to those of other biosynthetic HMG-CoA reductases. Unlike other HMG-CoA reductases, the enzyme was stable at 90 degrees C and was optimally active at pH 5.5 and 85 degrees C.     

Rat hepatic glutaminase: purification and immunochemical characterization

A method for the purification of phosphate-activated glutaminase from the liver of streptozotocin-diabetic rats is described. The procedure involves solubilization of glutaminase activity from isolated mitochondria by sonication, followed by ammonium sulfate precipitation, polyethylene glycol precipitation, and sequential chromatography on DEAE, hydroxylapatite, and zinc-chelated resins. The enzyme was purified 600-fold to a specific activity of 31-57 U/mg protein. The purified enzyme has an apparent subunit molecular mass of 58,000-Da and is greater than 80% pure by scanning densitometry of sodium dodecyl sulfate-polyacrylamide gels. The purified enzyme has an apparent Km for glutamine of 17 mM and a pH optimum between 7.8 and 8.2. The physical and kinetic properties of this enzyme are similar to those of the enzyme from normal rat liver. Polyclonal antibodies raised against the enzyme specifically inhibit hepatic glutaminase activity and react primarily with a 58,000-Da peptide in liver fractions on immunoblots. These antibodies were used in equivalence point titrations and immunoblots to provide evidence for increased concentration of glutaminase protein in the liver of diabetic rats with no change in specific activity of the enzyme. In addition, the antibodies cross-react, at low affinity, with kidney-type glutaminases. On immunoblots, the antibodies did not react with fetal liver, mammary gland, or lung. Antibodies to rat hepatic glutaminase should prove useful as tools to study the long-term regulation of the enzyme.     

Purification of thioredoxin, thioredoxin reductase, and glutathione reductase by affinity chromatography.

A scheme is described for the large scale purification of thioredoxin, thioredoxin reductase, and glutathione reductase. The scheme is based on an initial separation of thioredoxin from the two reductases by affinity chromatography on agarose-bound N6-(6-aminohexyl)-adenosine 2',5'-bisphosphate (agarose-2',5'-ADP). The two reductases were then separated by hydrophobic chromatography and purified separately to homogeneity. Thioredoxin was purified to homogeneity by immunoadsorption to agarose containing immobilized goat anti-thioredoxin. Overall yields for thioredoxin, thioredoxin reductase, and glutathione reductase exceeded 80% in each case. Both reductases exhibit an absorption band at approximately 320 nm which appears due to a residual amount of tightly bound NADP. Presence of this absorption band has no apparent effect on the specific activity of either enzyme.     

2,4-Dienoyl coenzyme A reductases from bovine liver and Escherichia coli. Comparison of properties

2,4-Dienoyl-CoA reductases, enzymes of the beta-oxidation of unsaturated fatty acids which were purified from bovine liver and oleate-induced cells of Escherichia coli, revealed very similar substrate specificities but distinctly different molecular properties. The subunit molecular weights, estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were 32,000 and 73,000 for the mammalian and the bacterial enzyme, respectively. The native molecular weights, calculated from sedimentation coefficients and Stokes radii yielded 124,000 for the bovine liver and 70,000 for the bacterial enzyme. Thus, bovine liver 2,4-dienoyl-CoA reductase is a tetramer consisting of four identical subunits. The E. coli 2,4-dienoyl-CoA reductase, however, possesses a monomeric structure. The latter enzyme contains 1 mol of FAD/mol of enzyme, whereas the former reductase is not a flavoprotein. The bovine liver reductase reduced 2-trans, 4-cis- and 2-trans,4-trans-decadienoyl-CoA to 3-trans-decenoyl-CoA. The E. coli reductase catalyzed the reduction of the same two substrates but in contrast yielded 2-trans-decenoyl-CoA as reaction product. Certain other properties of the two 2,4-dienoyl-CoA reductases are also presented. The localization of the reductase step within the degradation pathway of 4-cis-decenoyl-CoA, a metabolite of linoleic acid, is discussed.     

Purification of the high-Km aldehyde reductase from rat brain and liver and from ox brain.

A procedure is described that yields an apparently homogeneous preparation of the high-Km aldehyde reductase from rat brain. This procedure is also applicable to the purification of this enzyme from rat liver and ox brain. In the latter case, however, the purified preparation could be resolved into two protein bands, both of which had enzyme activity, by polyacrylamide-gel electrophoresis. Since a sample of the ox brain enzyme from an earlier step in the purification procedure only showed the presence of a single band of activity after electrophoresis, this apparent multiplicity probably results from modification of the enzyme, possibly by oxidation, during the final step of the purification. A number of properties of the rat brain enzyme were determined and these were compared with those of the enzyme from rat liver. The two preparations were similar in their stabilities, behaviour during purification, kinetic properties, electrophoretic mobilities and amino acid compositions. Antibodies to the rat liver enzyme cross-reacted with that from brain and the inhibition of both these preparations by the antiserum was similar, further supporting the view that the enzymes from these two sources were closely similar if not identical.     

Biochemical characteristics of purified beef liver NADPH-cytochrome P450 reductase

NADPH-cytochrome P450 reductase, an obligatory component of the cytochrome P450 dependent monooxygenase system, was purified to electrophoretic homogeneity from beef liver microsomes. The purification procedure involved the ion exchange chromatography of the detergent-solubilized microsomes on first and second DEAE-cellulose columns, followed by 2',5'-ADP Sepharose affinity chromatography. Further concentration of the enzyme and removal of Emulgen 913 and 2'-AMP were accomplished on the final hydroxylapatite column. The enzyme was purified 239-fold and the yield was 13.5%. Monomer molecular weight of the enzyme was estimated to be 76000 +/- 3000 (N = 5) by SDS-PAGE. The absolute absorption spectrum of beef reductase showed two peaks at 455 and 378 nm, with a shoulder at 478 nm, characteristics of flavoproteins. The effects of cytochrome c concentration, pH, and ionic strength on enzyme activity were studied. Reduction of cytochrome c with the enzyme followed Michaelis-Menten kinetics, and the apparent K(m) of the purified enzyme was found to be 47.7 microM for cytochrome c when the enzyme activity was measured in 0.3 M potassium phosphate buffer (pH 7.7). Stability of cytochrome c reductase activity was examined at 25 and 37 degrees C in the presence and absence of 20% glycerol. The presence of glycerol enhanced the stability of cytochrome c reductase activity at both temperatures. Sheep lung microsomal cytochrome P4502B and NADPH-cytochrome P450 reductase were also purified by the already existing methods developed in our laboratory. Both beef liver and sheep lung reductases were found to be effective in supporting benzphetamine and cocaine N-demethylation reactions in the reconstituted systems containing purified sheep lung cytochrome P4502B and synthetic lipid, phosphatidylcholine dilauroyl.     

Intramitochondrial proteolytic activity of rat liver]

Highly purified mitochondria and lysosomes are isolated from rat liver homogenate. pH optimum of proteolytic activity with respect to proteins of own structures and to mitochondrial structural protein is investigated. The purification of mitochondria from lysosomes is found to be accompanied by the change of proteolytic activity pH optimum from 5.0 to 6.0 in coarse and purified mitochondria respectively. Comparative study of structural protein hydrolysis products with enzyme preparations from purified mitochondria and lysosomes has revealed differences in the spectrum of the reaction products. The data obtained suggest a presence of a proteolytic enzyme in rat liver mitochondria.     

Comparison of highly purified sheep liver and lung NADPH-cytochrome P-450 reductases by the analysis of kinetic and catalytic properties

1. Reductase was purified to electrophoretic homogeneity from sheep liver and lung microsomes. The specific activity of both enzymes ranged from 55 to 66 mumol cytochrome c reduced/min/mg protein. 2. Liver and lung reductases appeared to have similar kinetic and spectral properties. Km (NADPH) and Km (cytochrome c) values were calculated to be 14.3 +/- 1.23 microM and 22.2 +/- 2.78 microM for liver and 11.1 +/- 0.70 microM and 20.0 +/- 2.15 microM for lung reductase, respectively. Kinetic studies showed that cytochrome c can bind the oxidized form of the enzyme as well as its reduced form and both reductases operated through a ping-pong type mechanism. 3. These reductases cannot be distinguished on the basis of monomer molecular weights (Mr 78,000) except that the liver reductase was found to be more susceptible to proteolytic attack. 4. Both reductases supported aniline 4-hydroxylation and ethylmorphine N-demethylation reactions to the same extent in the reconstituted systems. However, sheep lung reductase appeared only 36.5 and 14.8% as effective in catalyzing benzo[a]pyrene reaction as an equivalent amount of reductase from liver in the presence of liver cytochrome P-450 and 3MC-treated rat liver cytochrome P-448, respectively.     

Overexpression of wild type and SeCys/Cys mutant of human thioredoxin reductase in E. coli: the role of selenocysteine in the catalytic activity

In contrast to Escherichia coli and yeast thioredoxin reductases, the human placental enzyme contains an additional redox center consisting of a cysteine-selenocysteine pair that precedes the C-terminal glycine residue. This reactive selenocysteine-containing center imbues the enzyme with its unusually wide substrate specificity. For expression of the human gene in E. coli, the sequence corresponding to the SECIS element required for selenocysteine insertion in E. coli formate dehydrogenase H was inserted downstream of the TGA codon in the human thioredoxin reductase gene. Omission of this SECIS element from another construct resulted in termination at UGA. Change of the TGA codon to TGT gave a mutant enzyme form in which selenocysteine was replaced with cysteine. The three gene products were purified using a standard isolation protocol. Binding properties of the three proteins to the affinity resins used for purification and to NADPH were similar. The three proteins occurred as dimers in the native state and exhibited characteristic thiolate-flavin charge transfer spectra upon reduction. With DTNB as substrate, compared to native rat liver thioredoxin reductase, catalytic activities were 16% for the recombinant wild type enzyme, about 5% for the cysteine mutant enzyme, and negligible for the truncated enzyme form.