Rat erythrocyte NADH-cytochrome b5 reductase. Quantitation and comparison between the membrane-bound and soluble forms using an antibody against the rat liver enzyme

Structural Features of Cytochrome b5–Cytochrome b5 Reductase Complex Formation and Implications for the Intramolecular Dynamics of Cytochrome b5 Reductase

Membrane cytochrome b₅ reductase is a pleiotropic oxidoreductase that uses primarily soluble reduced nicotinamide adenine dinucleotide (NADH) as an electron donor to reduce multiple biological acceptors localized in cellular membranes. Some of the biological acceptors of the reductase and coupled redox proteins might eventually transfer electrons to oxygen to form reactive oxygen species. Additionally, an inefficient electron transfer to redox acceptors can lead to electron uncoupling and superoxide anion formation by the reductase. Many efforts have been made to characterize the involved catalytic domains in the electron transfer from the reduced flavoprotein to its electron acceptors, such as cytochrome b₅, through a detailed description of the flavin and NADH-binding sites. This information might help to understand better the processes and modifications involved in reactive oxygen formation by the cytochrome b₅ reductase. Nevertheless, more than half a century since this enzyme was first purified, the one-electron transfer process toward potential electron acceptors of the reductase is still only partially understood. New advances in computational analysis of protein structures allow predicting the intramolecular protein dynamics, identifying potential functional sites, or evaluating the effects of microenvironment changes in protein structure and dynamics. We applied this approach to characterize further the roles of amino acid domains within cytochrome b₅ reductase structure, part of the catalytic domain, and several sensors and structural domains involved in the interactions with cytochrome b₅ and other electron acceptors. The computational analysis results allowed us to rationalize some of the available spectroscopic data regarding ligand-induced conformational changes leading to an increase in the flavin adenine dinucleotide (FAD) solvent-exposed surface, which has been previously correlated with the formation of complexes with electron acceptors.

Human cytochrome b5 reductase: structure, function, and potential applications

Cytochrome b5 reductase is a flavoprotein that is produced as two different isoforms that have different localizations. The amphipathic microsomal isoform, found in all cell types with the exception of erythrocytes, consists of one hydrophobic membrane-anchoring domain and a larger hydrophilic flavin catalytic domain. The soluble cytochrome b5 reductase isoform, found in human erythrocytes, is a truncated protein that is encoded by an alternative transcript and consists of the larger domain only. Cytochrome b5 reductase is involved in the transfer of reducing equivalents from the physiological electron donor, NADH, via an FAD domain to the small molecules of cytochrome b5. This protein has received much attention from researchers due to its involvement in many oxidation and reduction reactions, such as the reduction of methemoglobin to hemoglobin. Autosomal cytochrome b5 reductase gene deficiency manifests with the accumulation of oxidized Fe+3 and recessive congenital methemoglobinemia in humans. In this article, we provide a comprehensive overview of the structure and function of cytochrome b5 reductase from different eukaryotic sources and its potential use in the food industry, biosensor, and diagnostic areas.

Trans-plasma membrane electron transport in mammals: functional significance in health and disease

Trans-plasma membrane electron transport (t-PMET) has been established since the 1960s, but it has only been subject to more intensive research in the last decade. The discovery and characterization at the molecular level of its novel components has increased our understanding of how t-PMET regulates distinct cellular functions. This review will give an update on t-PMET, with particular emphasis on how its malfunction relates to some diseases, such as cancer, abnormal cell death, cardiovascular diseases, aging, obesity, neurodegenerative diseases, pulmonary fibrosis, asthma, and genetically linked pathologies. Understanding these relationships may provide novel therapeutic approaches for pathologies associated with unbalanced redox state.

Cytochrome b₅ reductase-cytochrome b₅ as an active P450 redox enzyme system in Phanerochaete chrysosporium: atypical properties and in vivo evidence of electron transfer capability to CYP63A2

Two central redox enzyme systems exist to reduce eukaryotic P450 enzymes, the P450 oxidoreductase (POR) and the cyt b₅ reductase-cyt b₅. In fungi, limited information is available for the cyt b(5) reductase-cyt b(5) system. Here we characterized the kinetic mechanism of (cyt b₅r)-cyt b₅ redox system from the model white-rot fungus Phanerochaete chrysosporium (Pc) and made a quantitative comparison to the POR system. We determined that Pc-cyt b₅r followed a "ping-pong" mechanism and could directly reduce cytochrome c. However, unlike other cyt b₅ reductases, Pc-cyt b₅r lacked the typical ferricyanide reduction activity, a standard for cyt b₅ reductases. Through co-expression in yeast, we demonstrated that the Pc-cyt b₅r-cyt b₅ complex is capable of transferring electrons to Pc-P450 CYP63A2 for its benzo(a)pyrene monooxygenation activity and that the efficiency was comparable to POR. In fact, both redox systems supported oxidation of an estimated one-third of the added benzo(a)pyrene amount. To our knowledge, this is the first report to indicate that the cyt b₅r-cyt b₅ complex of fungi is capable of transferring electrons to a P450 monooxygenase. Furthermore, this is the first eukaryotic quantitative comparison of the two P450 redox enzyme systems (POR and cyt b₅r-cyt b₅) in terms of supporting a P450 monooxygenase activity.

Clustering of plasma membrane-bound cytochrome b5 reductase within 'lipid raft' microdomains of the neuronal plasma membrane

Plasma membrane redox centres play a major role in neuronal defence against oxidative stress and survival. In cerebellar granule neurons in culture (CGN) a large pool of the flavoproteins are associated with the plasma membrane, and the intensity of CGN green/orange autofluorescence correlated with the levels of expression of cytochrome b(5) reductase. Regionalization of cytochrome b(5) reductase in the plasma membrane of CGN by fluorescence resonance energy transfer points out the close proximity between cytochrome b(5) reductase and the 'lipid raft' markers cholera toxin B and caveolin-2. This study unravels that membrane-bound cytochrome b(5) reductase is largely enriched at interneuronal contact sites in the neuronal soma and associated with 'lipid rafts' of the CGN plasma membrane. We also show that cytochrome b(5) reductase makes a large contribution to the NADH oxidase activity and to the red-shifted flavine fluorescence of purified rat brain synaptic plasma membranes. In conclusion, membrane-bound cytochrome b(5) reductase forms a large mesh of redox centres associated with the neuronal plasma membrane.

Reduction of ascorbate free radical by the plasma membrane of synaptic terminals from rat brain

Synaptic plasma membranes (SPMV) decrease the steady state ascorbate free radical (AFR) concentration of 1mM ascorbate in phosphate/EDTA buffer (pH 7), due to AFR recycling by redox coupling between ascorbate and the ubiquinone content of these membranes. In the presence of NADH, but not NADPH, SPMV catalyse a rapid recycling of AFR which further lower the AFR concentration below 0.05 microM. These results correlate with the nearly 10-fold higher NADH oxidase over NADPH oxidase activity of SPMV. SPMV has NADH-dependent coenzyme Q reductase activity. In the presence of ascorbate the stimulation of the NADH oxidase activity of SPMV by coenzyme Q(1) and cytochrome c can be accounted for by the increase of the AFR concentration generated by the redox pairs ascorbate/coenzyme Q(1) and ascorbate/cytochrome c. The NADH:AFR reductase activity makes a major contribution to the NADH oxidase activity of SPMV and decreases the steady-state AFR concentration well below the micromolar concentration range.

The importance of plasma membrane coenzyme Q in aging and stress responses

The plasma membrane of eukaryotic cells is the limit to interact with the environment. This position implies receiving stress signals that affects its components such as phospholipids. Inserted inside these components is coenzyme Q that is a redox compound acting as antioxidant. Coenzyme Q is reduced by diverse dehydrogenase enzymes mainly NADH-cytochrome b(5) reductase and NAD(P)H:quinone reductase 1. Reduced coenzyme Q can prevent lipid peroxidation chain reaction by itself or by reducing other antioxidants such as alpha-tocopherol and ascorbate. The group formed by antioxidants and the enzymes able to reduce coenzyme Q constitutes a plasma membrane redox system that is regulated by conditions that induce oxidative stress. Growth factor removal, ethidium bromide-induced rho degrees cells, and vitamin E deficiency are some of the conditions where both coenzyme Q and its reductases are increased in the plasma membrane. This antioxidant system in the plasma membrane has been observed to participate in the healthy aging induced by calorie restriction. Furthermore, coenzyme Q regulates the release of ceramide from sphingomyelin, which is concentrated in the plasma membrane. This results from the non-competitive inhibition of the neutral sphingomyelinase by coenzyme Q particularly by its reduced form. Coenzyme Q in the plasma membrane is then the center of a complex antioxidant system preventing the accumulation of oxidative damage and regulating the externally initiated ceramide signaling pathway.

Purification and characterization of cytochrome b5 reductase from the house fly, Musca domestica

NADH-cytochrome b5 reductase (b5R) from the house fly was purified through solubilization of microsomes with Triton X-100 followed by DEAE, carboxylmethyl and 5'-ADP affinity column chromatography. Yields of 9% with a 320-fold increase in NADH-ferricyanide reductase specific activity and 2% with a 76-fold increase in NADH-cytochrome b5R specific activity were obtained. Two forms of b5R, a major form with the apparent molecular mass of 31 kDa and a minor form of 33 kDa, were obtained. Both forms of purified b5R could reduce cytochrome b5 and both could use NADH or NADPH as an electron donor, although NADH was more efficient. Kinetics of the b5R activities were also studied. The 31-kDa b5R consists of integral of 291 amino acids with the NH2-terminal sequence of Thr-Ala-Arg-Leu-Arg-Thr-Leu-Ile-Asp-Ala. An antiserum developed against the 31-kDa b5R recognized both forms of b5R. Using this polyclonal antiserum as a probe, immunologically reactive proteins were found in microsomes from five species of Diptera, mouse and rat liver but not in spider mites nor insects from other orders.

The pH-dependent reduction of Adriamycin catalysed by NADH:cytochrome b5 reductase

Adriamycin is a redox active antineoplastic antibiotic that upon reduction can, in the presence of oxygen, redox cycle to form reactive oxygen species, while in anaerobiosis can generate a reactive quinone methide. NADH:cytochrome b5 reductase catalysed the reduction of adriamycin at pH 6.6 with an apparent Km of 1.8 microM; at pH 7.6, no measurable reduction of adriamycin occurred. Aerobically, in the presence of enzyme and NADH, adriamycin stimulated oxygen consumption and concomitant accumulation of hydrogen peroxide. At pH 7.6, no discernible oxygen consumption nor detectable hydrogen peroxide generation was observed. The findings demonstrate that NADH:cytochrome b5 reductase is capable of reducing adriamycin, in a pH-dependent manner, to species that can redox cycle in the presence of oxygen to form reactive oxygen molecules and thus may contribute to the generation of oxidative stress, a phenomenon suggested to be involved in both the toxicity and the antineoplastic activity of adriamycin.

Heterogeneity of the rat NADH-cytochrome-b5-reductase transcripts resulting from multiple alternative first exons

In order to understand the mechanisms responsible for the generation of different isoforms (membrane-bound and soluble) of NADH-cytochrome b5 reductase, and the different clinical forms of recessive congenital methemoglobinemia due to the deficiency of this enzyme in humans (type I, without mental retardation; type II, with mental retardation), we have looked for mRNA heterogeneity in various rat tissues. We have found four types of mRNAs, each with a different first exon (1L, 1R, 1X and 1Y), all of which were precisely spliced to join the common second exon. Our results are consistent with a 5'-->3' 'scanning' mechanism for splice-site selection. The previously characterized 1L and 1R transcripts arise from the alternative use of either a ubiquitous promoter (Pr-L) or an erythroid-specific promoter (Pr-R). In addition, the X and Y RNA species are novel transcripts which are expressed ubiquitously and at a relatively low level. The first alternative exons 1X and 1Y are noncoding, such that the AUG codon present in the common second exon is functional, as it is in the R mRNA. Thus, the X and Y mRNAs are expected to be translated in vivo into a ubiquitous soluble enzyme. Consequently, the rat NADH-cytochrome-b5-reductase gene is expressed through the use of at least four different promoters, which are probably subjected to different forms of regulation. This model of gene expression in rat could be important in understanding the basis for the different types of the NADH-cytochrome-b5-reductase enzyme and their deficiency in man.

Monoclonal antibodies prepared against PAS-I butyrophilin and GP-55 from guinea-pig milk-fat-globule membrane bind specifically to the apical pole of secretory-epithelial cells in lactating mammary tissue

Monoclonal antibodies to the three major glycoproteins of guinea-pig milk-fat-globule membrane were isolated. The specificity of these antibodies was determined by solid-phase immunoassays and by immunoblotting and autoradiographic techniques after one- and two-dimensional gel electrophoresis. The antibodies bound to PAS-I, a sialoglycoprotein of Mr greater than or equal to 200 000 and the glycoproteins butyrophilin and GP-55, of Mr 63 000 and 55 000, respectively. Immunolocalization studies showed that all three proteins were highly concentrated in the apical pole of secretory-epithelial cells in mammary tissue during lactation. PAS-I, butyrophilin or GP-55, were not detected in either the basal cytoplasm of mammary epithelial cells or in myoepithelial cells, capillary endothelial cells or other cells found in the mammary gland. These proteins were either present in small amounts or were absent from mammary tissue taken in late pregnancy. The monoclonal antibodies characterized in this study will therefore be useful as probes for studies of the biogenesis of apical membrane proteins in mammary epithelial cells during lactation.

Erythrocytes-mediated metabolic activation of cyclophosphamide in yeast mutagenicity test

The degree of conversion of cyclophosphamide (CP) into mutagenic intermediates was studied using mouse erythrocytes as the metabolic activation system. The amount of mutagenic intermediates produced was measured indirectly in terms of induced frequencies of mitotic recombination, mitotic gene conversion, and reverse mutation in the diploid D7 strain of Saccharomyces cerevisiae. In the absence of S9 microsomal fraction or erythrocytes, CP did not induce any genetic response. In the presence of erythrocytes, on the other hand, CP clearly induced increases in the three genetic endpoints. The responses, however, were lower than those observed with the S9 activation system. The activating principle seems to be the oxyhemoglobin. In fact, neither p-nitroanisole O-demethylase activity nor genotoxic responses performed with red-blood cells from uninduced and PB-induced mice indicate that (possible) water-soluble forms of cytochrome P-450 were responsible for the activation of CP by erythrocytes.