NADH-cytochrome b5 reductase and cytochrome b5 isoforms as models for the study of post-translational targeting to the endoplasmic reticulum

Molecular Dynamic Simulation Analysis of a Novel Missense Variant in CYB5R3 Gene in Patients with Methemoglobinemia

Background and Objective: Mutations in the CYB5R3 gene cause reduced NADH-dependent cytochrome b5 reductase enzyme function and consequently lead to recessive congenital methemoglobinemia (RCM). RCM exists as RCM type I (RCM1) and RCM type II (RCM2). RCM1 leads to higher methemoglobin levels causing only cyanosis, while in RCM2, neurological complications are also present along with cyanosis. Materials and Methods: In the current study, a consanguineous Pakistani family with three individuals showing clinical manifestations of cyanosis, chest pain radiating to the left arm, dyspnea, orthopnea, and hemoptysis was studied. Following clinical assessment, a search for the causative gene was performed using whole exome sequencing (WES) and Sanger sequencing. Various variant effect prediction tools and ACMG criteria were applied to interpret the pathogenicity of the prioritized variants. Molecular dynamic simulation studies of wild and mutant systems were performed to determine the stability of the mutant CYB5R3 protein. Results: Data analysis of WES revealed a novel homozygous missense variant NM_001171660.2: c.670A > T: NP_001165131.1: p.(Ile224Phe) in exon 8 of the CYB5R3 gene located on chromosome 22q13.2. Sanger sequencing validated the segregation of the identified variant with the disease phenotype within the family. Bioinformatics prediction tools and ACMG guidelines predicted the identified variant p.(Ile224Phe) as disease-causing and likely pathogenic, respectively. Molecular dynamics study revealed that the variant p.(Ile224Phe) in the CYB5R3 resides in the NADH domain of the protein, the aberrant function of which is detrimental. Conclusions: The present study expanded the variant spectrum of the CYB5R3 gene. This will facilitate genetic counselling of the same and other similar families carrying mutations in the CYB5R3 gene.

Cytochrome b5 reductases: Redox regulators of cell homeostasis

The cytochrome-b5 reductase (CYB5R) family of flavoproteins is known to regulate reduction-oxidation (redox) balance in cells. The five enzyme members are highly compartmentalized at the subcellular level and function as "redox switches" enabling the reduction of several substrates, such as heme and coenzyme Q. Critical insight into the physiological and pathophysiological significance of CYB5R enzymes has been gleaned from several human genetic variants that cause congenital disease and a broad spectrum of chronic human diseases. Among the CYB5R genetic variants, CYB5R3 is well-characterized and deficiency in expression and activity is associated with type II methemoglobinemia, cancer, neurodegenerative disorders, diabetes, and cardiovascular disease. Importantly, pharmacological and genetic-based strategies are underway to target CYB5R3 to circumvent disease onset and mitigate severity. Despite our knowledge of CYB5R3 in human health and disease, the other reductases in the CYB5R family have been understudied, providing an opportunity to unravel critical function(s) for these enzymes in physiology and disease. In this review, we aim to provide the broad scientific community an up-to-date overview of the molecular, cellular, physiological, and pathophysiological roles of CYB5R proteins.

Loss of cardiomyocyte CYB5R3 impairs redox equilibrium and causes sudden cardiac death

Sudden cardiac death (SCD) in patients with heart failure (HF) is allied with an imbalance in reduction and oxidation (redox) signaling in cardiomyocytes; however, the basic pathways and mechanisms governing redox homeostasis in cardiomyocytes are not fully understood. Here, we show that cytochrome b5 reductase 3 (CYB5R3), an enzyme known to regulate redox signaling in erythrocytes and vascular cells, is essential for cardiomyocyte function. Using a conditional cardiomyocyte-specific CYB5R3-knockout mouse, we discovered that deletion of CYB5R3 in male, but not female, adult cardiomyocytes causes cardiac hypertrophy, bradycardia, and SCD. The increase in SCD in CYB5R3-KO mice is associated with calcium mishandling, ventricular fibrillation, and cardiomyocyte hypertrophy. Molecular studies reveal that CYB5R3-KO hearts display decreased adenosine triphosphate (ATP), increased oxidative stress, suppressed coenzyme Q levels, and hemoprotein dysregulation. Finally, from a translational perspective, we reveal that the high-frequency missense genetic variant rs1800457, which translates into a CYB5R3 T117S partial loss-of-function protein, associates with decreased event-free survival (~20%) in Black persons with HF with reduced ejection fraction (HFrEF). Together, these studies reveal a crucial role for CYB5R3 in cardiomyocyte redox biology and identify a genetic biomarker for persons of African ancestry that may potentially increase the risk of death from HFrEF.

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.

Inter-organelle interactions between the ER and mitotic spindle facilitates Zika protease cleavage of human Kinesin-5 and results in mitotic defects

Here we identify human Kinesin-5, Kif11/HsEg5, as a cellular target of Zika protease. We show that Zika NS2B-NS3 protease targets several sites within the motor domain of HsEg5 irrespective of motor binding to microtubules. The native integral ER-membrane protease triggers mitotic spindle positioning defects and a prolonged metaphase delay in cultured cells. Our data support a model whereby loss of function of HsEg5 is mediated by Zika protease and is spatially restricted to the ER-mitotic spindle interface during mitosis. The resulting phenotype is distinct from the monopolar phenotype that typically results from uniform inhibition of HsEg5 by RNAi or drugs. In addition, our data reveal novel inter-organelle interactions between the mitotic apparatus and the surrounding reticulate ER network. Given that Kif11 is haplo-insufficient in humans, and reduced dosage results in microcephaly, we propose that Zika protease targeting of HsEg5 may be a key event in the etiology of Zika syndrome microcephaly.

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Zika protease cleavage of Kinesin-5 impairs mitotic progression

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Inter-organelle interactions spatially control Zika proteolysis of Kinesin-5

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Native Zika protease affects mitosis differently than soluble Zika protease

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Zika protease may elicit fetal microcephaly and blindness via Kif11/Kinesin-5

GGPP depletion initiates metaflammation through disequilibrating CYB5R3-dependent eicosanoid metabolism

Metaflammation is a primary inflammatory complication of metabolic disorders characterized by altered production of many inflammatory cytokines, adipokines, and lipid mediators. Whereas multiple inflammation networks have been identified, the mechanisms by which metaflammation is initiated have long been controversial. As the mevalonate pathway (MVA) produces abundant bioactive isoprenoids and abnormal MVA has a phenotypic association with inflammation/immunity, we speculate that isoprenoids from the MVA may provide a causal link between metaflammation and metabolic disorders. Using a line with the MVA isoprenoid producer geranylgeranyl diphosphate synthase (GGPPS) deleted, we find that geranylgeranyl pyrophosphate (GGPP) depletion causes an apparent metaflammation as evidenced by abnormal accumulation of fatty acids, eicosanoid intermediates, and proinflammatory cytokines. We also find that GGPP prenylate cytochrome b ₅ reductase 3 (CYB5R3) and the prenylated CYB5R3 then translocate from the mitochondrial to the endoplasmic reticulum (ER) pool. As CYB5R3 is a critical NADH-dependent reductase necessary for eicosanoid metabolism in ER, we thus suggest that GGPP-mediated CYB5R3 prenylation is necessary for metabolism. In addition, we observe that pharmacological inhibition of the MVA pathway by simvastatin is sufficient to inhibit CYB5R3 translocation and induces smooth muscle death. Therefore, we conclude that the dysregulation of MVA intermediates is an essential mechanism for metaflammation initiation, in which the imbalanced production of eicosanoid intermediates in the ER serve as an important pathogenic factor. Moreover, the interplay of MVA and eicosanoid metabolism as we reported here illustrates a model for the coordinating regulation among metabolite pathways.

Genes underlying species differences in cuticular hydrocarbon production between Drosophila melanogaster and D. simulans

Surface chemical compounds are key components of survival and reproduction in many species. Cuticular hydrocarbons (CHCs) are chemical compounds produced by all insects that are used for both desiccation resistance and chemical communication, including communication related to mating. In the species pair of Drosophila melanogaster and D. simulans, female CHCs stimulate conspecific males to mate and repel heterospecific males. While CHCs are a critical contributor to both reproductive success within a species and isolation between species, few genes underlying species variation in CHC profiles are known. Here, we use genetic mapping of the 3rd chromosome to test a suite of candidate genes for interspecies variation in CHCs. Candidate gene CG5946 was found to be involved in species differences in the production of 7,11-heptacosadiene and 7-tricosene between D. melanogaster and D. simulans. This is therefore a new candidate locus contributing to species-specific variation in the CHC profile. In the process of mapping genes for CHCs, we also identified 29 candidate genes for the reduced survival or inviability of interspecies hybrids.

The Zebrafish Cytochrome b5/Cytochrome b5 Reductase/NADH System Efficiently Reduces Cytoglobins 1 and 2: Conserved Activity of Cytochrome b5/Cytochrome b5 Reductases during Vertebrate Evolution

Cytoglobin is a heme protein evolutionarily related to hemoglobin and myoglobin. Cytoglobin is expressed ubiquitously in mammalian tissues; however, its physiological functions are yet unclear. Phylogenetic analyses indicate that the cytoglobin gene is highly conserved in vertebrate clades, from fish to reptiles, amphibians, birds, and mammals. Most proposed roles for cytoglobin require the maintenance of a pool of reduced cytoglobin (Fe^II). We have shown previously that the human cytochrome b₅/cytochrome b₅ reductase system, considered a quintessential hemoglobin/myoglobin reductant, can reduce human and zebrafish cytoglobins ≤250-fold faster than human hemoglobin or myoglobin. It was unclear whether this reduction of zebrafish cytoglobins by mammalian proteins indicates a conserved pathway through vertebrate evolution. Here, we report the reduction of zebrafish cytoglobins 1 and 2 by the zebrafish cytochrome b₅ reductase and the two zebrafish cytochrome b₅ isoforms. In addition, the reducing system also supports reduction of Globin X, a conserved globin in fish and amphibians. Indeed, the zebrafish reducing system can maintain a fully reduced pool for both cytoglobins, and both cytochrome b₅ isoforms can support this process. We determined the P₅₀ for oxygen to be 0.5 Torr for cytoglobin 1 and 4.4 Torr for cytoglobin 2 at 25 °C. Thus, even at low oxygen tensions, the reduced cytoglobins may exist in a predominant oxygen-bound form. Under these conditions, the cytochrome b₅/cytochrome b₅ reductase system can support a conserved role for cytoglobins through evolution, providing electrons for redox signaling reactions such as nitric oxide dioxygenation, nitrite reduction, and phospholipid oxidation.

Defining the Role of the NADH-Cytochrome-b5 Reductase 3 in the Mitochondrial Amidoxime Reducing Component Enzyme System

The importance of the mitochondrial amidoxime reducing component (mARC)-containing enzyme system in N-reductive metabolism has been studied extensively. It catalyzes the reduction of various N-hydroxylated compounds and therefore acts as the counterpart of cytochrome P450- and flavin-containing monooxygenase-catalyzed oxidations at nitrogen centers. This enzyme system was found to be responsible for the activation of amidoxime and N-hydroxyguanidine prodrugs in drug metabolism. The synergy of three components (mARC, cytochrome b5, and the appropriate reductase) is crucial to exert the N-reductive catalytic effect. Previous studies have demonstrated the involvement of the specific isoforms of the molybdoenzyme mARC and the electron transport protein cytochrome b5 in N-reductive metabolism. To date, the corresponding reductase involved in N-reductive metabolism has yet to be defined because previous investigations have presented ambiguous results. Using small interfering RNA-mediated knockdown in human cells and assessing the stoichiometry of the enzyme system reconstituted in vitro, we provide evidence that NADH-cytochrome-b5 reductase 3 is the principal reductase involved in the mARC enzyme system and is an essential component of N-reductive metabolism in human cells. In addition, only minimal levels of cytochrome-b5 reductase 3 protein are sufficient for catalysis, which impeded previous attempts to identify the reductase.

Stimulation by pro-apoptotic valinomycin of cytosolic NADH/cytochrome c electron transport pathway-Effect of SH reagents

Intrinsic and extrinsic apoptosis are both characterised by the presence of cytochrome c (cyto-c) in the cytosol. We present data on the extra-mitochondrial NADH oxidation catalysed by exogenous (cytosolic) cyto-c, as a possible answer to the paradox of apoptosis being an energy-dependent program but characterized by the impairment of the respiratory chain. The reduction of molecular oxygen induced by the cytosolic NADH/cyto-c pathway is coupled to the generation of an electrochemical proton gradient available for ATP synthesis. Original findings show that SH reagents inhibit the NADH/cyto-c system with a conformational change mechanism. The mitochondrial integrity-test of sulfite oxidase unequivocally demonstrates that this enzyme (120kDa) can be released outside but exogenous cyto-c (12.5kDa) does not permeate into mitochondria. Valinomycin at 2nM stimulates both the energy-dependent reversible mitochondrial swelling and the NADH/cyto-c oxidation pathway. The pro-apoptotic activity of valinomycin, as well as to the dissipation of membrane potential, can be also ascribed to the increased activity of the NADH/cyto-c oxidation pathway useful as an additional source of energy for apoptosis. It can be speculated that the activation of the NADH/cyto-c system coupled to valinomycin-induced mitochondrial osmotic swelling may represent a strategy to activate apoptosis in confined solid tumours.

Cytosolic localization of NADH cytochrome b₅ oxidoreductase (Ncb5or)

Acyl-CoA desaturation in the endoplasmic reticulum (ER) membrane depends on cytosolic NADH or NADPH, whereas NADPH in the ER lumen is utilized by prereceptor glucocorticoid production. It was assumed that NADH cytochrome b5 oxidoreductase (Ncb5or) might connect Acyl-CoA desaturation to ER luminal redox. We aimed to clarify the ambiguous compartmentalization of Ncb5or and test the possible effect of stearoyl-CoA on microsomal NADPH level. Amino acid sequence analysis, fluorescence microscopy of GFP-tagged protein, immunocytochemistry, and western blot analysis of subcellular fractions unequivocally demonstrated that Ncb5or, either endogenous or exogenous, is localized in the cytoplasm and not in the ER lumen in cultured cells and liver tissue. Moreover, the involvement of ER-luminal reducing equivalents in stearoyl-CoA desaturation was excluded.

The mammalian molybdenum enzymes of mARC

The "mitochondrial amidoxime reducing component" (mARC) is the most recently discovered molybdenum-containing enzyme in mammals. All mammalian genomes studied to date contain two mARC genes: MARC1 and MARC2. The proteins encoded by these genes are mARC-1 and mARC-2 and represent the simplest form of eukaryotic molybdenum enzymes, only binding the molybdenum cofactor. In the presence of NADH, mARC proteins exert N-reductive activity together with the two electron transport proteins cytochrome b5 type B and NADH cytochrome b5 reductase. This enzyme system is capable of reducing a great variety of N-hydroxylated substrates. It plays a decisive role in the activation of prodrugs containing an amidoxime structure, and in detoxification pathways, e.g., of N-hydroxylated purine and pyrimidine bases. It belongs to a group of drug metabolism enzymes, in particular as a counterpart of P450 formed N-oxygenated metabolites. Its physiological relevance, on the other hand, is largely unknown. The aim of this article is to summarize our current knowledge of these proteins with a special focus on the mammalian enzymes and their N-reductive activity.

The involvement of mitochondrial amidoxime reducing components 1 and 2 and mitochondrial cytochrome b5 in N-reductive metabolism in human cells

The mitochondrial amidoxime reducing component mARC is a recently discovered molybdenum enzyme in mammals. mARC is not active as a standalone protein, but together with the electron transport proteins NADH-cytochrome b5 reductase (CYB5R) and cytochrome b5 (CYB5), it catalyzes the reduction of N-hydroxylated compounds such as amidoximes. The mARC-containing enzyme system is therefore considered to be responsible for the activation of amidoxime prodrugs. All hitherto analyzed mammalian genomes code for two mARC genes (also referred to as MOSC1 and MOSC2), which share high sequence similarities. By RNAi experiments in two different human cell lines, we demonstrate for the first time that both mARC proteins are capable of reducing N-hydroxylated substrates in cell metabolism. The extent of involvement is highly dependent on the expression level of the particular mARC protein. Furthermore, the mitochondrial isoform of CYB5 (CYB5B) is clearly identified as an essential component of the mARC-containing N-reductase system in human cells. The participation of the microsomal isoform (CYB5A) in N-reduction could be excluded by siRNA-mediated down-regulation in HEK-293 cells and knock-out in mice. Using heme-free apo-CYB5, the contribution of mitochondrial CYB5 to N-reductive catalysis was proven to strictly depend on heme. Finally, we created recombinant CYB5B variants corresponding to four nonsynonymous single nucleotide polymorphisms (SNPs). Investigated mutations of the heme protein seemed to have no significant impact on N-reductive activity of the reconstituted enzyme system.

Molecular basis of two novel mutations found in type I methemoglobinemia

Congenital methemoglobinemia due to NADH-cytochrome b5 reductase 3 (CYB5R3) deficiency is an autosomal recessive disorder that occurs sporadically worldwide, although endemic clusters of this disorder have been identified in certain ethnic groups. It is present as two distinct phenotypes, type I and type II. Type I methemoglobinemia is characterized by CYB5R3 enzyme deficiency restricted to erythrocytes and is associated with benign cyanosis. The less frequent type II methemoglobinemia is associated with generalized CYB5R3 deficiency affecting all cells and is lethal in early infancy. Here we describe the molecular basis of type I methemoglobinemia due to CYB5R3 deficiency in four patients from three distinct ethnic backgrounds, Asian Indian, Mexican and Greek. The CYB5R3 gene of three probands with type I methemoglobinemia and their relatives were sequenced revealing several putative causative mutations; in one subject multiple mutations were present. Two novel mutations, S54R and F157C, were identified and the previously described A179T, V253M mutations were also identified. All these point mutations mapped to the NADH binding domain and or the FAD binding domain. Each has the potential to sterically hinder cofactor binding causing instability of the CYB5R3 protein. Wild-type CYB5R3, as well as two of these novel mutations, S54R and F157C, was amplified, cloned, and purified recombinant peptide obtained. Kinetic and thermodynamic studies of these proteins show that the above mutations lead to decreased thermal stability.

Detection of alternative splice variants at the proteome level in Aspergillus flavus

Identification of proteins from proteolytic peptides or intact proteins plays an essential role in proteomics. Researchers use search engines to match the acquired peptide sequences to the target proteins. However, search engines depend on protein databases to provide candidates for consideration. Alternative splicing (AS), the mechanism where the exon of pre-mRNAs can be spliced and rearranged to generate distinct mRNA and therefore protein variants, enable higher eukaryotic organisms, with only a limited number of genes, to have the requisite complexity and diversity at the proteome level. Multiple alternative isoforms from one gene often share common segments of sequences. However, many protein databases only include a limited number of isoforms to keep minimal redundancy. As a result, the database search might not identify a target protein even with high quality tandem MS data and accurate intact precursor ion mass. We computationally predicted an exhaustive list of putative isoforms of Aspergillus flavus proteins from 20 371 expressed sequence tags to investigate whether an alternative splicing protein database can assign a greater proportion of mass spectrometry data. The newly constructed AS database provided 9807 new alternatively spliced variants in addition to 12 832 previously annotated proteins. The searches of the existing tandem MS spectra data set using the AS database identified 29 new proteins encoded by 26 genes. Nine fungal genes appeared to have multiple protein isoforms. In addition to the discovery of splice variants, AS database also showed potential to improve genome annotation. In summary, the introduction of an alternative splicing database helps identify more proteins and unveils more information about a proteome.

Plant-based strategies aimed at expressing HIV antigens and neutralizing antibodies at high levels. Nef as a case study

The first evidence that plants represent a valid, safe and cost-effective alternative to traditional expression systems for large-scale production of antigens and antibodies was described more than 10 years ago. Since then, considerable improvements have been made to increase the yield of plant-produced proteins. These include the use of signal sequences to target proteins to different cellular compartments, plastid transformation to achieve high transgene dosage, codon usage optimization to boost gene expression, and protein fusions to improve recombinant protein stability and accumulation. Thus, several HIV/SIV antigens and neutralizing anti-HIV antibodies have recently been successfully expressed in plants by stable nuclear or plastid transformation, and by transient expression systems based on plant virus vectors or Agrobacterium-mediated infection. The current article gives an overview of plant expressed HIV antigens and antibodies and provides an account of the use of different strategies aimed at increasing the expression of the accessory multifunctional HIV-1 Nef protein in transgenic plants.

Anchorage to the cytosolic face of the endoplasmic reticulum membrane: a new strategy to stabilize a cytosolic recombinant antigen in plants

The levels of accumulation of recombinant vaccines in transgenic plants are protein specific and strongly influenced by the subcellular compartment of destination. The human immunodeficiency virus protein Nef (negative factor), a promising target for the development of an antiviral vaccine, is a cytosolic protein that accumulates to low levels in transgenic tobacco and is even more unstable when introduced into the secretory pathway, probably because of folding defects in the non-cytosolic environment. To improve Nef accumulation, a new strategy was developed to anchor the molecule to the cytosolic face of the endoplasmic reticulum (ER) membrane. For this purpose, the Nef sequence was fused to the C-terminal domain of mammalian ER cytochrome b5, a long-lived, tail-anchored (TA) protein. This consistently increased Nef accumulation by more than threefold in many independent transgenic tobacco plants. Real-time polymerase chain reaction of mRNA levels and protein pulse-chase analysis indicated that the increase was not caused by higher transcript levels but by enhanced protein stability. Subcellular fractionation and immunocytochemistry indicated that Nef-TA accumulated on the ER membrane. Over-expression of mammalian or plant ER cytochrome b5 caused the formation of stacked membrane structures, as observed previously in similar experiments performed in mammalian cells; however, Nef-TA did not alter membrane organization in tobacco cells. Finally, Nef could be removed in vitro by its tail-anchor, taking advantage of an engineered thrombin cleavage site. These results open up the way to use tail-anchors to improve foreign protein stability in the plant cytosol without perturbing cellular functions.

Discovery and characterization of a cytochrome b5 variant in humans with impaired hydroxylamine reduction capacity

OBJECTIVES

We have shown that cytochrome b5 (cyt b5), along with its reductase, NADH cytochrome b5 reductase (b5R), is capable of direct xenobiotic biotransformation. We hypothesized that functionally significant genetic variability in cyt b5 could be found in healthy individuals.

BASIC METHODS

Cyt b5 cDNAs were prepared from peripheral blood mononuclear cells from 63 individuals.

MAIN RESULTS

One individual was heterozygous for a sequence variant in cyt b5 (A178G), with a predicted amino acid substitution of T60A. This variant, when expressed in Escherichia. coli, maintained a similar Vmax for the hydroxylamines of sulfamethoxazole, 4-aminobiphenyl, and 2-amino-l-methyl-6-phenylimidazo[4,5-b] pyridine (PhIP), compared with wild type cyt b5, with a modestly increased Km (2 to 3.5-fold) for each substrate. When expressed in a mammalian system (HeLa cells), however, T60A was associated with a 70% reduction in cyt b5 protein expression compared with wild type. mRNA expression for both isoforms were comparable in HeLa cells, and translation of these mRNAs in a rabbit reticulocyte lysate system with inhibited proteasomal machinery were also similar. Incubation of these translated enzymes with uninhibited rabbit reticulocyte lysate, however, indicated greater susceptibility of T60A to proteasomal degradation.

CONCLUSIONS

These data indicate that a naturally occurring variant in cyt b5, T60A, leads to modestly altered affinity for hydroxylamine substrates and dramatically reduced cyt b5 expression. Work is underway to determine the prevalence of this and other variants in cyt b5 or b5R in a larger population, and to determine the association of such variants with differences in hydroxylamine reduction in vivo.

Vitamin C. Biosynthesis, recycling and degradation in mammals

Vitamin C, a reducing agent and antioxidant, is a cofactor in reactions catalyzed by Cu(+)-dependent monooxygenases and Fe(2+)-dependent dioxygenases. It is synthesized, in vertebrates having this capacity, from d-glucuronate. The latter is formed through direct hydrolysis of uridine diphosphate (UDP)-glucuronate by enzyme(s) bound to the endoplasmic reticulum membrane, sharing many properties with, and most likely identical to, UDP-glucuronosyltransferases. Non-glucuronidable xenobiotics (aminopyrine, metyrapone, chloretone and others) stimulate the enzymatic hydrolysis of UDP-glucuronate, accounting for their effect to increase vitamin C formation in vivo. Glucuronate is converted to l-gulonate by aldehyde reductase, an enzyme of the aldo-keto reductase superfamily. l-Gulonate is converted to l-gulonolactone by a lactonase identified as SMP30 or regucalcin, whose absence in mice leads to vitamin C deficiency. The last step in the pathway of vitamin C synthesis is the oxidation of l-gulonolactone to l-ascorbic acid by l-gulonolactone oxidase, an enzyme associated with the endoplasmic reticulum membrane and deficient in man, guinea pig and other species due to mutations in its gene. Another fate of glucuronate is its conversion to d-xylulose in a five-step pathway, the pentose pathway, involving identified oxidoreductases and an unknown decarboxylase. Semidehydroascorbate, a major oxidation product of vitamin C, is reconverted to ascorbate in the cytosol by cytochrome b(5) reductase and thioredoxin reductase in reactions involving NADH and NADPH, respectively. Transmembrane electron transfer systems using ascorbate or NADH as electron donors serve to reduce semidehydroascorbate present in neuroendocrine secretory vesicles and in the extracellular medium. Dehydroascorbate, the fully oxidized form of vitamin C, is reduced spontaneously by glutathione, as well as enzymatically in reactions using glutathione or NADPH. The degradation of vitamin C in mammals is initiated by the hydrolysis of dehydroascorbate to 2,3-diketo-l-gulonate, which is spontaneously degraded to oxalate, CO(2) and l-erythrulose. This is at variance with bacteria such as Escherichia coli, which have enzymatic degradation pathways for ascorbate and probably also dehydroascorbate.

Reductive detoxification of arylhydroxylamine carcinogens by human NADH cytochrome b5 reductase and cytochrome b5

Heterocyclic and aromatic amine carcinogens are thought to lead to tumor initiation via the formation of DNA adducts, and bioactivation to arylhydroxylamine metabolites is necessary for reactivity with DNA. Carcinogenic arylhydroxylamine metabolites are cleared by a microsomal, NADH-dependent, oxygen-insensitive reduction pathway in humans, which may be a source of interindividual variability in response to aromatic amine carcinogens. The purpose of this study was to characterize the identity of this reduction pathway in human liver. On the basis of our findings with structurally similar arylhydroxylamine metabolites of therapeutic drugs, we hypothesized that the reductive detoxification of arylhydroxylamine carcinogens was catalyzed by NADH cytochrome b5 reductase (b5R) and cytochrome b5 (cyt b5). We found that reduction of the carcinogenic hydroxylamines of the aromatic amine 4-aminobiphenyl (4-ABP; found in cigarette smoke) and the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP; found in grilled meats) was indeed catalyzed by a purified system containing only human b5R and cyt b5. Specific activities were 56-346-fold higher in the purified system as compared to human liver microsomes (HLM), with similar Michaelis-Menten constants (K(m) values) in both systems. The stoichiometry for b5R and cyt b5 that yielded the highest activity in the purified system was also similar to that found in native HLM ( approximately 1:8 to 1:10). Polyclonal antisera to either b5R or cyt b5 significantly inhibited N-hydroxy-4-aminobiphenyl (NHOH-4-ABP) reduction by 95 and 89%, respectively, and immunoreactive cyt b5 protein content in individual HLM was significantly correlated with individual reduction of both NHOH-4-ABP and N-hydroxy-PhIP (NHOH-PhIP). Finally, titration of HLM into the purified b5R/cyt b5 system did not enhance the efficiency of reduction activity. We conclude that b5R and cyt b5 are together solely capable of the reduction of arylhydroxylamine carcinogens, and we further hypothesize that this pathway may be a source of individual variability with respect to cancer susceptibility following 4-ABP or PhIP exposure.

Intracellular sorting of the tail-anchored protein cytochrome b5 in plants: a comparative study using different isoforms from rabbit and Arabidopsis

Tail-anchored (TA) proteins are bound to membranes by a hydrophobic sequence located very close to the C-terminus, followed by a short luminal polar region. Their active domains are exposed to the cytosol. TA proteins are synthesized on free cytosolic ribosomes and are found on the surface of every subcellular compartment, where they play various roles. The basic mechanisms of sorting and targeting of TA proteins to the correct membrane are poorly characterized. In mammalian cells, the net charge of the luminal region determines the sorting to the correct target membrane, a positive charge leading to mitochondria and negative or null charge to the endoplasmic reticulum (ER). Here sorting signals of TA proteins were studied in plant cells and compared with those of mammalian proteins, using in vitro translation-translocation and in vivo expression in tobacco protoplasts or leaves. It is shown that rabbit cytochrome b5 (cyt b5) with a negative charge is faithfully sorted to the plant ER, whereas a change to a positive charge leads to chloroplast targeting (instead of to mitochondria as observed in mammalian cells). The subcellular location of two cyt b5 isoforms from Arabidopsis thaliana (At1g26340 and At5g48810, both with positive net charge) was then determined. At5g48810 is targeted to the ER, and At1g26340 to the chloroplast envelope. The results show that the plant ER, unlike the mammalian ER, can accommodate cytochromes with opposite C-terminal net charge, and plant cells have a specific and as yet uncharacterized mechanism to sort TA proteins with the same positive C-terminal charge to different membranes.

NADH cytochrome b5 reductase and cytochrome b5 catalyze the microsomal reduction of xenobiotic hydroxylamines and amidoximes in humans

Hydroxylamine metabolites, implicated in dose-dependent and idiosyncratic toxicity from arylamine drugs, and amidoximes, used as pro-drugs, are metabolized by an as yet incompletely characterized NADH-dependent microsomal reductase system. We hypothesized that NADH cytochrome b5 reductase and cytochrome b5 were responsible for this enzymatic activity in humans. Purified human soluble NADH cytochrome b5 reductase and cytochrome b5, expressed in Escherichia coli, efficiently catalyzed the reduction of sulfamethoxazole hydroxylamine, dapsone hydroxylamine, and benzamidoxime, with apparent Km values similar to those found in human liver microsomes and specific activities (Vmax) 74 to 235 times higher than in microsomes. Minimal activity was seen with either protein alone, and microsomal protein did not enhance activity other than additively. All three reduction activities were significantly correlated with immunoreactivity for cytochrome b5 in individual human liver microsomes. In addition, polyclonal antibodies to both NADH cytochrome b5 reductase and cytochrome b5 significantly inhibited reduction activity for sulfamethoxazole hydroxylamine. Finally, fibroblasts from a patient with type II hereditary methemoglobinemia (deficient in NADH cytochrome b5 reductase) showed virtually no activity for hydroxylamine reduction, compared with normal fibroblasts. These results indicate a novel direct role for NADH cytochrome b5 reductase and cytochrome b5 in xenobiotic metabolism and suggest that pharmacogenetic variability in either of these proteins may effect drug reduction capacity.

Combined analysis of oligonucleotide microarray data from transgenic and knockout mice identifies direct SREBP target genes

The synthesis of fatty acids and cholesterol, the building blocks of membranes, is regulated by three membrane-bound transcription factors: sterol regulatory element-binding proteins (SREBP)-1a, -1c, and -2. Their function in liver has been characterized in transgenic mice that overexpress each SREBP isoform and in mice that lack all three nuclear SREBPs as a result of gene knockout of SREBP cleavage-activating protein (SCAP), a protein required for nuclear localization of SREBPs. Here, we use oligonucleotide arrays hybridized with RNA from livers of three lines of mice (transgenic for SREBP-1a, transgenic for SREBP-2, and knockout for SCAP) to identify genes that are likely to be direct targets of SREBPs in liver. A total of 1,003 genes showed statistically significant increased expression in livers of transgenic SREBP-1a mice, 505 increased in livers of transgenic SREBP-2 mice, and 343 showed decreased expression in Scap-/- livers. A subset of 33 genes met the stringent combinatorial criteria of induction in both SREBP transgenics and decreased expression in SCAP-deficient mice. Of these 33 genes, 13 were previously identified as direct targets of SREBP action. Of the remaining 20 genes, 13 encode enzymes or carrier proteins involved in cholesterol metabolism, 3 participate in fatty acid metabolism, and 4 have no known connection to lipid metabolism. Through application of stringent combinatorial criteria, the transgenic/knockout approach allows identification of genes whose activities are likely to be controlled directly by one family of transcription factors, in this case the SREBPs.

The Opitz syndrome gene Mid1 is transcribed from a human endogenous retroviral promoter

Human endogenous retroviruses (HERVs) and other long terminal repeat (LTR)-containing elements comprise a significant portion (8%) of the human genome and are likely vestiges of retroviral infections during primate evolution. Many of the HERVs present in human DNA have retained functional promoter, enhancer, and polyadenylation signals, and these regulatory sequences have the potential to modify the expression of nearby genes. To identify retroviral elements that contribute to the transcription of human genes, we screened sequence databases for chimeric (viral-cellular) transcripts. These searches revealed a fusion transcript containing the LTR of an HERV-E element linked to the Opitz syndrome gene Mid1. We confirmed the authenticity of the chimeric transcript by 5' rapid amplification of cDNA ends (RACE) and established that the Mid1 mRNA isoform was transcribed from a retroviral LTR. The identification of a retroviral first exon suggested the existence of alternative promoters for Mid1 because nonretroviral (native) 5' untranslated regions (UTRs) had been reported previously for this gene. Although Mid1 transcripts could be detected in all tissues tested, quantitative real-time reverse transcription-polymerase chain reaction indicated that the retroviral promoter contributes significantly to the level of Mid1 transcripts in placenta and embryonic kidney, where chimeric mRNAs were found to represent 25% and 22% of overall Mid1 mRNAs, respectively. Transient transfection studies supported a role for the LTR as a strong tissue-specific promoter in placental and embryonic kidney cell lines and suggested a function for the LTR as an enhancer. These findings provide further evidence that some endogenous retroviruses have evolved a biological function by contributing transcriptional regulatory elements to cellular genes.

Probing the differences between rat liver outer mitochondrial membrane cytochrome b5 and microsomal cytochromes b5

Two distinct forms of cytochrome b5 exist in the rat hepatocyte. One is associated with the membrane of the endoplasmic reticulum (microsomal, or Mc, cyt b5) while the other is associated with the outer membrane of liver mitochondria (OM cyt b5). Rat OM cyt b5, the only OM cyt b5 identified so far, has a significantly more negative reduction potential and is substantially more stable toward chemical and thermal denaturation than Mc cytochromes b5. In addition, hemin is kinetically trapped in rat OM cyt b5 but not in the Mc proteins. As a result, no transfer of hemin from rat OM cyt b5 to apomyoglobin is observed at pH values as low as 5.2, nor can the thermodyamically favored ratio of hemin orientational isomers be achieved under physiologically relevant conditions. These differences are striking given the similarity of the respective protein folds. A combined theoretical and experimental study has been conducted in order to probe the structural basis behind the remarkably different properties of rat OM and Mc cytochromes b5. Molecular dynamics (MD) simulations starting from the crystal structure of bovine Mc cyt b5 revealed a conformational change that exposes several internal residues to the aqueous environment. The new conformation is equivalent to the "cleft-opened" intermediate observed in a previously reported MD simulation of bovine Mc cyt b5 [Storch, E. M., and Daggett, V. (1995) Biochemistry 34, 9682-9693]. The rat OM protein does not adopt a comparable conformation in MD simulations, thus restricting access of water to the protein interior. Subsequent comparisons of the protein sequences and structures suggested that an extended hydrophobic network encompassing the side chains of Ala-18, Ile-32, Leu-36, and Leu-47 might contribute to the inability of rat OM cyt b5 to adopt the cleft-opened conformation and, hence, stabilize its fold relative to the Mc isoforms. A corresponding network is not present in bovine Mc cyt b5 because positions 18, 32, and 47, are occupied by Ser, Leu, and Arg, respectively. To probe the roles played by Ala-18, Ile-32, and Leu-47 in endowing rat OM cyt b5 with its unusual structural properties, we have replaced them with the corresponding residues in bovine Mc cyt b5. Hence, the I32L (single), A18S/L47R (double), and A18S/L47R/I32L (triple) mutants of rat OM cyt b5 were prepared. The stability of these proteins was found to decrease in the following order: WT rat OM > rat OM I32L > rat OM A18S/L47R > rat OM A18S/L47R/I32L > bovine Mc cyt b5. The decrease in stability of the rat OM protein correlates with the extent to which the hydrophobic cluster involving the side chains of residues 18, 32, 36, and 47 has been disrupted. Complete disruption of the hydrophobic network in the triple mutant is confirmed in a 2.0 A resolution crystal structure of the protein. Disruption of the hydrophobic network also facilitates hemin loss at pH 5.2 for the double and triple mutants, with the less stable triple mutant exhibiting the greater rate of hemin transfer to apomyoglobin. Finally, 1H NMR spectroscopy and side-by-side comparisons of the crystal structures of bovine Mc, rat OM, and rat OM A18S/L47R/I32L cyt b5 allowed us to conclude that the nature of residue 32 plays a key role in controlling the relative stability of hemin orientational isomers A and B in rat OM cyt b5. A similar analysis led to the conclusion that Leu-70 and Ser-71 play a pivotal role in stabilizing isomer A relative to isomer B in Mc cytochromes b5.

Transcriptional and translational mechanisms of cytochrome b5 reductase isoenzyme generation in humans

Cytochrome b5 reductase (b5R) is an essential enzyme that exists in soluble and membrane-bound isoforms, each with specific functions. In the rat, the two forms are generated from alternative transcripts differing in the first exons. In contrast, the biogenesis of b5R isoforms in the human is not yet well understood. In the present study we have detected three novel alternative exons, designated 1S, S' and 1B, located between the first alternative exon 1M and the common second exon in the human b5R gene. Accordingly, multiple M-type, S-type and SS'-type and B-type transcripts are generated. All types of human b5R transcript are expressed ubiquitously. An analysis of in vitro translation products demonstrated an alternative use of different AUG initiators resulting in the production of various human b5R protein isoforms. Our results indicate that the organization of the 5' region of the b5R gene is not conserved between rodents and humans. Insertion of Alu elements into the human b5R gene, in particular just upstream of the S/S' region, could be responsible for dynamic events of gene rearrangement during evolution.

Distribution and localization of CMP-N-acetylneuraminic acid hydroxylase and N-glycolylneuraminic acid-containing glycoconjugates in porcine lymph node and peripheral blood lymphocytes

An immunohistochemical analysis was performed on paraplast-embedded sections of porcine lymph node with antibodies specific for CMP-N-acetylneuraminic acid hydroxylase (h-3 antibody) and glycoconjugate-bound N-glycolylneuraminic acid (Neu5Gc), which appears as a result of the hydroxylase reaction (a-Gc antibody). The observed localization of the enzyme in cells of the perifollicular zone, including lymphocytes, was reflected in a similar distribution of glycoconjugate-bound Neu5Gc. This result confirms previous biochemical investigations on the role of the hydroxylase in regulating Neu5Gc biosynthesis in vitro on a histological level. An analysis of lymphocytes isolated from porcine thymus, spleen, lymph node and peripheral blood revealed differences in the amount of Neu5Gc in the various lymphocytes that correlated well with the activity of the hydroxylase determined in these cells. The largest amount of Neu5Gc and highest activity of the enzyme were detected in the peripheral blood lymphocytes (PBL). Immunohistochemical studies with a-Gc and h-3 antibodies on sections of paraplast-embedded PBL showed that these antigens were located at the cell surface and in the cytosol, respectively. Ultrastructural immunocytochemistry with the h-3 antibody and immunogold labelling was used to investigate the subcellular localization of the hydroxylase. The enzyme was detected in the cytosol in the vicinity of the nuclear membrane and the outer membrane of mitochondria, in particular those close to the nucleus. The antigen was also detected on cytoplasmic tubular structures. In addition, a weak labelling of the Golgi apparatus was also observed occasionally. The possibility that this localization may be related to the availability of the substrate CMP-Neu5Ac and the redox partner cytochrome b5 is discussed.

Tonoplast subcellular localization of maize cytochrome b5 reductases

Plant cytochrome b5 reductases (b5R) are assumed to be part of an ER-associated redox chain that oxidizes NADH to provide electrons via cytochrome b5 (cyt b5) to ER-associated fatty acyl desaturase and related hydroxylases, as in mammalian cells. Here we report on cDNA cloning of a novel maize b5R, NFR II, strongly related to a previously cloned cDNA, NFR I (Bagnaresi et al., 1999, Biochem. J. 338, 499-505). Maize b5R isoforms are produced by a small multi-gene family. The NFR cDNAs were shown to encode active b5Rs by heterologous expression in yeast. Both reductases, in addition to Fe3+-chelates, efficiently reduced Cu2+-chelates. Using a polyclonal antibody able to recognize both NFR I and NFR II isoforms, no ER or mitochondrial localization could be detected in maize roots. Unexpectedly, maize b5Rs were found to be targeted to the tonoplast. Using the most specific assay to measure NFR activity, we confirmed that the highest NFR specific activity is associated with tonoplast-enriched maize root fractions. Tonoplast targeting is not consistent with a role in desaturase reactions or with the other functions ascribed to date to plant b5R. This indicates that alternative ER-associated electron donors for desaturases need to be sought, and that plant b5Rs may have previously unexpected functions.

Expression and translocation of Drosophila nuclear encoded cytochrome b(5) proteins to mitochondria

DNA sequence studies of cytochrome b(5) (Cyt-b) genes from Drosophila melanogaster and Drosophila virilis predict that the Drosophila Cyt-b proteins are extremely hydrophobic and have at least eight potential transmembrane spanning domains. Primary protein sequence analysis also predicts that the Cyt-b proteins have mitochondrial targeting sequences and they contain sites for potential post-translational modification similar to other cytochrome proteins. We report the characterization of the cytochrome b(5) proteins from Drosophila melanogaster and Drosophila virilis. We have used a Drosophila cytochrome b(5) specific antibody to demonstrate that cytochrome b(5) proteins are expressed in muscle-containing tissues in the fly. We also provide evidence that the nuclear encoded cytochrome b(5) protein that contains a mitochondrial targeting sequence is translocated to mitochondria.

Determination of concentration of cytosolic NADH-cytochrome b5 reductase in erythrocytes from normal Chinese adults, neonates and patients with hereditary methemoglobinemia by double-antibody sandwich ELISA

NADH-cytochrome b5 reductase (b5R), present in various tissues of the body, is a redox enzyme of multiple functions. The deficiency of the enzyme leads to hereditary methemoglobinemia. With rabbit anti-b5R antibody for plate coating and enzyme-labeled anti-b5R monoclonal antibody as reporter, we have developed a sandwich ELISA procedure for the determination of b5R concentration. This procedure is sensitive to a wide range of linearity, and convenient in coping with large numbers of samples. Using this novel method, cytosolic b5R concentration in the erythrocytes of 30 normal Chinese adults was estimated to be 25.63+/-8.54 ng/mg Hb. It was found that the concentration of red cell soluble b5R of five newborns was significantly lower than that of normal adults and soluble b5R was undetectable in the erythrocytes of 4 patients with type I hereditary methemoglobinemia. Our results demonstrated that the reduced b5R activity in red cell cytosol of both neonates and type I hereditary methemoglobinemic patients results largely or mainly from the lowered b5R concentration. Our novel method might be further exploited for in-depth investigation of the relationship between the qualitative and quantitative changes of b5R with regard to physiological and pathological conditions.

Alternative splicing determines the intracellular localization of the novel nuclear protein Nop30 and its interaction with the splicing factor SRp30c

We report on the molecular cloning of a novel human cDNA by its interaction with the splicing factor SRp30c in a yeast two-hybrid screen. This cDNA is predominantly expressed in muscle and encodes a protein that is present in the nucleoplasm and concentrated in nucleoli. It was therefore termed Nop30 (nucleolar protein of 30 kDa). We have also identified a related cDNA with a different carboxyl terminus. Sequencing of the NOP gene demonstrated that both cDNAs are generated by alternative 5' splice site usage from a single gene that consists of four exons, spans at least 1800 nucleotides, and is located on chromosome 16q21-q23. The alternative 5' splice site usage introduces a frameshift creating two different carboxyl termini. The carboxyl terminus of Nop30 is rich in serines and arginines and has been found to target the protein into the nucleus, whereas its isoform is characterized by proline/glutamic acid dipeptides in its carboxyl terminus and is predominantly found in the cytosol. Interaction studies in yeast, in vitro protein interaction assays, and co-immunoprecipitations demonstrated that Nop30 multimerizes and binds to the RS domain of SRp30c but not to other splicing factors tested. Overexpression of Nop30 changes alternative exon usage in preprotachykinin and SRp20 reporter genes, suggesting that Nop30 influences alternative splice site selection in vivo.

Characterization of a novel NADH-specific, FAD-containing, soluble reductase with ferric citrate reductase activity from maize seedlings

A novel NADH-dependent, soluble flavoreductase of 60 kDa, active toward ferric chelates and quinones, has been purified from maize seedlings. Two closely related isoforms were separated. The two isoforms are similar in several biochemical features, with the exception of the apparent molecular mass of their subunits (29 and 31 kDa, respectively). They are homodimers in the native state, they bind FAD as the prosthetic group and show strong preference for NADH over NADPH as the electron donor. Ferric chelates (chiefly ferric citrate, Km 3-5 x 10(-5) M; kcat/Km 3.4-3.7 x 10(5) M-1 s-1), and some quinones (benzoquinone, coenzyme Q-0, and juglone) are used as electron acceptors. Enzymatic reduction of benzoquinone occurs with formation of radical semiquinones. Both soluble ferric chelate reductase isoforms are strongly inhibited by p-hydroxymercuribenzoic acid (I50 5 nM) and by cibachron blue, the latter giving nonlinear inhibition. It is suggested that soluble ferric chelate reductase might be involved in the symplastic reduction of iron chelates which is required for the assembly of iron-containing macromolecules such as cytochromes and ferritin.

Microsomal electron transfer in higher plants: cloning and heterologous expression of NADH-cytochrome b5 reductase from Arabidopsis

AtCBR, a cDNA encoding NADH-cytochrome (Cyt) b5 reductase, and AtB5-A and AtB5-B, two cDNAs encoding Cyt b5, were isolated from Arabidopsis. The primary structure deduced from the AtCBR cDNA was 40% identical to those of the NADH-Cyt b5 reductases of yeast and mammals. A recombinant AtCBR protein prepared using a baculovirus system exhibited typical spectral properties of NADH-Cyt b5 reductase and was used to study its electron-transfer activity. The recombinant NADH-Cyt b5 reductase was functionally active and displayed strict specificity to NADH for the reduction of a recombinant Cyt b5 (AtB5-A), whereas no Cyt b5 reduction was observed when NADPH was used as the electron donor. Conversely, a recombinant NADPH-Cyt P450 reductase of Arabidopsis was able to reduce Cyt b5 with NADPH but not with NADH. To our knowledge, this is the first evidence in higher plants that both NADH-Cyt b5 reductase and NADPH-Cyt P450 reductase can reduce Cyt b5 and have clear specificities in terms of the electron donor, NADH or NADPH, respectively. This substrate specificity of the two reductases is discussed in relation to the NADH- and NADPH-dependent activities of microsomal fatty acid desaturases.

NADH:Fe(III)-chelate reductase of maize roots is an active cytochrome b5 reductase

Microsomal NADH:Fe(III)-chelate reductase (NFR) of maize roots has been purified as a monomeric flavoprotein of 32 kDa with non-covalently bound FAD. In the presence of NADH, NFR efficiently reduced the physiological iron-chelate Fe(III)-citrate (K[cat]/K[m](Fe(III)-citrate) = 6.0 X 10[6] M[-1] S[-1]) with a sequential reaction mechanism. Purified NFR was totally inhibited by the sulfhydryl reagent PHMB at 10(-9) M, and it could use cyt b5 as alternative electron acceptor with a maximal reduction rate as high as with Fe(III)-citrate. We conclude that in maize roots the reduction of Fe(III)-citrate is chiefly performed by a cytochrome b5 reductase, mostly associated with intracellular membranes and in part with the plasma membrane.

Evolutionary divergence of the cytochrome b5 gene of Drosophila

Cytochrome proteins perform a broad spectrum of biological functions ranging from oxidative metabolism to electron transport and are thus essential to all organisms. The b-type cytochrome proteins bind heme noncovalently, are expressed in many different forms and are localized to various cellular compartments. We report the characterization of the cytochrome b5 (Cyt-b) gene of Drosophila virilis and compare its structure to the Cyt-b gene of Drosophila melanogaster. As in D. melanogaster, the D. virilis gene is nuclear encoded and single copy. Although the intron/exon structures of these homologues differ, the Cyt-b proteins of D. melanogaster and D. virilis are approximately 75% identical and share the same size coding regions (1,242 nucleotides) and protein products (414 amino acids). The Drosophila Cyt-b proteins show sequence similarity to other b-type cytochromes, especially in the N-terminal heme-binding domain, and may be targeted to the mitochondrial membrane. The greatest levels of similarity are observed in areas of potential importance for protein structure and function. The exon sequences of the D. virilis Cyt-b gene differ by a total of 292 base changes. However, 62% of these changes are silent. The high degree of conservation between species separated by 60 million years of evolution in both the DNA and amino acid sequences suggests this nuclear cytochrome b5 locus encodes an essential product of the Drosophila system.

The carboxyl terminus of the membrane-binding domain of cytochrome b5 spans the bilayer of the endoplasmic reticulum

Preliminary studies (Vergères, G., and Waskell, L. (1992) J. Biol. Chem. 267, 12583-12591) have suggested that the carboxyl-terminal membrane-binding domain of cytochrome b5 traverses the membrane and that the carboxyl terminus is in the lumen of the endoplasmic reticulum. In order to confirm and extend these studies, additional experiments were conducted. The gene coding for rat cytochrome b5 was transcribed and the resulting mRNA was translated in vitro in a rabbit reticulocyte lysate in the presence of microsomes. The binding and topology of cytochrome b5 were investigated by treating microsomes containing the newly incorporated cytochrome b5 with carboxypeptidase Y and trypsin. Our studies indicate that cytochrome b5 is inserted both co- and post-translationally into microsomes in a topology in which the membrane-binding domain spans the bilayer with its COOH terminus in the lumen. Cytochrome b5 is also incorporated into microsomes pretreated with trypsin in a topology indistinguishable from the one resulting from the insertion of the protein into untreated microsomes, reconfirming that cytochrome b5 does not use the signal recognition particle-dependent translocation machinery. Our results do not allow a distinction to be made between a spontaneous insertion mode or some other trypsin-resistant receptor-mediated mechanism. A role for Pro115 in the middle of the membrane-binding domain of cytochrome b5 was also examined by mutating it to an alanine and subsequently characterizing the ability of the mutant protein to be incorporated into membranes. The mutant protein inserted more slowly in vitro into microsomes as well as into pure lipid bilayers by a factor of 2 to 3.

Cytochrome b5, its functions, structure and membrane topology

The first part of the present communication reviews recent advances in our understanding of the known physiological functions of cytochrome b5. In addition, one section is devoted to a description of a recently discovered function of cytochrome b5, namely its involvement in the synthesis of the oncofetal antigen N-glycolylneuraminic acid. The second part of the article summarizes site-directed mutagenesis studies, primarily conducted in the author's laboratory, in both the catalytic heme-binding and membrane-binding domain of cytochrome b5. These studies have shown that: 1) the membrane binding domain of cytochrome b5 spans the bilayer; 2) cytochrome b5 lacking 19 COOH-terminal amino acids does not bind to membrane bilayers; and 3) specific amino acids in the membrane binding domain have been mutated and shown not to be essential for the function of cytochrome b5 with its redox partners.

Tobacco cytochrome b5: cDNA isolation, expression analysis and in vitro protein targeting

A full-length clone encoding cytochrome b5 has been isolated from a tobacco leaf cDNA library in lambda gt11 by PCR using degenerate primers. This cDNA encodes a protein of 139 residues which exhibits a high degree of homology to other cytochrome b5s, the message for which is expressed predominantly in developing seeds and in pigmented flower tissue. In the developing tobacco seed the mRNA is abundant at very early stages (< 10 days after flowering). Southern analysis indicated that more than one gene encodes cytochrome b5 in the tobacco genome. In vitro transcription and translation studies of the cDNA indicated that the protein inserts into the ER membrane by a non-SRP-mediated pathway and that the C-terminus of the protein is required for targeting and insertion.