A single mRNA, transcribed from an alternative, erythroid-specific, promoter, codes for two non-myristylated forms of NADH-cytochrome b5 reductase

Mitophagy in Human Diseases

Mitophagy is a selective autophagic process, essential for cellular homeostasis, that eliminates dysfunctional mitochondria. Activated by inner membrane depolarization, it plays an important role during development and is fundamental in highly differentiated post-mitotic cells that are highly dependent on aerobic metabolism, such as neurons, muscle cells, and hepatocytes. Both defective and excessive mitophagy have been proposed to contribute to age-related neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases, metabolic diseases, vascular complications of diabetes, myocardial injury, muscle dystrophy, and liver disease, among others. Pharmacological or dietary interventions that restore mitophagy homeostasis and facilitate the elimination of irreversibly damaged mitochondria, thus, could serve as potential therapies in several chronic diseases. However, despite extraordinary advances in this field, mainly derived from in vitro and preclinical animal models, human applications based on the regulation of mitochondrial quality in patients have not yet been approved. In this review, we summarize the key selective mitochondrial autophagy pathways and their role in prevalent chronic human diseases and highlight the potential use of specific interventions.

The multiple roles of coenzyme Q in cellular homeostasis and their relevance for the pathogenesis of coenzyme Q deficiency

Coenzyme Q (CoQ) is a redox active lipid that plays a central role in cellular homeostasis. It was discovered more than 60 years ago because of its role as electron transporter in the mitochondrial respiratory chain. Since then it has become evident that CoQ has many other functions, not directly related to bioenergetics. It is a cofactor of several mitochondrial dehydrogenases involved in the metabolism of lipids, amino acids, and nucleotides, and in sulfide detoxification. It is a powerful antioxidant and it is involved in the control of programmed cell death by modulating both apoptosis and ferroptosis. CoQ deficiency is a clinically and genetically heterogeneous group of disorders characterized by the impairment of CoQ biosynthesis. CoQ deficient patients display defects in cellular bioenergetics, but also in the other pathways in which CoQ is involved. In this review we will focus on the functions of CoQ not directly related to the respiratory chain, and on how their impairment is relevant for the pathophysiology of CoQ deficiency. A better understanding of the complex set of events triggered by CoQ deficiency will allow to design novel approaches for the treatment of this condition.

Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia)

Type I congenital methemoglobinemia is an autosomal recessive disorder. A high frequency of congenital methemoglobinemia has been reported among Native Americans inhabiting the Yukon-Kuskokwim Delta. Other rare cases of congenital methemoglobinemia of types I and II have been reported in Japan and other countries. In Russia-namely, in Yakutia-a high frequency of type I congenital methemoglobinemia has been reported. In 2009, the Consultation Polyclinic of the Pediatric Center in Yakutsk city established a registry of children with congenital methemoglobinemia. In total, 43 patients were registered between 2005 and 2009. The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%) and physical examination revealed cyanosis of the skin and mucus membranes. There were significant positive relationships between percentage of methemoglobin and erythrocyte count, hemoglobin concentration, and hematocrit among male patients, consistent with an upregulation of the hypoxic response. The prevalence per 100,000 children ranged from 12.7 to 47.0 in 3 geographic regions of Yakutia. Further research is needed to clarify the clinical consequences of congenital methemoglobinemia in the children of Yakutia and the reasons for the high variability in the prevalence of the condition.

PI(4,5)P(2)-dependent and Ca(2+)-regulated ER-PM interactions mediated by the extended synaptotagmins

Most available information on endoplasmic reticulum (ER)-plasma membrane (PM) contacts in cells of higher eukaryotes concerns proteins implicated in the regulation of Ca(2+) entry. However, growing evidence suggests that such contacts play more general roles in cell physiology, pointing to the existence of additionally ubiquitously expressed ER-PM tethers. Here, we show that the three extended synaptotagmins (E-Syts) are ER proteins that participate in such tethering function via C2 domain-dependent interactions with the PM that require PI(4,5)P2 in the case of E-Syt2 and E-Syt3 and also elevation of cytosolic Ca(2+) in the case of E-Syt1. As they form heteromeric complexes, the E-Syts confer cytosolic Ca(2+) regulation to ER-PM contact formation. E-Syts-dependent contacts, however, are not required for store-operated Ca(2+) entry. Thus, the ER-PM tethering function of the E-Syts (tricalbins in yeast) mediates the formation of ER-PM contacts sites, which are functionally distinct from those mediated by STIM1 and Orai1.

Unique expression pattern of human nicotinamide mononucleotide adenylyltransferase isozymes in red blood cells

Nicotinamide mononucleotide adenylyltransferase (NMNAT) catalyzes the formation of nicotinamide adenine dinucleotide (NAD). In humans, three isozymes have been identified: NMNAT1, which is widely expressed in all tissues, NMNAT2 and NMNAT3, which show a tissue-specific expression and whose mRNA levels are generally lower compared to NMNAT1. In the present study we determined the individual NMNAT isozymes activity in human red blood cells (RBCs) by using a biochemical discrimination assay based on the distinctive catalytic properties of the three proteins. We found that isozyme 3 predominates over isozyme 1, whereas isozyme 2 is absent. This high prevalence of NMNAT3 is cell-aging independent and was also confirmed by analyzing the mRNA and protein levels. RBC represent the first human cell type with a remarkable predominance of NMNAT3, and this unique expression pattern is discussed in light of the catalytic properties of the isozymes and in consideration of the biochemical microenvironment of RBC.

Cytochrome b5 and NADH cytochrome b5 reductase: genotype-phenotype correlations for hydroxylamine reduction

OBJECTIVES

NADH cytochrome b5 reductase (b5R) and cytochrome b5 (b5) catalyze the reduction of sulfamethoxazole hydroxylamine (SMX-HA), which can contribute to sulfonamide hypersensitivity, to the parent drug sulfamethoxazole. Variability in hydroxylamine reduction could thus play a role in adverse drug reactions. The aim of this study was to characterize variability in SMX-HA reduction in 111 human livers, and investigate its association with single nucleotide polymorphisms (SNPs) in b5 and b5R cDNA.

METHODS

Liver microsomes were assayed for SMX-HA reduction activity, and b5 and b5R expression was semiquantified by immunoblotting. The coding regions of the b5 (CYB5A) and b5R (CYB5R3) genes were resequenced.

RESULTS

Hepatic SMX-HA reduction displayed a 19-fold range of individual variability (0.06-1.11 nmol/min/mg protein), and a 17-fold range in efficiency (Vmax/Km) among outliers. SMX-HA reduction was positively correlated with b5 and b5R protein content (P<0.0001, r=0.42; P=0.01, r=0.23, respectively), and expression of both proteins correlated with one another (P<0.0001; r=0.74). A novel cSNP in CYB5A (S5A) was associated with very low activity and protein expression. Two novel CYB5R3 SNPs, R59H and R297H, displayed atypical SMX-HA reduction kinetics and decreased SMX-HA reduction efficiency.

CONCLUSION

These studies indicate that although novel cSNPs in CYB5A and CYB5R3 are associated with significantly altered protein expression and/or hydroxylamine reduction activities, these low-frequency cSNPs seem to only minimally impact overall observed phenotypic variability. Work is underway to characterize polymorphisms in other regions of these genes to further account for individual variability in hydroxylamine reduction.

A novel mutation in type II methemoglobinemia

Type II methemoglobinemia is a somatic deficiency of cytochrome b5 reductase with severe global neurologic impairment. We report a novel mutation in exon 3 of the CYB5R3 gene on chromosome 22 consisting of homozygous 1-base pair (bp) deletion noted as c.215delG; p.Gly72AlafsX100. The patient had improvement of gross motor skills, chewing, and swallowing that may be due to the initiation of daily ascorbic acid therapy. We hypothesize that a possible response to ascorbic acid may be related to the effect of making additional ferrous iron available for its role as a cofactor in carnitine synthesis.

Loss of Ncb5or results in impaired fatty acid desaturation, lipoatrophy, and diabetes

Targeted ablation of the novel flavoheme reductase Ncb5or knock-out (KO) results in progressive loss of pancreatic beta-cells and white adipose tissue over time. Lipoatrophy persisted in KO animals in which the confounding metabolic effects of diabetes were eliminated by islet transplantation (transplanted knockout (TKO)). Lipid profiles in livers prepared from TKO animals were markedly deficient in triglycerides and diacylglycerides. Despite enhanced expression of stearoyl-Co-A desaturase-1, levels of palmitoleic and oleic acids (Delta9 fatty acid desaturation) were decreased in TKO relative to wild type controls. Treatment of KO hepatocytes with palmitic acid reduced cell viability and increased apoptosis, a response blunted by co-incubation with oleic acid. The results presented here support the hypothesis that Ncb5or supplies electrons for fatty acid desaturation, offer new insight into the regulation of a crucial step in fatty acid biosynthesis, and provide a plausible explanation for both the diabetic and the lipoatrophic phenotype in Ncb5or(-/-) mice.

Simulation study of methemoglobin reduction in erythrocytes. Differential contributions of two pathways to tolerance to oxidative stress

Methemoglobin (metHb), an oxidized form of hemoglobin, is unable to bind and carry oxygen. Erythrocytes are continuously subjected to oxidative stress and nitrite exposure, which results in the spontaneous formation of metHb. To avoid the accumulation of metHb, reductive pathways mediated by cytochrome b5 or flavin, coupled with NADH-dependent or NADPH-dependent metHb reductases, respectively, keep the level of metHb in erythrocytes at less than 1% of the total hemoglobin under normal conditions. In this work, a mathematical model has been developed to quantitatively assess the relative contributions of the two major metHb-reducing pathways, taking into consideration the supply of NADH and NADPH from central energy metabolism. The results of the simulation experiments suggest that these pathways have different roles in the reduction of metHb; one has a high response rate to hemoglobin oxidation with a limited reducing flux, and the other has a low response rate with a high capacity flux. On the basis of the results of our model, under normal oxidative conditions, the NADPH-dependent system, the physiological role of which to date has been unclear, is predicted to be responsible for most of the reduction of metHb. In contrast, the cytochrome b5-NADH pathway becomes dominant under conditions of excess metHb accumulation, only after the capacity of the flavin-NADPH pathway has reached its limit. We discuss the potential implications of a system designed with two metHb-reducing pathways in human erythrocytes.

N-myristoylation determines dual targeting of mammalian NADH-cytochrome b5 reductase to ER and mitochondrial outer membranes by a mechanism of kinetic partitioning

Mammalian NADH-cytochrome b(5) reductase (b5R) is an N-myristoylated protein that is dually targeted to ER and mitochondrial outer membranes. The N-linked myristate is not required for anchorage to membranes because a stretch of hydrophobic amino acids close to the NH₂ terminus guarantees a tight interaction of the protein with the phospholipid bilayer. Instead, the fatty acid is required for targeting of b5R to mitochondria because a nonmyristoylated mutant is exclusively localized to the ER. Here, we have investigated the mechanism by which N-linked myristate affects b5R targeting. We find that myristoylation interferes with interaction of the nascent chain with signal recognition particle, so that a portion of the nascent chains escapes from cotranslational integration into the ER and can be post-translationally targeted to the mitochondrial outer membrane. Thus, competition between two cotranslational events, binding of signal recognition particle and modification by N-myristoylation, determines the site of translation and the localization of b5R.

Nuclear localization of NADPH:cytochrome c (P450) reductase enhances the cytotoxicity of mitomycin C to Chinese hamster ovary cells

Overexpression of endoplasmic reticulum-localized NADPH: cytochrome c (P450) reductase (NPR) in Chinese hamster ovary cells increases the hypoxic/aerobic differential toxicity of the mitomycins. Because considerable evidence indicates that DNA cross-links are the major cytotoxic lesions generated by the mitomycins, we proposed that bioactivation of the mitomycins in the nucleus close to the DNA target would influence the cytotoxicity of these drugs. The simian virus 40 large T antigen nuclear localization signal was fused to the amino-terminal end of a human NPR protein that lacked its membrane anchor sequence. Immunofluorescent imaging of transfected cell lines expressing the fusion protein confirmed the nuclear location of the enzyme. Regardless of the oxygenation state of the cell, mitomycin C (MC) cytotoxicity was enhanced in cells with overexpressed NPR localized to the nuclear compartment compared with cells overexpressing an endoplasmic reticulum localized enzyme. Enhanced cytotoxicity in cells treated under hypoxic conditions correlated with increases in genomic DNA alkylations, with more MC-DNA adducts being formed when the enzyme was expressed closer to its DNA target. No change was observed in the hypoxic/aerobic differential toxicity as a function of enzyme localization. These findings indicate that drug efficacy is increased when the subcellular site of drug activation corresponds to its site of action.

Severe neurological impairment in hereditary methaemoglobinaemia type 2

Recessive congenital methaemoglobinaemia (RCM) due to NADH-cytochrome b5 reductase (cytb5r) deficiency is a very rare disorder. We report on two unrelated patients (4 and 2.5 years old) with RCM type 2. Developmental delay was obvious at the age of 4 months. On follow-up, both children showed severe tetraspastic cerebral palsy, profound cognitive impairment, strabismus, impressive secondary microcephaly and failure to thrive. One novel mutation in the DIA1gene was identified. Prenatal diagnosis was successfully done in both families by mutation analysis in chorionic villi or measurement of cytb5r in fetal amniotic cells.

CONCLUSION

Due to the severity of this disease and its 25% recurrence risk, prenatal diagnosis should be made available to all affected families.

Hydrogen peroxide- and cell-density-regulated expression of NADH-cytochrome b5 reductase in HeLa cells

Environmental conditions regulate the expression of different antioxidant enzymes in cell culture. We have studied the effect of cell density and hydrogen peroxide on the expression of NADH-cytochrome b5 reductase in HeLa cells. Polypeptide levels of the NADH-cytochrome b5 reductase increased about three fold in confluent HeLa cells compared to sparse cells. Addition of H2O2 to HeLa cells altered expression levels of the NADH-cytochrome b5 reducatase in a concentration-dependent way, being sparse cells more sensitive to H2O2 addition than confluent cells. The presence of pyruvate, a H2O2 scavenger, produced a significant increment (200%) in the levels of NADH-cytochrome b5 reductase in sparse cells, but less increase (25%) in confluent cells, suggesting that generation of endogenous H2O2 could repress NADH-cytochrome b5 reductase expression, particularly in sparse cultures. Accordingly, confluent HeLa cells showed significantly lower levels of reactive oxygen species than cells in sparse cultures. Addition of tert-butylhydroquinone, a compound which generates reactive oxygen species through redox cycling, also reduced expression of the NADH-cytochrome b5 reductase. Increments in several antioxidant enzymes taking place during confluency could participate in the increase of NADH-cytochrome b5 reductase expression by reducing reactive oxygen species levels in cells. Overall, our results support that acute oxidative stress caused by H2O2 inhibits the expression levels of NADH-cytochrome b5 reductase, most likely due to inhibition of SP1 transcriptional activity. On the other hand, adaptation to H2O2 involved increased expression of the cytochrome b5 reductase, supporting the existence of additional regulatory mechanisms.

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.

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.

A case of methemoglobinemia type II due to NADH-cytochrome b5 reductase deficiency: determination of the molecular basis

Clinical, biochemical and molecular findings in a patient with methemoglobinemia type II are described. Furthermore, a comparison between methemoglobinemia type I and type II, both caused by a deficiency of NADH-cytochrome b5 reductase (b5R), is made. Although the clinical pictures of type I and II are strikingly different, mutations in the diaphorase (DIA1) gene located on chromosome 22 have been described in both types. In the present patient, two newly identified mutations, both leading to a stop codon in exon 4 (Gln77Ter) and in exon 6 (Arg160Ter), were found. Identification of different mutations at different positions in the DIA1 gene might shed light on the clinical and biochemical differences between methemoglobinemia type I and type II.

Large-scale identification of secreted and membrane-associated gene products using DNA microarrays

Membrane-associated and secreted proteins are an important class of proteins and include receptors, transporters, adhesion molecules, hormones and cytokines. Although algorithms have been developed to recognize potential amino-terminal membrane-targeting signals or transmembrane domains in protein sequences, their accuracy is limited and they require knowledge of the entire coding sequence, including the N terminus, which is not currently available for most of the genes in most organisms, including human. Several experimental approaches for identifying secreted and membrane proteins have been described, but none have taken a comprehensive genomic approach. Furthermore, none of these methods allow easy classification of clones from arrayed cDNA libraries, for which large-scale gene-expression data are now becoming available through the use of DNA microarrays. We describe here a rapid and efficient method for identifying genes that encode secreted or membrane proteins. mRNA species bound to membrane-associated polysomes were separated from other mRNAs by sedimentation equilibrium or sedimentation velocity. The distribution of individual transcripts in the 'membrane-bound' and 'cytosolic' fractions was quantitated for thousands of genes by hybridization to DNA microarrays. Transcripts known to encode secreted or membrane proteins were enriched in the membrane-bound fractions, whereas those known to encode cytoplasmic proteins were enriched in the fractions containing mRNAs associated with free and cytoplasmic ribosomes. On this basis, we identified over 275 human genes and 285 yeast genes that are likely to encode previously unrecognized secreted or membrane proteins.

Identification of an NADH-cytochrome b(5) reductase gene from an arachidonic acid-producing fungus, Mortierella alpina 1S-4, by sequencing of the encoding cDNA and heterologous expression in a fungus, Aspergillus oryzae

Based on the sequence information for bovine and yeast NADH-cytochrome b(5) reductases (CbRs), a DNA fragment was cloned from Mortierella alpina 1S-4 after PCR amplification. This fragment was used as a probe to isolate a cDNA clone with an open reading frame encoding 298 amino acid residues which show marked sequence similarity to CbRs from other sources, such as yeast (Saccharomyces cerevisiae), bovine, human, and rat CbRs. These results suggested that this cDNA is a CbR gene. The results of a structural comparison of the flavin-binding beta-barrel domains of CbRs from various species and that of the M. alpina enzyme suggested that the overall barrel-folding patterns are similar to each other and that a specific arrangement of three highly conserved amino acid residues (i.e., arginine, tyrosine, and serine) plays a role in binding with the flavin (another prosthetic group) through hydrogen bonds. The corresponding genomic gene, which was also cloned from M. alpina 1S-4 by means of a hybridization method with the above probe, had four introns of different sizes. These introns had GT at the 5' end and AG at the 3' end, according to a general GT-AG rule. The expression of the full-length cDNA in a filamentous fungus, Aspergillus oryzae, resulted in an increase (4.7 times) in ferricyanide reduction activity involving the use of NADH as an electron donor in the microsomes. The M. alpina CbR was purified by solubilization of microsomes with cholic acid sodium salt, followed by DEAE-Sephacel, Mono-Q HR 5/5, and AMP-Sepharose 4B affinity column chromatographies; there was a 645-fold increase in the NADH-ferricyanide reductase specific activity. The purified CbR preferred NADH over NADPH as an electron donor. This is the first report of an analysis of this enzyme in filamentous fungi.

The short 5' untranslated region of the betaA3/A1-crystallin mRNA is responsible for leaky ribosomal scanning

Leaky ribosomal scanning allows the expression of multiple proteins from a single mRNA by occasionally skipping the first start codon, and initiating translation at a subsequent one. BetaA3- and betaA1-crystallin, two members of the beta-crystallin family of vertebrate eye lens proteins, are produced via this mechanism, of which, until now, only very few examples have been found in eukaryotic genes. Since the two start codons on the betaA3/A1 messenger lie in the same reading frame, the two translated proteins are identical, except for the 17 residues shorter N-terminal extension of betaA1-crystallin. It has been suggested that the very short leader (5-7 nucleotides) of the betaA3/A1 messenger might cause slippage at the first start codon, although the unfavorable context of this start codon might also be responsible. Using transient transfections, we now demonstrate that increasing the length of the leader sequence to 67 nucleotides indeed completely abolishes translation initiation at the second start codon, and thus expression of the betaA1-crystallin protein. Messengers having a leader of 5, 7 or 14 nucleotides all express both betaA3- and betaA1-crystallin at very similar relative levels.

Cloning and characterization of a maize cytochrome-b5 reductase with Fe3+-chelate reduction capability

We previously purified an NADH-dependent Fe3+-chelate reductase (NFR) from maize roots with biochemical features of a cytochrome-b5 reductase (b5R) [Sparla, Bagnaresi, Scagliarini and Trost (1997) FEBS Lett. 414, 571-575]. We have now cloned a maize root cDNA that, on the basis of sequence information, calculated parameters and functional assay, codes for NFR. Maize NFR has 66% and 65% similarity to mammal and yeast b5R respectively. It has a deduced molecular mass of 31.17 kDa and a pI of 8.53. An uncharged region is observed at its N-terminus but no myristoylation consensus site is present. Taken together, these results, coupled with previous biochemical evidence, prove that NFR belongs to the b5R class and document b5R from a plant at the molecular level for the first time. We have also identified a putative Arabidopsis thaliana NFR gene. Its organization (nine exons) closely resembles mammalian b5Rs. Several NFR isoforms are expected to exist in maize. They are probably not produced by alternative translational mechanisms as occur in mammals, because of specific constraints observed in the maize NFR cDNA sequence. In contrast with yeast and mammals, tissue-specific and various subcellular localizations of maize b5R isoforms could result from differential expression of the various members of a multigene family. The first molecular characterization of a plant b5R indicates an overall remarkable evolutionary conservation for these versatile reductase systems. In addition, the well-characterized Fe3+-chelate reduction capabilities of NFR, in addition to known Fe3+-haemoglobin reduction roles for mammal b5R isoforms, suggest further and more generalized roles for the b5R class in endocellular iron reduction.

Molecular basis of hereditary methaemoglobinaemia, types I and II: two novel mutations in the NADH-cytochrome b5 reductase gene

Hereditary methaemoglobinaemia, caused by deficiency of NADH-cytochrome b5 reductase (b5R), has been classified into two types, an erythrocyte (type I) and a generalized (type II). We analysed the b5R gene of two Thai patients and found two novel mutations. The patient with type II was homozygous for a C-to-T substitution in codon 8 3 that changes Arg (CGA) to a stop codon (TGA), resulting in a truncated b5R without the catalytic portion. The patient with type I was homozygous for a C-to-T substitution in codon 178 causing replacement of Ala (GCG) with Val (GTG). To characterize effects of this missense mutation, we investigated enzymatic properties of mutant b5R (Ala 178 Val). Although the mutant enzyme showed normal catalytic activity, less stability and different spectra were observed. These results suggest that this substitution influenced enzyme stability due to the slight change of structure. In conclusion, the nonsense mutation led to type II because of malfunction of the truncated protein. On the other hand, the missense mutation caused type I, due to degradation of the unstable mutant enzyme with normal activities in patient's erythrocytes, because of the lack of compensation by new protein synthesis during the long life-span of erythrocytes.

An Erythroid-Specific Transcript Generates the Soluble Form of NADH-Cytochrome b5 Reductase in Humans

Two forms of NADH-cytochrome b5 reductase (b5R), an erythrocyte-restricted soluble form, active in methemoglobin reduction, and a ubiquitous membrane-associated form involved in lipid metabolism, are produced from one gene. In the rat, the two forms are generated from alternative transcripts differing in the first exon, however, biogenesis of human b5R was less understood. Recently, two different transcripts (M and S), differing in the first exon were also described in humans. Here, we have investigated the tissue-specificity and the role of the S-transcript in the generation of soluble b5R. By RNase protection assays designed to simultaneously detect alternative b5R transcripts in the same sample, the S transcript was undetectable in nonerythroid and in erythroleukemic K562 cells induced to differentiate, but was present in terminal erythroblast cultures, and represented a major b5R transcript in reticulocytes. Analysis of the translation products of the M- and S-transcripts in HeLa cells transfected with the corresponding cDNAs demonstrated that the S-transcript generates soluble b5R, presumably from an internal initiation codon. Our results indicate that the S-transcript is expressed at late stages of erythroid maturation to generate soluble b5R.

Structure of the human hexokinase type I gene and nucleotide sequence of the 5' flanking region

This study reports the precise intron/exon boundaries and intron/exon composition of the human hexokinase type I gene. A yeast artificial chromosome containing the hexokinase type I gene was isolated from the yeast artificial chromosome library of the Centre d'Etude du Polymorphisme Humaine. A cosmid sublibrary was created and direct sequencing of the individual cosmids was used to provide the exon/intron organization. The human hexokinase type I gene was found to be composed of 18 exons ranging in size from 63 to 305 bp. Intron 1 is at least 15 kb in length, whereas intron 2 spans at least 10 kb. Overall, the length of the 17 introns ranges from 104 to greater than 15 kb. The entire coding region is contained in at least 75 kb of the gene. The structure of the gene reveals a remarkable conservation of the size of the exons compared with glucokinase and hexokinase type II. Isolation of the 5' flanking region of the gene revealed a 75-90% identity with the rat sequence. Direct evidence of an alternative red-blood-cell-specific exon 1 located upstream of the 5' flanking region of the gene is also provided.

The intracellular location of NADH:cytochrome b5 reductase modulates the cytotoxicity of the mitomycins to Chinese hamster ovary cells

NADH:cytochrome b5 reductase activates the mitomycins to alkylating intermediates in vitro. To investigate the intracellular role of this enzyme in mitomycin bioactivation, Chinese hamster ovary cell transfectants overexpressing rat NADH:cytochrome b5 reductase were generated. An NADH:cytochrome b5 reductase-transfected clone expressed 9-fold more enzyme than did parental cells; the levels of other mitomycin-activating oxidoreductases were unchanged. Although this enzyme activates the mitomycins in vitro, its overexpression in living cells caused decreases in sensitivity to mitomycin C in air and decreases in sensitivity to porfiromycin under both air and hypoxia. Mitomycin C cytotoxicity under hypoxia was similar to parental cells. Because NADH:cytochrome b5 reductase resides predominantly in the mitochondria of these cells, this enzyme may sequester these drugs in this compartment, thereby decreasing nuclear DNA alkylations and reducing cytotoxicity. A cytosolic form of NADH:cytochrome b5 reductase was generated. Transfectants expressing the cytosolic enzyme were restored to parental line sensitivity to both mitomycin C and porfiromycin in air with marked increases in drug sensitivity under hypoxia. The results implicate NADH:cytochrome b5 reductase in the differential bioactivation of the mitomycins and indicate that the subcellular site of drug activation can have complex effects on drug cytotoxicity.

Identification of alternative first exons of NADH-cytochrome b5 reductase gene expressed ubiquitously in human cells

Two forms of NADH-cytochrome b5 reductase (b5R), soluble and membrane-bound, are known. A hypothesis that the human soluble form b5R is generated through post-translational processing of the membrane-bound form was previously proposed. In this study, the 5'-rapid amplification of cDNA ends for human reticulocyte, liver, brain, and HL-60 cell mRNAs revealed the ubiquitous presence of an alternative type of human b5R mRNA which can probably be translated into the soluble form b5R directly; however, the erythroid-specific transcript of the b5R gene was not found. This type of b5R mRNA initiating from at least two sites contains a non-coding new first exon located between the first two exons of the human b5R gene identified before. In addition, this new first exon shares 62% homology with the first exon and its 3'-flanking intron sequences of rat erythroid-specific b5R mRNA, whereas the 5'-flanking region of the new first exon possesses features of house-keeping gene. These results might be important to understand the regulation mechanism of human b5R biosynthesis and divergent evolution of the gene regulation.

Identification of the cDNA for Human Red Blood Cell–Specific Hexokinase Isozyme

A unique cDNA for hexokinase (HK) was identified from poly(A)+ RNA of human reticulocytes by anchored polymerase chain reaction. This appeared to represent the cDNA for the red blood cell (RBC)–specific HK isozyme (HKR ) described in our previous study (Murakami et al: Blood 75:770, 1990). Its nucleotide sequence was identical to HKI cDNA except for the 5′ extreme end. It lacked the first 62 nucleotides of the HKI coding region: instead, it contained a unique sequence of 60 nucleotides at the beginning of the coding sequence as well as another unique sequence upstream of the putative translation initiation site. It lacked the porin-binding domain which facilitates binding to the mitochondria, thus explaining the exclusive cytoplasmic localization of HKR . It was the major cDNA derived from reticulocytes, consistent with the observation that HKR activity is predominant in reticulocytes. Northern blot analysis showed that it was expressed in the reticulocytes and in the K562 erythroleukemic cell line, but not in a lymphocytic cell line. In the extract of K562 cells, HKR activity co-eluted with the HKR of human RBCs on a MonoQ column (Pharmacia, Piscataway, NJ) chromatography, using a salt gradient elution. The separate genetic control of the RBC-specific HK isozyme explains the clinical reports of two types of HK deficiency, one in which the HK activity was reduced exclusively in the RBC (HKR defect) and another with general decrease of HK activity in several tissues (HKI defect).

A role for N-myristoylation in protein targeting: NADH-cytochrome b5 reductase requires myristic acid for association with outer mitochondrial but not ER membranes

N-myristoylation is a cotranslational modification involved in protein-protein interactions as well as in anchoring polypeptides to phospholipid bilayers; however, its role in targeting proteins to specific subcellular compartments has not been clearly defined. The mammalian myristoylated flavoenzyme NADH-cytochrome b5 reductase is integrated into ER and mitochondrial outer membranes via an anchor containing a stretch of 14 uncharged amino acids downstream to the NH2-terminal myristoylate glycine. Since previous studies suggested that the anchoring function could be adequately carried out by the 14 uncharged residues, we investigated a possible role for myristic acid in reductase targeting. The wild type (wt) and a nonmyristoylatable reductase mutant (gly2-->ala) were stably expressed in MDCK cells, and their localization was investigated by immunofluorescence, immuno-EM, and cell fractionation. By all three techniques, the wt protein localized to ER and mitochondria, while the nonmyristoylated mutant was found only on ER membranes. Pulse-chase experiments indicated that this altered steady state distribution was due to the mutant's inability to target to mitochondria, and not to its enhanced instability in that location. Both wt and mutant reductase were resistant to Na2CO3 extraction and partitioned into the detergent phase after treatment of a membrane fraction with Triton X-114, demonstrating that myristic acid is not required for tight anchoring of reductase to membranes. Our results indicate that myristoylated reductase localizes to ER and mitochondria by different mechanisms, and reveal a novel role for myristic acid in protein targeting.

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.

A novel point mutation in a 3' splice site of the NADH-cytochrome b5 reductase gene results in immunologically undetectable enzyme and impaired NADH-dependent ascorbate regeneration in cultured fibroblasts of a patient with type II hereditary methemoglobinemia

Hereditary methemoglobinemia with generalized deficiency of NADH-cytochrome b5 reductase (b5R) (type II) is a rare disease characterized by severe developmental abnormalities, which often lead to premature death. Although the molecular relationship between the symptoms of this condition and the enzyme deficit are not understood, it is thought that an important cause is the loss of the lipid metabolizing activities of the endoplasmic reticulum-located reductase. However, the functions of the form located on outer mitochondrial membranes have not been considered previously. In this study, we have analyzed the gene of an Italian patient and identified a novel G-->T transversion at the splice-acceptor site of the 9th exon, which results in the complete absence of immunologically detectable b5R in blood cells and skin fibroblasts. In cultured fibroblasts of the patient, NADH-dependent cytochrome c reductase, ferricyanide reductase, and semidehydroascorbate reductase activities were severely reduced. The latter activity is known to be due to b5R located on outer mitochondrial membranes. Thus, our results demonstrate that the reductase in its two membrane locations, endoplasmic reticulum and outer mitochondrial membranes, is the product of the same gene and suggest that a defect in ascorbate regeneration may contribute to the phenotype of hereditary methemoglobinemia of the generalized type.

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.

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

Cytochrome b5 and NADH-cytochrome b5 reductase are integral membrane proteins with cytosolic active domains and short membrane anchors, which are inserted post-translationally into their target membranes. Both are produced as different isoforms, with different localizations, in mammalian cells. In the rat, the reductase gene generates two transcripts by an alternative promoter mechanism: a ubiquitous mRNA coding for the myristylated membrane-bound form, and an erythroid mRNA which generates both the soluble form and a nonmyristylated membrane-binding form. The available evidence indicates that the ubiquitous myristylated form binds to the cytosolic face of both outer mitochondrial membranes and ER. In contrast, two genes code for two homologous forms of cytochrome b5, one of which is found on outer mitochondrial membranes, the other on the ER. The gene specifying the ER form probably also generates an erythroid-specific mRNA by alternative splicing, which codes for soluble cytochrome b5. Possible molecular mechanisms responsible for the observed localizations of these different enzyme isoforms are discussed.