Soluble cytochrome b 5 from human erythrocytes

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.

Redox-Regulation of α-Globin in Vascular Physiology

Interest in the structure, function, and evolutionary relations of circulating and intracellular globins dates back more than 60 years to the first determination of the three-dimensional structure of these proteins. Non-erythrocytic globins have been implicated in circulatory control through reactions that couple nitric oxide (NO) signaling with cellular oxygen availability and redox status. Small artery endothelial cells (ECs) express free α-globin, which causes vasoconstriction by degrading NO. This reaction converts reduced (Fe2+) α-globin to the oxidized (Fe3+) form, which is unstable, cytotoxic, and unable to degrade NO. Therefore, (Fe3+) α-globin must be stabilized and recycled to (Fe2+) α-globin to reinitiate the catalytic cycle. The molecular chaperone α-hemoglobin-stabilizing protein (AHSP) binds (Fe3+) α-globin to inhibit its degradation and facilitate its reduction. The mechanisms that reduce (Fe3+) α-globin in ECs are unknown, although endothelial nitric oxide synthase (eNOS) and cytochrome b5 reductase (CyB5R3) with cytochrome b5 type A (CyB5a) can reduce (Fe3+) α-globin in solution. Here, we examine the expression and cellular localization of eNOS, CyB5a, and CyB5R3 in mouse arterial ECs and show that α-globin can be reduced by either of two independent redox systems, CyB5R3/CyB5a and eNOS. Together, our findings provide new insights into the regulation of blood vessel contractility.

Modulation of CYP2C9 activity and hydrogen peroxide production by cytochrome b5

Cytochromes P450 (CYP) play a major role in drug detoxification, and cytochrome b₅ (cyt b5) stimulates the catalytic cycle of mono-oxygenation and detoxification reactions. Collateral reactions of this catalytic cycle can lead to a significant production of toxic reactive oxygen species (ROS). One of the most abundant CYP isoforms in the human liver is CYP2C9, which catalyzes the metabolic degradation of several drugs including nonsteroidal anti-inflammatory drugs. We studied modulation by microsomal membrane-bound and soluble cyt b5 of the hydroxylation of salicylic acid to gentisic acid and ROS release by CYP2C9 activity in human liver microsomes (HLMs) and by CYP2C9 baculosomes. CYP2C9 accounts for nearly 75% of salicylic acid hydroxylation in HLMs at concentrations reached after usual aspirin doses. The anti-cyt b5 antibody SC9513 largely inhibits the rate of salicylic acid hydroxylation by CYP2C9 in HLMs and CYP2C9 baculosomes, increasing the K_M approximately threefold. Besides, soluble human recombinant cyt b5 stimulates the Vmax nearly twofold while it decreases nearly threefold the Km value in CYP2C9 baculosomes. Regarding NADPH-dependent ROS production, soluble recombinant cyt b5 is a potent inhibitor both in HLMs and in CYP2C9 baculosomes, with inhibition constants of 1.04 ± 0.25 and 0.53 ± 0.06 µM cyt b5, respectively. This study indicates that variability in cyt b5 might be a major factor underlying interindividual variability in the metabolism of CYP2C9 substrates.

Peroxidase-like activity of cytochrome b5 is triggered upon hemichrome formation in alkaline pH

In alkaline media (pH12) a catalytic peroxidase activity of cytochrome b₅ was found associated to a different conformational state. Upon incubation at this pH, cytochrome b₅ electronic absorption spectrum was altered, with disappearance of characteristic bands of cytochrome b₅ at pH7.0. The appearance of new electronic absorption bands and EPR measurements support the formation of a cytochrome b₅ class B hemichrome with an acquired ability to bind polar ligands. This hemichrome is characterized by a negative formal redox potential and the same folding properties than cytochrome b₅ at pH7. The acquired peroxidase-like activity of cytochrome b₅ found at pH12, driven by a hemichrome formation, suggests a role of this protein in peroxidation products propagation.

Efficient Reduction of Vertebrate Cytoglobins by the Cytochrome b5/Cytochrome b5 Reductase/NADH System

Cytoglobin is a heme-containing protein ubiquitous in mammalian tissues. Unlike the evolutionarily related proteins hemoglobin and myoglobin, cytoglobin shows a six-coordinated heme binding, with the heme iron coordinated by two histidine side chains. Cytoglobin is involved in cytoprotection pathways through yet undefined mechanisms, and it has recently been demonstrated that cytoglobin has redox signaling properties via nitric oxide (NO) and nitrite metabolism. The reduced, ferrous cytoglobin can bind oxygen and will react with NO in a dioxygenation reaction to form nitrate, which dampens NO signaling. When deoxygenated, cytoglobin can bind nitrite and reduce it to NO. This oxidoreductase activity could be catalytic if an effective reduction system exists to regenerate the reduced heme species. The nature of the physiological cytoglobin reducing system is unknown, although it has been proposed that ascorbate and cytochrome b₅ could fulfill this role. Here we describe that physiological concentrations of cytochrome b₅ and cytochrome b₅ reductase can reduce human and fish cytoglobins at rates up to 250-fold higher than those reported for their known physiological substrates, hemoglobin and myoglobin, and up to 100-fold faster than 5 mM ascorbate. These data suggest that the cytochrome b₅/cytochrome b₅ reductase system is a viable reductant for cytoglobin in vivo, allowing for catalytic oxidoreductase activity.

Direct electrochemical analyses of human cytochromes b5 with a mutated heme pocket showed a good correlation between their midpoint and half wave potentials

Background

Cytochrome b₅ performs central roles in various biological electron transfer reactions, where difference in the redox potential of two reactant proteins provides the driving force. Redox potentials of cytochromes b₅ span a very wide range of ~400 mV, in which surface charge and hydrophobicity around the heme moiety are proposed to have crucial roles based on previous site-directed mutagenesis analyses.

Methods

Effects of mutations at conserved hydrophobic amino acid residues consisting of the heme pocket of cytochrome b₅ were analyzed by EPR and electrochemical methods. Cyclic voltammetry of the heme-binding domain of human cytochrome b₅ (HLMWb₅) and its site-directed mutants was conducted using a gold electrode pre-treated with β-mercarptopropionic acid by inclusion of positively-charged poly-L-lysine. On the other hand, static midpoint potentials were measured under a similar condition.

Results

Titration of HLMWb₅ with poly-L-lysine indicated that half-wave potential up-shifted to -19.5 mV when the concentration reached to form a complex. On the other hand, midpoint potentials of -3.2 and +16.5 mV were obtained for HLMWb₅ in the absence and presence of poly-L-lysine, respectively, by a spectroscopic electrochemical titration, suggesting that positive charges introduced by binding of poly-L-lysine around an exposed heme propionate resulted in a positive shift of the potential. Analyses on the five site-specific mutants showed a good correlation between the half-wave and the midpoint potentials, in which the former were 16~32 mV more negative than the latter, suggesting that both binding of poly-L-lysine and hydrophobicity around the heme moiety regulate the overall redox potentials.

Conclusions

Present study showed that simultaneous measurements of the midpoint and the half-wave potentials could be a good evaluating methodology for the analyses of static and dynamic redox properties of various hemoproteins including cytochrome b₅. The potentials might be modulated by a gross conformational change in the tertiary structure, by a slight change in the local structure, or by a change in the hydrophobicity around the heme moiety as found for the interaction with poly-L-lysine. Therefore, the system consisting of cytochrome b₅ and its partner proteins or peptides might be a good paradigm for studying the biological electron transfer reactions.

Heterologous expression of Ascaris suum cytochrome b5 precursor protein: a histidine-tagged full-length presequence is correctly processed to transport the mature protein to the periplasm of Escherichia coli

The cytochrome b(5) of the body wall of adult Ascaris suum, a porcine parasitic nematode, is a novel type of cytochrome b(5). It is a soluble protein that lacks the COOH-terminal membrane-anchoring domain found in erythrocyte cytochrome b(5), but possesses an NH(2)-terminal extension (presequence) of 30 amino acids that are missing from the 82-residue protein purified from the nematode tissues [Yu, Y., Yamasaki, H., Kita, K., and Takamiya, S., 1996, Arch. Biochem. Biophys. 328, 165-172]. The nematode cytochrome b(5) is, therefore, probably synthesized as a precursor protein whose presequence is cleaved to form a mature protein, but the localization of the mature protein is still unknown. To investigate the processing of the putative precursor protein, the wild-type precursor of nematode cytochrome b(5) with a complete presequence (b5wt) and its NH(2) terminus-truncated derivatives, b5Delta18 and b5Delta28, with 18 and 28 residues deleted, respectively, were expressed using pET-28a(+) vector in Escherichia coli. As expected, all transformants, tb5wt, tb5Delta18, and tb5Delta28, produced recombinant proteins with a histidine-tagged NH(2)-terminal extension. However, only the recombinant protein with the full-length presequence, produced in tb5wt, was correctly processed and transported to the periplasm, from which the majority of the induced product was purified as a mature protein chemically and functionally identical to the native protein purified from the nematode body wall. These results clearly show that the nematode histidine-tagged presequence functions as a signal peptide in E. coli.

Purification of soluble cytochrome b5 as a component of the reductive activation of porcine methionine synthase

In mammals, methionine synthase plays a central role in the detoxification of the rogue metabolite homocysteine. It catalyzes a transmethylation reaction in which a methyl group is transferred from methyltetrahydrofolate to homocysteine to generate tetrahydrofolate and methionine. The vitamin B12 cofactor cobalamin plays a direct role in this reaction by alternately accepting and donating the methyl group that is in transit from one substrate (methyltetrahydrofolate) to another (homocysteine). The reactivity of the cofactor intermediate cob(I)alamin renders the enzyme susceptible to oxidative damage. The oxidized enzyme may be returned to the catalytic turnover cycle via a reductive methylation reaction that requires S-adenosylmethionine as a methyl group donor, and a source of electrons. In this study, we have characterized an NADPH-dependent pathway for the reductive activation of porcine methionine synthase. Two proteins are required for the transfer of electrons from NADPH, one of which is microsomal and the other cytoplasmic. The cytoplasmic protein has been purified to homogeneity and is soluble cytochrome b5. It supports methionine synthase activity in the presence of NADPH and the microsomal component in a saturable manner. In addition, purified microsomal cytochrome P450 reductase and soluble cytochrome b5 reconstitute the activity of the porcine methionine synthase. Identification of soluble cytochrome b5 as a member of the reductive activation system for methionine synthase describes a function for this protein in non-erythrocyte cells. In erythrocytes, soluble cytochrome b5 functions in methemoglobin reduction. In addition, it identifies an additional locus in which genetic polymorphisms may play a role in the etiology of hyperhomocysteinemia, which is correlated with cardiovascular diseases.

13C-NMR studies of transmembrane electron transfer to extracellular ferricyanide in human erythrocytes

Human erythrocytes are known to reduce ferricyanide (hexacyanoferrate) [Fe(CN)6]3- to ferrocyanide [Fe(CN)6]2- in an extracellular reaction that involves the transmembrane transfer of reducing equivalents; potentially these could be either electrons from NADH, formed in glycolysis inside the cells or transmembrane exchange of reduced solutes. The 13C-NMR resonance of [Fe(13CN)6]3- (which was synthesised in our laboratory) was seen to be very broad while that of ferrocyanide was narrow. This phenomenon formed the basis of a simple non-invasive procedure to study ferricyanide reduction in high-haematocrit suspensions of erythrocytes. The method should be directly applicable to other cell types. In a series of experiments, erythrocyte metabolism was studied in the presence of ferricyanide, using 1H, 13C, and 31P NMR spectroscopy. Incubating the cells with 13C-labelled glucose enabled the rate of ferricyanide reduction, glucose utilisation, and lactate and bicarbonate production to be measured simultaneously. Various metabolic states were imposed as follows: glycolysis was inhibited with F- and iodoacetate; glucose transport was inhibited with phloretin and cytochalasin B; and anion transport was inhibited with dinitrostilbene 2,2'-disulfonate and p-chloromercuriphenyl sulfonate. Earlier work was confirmed, showing that ascorbate is intimately involved in the reduction reaction; but its main action appears not to be mediated by membrane transport but in a membrane-associated redox-protein complex that is functionally linked to glycolysis. Also, large differences (factors of three) in the rate of the reduction reaction were recorded in erythrocytes from different, apparently healthy, donors.

Molecular cloning, overexpression in Escherichia coli, structural and functional characterization of house fly cytochrome b5

A microsomal cytochrome b5 cDNA from the house fly, Musca domestica, was cloned and sequenced. The deduced amino acid sequence of the full-length house fly cytochrome b5 (134 residues) is 48% identical to that of rat microsomal cytochrome b5. The house fly cytochrome b5 protein was overexpressed in Escherichia coli, purified, and characterized. Absorption and EPR spectroscopy reveal properties very similar to cytochromes b5 from vertebrates. NMR spectra indicate that the orientation of the heme in the protein relative to its alpha,gamma meso axis is about 1:1. A redox potential of -26 mV versus standard hydrogen electrode was measured by cyclic voltammetry on a modified gold electrode in the presence of hexamminechromium(III) chloride. The cytochrome b5 is reduced by house fly cytochrome P450 reductase in a reconstituted system at a high rate (5.5 s-1), and it stimulates heptachlor epoxidation when reconstituted with house fly cytochrome P450 reductase, cytochrome P450 6A1, phospholipid, and detergent. Cytochrome b5 decreases the apparent Km for P450 reductase and increases the Vmax for heptachlor epoxidation at constant cytochrome P450 6A1 concentrations. The results indicate that cytochrome b5 stimulates a step following the first electron transfer during cytochrome P450 6A1 turnover.

CMP-N-acetylneuraminic acid hydroxylase from mouse liver and pig submandibular glands. Interaction with membrane-bound and soluble cytochrome b5-dependent electron transport chains

In this report, the nature of the protein components involved in the functioning of cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5 Ac) hydroxylase in high-speed supernatants of mouse liver has been investigated. Fractionation and reconstitution experiments showed that this enzyme system consists of NADH-cytochrome b5 reductase, cytochrome b5 and a 56-kDa terminal electron acceptor having the CMP-Neu5 Ac hydroxylase activity. This enzyme system is extracted in a soluble protein fraction; however, the amphipathic, usually membrane-associated, forms of cytochrome b5 and the reductase were found to predominate and are presumably the forms which support the turnover of the hydroxylase in vivo. Although the majority of cellular cytochrome b5 and cytochrome b5 reductase is membrane-bound, the addition of intact microsomes elicited no significant increase in the hydroxylase activity of supernatants. Detergent-solubilised microsomes, however, potently activated the hydroxylase, probably due to the greater accessibility of the cytochrome b5. Accordingly, in reconstitution experiments, pure hydrophilic cytochrome b5 interacts more effectively with the hydroxylase than isolated amphipathic cytochrome b5. Studies on the CMP-Neu5 Ac hydroxylase system in fractionated porcine submandibular glands and bovine liver suggest that the composition of this enzyme system is conserved in all mammals possessing sialoglycoconjugates containing N-glycolylneuraminic acid.

Differential expression of the mRNAs for the soluble and membrane-bound forms of rabbit cytochrome b5

Total RNA was extracted from a variety of rabbit tissues and reverse transcribed for use in the polymerase chain reaction technique. Using primers designed to amplify the membrane-bound liver cytochrome b5 cDNA, products of two sizes were observed. Both hybridized strongly to a radiolabelled liver cytochrome b5 probe. Sequencing confirmed that the two types of cDNA product encoded the membrane-bound and the soluble forms of b5. Messenger RNA corresponding to the soluble cytochrome was detected in the lung, gallbladder and the adrenal gland, as well as in reticulocytes and bone marrow. This was an unexpected finding since the protein has been isolated only from erythrocytes. In contrast, membrane-bound cytochrome b5 mRNA was detected in all tissues tested, suggesting that the corresponding protein is ubiquitous in tissue distribution.

Gene synthesis, bacterial expression, and 1H NMR spectroscopic studies of the rat outer mitochondrial membrane cytochrome b5

The gene coding for the water-soluble domain of the outer mitochondrial membrane cytochrome b5 (OM cytochrome b5) from rat liver has been synthetized and expressed in Escherichia coli. The DNA sequence was obtained by back-translating the known amino acid sequence [Lederer, F., Ghrir, R., Guiard, B., Cortial, S., & Ito, A. (1983) Eur. J. Biochem. 132, 95-102]. The recombinant OM cytochrome b5 was characterized by UV-visible, EPR, and 1H NMR spectroscopy. The UV-visible and EPR spectra of the OM cytochrome b5 are almost identical to the ones obtained from the overexpressed rat microsomal cytochrome b5 [Bodman, S. B. V., Schyler, M. A., Jollie, D. R., & Sligar, S. G. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 9443-9447]. The one-dimensional 1H NMR spectrum of the OM cytochrome b5 indicates that the rhombic perturbation of the ferric center is essentially identical to that in the microsomal beef, rabbit, chicken, and rat cytochromes b5. Two-dimensional 1H NMR spectroscopy (NOESY) and one-dimensional NOE difference spectroscopy were used to assign the contact-shifted resonances that correspond to each of the two isomers that result from the rotation of the heme around its alpha-gamma-meso axis. The assignment of the resonances allowed the determination of the heme orientation ratio in the OM cytochrome b5, which was found to be 1.0 +/- 0.1. It is noteworthy that the two cytochromes b5 that have similar populations of the two heme isomers (large heme disorder) originate from the rat liver.

Soluble and membrane-bound forms of cytochrome b5 are the products of a single gene in chicken

In order to determine the relationship of the soluble cytochrome b5 found in erythrocytes to the membrane-bound form found in other tissues, a cDNA clone encoding cytochrome b5 in chicken erythrocytes was isolated by using mixed oligonucleotides based on a partial amino acid sequence of the protein. Complete nucleotide sequence identity between the erythrocyte cDNA and the sequence of a cDNA clone of the liver protein suggests that they are transcribed from the same gene. The isolation and structural analysis of genomic clones was also consistent with the presence of only one cytochrome b5 gene in chicken. These results suggest that the formation of soluble erythrocyte cytochrome b5 occurs by proteolytic processing of the membrane-bound form. Thus, previous reports indicating that the carboxyl terminal amino acid residue of the erythrocyte form differs from the corresponding residue of the membrane-bound form may suggest the existence of a novel post-translational modification.

Methemoglobin reduction under near physiological conditions

Pure methemoglobin was prepared from fresh red cells and was used as substrate for methemoglobin reduction reaction. Two sources of methemoglobin reductase were used: (a) red cell hemolysate which was prepared by freezing and thawing of unwashed red cells; (b) purified methemoglobin reductase from bank blood. Methemoglobin reduction rate was measured in a mixture of pure methemoglobin (substrate) and hemolysate (enzyme). In other experiments the rate of methemoglobin reduction was measured in the above mixture with the addition of various other compounds such as NADH, cytochrome b5, and pure methemoglobin reductase. Only the addition of pure enzyme accelerated the rate of methemoglobin reduction. In other experiments, the rate of methemoglobin reduction was measured when the reduction reaction was carried out in the presence of various amounts of deoxyhemoglobin, globin, or albumin. It was shown that all proteins tested here decreased the reduction rate. It is concluded that (a) in the red cell, under normal conditions, only the activity of the methemoglobin reductase controls the speed of methemoglobin reduction, and (b) the inhibition of methemoglobin reduction by reduced hemoglobin is mostly nonspecific suggesting a noncompetitive reaction.

Purification by hydrophobic chromatography of soluble cytochrome b5 of human erythrocytes

Soluble cytochrome b5 of human erythrocytes was purified very effectively by hydrophobic chromatography using a butyl-Toyopearl 650 column. Cytochrome b5 was adsorbed tightly on the column in the presence of 60% saturated ammonium sulfate, and was eluted at 40% saturation of ammonium sulfate in the elution buffer. The chromatography gave a good yield of cytochrome b5 of the highest purity so far reported as estimated from the 414 nm to 280 nm absorbance ratio of the oxidized form of the cytochrome b5. The value obtained with the cytochrome b5 purified in this study was 6.57, and is higher than the previously reported highest value of 6.4 (Hultquist, D.E., Dean, R.T. and Douglas, R.H. (1974) Biochem. Biophys. Res. Commun. 60, 28-34). Spectral properties including molecular absorption coefficients were determined using the cytochrome b5 purified by this method.

Cytochrome b5 and NADH-cytochrome-b5 reductase from sipunculan erythrocytes; a methemerythrin reduction system from Phascolopsis gouldii

We report the purification and characterization of a soluble cytochrome b5 from coelomic erythrocytes of the sipunculan worm, Phascolopsis gouldii. We also report the isolation and purification of a membrane-bound NADH-cytochrome-b5 reductase from these erythrocytes. The non-heme iron protein, hemerythrin (Hr), is known to be the oxygen carrier in these erythrocytes. The aforementioned purified cytochrome b5 and reductase together catalyze the reduction of P. gouldii [Fe(III),Fe(III)]metHr to [Fe(II),Fe(II)deoxyHr by NADH. EPR spectroscopy demonstrates that a redox process involving formation of the intermediate [Fe(II),Fe(III)]semi-metHr occurs within intact sipunculan erythrocytes as well as in the system of purified components. The rhombic g-tensor of the EPR signal in both cases resembles that of (semi-met)RHr, the form obtained by one-electron reduction of metHr. These observations suggest that cytochrome b5 and NADH-cytochrome-b5 reductase in sipunculan erythrocytes function to counteract autoxidation of oxyHr. The sequence of electron flow in the system of purified components is: NADH----NADH-cytochrome-b5 reductase----cytochrome b5----metHr. At pH 7.5, the reduction of metHr in this system occurs in two phases, only the first of which is dependent on concentration of cytochrome b5. From an analysis of the kinetics and the EPR time-course, we propose that the two phases represent sequential reduction of met- to semi-metHr and reduction of semi-metHr to deoxyHr. This report represents the first demonstration of a physiological system for reduction of metHr.

Differences in C-terminal amino acid sequences between erythrocyte and liver cytochrome b5 isolated from pig and human. Evidence for two tissue-specific forms of cytochrome b5

Two forms of cytochrome b5, a soluble erythrocyte form and a membrane-bound liver form, were purified from pig and human, and structural differences between them were analyzed. Porcine and human erythrocyte cytochrome b5 consisted of 97 amino acid residues and contained the same catalytic domain structure (residues 1-96) as that of the corresponding liver cytochrome b5, but had one amino acid replacement at the C-terminus (residue 97). These results suggest that erythrocyte cytochrome b5 is not derived from the liver protein by proteolysis but a translational product from another distinct mRNA of cytochrome b5.

Purification and structural studies of rabbit erythrocyte cytochrome b5

A single form of cytochrome b5 has been isolated in highly purified form from the cytosolic fraction of rabbit erythrocytes by sequential chromatography on DE-52 cellulose, Sephadex G-75, and DEAE-Sephadex A50. The cytochrome is structurally similar to the N-terminal, heme-binding domain of rabbit liver microsomal cytochrome b5. Like the liver protein, it is blocked at the amino terminus. Its amino acid composition is similar to that of residues 1-97 of the microsomal protein. With one exception, tryptic peptides derived from apo-cytochrome b5 of rabbit erythrocytes co-elute with the tryptic peptides obtained from a soluble hemepeptide fragment of microsomal cytochrome b5. These findings, together with amino acid sequence analysis of the carboxyl terminal tryptic peptides, identify the erythrocyte cytochrome b5 as a 97-residue peptide.

Alteration of NADH-diaphorase and cytochrome b5 reductase activities of erythrocytes, platelets, and leucocytes in hereditary methaemoglobinaemia with and without mental retardation

NADH-diaphorase and cytochrome b5 reductase activities of platelets and leucocytes, as well as erythrocytes, were found to be deficient in a patient with hereditary methaemoglobinaemia associated with moderate mental retardation and non-progressive neurological disturbance, in which hyperactive reflexes and involuntary movements were notable. In another methaemoglobinaemic patient with no mental or neurological abnormalities, these enzyme activities were defective in erythrocytes but normal in platelets and leucocytes. The first case was a generalised cytochrome b5 reductase deficiency with non-progressive encephalopathy. It is suggested that the detection of cytochrome b5 reductase activity in platelets, in addition to that in leucocytes, is useful for the assessment of a generalised enzyme defect. Genetical involvement of the present cases is discussed in association with the diaphorase gene loci in humans.

Age-dependent decay of cytochrome b5 and cytochrome b5 reductase in human erythrocytes

Age-dependent decrease in cytochrome b5 was observed in erythrocytes from both a normal person and a patient with hereditary methaemoglobinaemia without neurological symptoms. With aging, concentrations of cytochrome b5 in erythrocytes from the patient were almost the same as those in the control. Age-dependent decrease in cytochrome b5 reductase activity in the control erythrocytes was also shown; however, the reductase activity was very low in erythrocytes from the patient over the whole age range. Our studies show that methaemoglobin content of erythrocytes seems to be dependent on the content of cytochrome b5 in the cells, both in the control subject and in the patient.

Plasma membranes from intestinal microvilli and erythrocytes contain cytochromes b5 and P-420

The presence of cytochromes b5, P-450 and P-420 and activities of NADH- and NADPH-cytochrome c redutases were determined in plasma membranes isolated from microvilli of the chick and rat intestinal epithelium and erythrocyte membranes from chick, rat and man. The results are compared with the amounts of these components found in microsomal fractions from intestinal epithelium and in nuclear membranes from chick erythrocytes. Plasma membranes from intestinal microvilli and from erythrocytes contained significant amounts of NADH-cytochrome c reductase activity and of a pigment spectrophotometrically indistinguishable from rat liver microsomal cytochrome b5. In addition, cytochrome b5 fragments were prepared from the membranes by limited trypsin digestion and consisted of two to four components with Mr values in the range 10 000-13 500. In low-temperature difference spectra, the presence of a second cytochrome was noted which was similar to cytochrome P-420. Cytochrome P-450 and NADPH-cytochrome c reductase activities were not detected in plasma membrane fractions in significant concentrations but were present in the corresponding endomembrane fractions. These findings in highly purified, well defined plasma membrane fractions, in which contamination by endomembranes is minimal, strengthen the evidence for the existence of cytochrome-containing redox systems in plasma membranes of various cells and suggest that such redox components are general components of the cell surface. Possible functions and origins of these redox components in plasma membranes are discussed.

Effects of hemolysate concentration, ionic strength and cytochrome b5 concentration on the rate of methemoglobin reduction in hemolysates of human erythrocytes

An assay for determining the rate of methemoglobin reduction in hemolysates of human erythrocytes has been developed. The rates obtained by this assay, when corrected for dilution, are comparable to those obtained with intact cells. Increased ionic strength inhibits the reaction, whereas EDTA increases the rate of reduction. The rate with NADPH as electron donor is 65-70% of the rate with NADH. Added cytochrome b5 stimulates the reaction. The assay has been used to examine erythrocytes from two methemoglobinemic sisters and their asymptomatic mother. Hemolysates of the two patients have both decreased dichlorophenolindophenol reductase activity and decreased ability to reduce methemoglobin. Hemolysates from the heterozygous mother have intermediate dichlorophenolindophenol reductase activity and intermediate methemoglobin reduction ability. The data presented in this paper indicate that the concentrations of cytochrome b5 and cytochrome b5 reductase determine the rate of methemoglobin reduction in hemolysates.

Isolation and properties of a new, soluble, hemoprotein (H-450) from pig liver

A new soluble hemoprotein, designated as H-450, has been purified from pig liver. The absolute absorption spectrum of H-450 shows maxima at 550 and 428 nm. The dithionite-reduced H-450 has absorption peaks at 572, 540, and 450 nm; the unique Soret band at 450 nm is the basis for our tentative designation of this new hemoprotein as H-450 (hemoprotein 450). The spectrum of dithionite-reduced H-450 at 77 K gives two alpha peaks (571 and 566 nm), three beta peaks (546, 537, and 529 nm), and a Soret band at 449 nm. The prosthetic group of H-450 has been identified as protoheme IX. Gel electrophoresis experiments show that H-450 is composed of two nonidentical subunits, alpha and beta (mol wts = 61 000 and 45 000). H-450 contains 1 mol of heme/alphabeta dimer of 106 000 molecular weight. Preliminary sedimentation equilibrium experiments suggest a minimum molecular weight of 218 000 for the native protein. This corresponds to a tetramer, alpha2beta2 containing two heme groups. H-450 is not reduced by reduced nicotinamide adenine dinucleotide (NADH), NADH phosphate, ascorbate, or ferrocyanide. Neither reduced nor oxidized H-450 binds CO, 1 mM cyahide, or 1 mM azide. Dithionite-reduced H-450 is autoxidizable. The molar extinction coefficient of native H-450 is 261 X 103 at 280 nm and 263 X 103 at 428 nm. The purification procedure involves homogenization, high-speed centrifugation, ammonium sulfate fractionation, diethylaminoethylcellulose chromatography, density gradient centrifugation, a calcium phosphate gel step, and a second density gradient centrifugation. The procedure yeilds approximately 2 mg of purified protein from 750 g of pig liver.

Immunological similarity between NADH-cytochrome b5 reductase of erythrocytes and liver microsomes

In a number of animal species soluble NADH-cytochrome b5 reductase of erythrocytes was compared with membrane-bound NADH-cytochrome b5 reductase of liver microsomes by using an antibody to purified NADH-cytochrome b5 reductase from rat liver microsomes. The results obtained indicated clearly that they are immunologically very similar to each other. The data with erythrocyte ghosts suggested that cytochrome b5 and NADH-cytochrome b5 reductase are also present in the ghost.

Homology between bakers' yeast cytochrome b2 and liver microsomal cytochrome b5

The amino-acid sequence of the hemebinding region of bakers' yeast cytochrome b(2) [L-(+)-lactate dehydrogenase, EC 1.1.2.3] has been determined. It shows a strong similarity with the sequence of microsomal cytochrome b(5), and appears to be compatible with the same kind of peptide-chain folding, in agreement with data obtained previously by various physiochemical methods. The comparison shows that the fifth and sixth heme ligands must be histidine residues, thus substantiating previous conclusions drawn in particular from photooxidation experiments and nuclear magnetic resonance studies. The data reported in this paper suggest a common origin for the two proteins. Implications for their biochemical evolution are presented.