Evidence for the Participation of Cytochrome b5 in Plasmalogen Biosynthesis

Systematic crosstalk in plasmalogen and diacyl lipid biosynthesis for their differential yet concerted molecular functions in the cell

Plasmalogen is a major phospholipid of mammalian cell membranes. Recently it is becoming evident that the sn-1 vinyl-ether linkage in plasmalogen, contrasting to the ester linkage in the counterpart diacyl glycerophospholipid, yields differential molecular characteristics for these lipids especially related to hydrocarbon-chain order, so as to concertedly regulate biological membrane processes. A role played by NMR in gaining information in this respect, ranging from molecular to tissue levels, draws particular attention. We note here that a broad range of enzymes in de novo synthesis pathway of plasmalogen commonly constitute that of diacyl glycerophospholipid. This fact forms the basis for systematic crosstalk that not only controls a quantitative balance between these lipids, but also senses a defect causing loss of lipid in either pathway for compensation by increase of the counterpart lipid. However, this inherent counterbalancing mechanism paradoxically amplifies imbalance in differential effects of these lipids in a diseased state on membrane processes. While sharing of enzymes has been recognized, it is now possible to overview the crosstalk with growing information for specific enzymes involved. The overview provides a fundamental clue to consider cell and tissue type-dependent schemes in regulating membrane processes by plasmalogen and diacyl glycerophospholipid in health and disease.

Cytochrome b5 modulates multiple reactions in steroidogenesis by diverse mechanisms

Cytochrome b5 (cyt-b5) is a relatively small haemoprotein which plays an important role in the regulation of mammalian steroidogenesis. This unique protein has the ability to modulate the activity of key steroidogenic enzymes via a number of diverse reaction mechanisms. Cyt-b5 can augment the 17,20-lyase activity of CYP17A1 by promoting the interaction of CYP17A1 and POR; enhance the 16-ene-synthase activity of CYP17A1 by acting as an electron donor; and enhance the activity of 3βHSD by increasing the affinity of 3βHSD for its cofactor NAD(+). We review the modulation of CYP17A1 and 3βHSD activity by cyt-b5 and discuss the reaction mechanisms associated with each activity. The physiological importance of cyt-b5 in regulating mammalian steroidogenesis is presented and the impact of inactivating cyt-b5 mutations are reviewed. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

The role of cytochrome b5 structural domains in interaction with cytochromes P450

To understand the role of the structural elements of cytochrome b5 in its interaction with cytochrome P450 and the catalysis performed by this heme protein, we carried out comparative structural and functional analysis of the two major mammalian forms of membrane-bound cytochrome b5 - microsomal and mitochondrial, designed chimeric forms of the heme proteins in which the hydrophilic domain of one heme protein is replaced by the hydrophilic domain of another one, and investigated the effect of the highly purified native and chimeric heme proteins on the enzymatic activity of recombinant cytochromes P4503A4 and P45017A1 (CYP3A4 and CYP17A1). We show that the presence of a hydrophobic domain in the structure of cytochrome b5 is necessary for its effective interaction with its redox partners, while the nature of the hydrophobic domain has no significant effect on the ability of cytochrome b5 to stimulate the activity of cytochrome P450-catalyzed reactions. Thus, the functional properties of cytochrome b5 are mainly determined by the structure of the heme-binding domain.

Cytochrome b5: novel roles in steroidogenesis

Cytochrome b(5) (cyt-b(5)) is essential for the regulation of steroidogenesis and as such has been implicated in a number of clinical conditions. It is well documented that this small hemoprotein augments the 17,20-lyase activity of cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). Studies have revealed that this augmentation is accomplished by cyt-b(5) enhancing the interaction between cytochrome P450 reductase (POR) and CYP17A1. In this paper we present evidence that cyt-b(5) induces a conformational change in CYP17A1, in addition to facilitating the interaction between CYP17A1 and POR. We also review the recently published finding that cyt-b(5) allosterically augments the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD), a non cytochrome P450 enzyme, by increasing the enzymes affinity for its cofactor, NAD(+). The physiological importance of this finding, in terms of understanding adrenal androstenedione production, is examined. Finally, evidence that cyt-b(5) is able to form homomeric complexes in living cells is presented and discussed.

NAD(P)H cytochrome b5 oxidoreductase deficiency in Leishmania major results in impaired linoleate synthesis followed by increased oxidative stress and cell death

NAD(P)H cytochrome b(5) oxidoreductase (Ncb5or), comprising cytochrome b(5) and cytochrome b(5) reductase domains, is widely distributed in eukaryotic organisms. Although Ncb5or plays a crucial role in lipid metabolism of mice, so far no Ncb5or gene has been reported in the unicellular parasitic protozoa Leishmania species. We have cloned, expressed, and characterized Ncb5or gene from Leishmania major. Steady state catalysis and spectral studies show that NADH can quickly reduce the ferric state of the enzyme to the ferrous state and is able to donate an electron(s) to external acceptors. To elucidate its exact physiological role in Leishmania, we attempted to create NAD(P)H cytochrome b(5) oxidoreductase from L. major (LmNcb5or) knock-out mutants by targeted gene replacement technique. A free fatty acid profile in knock-out (KO) cells reveals marked deficiency in linoleate and linolenate when compared with wild type (WT) or overexpressing cells. KO culture has a higher percentage of dead cells compared with both WT and overexpressing cells. Increased O(2) uptake, uncoupling and ATP synthesis, and loss of mitochondrial membrane potential are evident in KO cells. Flow cytometric analysis reveals the presence of a higher concentration of intracellular H(2)O(2), indicative of increased oxidative stress in parasites lacking LmNcb5or. Cell death is significantly reduced when the KO cells are pretreated with BSA bound linoleate. Real time PCR studies demonstrate a higher Δ12 desaturase, superoxide dismutase, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNA with a concomitant fall in Δ9 desaturase mRNA expression in LmNcb5or null cell line. Together these findings suggest that decreased linoleate synthesis, and increased oxidative stress and apoptosis are the major consequences of LmNcb5or deficiency in Leishmania.

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.

Deletion of microsomal cytochrome b5 profoundly affects hepatic and extrahepatic drug metabolism

We demonstrated recently that cytochrome b(5) plays an important in vivo role in hepatic cytochrome P450 (P450) function [J Biol Chem 283:31385-31393, 2008]. We have now generated a model in which cytochrome b(5) has been deleted in all tissues [cytochrome b(5) complete null (BCN)], which surprisingly results in a viable mouse despite the putative in vivo roles of this protein in lipid and steroid hormone metabolism and the reduction of methemoglobin. In contrast to the liver-specific deletion, complete deletion of cytochrome b(5) leads to a neonatal increase in the expression of many hepatic P450s at both the protein and mRNA level. In extrahepatic tissues, some changes in P450 expression were also observed that were isoform-dependent. In vitro cytochrome P450 activities in liver, kidney, lung, and small intestine of BCN mice were determined for a range of model substrates and probe drugs; a profound reduction in the metabolism of some substrates, particularly in lung, kidney, and small intestine, was observed. In vivo, the metabolism of metoprolol was significantly altered in BCN mice, in contrast to the previous finding in the liver-specific cytochrome b(5) deletion, suggesting that extrahepatic cytochrome b(5) plays a significant role in its disposition. Testicular Cyp17 hydroxylase and lyase activities were also significantly reduced by cytochrome b(5) deletion, leading to significantly lower levels of testicular testosterone. The BCN mouse provides an additional model system with which to further investigate the functions of cytochrome b(5), particularly in extrahepatic tissues.

Preparation of a biologically active apo-cytochrome b5 via heterologous expression in Escherichia coli

Cytochrome b(5) (b(5)) has been shown to modulate many cytochrome P450 (CYP)-dependent reactions. In order to elucidate the mechanism of such modulations, it is necessary to evaluate not only the effect of native b(5) on CYP-catalyzed reactions, but also that of the apo-cytochrome b(5) (apo-b(5)). Therefore, the apo-b(5) protein was prepared using a heterologous expression in Escherichia coli. The gene for rabbit b(5) was constructed from synthetic oligonucleotides using polymerase chain reaction (PCR), cloned into pUC19 plasmid and amplified in DH5 alpha cells. The gene sequence was verified by DNA sequencing. The sequence coding b(5) was cleaved from pUC19 by NdeI and XhoI restriction endonucleases and subcloned to the expression vector pET22b. This vector was used to transform E. coli BL-21 (DE3) Gold cells by heat shock. Expression of b(5) was induced with isopropyl beta-D-1-thiogalactopyranoside (IPTG). The b(5) protein, produced predominantly in its apo-form, was purified from isolated membranes of E. coli cells by chromatography on a column of DEAE-Sepharose. Using such procedures, the homogenous preparation of apo-b(5) protein was obtained. Oxidized and reduced forms of the apo-b(5) reconstituted with heme exhibit the same absorbance spectra as native b(5). The prepared recombinant apo-b(5) reconstituted with heme can be reduced by NADPH:CYP reductase. The reconstituted apo-b(5) is also fully biologically active, exhibiting the comparable stimulation effect on the CYP3A4 enzymatic activity towards oxidation of 1-phenylazo-2-hydroxynaphthalene (Sudan I) as native rabbit and human b(5).

Analysis of the interactions of cytochrome b5 with flavocytochrome P450 BM3 and its domains

Interactions between a soluble form of microsomal cytochrome b(5) (b(5)) from Musca domestica (housefly) and Bacillus megaterium flavocytochrome P450 BM3 and its component reductase (CPR), heme (P450) and FAD/NADPH-binding (FAD) domains were analyzed by a combination of steady-state and stopped-flow kinetics methods, and optical spectroscopy techniques. The high affinity binding of b(5) to P450 BM3 induced a low-spin to high-spin transition in the P450 heme iron (K(d) for b(5) binding = 0.44 microM and 0.72 microM for the heme domain and intact flavocytochrome, respectively). The b(5) had modest inhibitory effects on steady-state turnover of P450 BM3 with fatty acids, and the ferrous-carbon monoxy P450 complex was substantially stabilized on binding b(5). Single turnover reduction of b(5) by BM3 using stopped-flow absorption spectroscopy (k(lim) = 116 s(-1)) was substantially faster than steady-state reduction of b(5) by P450 BM3 (or its CPR and FAD domains), indicating rate-limiting step(s) other than BM3 flavin-to-b(5) heme electron transfer in the steady-state reaction. Steady-state b(5) reduction by P450 BM3 was considerably accelerated at high ionic strength. Pre-reduction of P450 BM3 by NADPH decreased the k(lim) for b(5) reduction approximately 10-fold, and also resulted in a lag phase in steady-state b(5) reduction that was likely due to BM3 conformational perturbations sensitive to the reduction state of the flavocytochrome. Ferrous b(5) could not reduce the ferric P450 BM3 heme domain under anaerobic conditions, consistent with heme iron reduction potentials of the two proteins. However, rapid oxidation of both hemoproteins occurred on aeration of the ferrous protein mixture (and despite the much slower autoxidation rate of b(5) in isolation), consistent with electron transfer occurring from b(5) to the oxyferrous P450 BM3 in the complex. The results demonstrate that strong interactions occur between a eukaryotic b(5) and a model prokaryotic P450. Binding of b(5) perturbs BM3 heme iron spin-state equilibrium, as is seen in many physiologically relevant b(5) interactions with eukaryotic P450s. These results are consistent with the conservation of structure of P450s (particularly at the heme proximal face) between prokaryotes and eukaryotes, and may point to as yet undiscovered roles for b(5)-like proteins in the control of activities of certain prokaryotic P450s.

Reduction of cytochrome b5 by NADPH-cytochrome P450 reductase

The reduction of mammalian cytochrome b5 (b5) by NADPH-cytochrome P450 (P450) reductase is involved in a number of biological reactions. The kinetics of the process have received limited consideration previously, and a combination of pre-steady-state (stopped-flow) and steady-state approaches was used to investigate the mechanism of b5 reduction. In the absence of detergent or lipid, a reductase-b5 complex is formed and rearranges slowly to an active form. Electron transfer to b5 is rapid within this complex (>30 s(-1) at 23 degrees C), as fast as to cytochrome c. With excess b5 present, a burst of reduction is observed, consistent with rapid electron transfer to one or two b5 molecules per reductase, followed by a subsequent rate-limiting event. In detergent vesicles, the reductase and b5 interact rapidly but electron transfer is slower (approximately 3 s(-1) at 23 degrees C). Experiments with dimyristyl lecithin vesicles yielded results intermediate between the non-vesicle and detergent systems. These steady-state and pre-steady-state kinetics provide views of the different natures of the reduction of b5 by the reductase in the absence and presence of vesicles. Without vesicles, the encounter of the reductase and b5 is rapid, followed by a slow reorganization of the initial complex (approximately 0.07 s(-1)), very fast reduction, and dissociation. In vesicles, encounter is rapid and the slow step (approximately 3 s(-1)) is reduction within a complex less favorable for reduction than in the non-vesicle systems.

The structure of the S127P mutant of cytochrome b5 reductase that causes methemoglobinemia shows the AMP moiety of the flavin occupying the substrate binding site

Methemoglobinemia, the first hereditary disease to be identified that involved an enzyme deficiency, has been ascribed to mutations in the enzyme cytochrome b(5) reductase. A variety of defects in either the erythrocytic or microsomal forms of the enzyme have been identified that give rise to the type I or type II variant of the disease, respectively. The positions of the methemoglobinemia-causing mutations are scattered throughout the protein sequence, but the majority of the nontruncated mutants that produce type II symptoms occur close to the flavin adenine dinucleotide (FAD) cofactor binding site. While X-ray structures have been determined for the soluble, flavin-containing diaphorase domains of the rat and pig enzymes, no X-ray or NMR structure has been described for the human enzyme or any of the methemoglobinemia variants. S127P, a mutant that causes type II methemoglobinemia, was the first to be positively identified and have its spectroscopic and kinetic properties characterized that revealed altered nicotinamide adenine dinucleotide hydride (NADH) substrate binding behavior. To understand these changes at a structural level, we have determined the structure of the S127P mutant of rat cytochrome b(5) reductase to 1.8 A resolution, providing the first structural snapshot of a cytochrome b(5) reductase mutant that causes methemoglobinemia. The high-resolution structure revealed that the adenosine diphosphate (ADP) moiety of the FAD prosthetic group is displaced into the corresponding ADP binding site of the physiological substrate, NADH, thus acting as a substrate inhibitor which is consistent with both the spectroscopic and kinetic data.

The many roles of cytochrome b5

Four distinct suggestions have been made to explain the mechanism of the cytochrome b(5)-imposed positive modifier action of the cytochrome P450 monooxygenase reaction. The first mechanism involves a direct input of an electron into the monooxygenase cycle. This is the second of the two electrons necessary for activation of molecular oxygen, and appears to be a rate-limiting step in the monooxygenase reaction. P450 monooxygenases all appear to be uncoupled to varying extents, releasing superoxide and hydrogen peroxide instead of oxidized substrate. A second mechanism suggests that cytochrome b(5) acts as a positive modifier of the monooxygenase by decreasing the extent of uncoupling of the monooxygenase reaction. The implication is that a slow input of the second electron allows uncoupling of a superoxide anion instead of formation of two-electron reduced oxygen. Faster input of the second electron via cytochrome b(5) would result in formation of more of the activated oxygen that reacts with substrate to form product. A third suggestion involves formation of a two-hemoprotein complex between cytochrome b(5) and cytochrome P450 that allows acceptance of two electrons from NADPH-cytochrome P450 reductase. Uncomplexed cytochrome P450 accepts an electron from the reductase, dissociates from it, binds oxygen, and re-associates with the reductase to accept another electron. Complexation with cytochrome b(5) enhances the rate of formation of the active oxygen by obviating the need for two interactions with reductase. The fourth mechanism has cytochrome b(5) serving as an effector without a reduction-oxidation role in the monooxygenation reaction. This effector function may be to enhance the breakdown of the oxygenated hemoprotein to products or to facilitate flow of electrons through the system.

Heterologous expression of an endogenous rat cytochrome b(5)/cytochrome b(5) reductase fusion protein: identification of histidines 62 and 85 as the heme axial ligands

The gene coding for expression of an endogenous soluble fusion protein comprising a b-type cytochrome-containing domain and a FAD-containing domain has been cloned from rat liver mRNA. The 1461-bp hemoflavoprotein gene corresponded to a protein of 493 residues with the heme- and FAD-containing domains comprising the amino and carboxy termini of the protein, respectively. Sequence analysis indicated the heme and flavin domains were directly analogous to the corresponding domains in microsomal cytochrome b(5) (cb5) and cytochrome b(5) reductase (cb5r), respectively. The full-length fusion protein was purified to homogeneity and demonstrated to contain both heme and FAD prosthetic groups by spectroscopic analyses and MALDI-TOF mass spectrometry. The cb5/cb5r fusion protein was able to utilize both NADPH and NADH as reductants and exhibited both NADPH:ferricyanide (k(cat) = 21.7 s(-1), K(NADPH)(m) = 1 microM. K(FeCN6)(m) = 8 microM) and NADPH:cytochrome c (k(cat) = 8.3 s(-1), K(NADPH)(m) = 1 microM. K(cyt c)(m) = 7 microM) reductase activities with a preference for NADPH as the reduced pyridine nucleotide substrate. NADPH-reduction was stereospecific for transfer of the 4R-proton and involved a hydride transfer mechanism with a kinetic isotope effect of 3.1 for NADPH/NADPD. Site-directed mutagenesis was used to examine the role of two conserved histidine residues, H62 and H85, in the heme domain segment. Substitution of either residue by alanine or methionine resulted in the production of simple flavoproteins that were effectively devoid of both heme and NAD(P)H:cytochrome c reductase activity while retaining NAD(P)H:ferricyanide activity, confirming that the former activity required a functional heme domain. These results have demonstrated that the rat cb5/cb5r fusion protein is homologous to the human variant and has identified the heme and FAD as the sites of interaction with cytochrome c and ferricyanide, respectively. Mutagenesis has confirmed the identity of both axial heme ligands which are equivalent to the corresponding residues in microsomal cytochrome b(5).

Simultaneous purification and characterization of cytochrome b5 reductase and cytochrome b5 from sheep liver

Cytochrome b5 was purified from detergent solubilized sheep liver microsomes by using three successive DEAE-cellulose, and Sephadex G-100 column chromatographies. It was purified 54-fold and the yield was 23.5% with respect to microsomes. The apparent Mr of cytochrome b5 was estimated to be 16,200 +/- 500 by SDS-PAGE. Absolute absorption spectrum of the purified cytochrome b5 showed maximal absorption at 412 nm and dithionite-reduced cytochrome b5 gave peaks at 557, 526.5 and 423 nm. The ability of the purified sheep liver cytochrome b5 to transfer electrons from NADH-cytochrome b5 reductase to cytochrome c was investigated. The K(m) and Vmax values were calculated to be 0.088 microM cytochrome b5 and 315.8 microM cytochrome c reduced/min/mg enzyme, respectively. Also the reduction of cytochrome b5 by reductase was studied and K(m) and Vmax values were determined to be 5 microM cytochrome b5 and 5200 nmol cytochrome b5 reduced/min/mg enzyme, respectively. The K(m) and Vmax values for the cofactor NADH in the presence of saturating concentration of cytochrome b5 were found to be 0.0017 mM NADH and 6944 nmol cytochrome b5 reduced/min/mg enzyme, respectively. NADH-cytochrome b5 reductase was also partially purified from the same source, detergent solubilized sheep liver microsomes, by using two successive DEAE-cellulose, and 5'-ADP-agarose affinity column chromatographies. It was purified 144-fold and the yield was 7% with respect to microsomes. The apparent monomer Mr of reductase was estimated to be 34,000 by SDS-PAGE. When ferricyanide was used as an electron acceptor, reductase showed maximum activity between 6.8 and 7.5. The K(m) and Vmax values of the enzyme for ferricyanide were calculated as 0.024 mM ferricyanide and 673 mumol ferricyanide reduced/min/mg enzyme, respectively. The K(m) and Vmax values for the cofactor NADH in the presence of saturating amounts of ferricyanide were found to be 0.020 mM NADH and 699 mumol ferricyanide reduced/min/mg enzyme, respectively.

Characterization of ceramide synthesis. A dihydroceramide desaturase introduces the 4,5-trans-double bond of sphingosine at the level of dihydroceramide

Ceramide (N-acylsphingosine) biosynthesis has been proposed to involve introduction of the 4,5-trans-double bond of sphingosine after synthesis of dihydroceramide (i.e. N-acylsphinganine). For the first time, the in vitro conversion of dihydroceramide to ceramide has been demonstrated using rat liver microsomes and N-[1-14C]octanoyl-D-erythro-sphinganine (st-H2Cer) and either NADH or NADPH as co-substrate; the apparent Km values for st-H2Cer and NADH were 340 and 120 microM, respectively. Molecular oxygen is required for enzymatic activity, and cyanide, divalent copper, as well as antibodies raised against cytochrome b5 are inhibitory, which suggests that this enzyme should be named dihydroceramide desaturase based on these similarities with the mechanism of delta9-desaturase (stearoyl-CoA desaturase). Factors that influenced the activity of dihydroceramide desaturase include the alkyl chain length of the sphingoid base (in the order C18 > C12 > C8) and fatty acid (C8 > C18); the stereochemistry of the sphingoid base (D-erythro- > L-threo-dihydroceramides); the nature of the headgroup, with the highest activity with dihydroceramide, but some (approximately 20%) activity with dihydroglucosylceramide, however); and the ability to utilize alternative reductants (ascorbic acid could substitute for a reduced pyridine nucleotide, but was inhibitory at higher concentrations). Dihydroceramide desaturase was inhibited by dithiothreitol, which suggests that it might be possible to alter ceramide synthesis by varying the thiol status of hepatocytes. Consistent with this hypothesis, when rat hepatocytes were cultured in varying concentrations of N-acetylcysteine (5 and 10 mM), there was a decrease in the relative incorporation of [14C]serine into [14C]ceramide. These studies have conclusively established the pathway of ceramide synthesis via desaturation of dihydroceramide and have uncovered several properties of this reaction that warrant further consideration for their relevance to both sphingolipid metabolism and signaling.

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.

Changes in glycerophospholipid profile in experimental nephrotic syndrome

We investigated changes in the glycerophospholipids in kidney tissue and its various intracellular fractions in rats with nephrotic syndrome induced by puromycin aminonucleoside. The ethanolamine plasmalogen, 1-O-alk-1'-enyl-2-acyl-GPE (EP), was increased in kidney tissue obtained from the puromycin-treated animals. A similar increase was found in the mitochondria and endoplasmic reticulum (microsomes) of this tissue. These increases were not found in the liver. Since platelet-activating factor (PAF) is known to be produced in increased amounts in inflammatory disorders, it is suggested that the higher plasmalogen found in rat kidneys during experimental nephrotic syndrome might be derived from increased levels of this autacoid. The increase in PAF may also result in the elevation of plasma PAF-acetylhydrolase (AH) activity observed in these animals.

Hydrocarbon chain distribution of ether phospholipids of the ascidian Halocynthia roretzi and the sea urchin Strongylocentrotus intermedius

The contents and compositions of the 1-O-alk-1'-enyl-2-acyl, 1-O-alkyl-2-acyl, and 1,2-diacyl glycerophospholipids in the muscle and viscera of the ascidian Halocynthia roretzi, and of the gonad of the sea urchin Strongylocentrotus intermedius, which are eaten to some extent in Alaska and in Asia, were analyzed by gas-liquid chromatography. 1-O-Alk-1'-enyl-2-acyl glycerophospholipids were found in all of the samples, accounting for 64.4-69.0% of the ethanolamine glycerophospholipid (EPL). By contrast, the levels of the 1-O-Alk-1'-enyl-2-acyl choline glycerophospholipids (CPL) were low (3.1-5.7%). CPL was rich in the 1-O-alkyl-2-acyl subclass amounting to 12.5-23.9% in the ascidian sample. The level of CPL in the sea urchin gonad was extremely high, amounting to 46.1%. The most prominent alkyl chains in the sn-1 position of CPL from the ascidian muscle were 16:0 (44.6%), 18:1 (26.5%), and 18:0 (10.7%), and of CPL from the sea urchin gonad were 18:0 (36.2%), 16:0 (33.0%), and 18:1 (17.8%). Unusually high levels of odd-numbered alkyl chains, e.g., 19:0 and anteiso 17:0, were detected in the CPL of all samples. The prominent alkenyl chains of EPL were 18:0 (69.4%), 16:0 (10.0%), and 18:1 (8.54%) (not counting the vinyl double bond) for the sea urchin gonad. Relatively high levels of 20:1 alkenyl chains were also present. The glycerol sn-2 positions contained high proportions of polyunsaturated fatty acids. Thus, 20:5n-3 (43.6%) and 22:6n-3 (20.1%) were most abundant in the alkylacyl CPL from the ascidian muscle and 20:5n-3 (54.9%) and 20:4n-6 (30.1%) in alkylacyl CPL from the sea urchin gonad. Despite a possible interconversion of the alkyl and alkenyl chains of each class of the ether phospholipids, they showed few features in common.

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.

Fatty acid desaturation in an animal cell mutant defective in plasmanylethanolamine desaturase

We have recently reported the isolation of three plasmalogen-deficient mutants in a murine, macrophage-like cell line, RAW 264.7 (Zoeller et al. (1992) J. Biol. Chem. 267, 8299-8306). One of these mutant strains, RAW.12, is deficient in delta 1'-desaturase (plasmanylethanolamine desaturase, EC 1.14.99.19), the activity responsible for introducing the vinyl-ether double bond found in plasmalogens. We have examined these mutant cells to determine whether any of the desaturase activities involved in the desaturation of fatty acyl-CoAs were affected and found no evidence to suggest this. Stearoyl-CoA desaturase (delta 9-desaturase) activity was normal when measured in microsomes from RAW.12 cells and the conversion of stearate to oleate (which requires the delta 9-desaturase system) by intact RAW.12 cells was unaltered compared to wild-type cells. The conversion of linoleate to arachidonate by intact cells (which requires the delta 5 and delta 6 desaturase activities) was also normal in the mutant cells. Fatty acid analyses showed no decreases in the relative levels of the unsaturated fatty acids that require the delta 9, delta 6 and delta 5 desaturase activities for biosyntheses of 18:1, 18:3, and 20:4 respectively. Analysis of the cytochrome b5/cytochrome b5 reductase electron transport system, which supports delta 1'-desaturase activity, showed only a modest (30%) decrease in activity. These data suggest that the delta 1'-desaturase system contains at least one component (possibly the terminal desaturase) that is not shared by the acyl-CoA desaturases examined and that RAW.12 is deficient in this component.

Biosynthesis of choline plasmalogens in neonatal rat myocytes

The alk-1-enyl bond in plasmenylethanolamine is formed from plasmanylethanolamine by the action of a microsomal cytochrome b5-dependent desaturase. However, the origin of the alk-1-enyl linkage in plasmenylcholine, a significant subclass of phospholipids in heart tissues of certain animal species, is not yet known. We have used neonatal rat myocytes as a model to study the biosynthesis of plasmenylcholine in the present studies since they have a phospholipid composition and subclasses of 1,2-diradyl-sn-glycero-3-phosphocholine (-GPC) similar to those of neonatal rat hearts. When equal concentrations of [3H]hexadecyllyso-GPC or [3H]hexadecyllyso-sn-glycero-3-phosphoethanolamine (-GPE) are incubated under identical conditions with myocytes for 4, 12, and 24 h, the rate of plasmenylcholine formation is faster from [3H]hexadecyllyso-GPE than from [3H]hexadecyllyso-GPC. Also, when [3H]alkyllyso-GPC and alkyllyso-[N-methyl-14C]GPC are incubated with rat myocytes for various times up to 24 h, the 3H/14C ratio in the diacyl-GPC plus alkylacyl-GPC fraction and alkyllyso-GPC remains relatively constant (3H/14C = 2.7), whereas the 3H/14C of plasmenylcholine increases from 0.3 at 2 h to 1.7 after 24 h. Finally, when the rat myocytes are prelabeled with [3H]alkyllyso-GPE for 4 h and then reincubated with either [14C]choline or [14C]methionine for 1 or 3 h, both [14C]choline and [14C]methionine are incorporated into plasmenylcholine, except the 14C/3H is much higher (5- to 15-fold) in the [14C]choline-labeled plasmenylcholine than in the [14C]methionine-labeled plasmenylcholine. Collectively, our data show plasmenylcholine is not directly derived from plasmanylcholine or lysoplasmanylcholine, but instead is formed from plasmenylethanolamine via some type of hydrolytic exchange mechanism, and the contribution of plasmenylethanolamine through methylation to the synthesis of plasmenylcholine is of limited capacity.

Kinetic properties of purified sheep lung microsomal NADH-cytochrome b5 reductase

1. Lung NADH-cytochrome b5 reductase was saturated with its artificial substrate, potassium ferricyanide at approximately 0.1 mM ferricyanide concentration, and the activity of the lung enzyme was inhibited by the higher concentrations of potassium ferricyanide. Ferricyanide at 0.5 and 1.0 mM inhibited the activity of the enzyme by about 20 and 61% respectively. The apparent Km value was calculated as 13.7 microM potassium ferricyanide and 4.3 microM NADH. 2. The Michaelis constants for cytochrome b5 and NADH were determined to be 1.67 and 7.7 microM from the Lineweaver-Burk plots. These results demonstrate that affinity of the lung reductase for its natural substrate is almost 10 times higher than that for potassium ferricyanide. 3. Addition of non-ionic detergent stimulated the rate of reductase-catalyzed reduction of lung cytochrome b5 up to 8.2-fold. 4. Kinetic studies performed with lung reductase by varying NADH and cytochrome b5 concentrations at different fixed concentrations at cytochrome b5 or NADH showed a series of parallel lines indicating a "ping-pong" type of kinetic mechanism for interaction of NADH and cytochrome b5 with lung cytochrome b5 reductase.

Methoxyflurane acts at the substrate binding site of cytochrome P450 LM2 to induce a dependence on cytochrome b5

Rabbit cytochrome P450 isozyme 2 requires cytochrome b5 to metabolize the volatile anesthetic methoxyflurane but not the substrate benzphetamine [E. Canova-Davis and L. Waskell (1984) J. Biol. Chem. 259, 2541-2546]. To determine whether the requirement for cytochrome b5 for methoxyflurane oxidation is mediated by an allosteric effect on cytochrome P450 LM2 or cytochrome P450 reductase, we have investigated whether this anesthetic can induce a role for cytochrome b5 in benzphetamine metabolism. Using rabbit liver microsomes and antibodies raised in guinea pigs against rabbit cytochrome b5, we found that methoxyflurane did not create a cytochrome b5 requirement for benzphetamine metabolism. Methoxyflurane also failed to induce a role for cytochrome b5 in benzphetamine metabolism in the purified, reconstituted mixed function oxidase system. Studies of the reaction kinetics established that in the absence of cytochrome b5, methoxyflurane and benzphetamine are competitive inhibitors, and that in the presence of cytochrome b5, benzphetamine and methoxyflurane are two alternate substrates in competition for a single site on the same enzyme. These results all indicate that the methoxyflurane-induced cytochrome b5 dependence of the mixed function oxidase cytochrome P450 LM2 system is a direct result of the interaction between methoxyflurane and the substrate binding site of cytochrome P450 LM2 and suggest the focus of future studies of this question.

Palmitoyl-CoA elongation in brain microsomes: dependence on cytochrome b5 and NADH-cytochrome b5 reductase

Experiments were performed to demonstrate the involvement of electron transport system in fatty acid elongation in rat brain microsomes. Mercuric chloride and p-chloromercuriphenylsulfonate, inhibitors on NADH-cytochrome b5 reductase, at 32 microM inhibited NADH-supported palmitoyl-CoA elongation to 30 and 60% of control activity, respectively, whereas NADPH-supported palmitoyl-CoA elongation was unaffected by these mercurials. An antibody to rat liver NADH-cytochrome b5 reductase inhibited brain microsomal NADH-cytochrome b5 reductase activity and NADH-dependent palmitoyl-CoA elongation. Treatment of brain microsomes with trypsin diminished the cytochrome b5 content; NADH- and NADPH-cytochrome c reductase activities were significantly decreased, but the decrease in NADH-cytochrome b5 reductase activity was relatively small. Whereas essentially no incorporation of malonyl-CoA into palmitoyl-CoA was observed with trypsin-treated microsomes, addition of detergent-solubilized cytochrome b5 resulted in a recovery of fatty acid elongation. These results indicate the presence of an electron transport system, NADH-NADH-cytochrome b5 reductase-cytochrome b5-fatty acid elongation, in brain microsomes.

Role of the electron transfer system in microsomal drug monooxygenase reaction catalyzed by cytochrome P-450

Four protein components of the hepatic microsomal electron transfer system, NADPH-cytochrome P-450 reductase, cytochrome P-450, NADH-cytochrome b5 reductase, and cytochrome b5, all purified from liver microsomes of phenobarbital-pretreated rabbits, were co-reconstituted into liposomes of egg yolk phosphatidylcholine. The electron transfer rates between the four protein components were measured directly by the stopped-flow method with the reconstituted systems of different compositions, and the effect of the change of the composition on the monooxygenase activity was simultaneously determined. The results obtained led to the following conclusions: (i) The first of the two electrons required for the monooxygenase reaction is exclusively supplied via NADPH-cytochrome P-450 reductase, whereas the second one is preferentially supplied via cytochrome b5. (ii) The rate-limiting step of the overall monooxygenase reaction is the introduction of the second electron, or a step later than that, if the second electron is sufficiently supplied. (iii) All four proteins seem to distribute randomly on the plane of liposomal membranes, and the interaction between them is caused by the lateral diffusion of the proteins.

On the role of long-chain aldehydes in mammalian plasmalogen biosynthesis

[1-3H, 1-14C]Palmitaldehyde (3H:14C = 15) was injected intracerebrally to 18-day-old rats and incorporation of radioactivity into brain lipids was followed over a 24-h period. The substrate was metabolized primarily by oxidation to palmitic acid with loss of tritium and, to a lesser extent, by reduction to hexadecanol. The alkyl moieties of the ethanolamine phospholipids showed considerably lower 3H:14C ratios than the substrate, indicating a substantial participation in either lipid synthesis by tritium-free alcohols derived from 14C-labeled fatty acids. Virtually no 3H radioactivity was found in alkenyl moieties, indicating stereospecificity of both reduction of aldehyde and dehydrogenation of alkyl to alkenyl glycerolipid. The data are consistent with the general concept that plasmalogen biosynthesis proceeds exclusively through fatty alcohols and alkyl glycerolipids and that fatty aldehydes cannot be utilized directly.