Purification of a cytochrome bd terminal oxidase encoded by the Escherichia coli app locus from a delta cyo delta cyd strain complemented by genes from Bacillus firmus OF4

Escherichia coli GK100, with deletions in the operons encoding its two terminal oxidases, cytochrome bo and ctyochrome bd, was complemented for growth on succinate by a recombinant plasmid (pMS100) containing a 3.4-kb region of DNA from alkaliphilic Bacillus firmus OF4. The complementing DNA was predicted to encode five proteins, but neither sequence analysis nor complementation experiments with subclones provided insight into the basis for the complementation. Cytochrome difference spectra of everted membrane vesicles from the transformed strain had characteristics of a cytochrome bd spectrum but with features different from those observed for alkaliphile membranes. To determine the bacterial source and identity of the structural genes for the cytochrome bd in the transformed mutant, the complex was extracted and partially purified. On sodium dodecyl sulfate-polyacrylamide gels, two polypeptides were resolved from the preparation, 43 (subunit I) and 27 (subunit II) kDa. An internal peptide from subunit I was sequenced, and it yielded the same primary sequence as is found in positions 496 to 510 of E. coli appC. Consistent with the microsequencing results pMS100 failed to complement a triple mutant of E. coli carrying a deletion in appB as well as in the cyo and cyd loci. The deduced sequence of AppBC had been predicted to be very similar to the sequence of CydAB (J. Dassa et al., Mol. Gen. Genet. 229:341-352, 1991) but this is the first demonstration that the former is indeed a cytochrome bd terminal oxidase. The enzyme catalyzed oxygen uptake coupled to quinol or N,N,N',N'-tetramethyl-p-phenylenediamine oxidation, and the activity was sensitive to cyanide. No cross-reactivity to subunit-specific polyclonal antibodies directed against the two individual subunits of cyd-encoded cytochrome bd was detected. Since this is the second cytochrome bd discovered in E. coli, it is proposed that the two complexes be designated cytochrome bd-I (cydAB-encoded enzyme) and cytochrome bd-II (appBC-encoded enzyme). In addition, cbdAB is suggested as a more appropriate gene designation for cytochrome bd than either appBC or cyxAB.

Extensive accumulation of an extracellular L-amino-acid oxidase during gametogenesis of Chlamydomonas reinhardtii

In a previous study [Bulté, L. & Wollman, F.-A. (1992) Eur. J. Biochem. 204, 327-336], we identified a novel gamete-specific polypeptide of Chlamydomonas reinhardtii, M alpha. This 66-kDa polypeptide reacts with antibodies to cytochrome f and accumulates in gametes only in conditions that promote destabilisation of the cytochrome b6/f complex. Here, we show that M alpha is not a modification product of cytochrome f, but is part of protein M, a high-molecular-mass L-amino-acid oxidase located in the periplasm. It catalyzes oxidation of all L-amino acids tested, except cysteine. Using phenylalanine as a substrate, saturation of the enzymatic rate is reached at 2 microM. These characteristics suggest that protein M may operate in vivo as an efficient scavanger of ammonium from extracellular amino acids. The enzyme contains non-covalently bound FAD. It exists in two forms with essentially similar enzymatic properties, of 1.2-1.3 MDa and 0.9-1.0 MDa, respectively. The lighter form is an oligomer of M alpha, while the heavier form contains, in addition to M alpha, a second polypeptide of 135 kDa, M beta, in a molar ratio of 3-4 M alpha/M beta. Both polypeptides are glycosylated.

The use of monoclonal antibodies to study the structure and function of cytochrome f

Monoclonal antibodies (MAbs) were prepared against native cytochrome f (cyt f) isolated from turnip leaves. The two MAbs obtained, designated MAb-JB2 and MAb-ED4, were Western blot positive to purified turnip cytochrome f and also reacted with inside-out (ISO) but not right-side-out (RSO) spinach thylakoid membranes. MAb-ED4 reacted with a covalent adduct formed by crosslinking cyt f and plastocyanin (PC), whereas MAb-JB2 did not. In contrast, MAb-JB2 reacted with the isolated cyt b6/f complex but MAb-ED4 did not. These results indicate that MAb-JB2 binds to cyt f at or near the PC binding site on f, whereas MAb-ED4 binds to a portion of cyt f which is not exposed in the cyt b6/f complex. The location of the epitopes in the primary sequence of cyt f was determined by trypsin hydrolysis, HPLC separation of tryptic peptides, and ELISA identification of the purified peptides. The molecular weights of the purified peptides, determined by gel exclusion chromatography, were found to be 5040 and 3130 Da for MAb-JB2 and MAb-ED4, respectively. Amino acid sequencing showed that the first eight amino acids of the MAb-ED4 positive peptide were L-D-Q-P-L-T-S-N. These results suggest that the 3130-Da peptide has 28 amino acids extending from Leu 223 to Arg 250. This peptide is located on the N-terminal (lumen) side of the postulated membrane-spanning sequence. The first eight amino acids of the MAb-JB2-positive peptide were N-I-L-V-I-G-P-V. This sequence and the peptide molecular weight indicate that the epitope for MAb-JB2 is located within a 44-amino acid peptide extending from Asn 111 to Arg 154.

Immunologically cross-reactive and redox-competent cytochrome b6/f-complexes in the chlorophyll-free plasma membrane of cyanobacteria

Plasma and thylakoid membranes were separated and purified from cell-free extracts of the cyanobacteria Anacystis nidulans, Synechocystis 6714, Anabaena variabilis and Nostoc sp. strain Mac. Immunoblots of the membrane proteins using antisera raised against subunits I-IV of the chloroplast b6/f-complex gave evidence for the presence of a homologous complex in both plasma and thylakoid membranes from the four species of cyanobacteria investigated. Both plasma and thylakoid membranes catalyzed the electron transfer from (exogenous) plastoquinol-9 and NADH to horse heart ferricytochrome c. However, while with plasma membranes these reactions were severely inhibited by low concentrations of antimycin A and rotenone, respectively, the inhibitors were without major effect on thylakoid membranes. The results will be discussed in terms of a possible similarity (analogy and/or homology?) of cyanobacterial plasma membranes to the inner mitochondrial membrane.

Epitopes of monoclonal antibodies which inhibit ubiquinol oxidase activity of Escherichia coli cytochrome d complex localize functional domain

The aerobic respiratory chain of Escherichia coli contains two terminal oxidases: the cytochrome d complex and the cytochrome o complex. Each of these enzymes catalyzes the oxidation of ubiquinol-8 within the cytoplasmic membrane and the reduction of molecular oxygen to water. Both oxidases are coupling sites in the respiratory chain; electron transfer from ubiquinol to oxygen results in the generation of a proton electrochemical potential difference across the membrane. The cytochrome d complex is a heterodimer (subunits I and II) that has three heme prosthetic groups. Previous studies characterized two monoclonal antibodies that bind to subunit I and specifically block the ability of the enzyme to oxidize ubiquinol. In this paper, the epitopes of both of these monoclonal antibodies have been mapped to within a single 11-amino acid stretch of subunit I. The epitope is located in a large hydrophilic loop between the fifth and sixth putative membrane-spanning segments. Binding experiments with these monoclonal antibodies show this polypeptide loop to be periplasmic. Such localization suggests that the loop may be close to His186, which has been identified as one of the axial ligands of cytochrome b558. Together, these data begin to define a functional domain in which ubiquinol is oxidized near the periplasmic surface of the membrane.

Interspecies homology of cytochrome P-450: toxicological significance of cytochrome P-450 cross-reactive with anti-rat P-448-H antibodies in liver microsomes from dogs, monkeys and humans

The mutagenic activation of various promutagens by liver microsomes from dogs, monkeys and humans was investigated. Dog liver microsomes efficiently catalyzed the mutagenic activation of Trp-P-2 and Glu-P-1 followed by IQ and AAF. Monkey liver microsomes were most active in the activation of IQ followed by Glu-P-1, AAF and Trp-P-2. Although there were remarkable individual differences, human liver microsomes were found to be most active in the mutagenic activation of IQ followed by Trp-P-2, Glu-P-1 and AAF. Antibodies against rat P-448-H inhibited the mutagenic activation of Glu-P-1, Trp-P-2 and IQ in rat and dog liver microsomes, and Glu-P-1 and Trp-P-2 in monkey liver microsomes. The activation of Glu-P-1 and IQ in human liver microsomes was also strongly inhibited by anti-P-448-H antibodies. The amounts of cytochrome P-450 cross-reactive with anti-P-448-H antibodies in human liver microsomes highly correlated with the capacity to activate Glu-P-1, Trp-P-2 and IQ but not AAF.

Metabolic activation of pyrolysate arylamines by human liver microsomes; possible involvement of a P-488-H type cytochrome P-450

Metabolic activating capacity of human livers for carcinogenic heterocyclic arylamines has been studied using a Salmonella mutagenesis test. A large individual variation was observed among 15 liver samples in the capacities of activation of Glu-P-1 (2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole), IQ (2-amino-3-methylimidazo[4,5-f]quinoline) and MeIQx (2-amino-3,8-dimethyl-3 H-imidazo[4,5-f]quinoxaline). The average numbers of revertants induced by the three heterocyclic arylamines were nearly the same or rather higher in the presence of hepatic microsomes from human than those from rat. In high-performance liquid chromatography, formation of N-hydroxy-Glu-P-1 was detected and accounted for more than 80% of the total mutagenicity observed in the human microsomal system with Glu-P-1, indicating that, similarly to experimental animals, N-hydroxylation is a major activating step for heterocyclic arylamines in human. Addition of flavone or 7,8-benzoflavone to human liver microsomes showed effective inhibition of the mutagenic activation of Glu-P-1, although the treatment rather enhanced microsomal benzo[a]pyrene hydroxylation in human livers. Mutagenic activation of Glu-P-1 by human liver microsomes was also decreased by the inclusion of anti-rat P-448-H IgG, and was well correlated with the content of immunoreactive P-448-H in livers, suggesting the involvement of a human cytochrome P-450, which shares immunochemical and catalytic properties with rat P-448-H, in the metabolic activation of heterocyclic arylamines in human livers.

Relationships between membrane-bound cytochrome o from Vitreoscilla and that of Escherichia coli

The cytochrome o terminal oxidases from the bacteria Vitreoscilla and Escherichia coli are structurally and functionally related. They have similar optical spectra, both exhibit ubiquinol-1 oxidase activity and are inhibited similarly. Both enzymes contain four subunits by SDS-polyacrylamide gel electrophoresis analysis and contain protoheme IX and Cu2+ prosthetic groups. Antibodies raised against the oxidase purified from E. coli crossreact with the Vitreoscilla oxidase.

Purification and characterization of a previously unreported form of cytochrome P-448 from the liver of 3-methylcholanthrene-pretreated rats

At least four hepatic isoenzymes of cytochrome P-450 were purified and characterized from rats treated with 3-methylcholanthrene. A monoclonal antibody developed against one of the forms (designated cytochrome P-450 MC-B) and polyclonal antibodies against others were used to demonstrate that form MC-B is immunologically distinct from other methylcholanthrene-inducible forms. Limited N-terminal amino acid sequencing showed that cytochrome P-450 MC-B has a primary structure that differs from the N-terminal sequences of other established rat isoenzymes. Cytochrome P-450 MC-B has a minimum Mr of 53,000, a CO-reduced spectral maximum at 448 nm, a Soret maximum of 417 nm in the absolute oxidized spectrum and a pattern of substrate preferences that differs from those of the other methylcholanthrene-induced forms. The other forms (MC-A, MC-C and MC-D) share characteristics with isoenzymes previously reported by other investigators.

Immunocytochemical localization of the cytochrome b/f complex of chloroplast thylakoid membranes

Antibodies directed against purified cytochrome f, isolated from the cytochrome b/f complex of spinach chloroplasts, were used in on-grid immunogold labelling studies of spinach leaf tissue. Our results show unambiguously that cytochrome f, and hence the cytochrome b/f complex, is located in both appressed and non-appressed thylakoid membranes.

Immunological characterization of an Escherichia coli strain which is lacking cytochrome d

The isolated membranes from an Escherichia coli mutant strain which lacks spectroscopically detectable levels of cytochromes d, a1, and b558 also have abnormally low levels of N,N,N',N'-tetramethyl-p-phenylenediamine oxidase activity. In this paper, it is shown that the material previously identified as the N,N,N',N'-tetramethyl-p-phenylenediamine oxidase is, in fact, the two-subunit cytochrome d complex. Antisera directed against the native cytochrome d complex as well as against each of two subunits apparent on sodium dodecyl sulfate-polyacrylamide gels were used to show that the mutant strain lacks both subunits of the cytochrome d complex. Introduction of F-prime F152 into the mutant strain restored the two subunits along with the spectroscopic and enzymatic activity associated with the cytochrome d complex.

Immunological characterization of the cytochrome o terminal oxidase from Escherichia coli

The cytochrome o terminal oxidase from Escherichia coli was immunochemically purified and monospecific antiserum toward cytochrome o was obtained. This antiserum is able to precipitate 100% of the ubiquinol-1 oxidase activity in Triton X-100 extracts of membranes from an E. coli strain in which cytochrome o is the only terminal oxidase. Cytochrome o was analyzed and quantitated using crossed immunoelectrophoresis, rocket immunoelectrophoresis, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that cytochrome o is composed of four subunits of approximate equimolar stoichiometry with molecular weights of 51,000, 28,500, 18,000, and 12,700. The low temperature (77 K) reduced - oxidized spectrum of the immunoprecipitate shows two peaks at 555 and 562 nm, indicating b-type cytochromes. With the anti-cytochrome o and antiserum toward the cytochrome d terminal oxidase complex which was previously obtained, it is possible to immunochemically assay for all the cytochromes in the cytoplasmic membrane of aerobically grown E. coli. Preliminary results indicate that the biosynthesis of cytochrome o is repressed when cytochrome d is induced by lowering the dissolved oxygen concentration during cell growth.

Comparative properties of purified cytochrome P-448 from beta-naphthoflavone treated rats and rainbow trout

1. Cytochrome P-448 from beta-naphthoflavone treated rainbow trout (Salmo gairdnerii) liver was purified and compared to purified P-448 from beta-naphthoflavone treated rats (Rattus rattus) and purified P-450 from phenobarbital induced rats. 2. The two P-448 forms had similar spectral properties, substrate specificity, sensitivity to inhibitors and regioselectivity in the metabolism of benzo(a)pyrene and testosterone. 3. Rat and trout P-448 differed in apparent monomeric mol. wt (Mr) by at least 2000 daltons, and did not share identical antigenic determinants. Both rat and trout P-448 were shown to be quite different from rat P-450 using all of the above criteria for distinguishing multiple forms.

Immunological characterization of cytochrome H-450 in rat tissues

Rabbit antiserum produced against rat liver cytochrome H-450 was specific for cytochrome H-450. The antiserum did not react with hemolysate, microsomal and mitochondrial fractions of liver, and tissue extracts from heart, lung skeletal muscle, and testis of rat. With the monospecific antiserum, a rocket immunoelectrophoretic assay method was developed for the quantitation of the antigen with a sensitivity of 25 ng. By using rocket immunoelectrophoresis, the total amounts of the antigen found in liver, kidney, and brain of 20 rats were 33.6, 3.6, and 1.3 mg, respectively. It appears that the antigens in liver, kidney, and brain are immunologically identical. From immunological studies with subcellular fractions of rat liver, the antigen was found only in the postmicrosomal fraction. This indicates that the antigen is not a precursor or a proteolytic product of known cytochromes in mitochondria or microsomes. Therefore, cytochrome H-450 is a unique cytosolic protein found in brain, kidney, and liver.

Molecular and antigenic properties of cytochrome b5 from slow-muscle sarcoplasmic reticulum

NADH--cytochrome b5 reductase and cytochrome b5 associated with slow-muscle sarcoplasmic reticulum and liver microsomal fraction were identified with discrete protein bands of molecular weights 33000 and 16700 by polyacrylamide-gel electrophoresis. Purified detergent-extracted cytochrome b5 from muscle sarcoplasmic reticulum is indistinguishable from liver microsomal cytochrome b5 with respect to spectral properties, pI values and immunological reactivity with antibody to the liver cytochrome b5. Reaction of the antibody with membrane-bound cytochrome b5 inhibits the sarcoplasmic-reticulum NADH--cytochrome c reductase activity.

Immunological characterization of cytochrome b9 in crude extracts from rat tissues

Rabbit antiserum against rat liver-soluble cytochrome b9 was produced. The antiserum was specific to cytochrome b9 and did not react with hemoglobin, cytochrome c, cytochrome H-450, serum, hemolysate, and tissue extracts from lung, brain, testis, and skeletal muscle. From Ouchterlony double immunodiffusion and tandem crossed immunoelectrophoresis, the antigen was found in liver, kidney, and heart tissue extracts of rat with complete identity. From crossed immunoelectrophoresis, the antiserum distinguished monomeric and dimeric forms of the antigen. Rocket immunoelectrophoresis for quantitating the antigen was developed with a minimum sensitivity of 5 ng. By using rocket immunoelectrophoresis, the concentration of the antigen in crude tissue extracts from liver, heart, and kidney was determined. The antigen was not detectable in crude extracts from lung, brain, testis, and skeletal muscle by immunological methods. Therefore, the immunological assay method was very useful for identifying and quantitating cytochrome b9 in crude extracts with high sensitivity.

[Purification and characterization of Linoleoyl-CoA desaturase from rat liver microsomes (author's transl)]

Rat liver microsomes catalyzed the oxidative delta 6-desaturation of linoleoyl-CoA (C18: 2, delta 9.12.) to gamma-linolenoyl-CoA (c18: 3, delta 6.9.12.) by using molecular oxygen and NADH or NADPH as the electron donors. The antibodies against cytochrome b5 inhibited markedly the delta 6-desaturation in the intact microsomes of the rat liver, suggesting that cytochrome b5 participated in the delta 6-desaturation. These experimental results led us to the hypothesis that the delta 6-desaturation of linoleoyl-CoA followed the scheme. (See formula in text). Terminal "delta 6-desaturase" was purified from rat liver microsomes for the first time by Triton X-100 solubilization, DEAE-cellulose, CM-Sephadex and cytochrome b5-Sepharose chromatography using its high affinity for cytochrome b5. The final enzyme preparation was homogeneous when applied to sodium dodecyl sulfate disc gel electrophoresis. delta 6-desaturase appeared as a single polypeptide of 66,000 daltons containing 49% nonpolar amino acid residues and one atom of non-heme iron. We confirmed that delta 6-desaturase differed from delta 9-desaturase, which converted stearoyl-CoA to oleoyl-CoA. The delta 6-desaturase activity required NADH (or NADPH), linoleoyl-CoA, oxygen, lipid or detergent and three enzymes, such as NADH-cytochrome b5 reductase (or NADPH-cytochrome P -450 reductase), cytochrome b5, and delta 6-desaturase. The reconstituted system of these components also confirmed the electron flow represented in Scheme 1. The delta 6-desaturase activity was inhibited by iron chelators, cyanine and p-chloromercuriphenyl sulfonate. In the reconstituted system of Km value for linoleoyl-CoA was 47 micro M, the maximal velocity was 83nmol/min/mg protein of delta 6-desaturase and the optimal pH was 7.0. Catalase, superoxide dismutase and t-butanol showed supportive effects on the delta 6-desaturation of the reconstituted system when purified enzymes were employed.

Localization of a nuclear envelope-associated protein by indirect immunofluorescence microscopy using antibodies against a major polypeptide from rat liver fractions enriched in nuclear envelope-associated material

The location of a specific major polypeptide present in nuclear pore complex-enriched fractions from rat liver was examined by indirect immunofluorescence microscopy using chicken antibodies against this polypeptide. In both whole cell preparations of cultured cells grown on cover slips (mouse 3 T 3, rat kangaroo PtK2) and in frozen sections through liver and mammary gland tissue a strongly preferential, if not exclusive, binding to the nuclear periphery of interphase cells was observed. The specificity of this localization was demonstrated in these cells by the decoration of chromatin with antibodies against histones and of elements of the endoplasmic reticulum--outer mitochondrial membrane--system with antibodies to cytochrome b5. In addition, the localization was examined by electron microscopy using frozen sections and "immunoperoxidase" techniques. The results suggest that this polypeptide is contained in a protein specific for the nuclear periphery, probably closely associated with the peripheral chromatin.

Demonstration of the display of components of the endoplasmic reticulum system by indirect immunofluorescence microscopy using antibodies against cytochrome b(5) from rat liver microsomes

Antibodies against the purified fragment of cytochrome b(5) released by trypsin treatment from rat liver microsomes were raised in rabbits and used for the demonstration of membranes rich in cytochrome b(5), in particular the endoplasmic reticulum (ER) system, by indirect immunofluorescence microscopy. The method allowed the demonstration of ER not only in frozen sections of various tissues, including liver and lactating mammary gland from different species, but also in cultured cells of a diversity of species and cell types. In the cultured cells the structures most prominently decorated with the antibodies against cytochrome b(5) were the nuclear envelope and the ER system which in many cells could be recognized as a system of smoothly bending, branching threads, extending from the perinuclear cytoplasm toward the cell periphery. Changes in the display of such elements during mitosis and cell plating and possible influences of the specific fixations used are discussed.

The synthesis of cytochrome b on mitochondrial ribosomes in baker's yeast

Antiserum against a major cytochrome b peptide isolated from yeast mitochondria as described previously (Lin, L.-F.H., and Beattie, D.S., J. Biol. Chem. 1978, 253, 2412--2418) was raised in rabbits and shown to be monospecific against the pure antigen. Mitochondria were isolated from yeast cells grown in [3H]leucine, extracted with Lubrol and treated with antiserum to cytochrome b. Analysis of the immunoprecipitates by sodium dodecyl sulfate/polyacrylamide gel electrophoresis revealed the presence of a single major band of molecular weight 31 000 corresponding to cytochrome b. In order to determine the intracellular site of translation of cytochrome b, yeast cells were labeled in vivo under non-growing conditions with [3H]leucine in the absence or presence of inhibitors of cytoplasmic and mitochondrial protein synthesis. The incorporation of radioactive leucine into the apoprotein of cytochrome b isolated by immunoprecipitation followed by gel electrophoresis was insensitive to cycloheximide (an inhibitor of cytoplasmic protein synthesis) and sensitive to acriflavin, erythromycin, and chloramphenicol (inhibitors of mitochondrial protein synthesis). Furthermore, no cytochrome b apoprotein was present in a cytoplasmic petite mutant which lacked mitochondrial protein synthesis. Cytochrome b is thus a product of protein synthesis on mitochondrial ribosomes.

Immunochemical study on the pathway of electron flow in reduced nicotinamide adenine dinucleotide-dependent microsomal lipid peroxidation

NADH could support the lipid peroxidation of rat liver microsomes in the presence of ferric ions chelated by ADP(ADP-Fe). The reaction had a broad pH optimum (pH 5.8--7.4) and was more active in the acidic pH range. Antibodies to NADH-cytochrome b5 reductase [EC 1.6.2.2] and cytochrome b5 inhibited NADH-dependent lipid peroxidation in the presence of ADP-Fe, whereas the antibody against NADPH-cytochrome c reductase [EC 1.6.2.4] showed no inhibition. These oberservations suggest that the electron from NADH was supplied to the lipid peroxidation reaction via NADH-cytochrome b5 reductase and cytochrome b5. On the other hand, NADPH-supported lipid peroxidation was strongly inhibited by the antibody against NADPH-cytochrome c reductase, confirming the participation of this this flavoprotein in the NADPH-dependent reaction. In the presence of both ADP-Fe and ferric ions chelated by EDTA(EDTA-Fe), NADH-dependent lipid peroxidation was highly stimulated up to the level of the NADPH-dependent reaction. In this case, the antibody against cytochrome b5 could not inhibit the reaction, while the antibody against NADH-cytochrome b5 reductase did inhibit it, suggesting the direct transfer of electrons from NADH-cytochrome b5 reductase to EDTA-Fe complex.

Evidence for participation of cytochrome b5 in microsomal delta-6 desaturation of fatty acids

The delta-6 desaturation of linoleic acid to gamma-linolenic acid and oleic acid to 6,9-octadecadienoic acid by rat liver microsomes was investigated. Using a specific antibody prepared against purified rat liver cytochrome b5, we demonstrated that cytochrome b5 participated in delta-6 desaturation of both fatty acids. The reaction products were identified as their methyl ester derivatives by argentation thin-layer chromatography, gas-liquid chromatography, and reductive ozonolysis followed by gas-liquid chromatography.

Subfractionation of rat liver microsomes by immunoprecipitation and immunoadsorption methods

Rabbit antisera were prepared against cytochrome b5 and NADPH-cytochrome c reductase [EC 1.6.2.4] purified from rat liver microsomes, and utilized in examining the distribution of these and other membrane-bound enzymes among the vesicles of rat liver microsomal preparations by immunoprecipitation and immunoadsorption methods. Smooth microsomes with an average vesicular size of 200 nm (diameter) and sonicated smooth microsomes with an average diameter of 40-60 nm were used in subfractionation experiments. Immunoprecipitation of microsomal vesicles with anti-cytochrome b5 immunoglobulin failed to show any separation of the microsomes into fractions having different enzyme compositions. Cytochrome b5 was apparently distributed among all vesicles even when sonicated microsomes were used. When the antibody against NADPH-cytochrome c reductase was used, however, immunoadsorption of microsomes on Sepharose-bound antibody produced some separation of NADPH-cytochrome c reductase and cytochrome P-450 from NADH-cytochrome b5 reductase and cytochrome b5. The separation was more pronounced when sonicated microsomes were used. These results indicate microheterogeneity of the microsomal membrane, and suggest the clustering of NADPH-cytochrome c reductase and cytochrome P-450 molecules in the membrane.

Involvement of cytochrome b5 in the oxidative desaturation of linoleic acid to gamma-linolenic acid in rat liver microsomes

The effects of antibodies against microsomal electron-transport components on the in vitro activity of delta6-desaturation of linoleic acid to gamma-linolenic acid have been studied in intact microsomal membranes of rat liver. Reduced nicotinamide adenine dinucleotide (NADH) or reduced nicotinamide adenine dinucleotide phosphate (NADPH) (0.87 mM) served as electron donors, and effectively prompted the delta6-desaturase activities with yields of about 1.1 to 1.3 nmol per mg of protein in 10 min. Of the two antibodies studied under the same in vitro conditions, i.e., rabbit antisera preparations against rat liver microsomal hydrophilic parts of cytochrome b5 and NADPH-cytochrome c reductase, only the antibody against cytochrome b5 demonstrated a marked ability to inhibit the delta6-desaturase activity. This evidence supports a participation of cytochrome b5 in the delta6-desaturation of linoleic acid and suggests a pathway analogous to the delta9-desaturation of stearyl-CoA.

The cytochrome bc) complex of yeast mitochondria. Site of translation of the polypeptides in vivo

1. Yeast cells were labelled with radioactive amino acids in the presence of cycloheximide and the cytochrome bc1 complex was isolated from them as described in the preceding paper (Katan, M.B.., Pool, L. & Groot, G.S.P. (1976)Eur. J. Biochem, 65, 95-105). After analysis of this preparation by sodium dodecylsulphate polyacrylamide gel electrophoresis only one band, with an apparent Mr of 32000, was found to have incorporated radioactivity. The amount of label in the band was low, but could be increased approximately 5-fold by preincubating the cells in erythromycin before the labelling period. 2. Cells were labelled in the presence of chloramphenicol and the cytochrome bc1 complex was isolated by (NH4)2SO4 fractionation. Upon electrophoresis in the presence of sodium dodecylsulphate only four of the six bands that belong to the complex were found to have incorporated radioactivity; no radioactivity was found in the bands with an Mr of 40000 and 17000. The same result was obtained after labelling in the presence of acriflavin. If, however, the cytochrome bc1 complex was isolated by immunoprecipitation, all bands were found to have incorporated radioactivity in the presence of chloramphenicol. The amount of radioactivity in the Mr 32000 band was now clearly depressed. 3. It is concluded that of the seven polypeptides of the cytochrome bc1 complex of yeast only one is made on mitochondrial ribosomes. This polypeptide has an Mr of 32000 and is probably associated with cytochrome b.

Analytical study of microsomes and isolated subcellular membranes from rat liver. VI. Electron microscope examination of microsomes for cytochrome b5 by means of a ferritin-labeled antibody

The distribution of cytochrome b5 in rat liver microsomes, and in two microsomal subfractions isolated by density equilibration in a linear sucrose gradient, was studied under the electron microscope by means of a ferritin-labeled hybrid anti-cytochrome b5/anti-ferritin antibody. Results of this study show that cytochrome b5 is present in essentially all microsomal vesicles derived from endoplasmic reticulum (ER), whether rough or smooth. Thus, the dissociation of ER constituents into two groups (b and c), achieved by subfractionating microsomes by isopycnic centrifugation (Beaufay, H., A. Amar-Costesec, D. Thines-Sempoux, M. Wibo, M. Robbi, and J. Berthet. 1974. J. Cell Biol. 61:213-231), does not reflect the association of each group with distinct microsomal particles but reflects rather an enzymatic heterogeneity of the ER: the ratio of group c to group b enzymes increasing with the density and ribosome load of the particles.