Intermembrane electron transport in the absence of added water-soluble carriers

Electron transport from untreated to mersalyzed microsomal vesicles at the level of NADH-cytochrome b₅ reductase or cytochrome b₅ has been demonstrated in the absence of added water-soluble electron carriers. A similar effect was shown in the systems “intact mitochondria — mersalyzed microsomes” and “mersalyzed mitochondria— untreated microsomes”. No measurable electron transport between intact and mersalyzed particles of inner mitochondrial membrane was found. The obtained data suggest that the capability to carry out intermembrane electron transfer is specific for NADH-cytochrome b₅ reductase and/or cytochrome b₅, localized in microsomal and outer mitochondrial membranes.     

ISOLATION AND PROPERTIES OF THE PLASMA MEMBRANE OF KB CELLS

Plasma membranes from KB cells were isolated by the method of latex bead ingestion and were compared with those obtained by the ZnCl₂ method. Optimal conditions for bead uptake and the isolation procedure employing discontinuous sucrose gradient centrifugation are described. All steps of preparative procedure were monitored by electron microscopy and specific enzyme activities. The plasma membrane fraction obtained by both methods is characterized by the presence of the Na⁺ + K⁺-activated ATPase and 5'-nucleotidase, and contains NADPH-cytochrome c reductase and cytochrome b₅. The latter two enzymes are also present in lower concentrations in the microsomal fraction. Unlike microsomes which are devoid of the Na⁺ + K⁺-activated ATPase and which contain only traces of 5'-nucleotidase activity, the plasma membrane fraction contains only trace amounts of the rotenone-insensitive NADH-cytochrome c reductase but no cytochrome P-450, both of which are mainly microsomal components. Morphologically the plasma membrane fraction isolated by the latex bead method is composed of vesicles of 0.1–0.3 µm in diameter. On the basis of the biochemical and morphological criteria presented, it is concluded that the plasma membrane fraction isolated by the above methods are of high degree of purity.     

Properties of the System for the Mixed Function Oxidation of Kaurene and Kaurene Derivatives in Microsomes of the Immature Seed of Marah macrocarpus: Electron Transfer Components

Cytochrome P-450 and cytochrome b₅ at levels of approximately 0.10 and 0.60 nanomole per milligram of microsomal protein were detected by spectral measurements in microsomes prepared from endosperm tissue of immature Marah macrocarpus seeds. TPNH-cytochrome c reductase, DPNH-cytochrome c reductase, andDPNH-cytochrome b₅ reductase activities were also present in these microsomes at levels of approximately 0.060, 0.22, and 0.52 unit per milligram of microsomal protein, respectively. (One unit of reductase is the amount of enzyme catalyzing the reduction of 1 micromole of electron acceptor per minute.) Treatments of microsomes with steapsin or trypsin were not effective in solubilizing any of these electron transport components in detectable form. However, treatment of a microsomal suspension in 25% glycerol with 1% sodium deoxycholate led to the release of about 60% of the protein and each of the above hemoproteins and electron transfer activities to the fraction which was not pelleted after centrifugation for 2 hours at 105,000g. Some ent-kaur-16-ene oxidase activity could be detected in the solubilized fraction after removal of the detergent. Cytochrome b₅ and DPNH-cytochrome b₅ reductase activity were largely separated from one another and from an overlapping mixture of TPNH-cytochrome c reductase and DPNH-cytochrome c reductase when the sodium deoxycholate-solubilized fraction was chromatographed on a DEAE-cellulose column. No cytochrome P-450 or cytochrome P-420 was detected in the column fractions and no ent-kaur-16-ene oxidase activity was detected when the column fractions were tested singly or in combination.     

Alteration of Mycobacterium phlei membrane structure by freezing and thawing: reversal by heating.

Membrane vesicles from Mycobacterium phlei which contain the electron transport chain, when subjected to freezing to ?70 °C followed by slow thawing, exhibited a decreased level of phosphorylation coupled to the oxidation of substrates. This loss in oxidative phosphorylation was restored following heat treatment (50 °C for 10 min) of the membranes. Freeze treatment (?70 °C for 10 min) of membrane vesicles also resulted in a decrease in membrane bound coupling factor-latent ATPase activity. The soluble coupling factor(s) or cryoprotective agents (i.e., glycerol or dimethyl sulfoxide) were found to protect the membrane vesicles from the effects of freezing. Membrane vesicles depleted of particulate bound coupling factor were sensitive to exposure to low temperatures; however, complete protection was afforded by the addition of coupling factor. In addition, prolonged sonication of electron transport particles resulted in lowered $\text{P}\text{O}$ ratios, and heat treatment of these sonicated particles restored $\text{P}\text{O}$ ratio. Therefore, it appears that the effects of heat treatment and freeze treatment on membrane vesicles are reversible. The steady state level of reduced cytochrome b was considerably higher (40%) in heat-treated electron transport particles as compared to untreated particles (28.5%); electron transport particles subjected to freeze treatment showed a lower steady state level of cytochrome b (16.6%) as compared to electron transport particles. The steady state level of cytochrome b in freeze-treated particles returned to the original level (27.5%) for electron transport particles when subjected to heat treatment. Nevertheless, the total amount of enzymatically reducible cytochrome b was the same for all membranes after subjection to the various types of treatment. In contrast to cytochrome b, the reduced steady state levels of cytochrome c, and a + a₃ were not altered by heat or freeze treatment.     

BIOGENESIS OF ENDOPLASMIC RETICULUM MEMBRANES : II. Synthesis of Constitutive Microsomal Enzymes in Developing Rat Hepatocyte

The constitutive enzymes of microsomal membranes were investigated during a period of rapid ER development (from 3 days before to 8 days after birth) in rat hepatocytes. The activities studied (electron transport enzymes and phosphatases) appear at different times and increase at different rates. The increase in the enzyme activities tested was inhibited by Actinomycin D and puromycin. G-6-Pase and NADPH-cytochrome c reductase activities appeared first in the rough microsomes, and subsequently in smooth microsomes, eventually reaching a uniform concentration as in adult liver. The evidence suggests that the enzymes are synthesized in the rough part, then transferred to the smooth part, of the ER. Changes in the fat supplement of the maternal diet brought about changes in the fatty acid composition of microsomal phospholipids but did not influence the enzymic pattern of the suckling. Microsomes from 8-day-old and adult rats lose 95% of PLP and 80% of NADH-cytochrome c reductase activity after acetone-H₂O (10:1) extraction. However, one-half the original activity could be regained by adding back phospholipid micelles prepared from purified phospholipid, or from lipid extracts of heart mitochondria, or of liver microsomes of 8-day or adult rats, thus demonstrating an activation of the enzyme by nonspecific phospholipid. The results suggest that during development the enzymic pattern is not influenced by the fatty acid or phospholipid composition of ER membranes.     

ENZYME-STRUCTURE RELATIONSHIPS IN THE ENDOPLASMIC RETICULUM OF RAT LIVER : A Morphological and Biochemical Study

Subfractionation of preparations of rat liver microsomes with a suitable concentration of sodium deoxycholate has resulted in the isolation of a membrane fraction consisting of smooth surfaced vesicles virtually free of ribonucleoprotein particles. The membrane fraction is rich in phospholipids, and contains the microsomal NADH-cytochrome c reductase, NADH diaphorase, glucose-6-phosphatase, and ATPase in a concentrated form. The NADPH-cytochrome c reductase, a NADPH (or pyridine nucleotide unspecific) diaphorase, and cytochrome b₅ are recovered in the clear supernatant fraction. The ribonucleoprotein particles are devoid of, or relatively poor in, the enzyme activities mentioned. Those enzymes which are bound to the membranes vary in activity according to the structural state of the microsomes, whereas those which appear in the soluble fraction are stable. From these findings the conclusion is reached that certain enzymes of the endoplasmic reticulum are tightly bound to the membranes, whereas others either are loosely bound or are present in a soluble form within the lumina of the system. Some implications of these results as to the enzymic organization of the endoplasmic reticulum are discussed.     

A comparative study of liver mixed function oxidases in camels (Camelus dromedarius), guinea pigs (Cavia porcellus) and rats (Rattus norvegicus)

1. The activities of the drug-metabolizing enzymes, benzphetamine N-demethylase, 7-ethoxy-coumarin O-deethylase and dicoumarol oxidation have been measured in vitro in the liver of camels, guinea pigs and rats.2. In these species, levels of hepatic microsomal parameters namely microsomal protein, cytochrome P₄₅₀, cytochrome b₅ and NADPH-cytochrome c reductase have also been determined.3. In general, camels seemed to have the lowest enzyme activity when compared to rats and guinea pigs.4. Some sex differences were observed in the levels of enzymes studied. In rats and guinea pigs, males had higher benzphetamine N-demethylase than females. However, in camels and guinea pigs, females had higher 7-ethoxycoumarin O-deethylase when compared to males.     

FURTHER STUDIES ON THE INDUCTION OF THE DRUG-HYDROXYLATING ENZYME SYSTEM OF LIVER MICROSOMES

Further studies of the induction of the liver microsomal drug-hydroxylating enzyme system by pretreatment of rats with various drugs are presented. The phenobarbital-induced increase in the microsomal content of CO-binding pigment and in the activities of TPNH-cytochrome c reductase and the oxidative demethylation of aminopyrine is proportional, within certain limits, to the amount of phenobarbital injected. Removal of the inducer results in a parallel decrease in the levels of CO-binding pigment, TPNH-cytochrome c reductase, and aminopyrine demethylation. Other inducing drugs have been investigated and shown to act similarly to phenobarbital. The early increase in these enzymes is found in the microsomal subfraction consisting of rough-surfaced vesicles, whereas repeated administration of the inducing drug results in a concentration of the enzymes in the smooth-surfaced vesicles. The phenobarbital-stimulated formation of endoplasmic membranes is reflected in increased amounts of the various microsomal phospholipid fractions as revealed by thin layer chromatography. There is no significant difference between the stimulated rates of P_i³² incorporation into phospholipids of the two different microsomal subfractions in response to phenobarbital treatment. The drug-induced enzyme synthesis is unaffected by adrenalectomy.     

Light-induced electron transport pathways in membrane preparations from Rhodopseudomonas capsulata

The photosynthetic electron transport chain in Rhodopseudomonas capsulata cells was investigated by studying light-induced noncyclic electron transport from external donors to O₂. Two membrane preparations with opposite membrane polarity, heavy chromatophores and regular chromatophores, were used to characterize this electron transport. It was shown that with lipophylic electron donors such as dichloroindophenol, diaminobenzidine, and phenazine methosulfate the electron transport activities were similar in both types of chromatophores, whereas horse heart cytochrome c, K₄Fe(CN)₆, 3-sulfonic acid phenazine methosulfate, and ascorbate, which cannot penetrate the membrane, were more active in the heavy chromatophores than in the regular chromatophores. Partial depletion of cytochrome c₂ from the heavy chromatophores caused a decrease in the light-induced O₂ uptake from reduced dichloroindophenol or ascorbate. The activity could be restored with higher concentrations of dichloroindophenol or with purified cytochrome c₂ from Rps. capsulata. It is assumed that in the heavy chromatophores the artificial electron donors are oxidized on the cytochrome c₂ level which faces the outside medium. However, cytochrome c₂ is not exposed to the outside medium in the regular chromatophores. Therefore, only lipophylic donors would interact with cytochrome c₂ in this system, while hydrophylic donors would be oxidized by another component of the electron transport chain which is exposed to the external medium. Studies with inhibitors of photophosphorylation show that antimycin A enhances the light-dependent electron transport to O₂ whereas 1:10 phenanthroline inhibited the reaction, but dibromothymoquinone did not affect it. It is assumed that a nonheme iron protein is taking part in this electron transport but not a dibromothymoquinone-sensitive quinone. The terminal oxidase of the light-dependent pathway is different from the two oxidases of the respiratory chain. The ratio between electrons entering the system and molecules of O₂ consumed is 4, which means that the end product of O₂ reduction is H₂O.     

PROTEOLYTIC MICRODISSECTION OF SMOOTH-SURFACED VESICLES OF LIVER MICROSOMES

Digestion of rabbit liver microsomal smooth vesicles with Bacillus subtilis protease released proteins and peptide fragments from the vesicles, without solubilizing phospholipids and cholesterol. The proteolysis was, however, limited when about 30% of the protein had been solubilized. The same limitation was observed when the vesicles were treated with trypsin, chymotrypsin, or their combinations with the bacterial protease. The limited proteolysis was accompanied by selective solubilization of cytochrome b₅ and microsomal NADPH-specific flavoprotein, leaving the CO-binding hemoprotein and some other enzymes still attached to the vesicular membranes. Sucrose density gradient centrifugation of protease-treated vesicles indicated that all the vesicles had been attacked by the protease to similar extents. The behavior of intact and digested vesicles in dextran density gradient centrifugation suggested that the vesicles, even after proteolytic digestion, existed in the form of closed sacs which were impermeable to macromolecules such as dextran and proteases. It was concluded that only the outside surface of the vesicles is susceptible to the proteolytic action and that cytochrome b₅ and the NADPH-specific flavoprotein are located in the susceptible area.     

Reduced nicotinamide adenine dinucleotide-cytochrome b5 reductase and cytochrome b5 as electron carriers in NADH-supported cytochrome P-450 -dependent enzyme activities in liver microsomes

The role of NADH-cytochrome b₅ reductase and cytochrome b₅ as electron carriers in NADH-supported electron transport reactions in rat liver microsomes has been examined by measuring three enzyme activities: NADH-cytochrome P-450 reductase, NADH-peroxidase, and NADH-cytochrome c reductase. The first two reactions are known to involve the participation of an NADH-specific reductase and cytochrome P-450 whereas the third requires the reductase and cytochrome b₅. Antibody prepared against NADH-cytochrome b₅ reductase markedly inhibited the NADH-peroxidase and NADH-cytochrome c reductase activities suggesting the involvement of this NADH-specific reductase in these reactions. Liver microsomes prepared from phenobarbital-pretreated rats were digested with subtilisin to remove cytochrome b₅ and the submicrosomal particles were collected by centrifugation. The specific content of cytochrome b₅ in the digested particles was about 5% of that originally present in liver microsomes and all three enzyme activities showed similar decreases whereas NADH-ferricyanide reductase activity (an activity associated with the flavoenzyme NADH-cytochrome b₅ reductase) remained virtually unchanged. Binding of an excess of detergent-purified cytochrome b₅ to the submicrosomal particles at 37 °C for 20 min followed by centrifugation and enzymic measurements revealed a striking increase in the three enzyme activities. Further evidence for cytochrome b₅ involvement in the NADH-peroxidase reaction was the marked inhibition by antibody prepared against the hemoprotein. These results suggest that in microsomal NADH-supported cytochrome P-450-dependent electron transport reactions, cytochrome b₅ functions as an intermediate electron carrier between NADH-cytochrome b₅ reductase and cytochrome P-450.     

Asymmetrical orientation of phospholipids and their interactions with marker enzymes in pig heart mitochondrial inner membrane

The transverse distribution of phospholipids and their interactions with marker enzymes were investigated in pig heart mitoplasts and inverted vesicles, using phospholipase A₂ from N. naja venom and chemical labeling with TNBS and FDNB. Morphological integrity was checked by freeze-fracturing. Fifty percent of phosphatidylcholine was hydrolyzed in mitoplasts as well as in inverted vesicles, suggesting an even distribution of this phospholipid on the two halves of the inner membrane; however, the fatty acid distribution did not appear the same in the two membrane fractions. Cardiolipin is exclusively hydrolyzed in inverted vesicles proving its location on the inner face of the inner membrane. The results obtained from phospholipase hydrolysis and TNBS labeling suggest that three different pools of phosphatidylethanolamine occur in the membrane: a first pool—about 50–60% of the total membrane phosphatidylethanolamine–is quickly accessible from the two sides of the membrane, a second pool—about 20–30% is slowly available, and finally 20–30% are buried within the membrane and inaccessible to the phospholipase and the probe. The cytochrome c oxidase activity increased in mitoplasts with the phospholipase attack suggesting a better accessibility of added cytochrome c after the attack. The rotenone-sensitive NADH-cytochrome c reductase was activated in mitoplasts but completely inactivated in inverted vesicles by the attack; the addition of cardiolipin liposomes restored the latter activity. The soluble matricial malate dehydrogenase was released, but the particulate form of this enzyme, strongly associated to the membrane, was detached only after attack of inverted vesicles.     

Reconstitution of the reduced nicotinamide adenine dinucleotide phosphate- and reduced nicotinamide adenine dinucleotide-peroxidase activities from solubilized components of rat liver microsomes.

The liver microsomal enzyme system that catalyzes the oxidation of NADPH by organic hydroperoxides has been solubilized and resolved by the use of detergents into fractions containing NADPH-cytochrome c reductase, cytochrome P-450 (or P-448), and microsomal lipid. Partially purified cytochromes P-450 and P-448, free of the reductase and of cytochrome b₅, were prepared from liver microsomes of rats pretreated with phenobarbital (PB) and 3-methylcholanthrene (3-MC), respectively, and reconstituted separately with the reductase and lipid fractions prepared from PB-treated animals to yield enzymically active preparations functional in cumene hydroperoxide-dependent NADPH oxidation. The reductase, cytochrome P-450 (or P-448), and lipid fractions were all required for maximal catalytic activity. Detergent-purified cytochrome b₅ when added to the complete system did not enhance the reaction rate. However, the partially purified cytochrome P-450 (or P-448) preparation was by itself capable of supporting the NADPH-peroxidase reaction but at a lower rate (25% of the maximal velocity) than the complete system. Other heme compounds such as hematin, methemoglobin, metmyoglobin, and ferricytochrome c could also act as comparable catalysts for the peroxidation of NADPH by cumene hydroperoxide and in these reactions, NADH was able to substitute for NADPH. The microsomal NADH-dependent peroxidase activity was also reconstituted from solubilized components of liver microsomes and was found to require NADH-cytochrome b₅ reductase, cytochrome P-450 (or P-448), lipid, and cytochrome b₅ for maximal catalytic activity. These results lend support to our earlier hypothesis that two distinct electron transport pathways operate in NADPH- and NADH-dependent hydroperoxide decomposition in liver microsomes.     

Analytical study of microsomes and isolated subcellular membranes from rat liver. 3. Subfractionation of the microsomal fraction by isopycnic and differential centrifugation in density gradients

Rat liver microsomal fractions have been equilibrated in various types of linear density gradients. 15 fractions were collected and assayed for 27 constituents. As a result of this analysis microsomal constituents have been classified, in the order of increasing median density, into four groups labeled a, b, c, and d. Group a includes: monoamine oxidase, galactosyltransferase, 5''-nucleotidase, alkaline phosphodiesterase I, alkaline phosphatase, and cholesterol; group b: NADH cytochrome c reductase, NADPH cytochrome c reductase, aminopyrine demethylase, cytochrome b ₅, and cytochrome P 450; group c: glucose 6-phosphatase, nucleoside diphosphatase, esterase, β-glucuronidase, and glucuronyltransferase; group d: RNA, membrane-bound ribosomes, and some enzymes probably adsorbed on ribosomes: fumarase, aldolase, and glutamine synthetase. Analysis of the microsomal fraction by differential centrifugation in density gradient has further dissociated group a into constituents which sediment more slowly (monoamine oxidase and galactosyltransferase) than those of groups b and c, and 5''-nucleotidase, alkaline phosphodiesterase I, alkaline phosphatase, and the bulk of cholesterol which sediment more rapidly (group a2). The microsomal monoamine oxidase is attributed, at least partially, to detached fragments of external mitochondrial membrane. Galactosyltransferase belongs to the Golgi complex. Group a2 constituents are related to plasma membranes. Constituents of groups b and c and RNA belong to microsomal vesicles derived from the endoplasmic reticulum. These latter exhibit a noticeable biochemical heterogeneity and represent at the most 80% of microsomal protein, the rest being accounted for by particles bearing the constituents of groups a and some contaminating mitochondria, lysosomes, and peroxisomes. Attention is called to the operational meaning of microsomal subfractions and to their cytological complexity.     

Microsomal electron transport. I. Reduced nicotinamide adenine dinucleotide phosphate-cytochrome c reductase and cytochrome P-450 as electron carriers in microsomal NADPH-peroxidase activity

An electron transport system that catalyzes the oxidation of NADPH by organic, hydroperoxides has been discovered in microsomal fractions. A tissue distribution study revealed that the microsomal fraction of rat liver was particularly effective in catalyzing the NADPH-peroxidase reaction whereas microsomes from adrenal cortex, lung, kidney, and testis were weakly active. The properties of the hepatic microsomal NADPH-peroxidase enzyme system were next examined in detail.The rate of NADPH oxidation by hydroperoxides was first-order with respect to microsomal protein concentration and a K_m value for NADPH of less than 3 μm was obtained. Examination of the hydroperoxide specificity revealed that cumene hydroperoxide and various steroid hydroperoxides were effective substrates for the enzyme system. Using cumene hydroperoxide as substrate, the reaction rate showed saturation kinetics with increasing concentrations of hydroperoxide and an apparent K_m of about 0.4 mm was obtained. The NADPH-peroxidase reaction was inhibited by potassium cyanide, half-maximal inhibition occurring at a cyanide concentration of 2.2 mm. NADH was able to support the NADPH-dependent peroxidase activity synergistically.Evidence compiled for the involvement of NADPH-cytochrome c reductase (NADPH-cytochrome c oxidoreductase, EC 1.6.2.3) in the NADPH-peroxidase reaction included: (1) an identical pH optimum for both activities; (2) stimulation of NADPH-peroxidase activity by increasing ionic strength; (3) inhibition by 0.05 mm, p-hydroxymercuribenzoate with partial protection by NADPH; (4) inhibition by NADP⁺; and (5) inactivation by antiserum to NADPH-cytochrome c reductase. In contrast, antibody to cytochrome b₅ did not inhibit the NADPH-peroxidase activity. Evidence for the participation of cytochrome P-450 in the NADPH-peroxidase reaction included inhibition by compounds forming type I, type II, and modified type II difference spectra with cytochrome P-450; inhibition by reagents converting cytochrome P-450 to cytochrome P-420; and marked stimulation by in vivo phenobarbital administration. The NADPH-reduced form of cytochrome P-450 was oxidized very rapidly by cumene hydroperoxide under a CO atmosphere.It was concluded that the NADPH-peroxidase enzyme system of liver microsomes is composed of the same electron transport components which function in substrate hydroxylation reactions.     

Cytochrome c interaction with membranes. II. Comparative study of the interaction of c cytochromes with the mitochondrial membrane

A comparative study of the interaction of various cytochromes c with phospholipid vesicles and with mitochondrial membranes was undertaken. Both mammalian and yeast types of cytochrome c bind preferentially in the oxidized form as evidenced by the midpoint redox potential (E_m 7.0) becoming more negative upon binding. Cytochrome c which is reincorporated into cytochrome c-depleted mitochondria is kinetically comparable with the native cytochrome c component; rate of cytochrome b oxidation is maximally restored at ratios of c₁:c:a of 1:1:1. Comparison between the electron paramagnetic spectrum of cytochrome c labeled at methionine 65 or cysteine 103 reveals that upon binding to the mitochondrial membrane, the former is immobilized and not the latter. This result suggests that cytochrome c binds to the membrane at the side at which methionine 65 is located.     

Purification, physicochemical properties, and localization of cytochrome c in energy-transducing membranes from Mycobacterium phlei.

Cytochrome c from Mycobacterium phlei has been isolated and purified to homogeneity using an isoelectric focusing technique. The purified cytochrome c has a molecular weight of 12,600 ± 400 and exhibits an isoelectric point (pI) of 4.7 ± 0.05. The amino acid composition of cytochrome c shows a higher proportion of valine and arginine residues and a greatly reduced content of lysine residues when compared to Bacillus subtilis cytochrome c. This imparts less acidic character to the cytochrome c from M. phlei. The cytochrome c from M. phlei acts as the most effective electron acceptor for M. phlei NADH-cytochrome c reductase, while yeast and horse heart cytochrome c are not as efficient electron acceptors. The absence of correlation between the oxidation-reduction potential with the observed activity of NADH-cytochrome c reductase activity indicates that the electrochemical potential is not a sufficient determinant for bacterial cytochrome c function. In order to obtain information concerning the topology of respiratory components, two membrane systems from M. phlei were used; ghost preparations in which the membrane is oriented rightside out as in whole cells and membrane vesicles in which membranes are oriented inside out. Labeling of protoplast ghosts and membrane vesicles with lactoperoxidase-catalyzed iodination reveals that cytochrome c is localized on the outer membrane of protoplast ghosts, which is similar to that observed in mammalian mitochondria. The results also show that cytochrome c from M. phlei binds preferentially to basic phospholipids and not to neutral or acidic phospholipids. Scatchard analysis of the binding of cytochrome c to phosphatidyl ethanolamine shows high affinity (K_a of 3.79 × 10⁵M^?1) and low affinity (K_a of 3.75 × 10⁴M^?1) binding.     

Internal charge transfer in cytochrome c oxidase at a limited proton supply: Proton pumping ceases at high pH

Background

In the membrane-bound enzyme cytochrome c oxidase, electron transfer from cytochrome c to O₂ is linked to proton uptake from solution to form H₂O, resulting in a charge separation across the membrane. In addition, the reaction drives pumping of protons across the membrane.

Methods

In this study we have measured voltage changes as a function of pH during reaction of the four-electron reduced cytochrome c oxidase from Rhodobacter sphaeroides with O₂. These electrogenic events were measured across membranes containing purified enzyme reconstituted into lipid vesicles.

Results

The results show that the pH dependence of voltage changes (primarily associated with proton transfer) during O₂ reduction does not match that of the previously studied absorbance changes (primarily associated with electron transfer). Furthermore, the voltage changes decrease with increasing pH.

Conclusions

The data indicate that cytochrome c oxidase does not pump protons at high pH (10.5) (or protons are taken from the “wrong” side of the membrane) and that at this pH the net proton-uptake stoichiometry is ∼ 1/2 of that at pH 8. Furthermore, the results provide a basis for interpretation of results from studies of mutant forms of the enzyme.

General significance

These results provide new insights into the function of cytochrome c oxidase.     

Properties of rat liver mitochondria with intermembrane Cytochrome c.

When rat liver mitochondria were suspended in 0.15 m KCl, the cytochrome c appeared to be solubilized from the binding site on the outside of the inner membrane and trapped in the intermembrane space. When the outer membrane of these mitochondria was disrupted with digitonin at a digitonin concentration of 0.15 mg/mg of protein, the solubilized cytochrome c could be released from mitochondria along with adenylate kinase. When mitochondria were suspended in 0.15 m KCl instead of 0.33 m sucrose, the $\text{ADP}\text{O}$ ratio observed with succinate, β-hydroxybutyrate, malate + pyruvate or glutamate as substrates was little affected. A number of cycles of State 4-State 3-State 4 with ADP was observed. The respiratory control ratios, however, were decreased, particularly when glutamate was used as the substrate. Cytochrome c oxidase activity was also decreased to 55% when assayed using ascorbate + N,N,N′,N′-tetramethyl-p-phenylene-diamine (TMPD) as substrates. Suspension of mitochondria in 0.15 m KCl resulted in an enhancement of the very low NADH oxidation by intact mitochondria and a twofold enhancement of sulfite oxidation. Trapped cytochrome c in outer membrane vesicles prepared from untreated and trypsin-treated intact mitochondria was found to be readily reduced by NADH and suggests that some cytochrome b₅ is located on the inner surface of the outer membrane. The enhanced NADH oxidase could therefore reflect the ability of cytochrome c to mediate intermembrane electron transport. The enhanced sulfite oxidase activity was sensitive to cyanide inhibition and coupled to oxidative phosphorylation ($\text{ADP}\text{O}\text{< 1}$) unlike the activity of mitochondria in sucrose medium. These results suggest that free cytochrome c in the intermembrane space can mediate electron transfer between the sulfite oxidase and the inner membrane.     

Incorporation of radioactive- -aminolevulinic acid into microsomal cytochrome P 450 : selective breakdown of the hemoprotein by allylisopropylacetamide and carbon tetrachloride

Administration of allylisopropylacetamide (AIA) or CCl₄ to rats previously treated with phenobarbital leads to a rapid decrease in cytochrome P₄₅₀ within 1 hr. The amount of cytochrome b₅ and NADPH cytochrome c reductase in liver microsomes remains unchanged following AIA treatment. In contrast, CCl₄ administration causes a decrease in total microsomal protein thus leading to a net loss in cytochrome b₅ and NADPH cytochrome c reductase. By using ³H-δ-aminolevulinic acid to label microsomal cytochrome P₄₅₀ heme, the effect of AIA and CCl₄ on this cytochrome was shown to be caused by destruction of preexisting CO-binding pigment and not from inhibition of synthesis. In addition, the breakdown products of cytochrome P₄₅₀ heme accumulate in the liver after AIA or CCl₄ treatment.