The possible involvement of cytochrome b5 in the oxidation of lauric acid by microsomes from kidney cortex and liver of rats

Antibody against NADPH-cytochrome $\text{c}$ reductase inhibited the NADPH-dependent omega and penultimate hydroxylation of lauric acid by microsomes from kidney cortex and liver of rats, but did not inhibit the NADH-dependent hydroxylation of lauric acid. By contrast, an antibody against cytochrome b₅ inhibited both the NADH and the NADPH-dependent hydroxylation of lauric acid by these microsomal preparations. Although the antibody against cytochrome b₅ did not inhibit NADPH-oxidation, this lack of inhibition could not be attributed to the presence of an endogenous substrate or an uncoupling inhibitor in the antibody preparation. These findings suggest that NADPH-cytochrome $\text{c}$ reductase mediates the NADPH-dependent hydroxylation of lauric acid but not its NADH-dependent hydroxylation, whereas cytochrome b₅ plays a role in both the NADPH and the NADH-dependent hydroxylation of the fatty acid.     

Effects of anti-NADPH-cytochrome c reductase and anti-cytochrome b5 antibodies on the hepatic and pulmonary microsomal metabolism and covalent binding of the pulmonary toxin 4-ipomeanol.

An antibody prepared against purified rat liver NADPH-cytochrome $\text{c}$ reductase inhibited both the pulmonary and hepatic microsomal covalent binding of 4-ipomeanol as well as the respective NADPH-cytochrome $\text{c}$ reductase activities, findings which are consistent with previous studies which indicated the participation of cytochrome P450 in the metabolic activation of the toxin. An antibody prepared against purified rat liver cytochrome b₅, which strongly inhibited both the rat hepatic and pulmonary NADH-dependent cytochrome $\text{c}$ reductases, and was inactive against the respective NADPH-dependent cytochrome $\text{c}$ reductases, had little effect on metabolic activation of 4-ipomeanol by hepatic microsomes, but strongly inhibited both the NADH-supported and the NADPH-supported pulmonary microsomal metabolism and covalent binding of the compound. These results suggest that metabolic activation of 4-ipomeanol involves a two-electron transfer in which transfer of the second electron via cytochrome b₅ is rate-limiting in lung microsomes.     

Tetrahydropterin: Reduction of cytochrome c and coupled phosphorylation at mitochondrial site 3

Incubation of rat liver mitochondria with tetrahydropterin results in ATP production with a P:O ratio of 0.85, consistent with the entry of reducing equivalents into the mitochondrial electron transport chain at cytochrome $\text{c}$. No evidence for an enzymatic reduction of cytochrome $\text{c}$ was found. The reduction of either soluble or mitochondrial cytochrome $\text{c}$ was not diminished by superoxide dismutase or anaerobic conditions, indicating that the reaction is not dependent on the autoxidation of the reduced pterin and the formation of an active species of oxygen. The experiments indicate a potential pathway for the production of ATP coupled to the oxidation of NADPH through the activity of NADPH-dependent pteridine reductases.     

On the existence of two populations of mitochondria in a single organ. Respiration, calcium transport and enzyme activities.

Mitochondria isolated from rat heart and kidney cortex by Polytron treatment of the tissues exhibit lower state 3 rates of respiration than mitochondria isolated by Nagarse method. Addition of cytochrome $\text{c}$ to Polytron mitochondria isolated from heart, but not from kidney, increases oxygen uptake to values approaching those of Nagarse-treated preparations. Similar results were observed for Ca²⁺ uptake. Kidney Polytron mitochondria exhibited lower mitochondrial, but higher non-mitochondrial enzyme activities compared to kidney Nagarse mitochondria. Enzyme activities were the same in Polytron and Nagarse mitochondria from heart. The differences between Polytron and Nagarse mitochondria appear to be mainly due to lower cytochrome $\text{c}$ content of Polytron mitochondria from heart and higher contamination of Polytron mitochondria from kidney.     

In vitro synthesis of monoamine oxidase of rat liver outer mitochondrial membrane

Monoamine oxidase, an intrinsic protein of outer mitochondrial membrane, was purified from bovine liver and rabbit antibody against the enzyme was prepared. The antibody could react with the monoamine oxidase of rat liver mitochondria. When rat liver RNA was translated $\text{in}\text{vitro}$ using rabbit reticulocyte lysate and monoamine oxidase peptide in the translation products was immunoprecipitated by the antibody, the peptide was detected in the products programmed by the messenger RNA's from total and free polysomes but not that from bound polysomes. The enzyme synthesized $\text{in}\text{vitro}$ had the same apparent molecular size as the mature protein in outer mitochondrial membrane.     

Immunohistochemical localization of NADPH-cytochrome c reductase in rat liver.

NADPH-cytochrome $\text{c}$ reductase (NADPH-cytochrome reductase, EC 1.6.2.4), the flavoprotein which is responsible for the NADPH-dependent reduction of cytochromes P-450 in hepatic microsomes, has been localized immunohistochemically at the light microscopic level in rat liver. Localization was achieved through the use of sheep antiserum to rat hepatic microsomal NADPH-cytochrome $\text{c}$ reductase in an unlabeled antibody peroxidase-antiperoxidase technique. Parenchymal cells throughout the liver lobule were found to be stained positively for NADPH-cytochrome $\text{c}$ reductase, although the intensity of immunostaining was slightly greater in the centrilobular regions. Immunostaining for NADPH-cytochrome $\text{c}$ reductase was not detected in Kupffer cells, connective tissue cells, or in cells of the hepatic vasculature.     

Binding of cytochrome c to the cytochrome bc1 complex (complex III) and its subunits cytochrome c1 and b1.

Cytochrome $\text{c}$₁, the electron donor for cytochrome $\text{c}$, is a subunit of the mitochondrial cytochrome $\text{bc}$₁ complex (complex III, cytochrome $\text{c}$ reductase). To test if cytochrome $\text{c}$₁ is the cytochrome $\text{c}$-binding subunit of the $\text{bc}$₁ complex, binding of cytochrome $\text{c}$ to the complex and to isolated cytochrome $\text{c}$₁ was compared by a gel-filtration method under non-equilibrium conditions (a $\text{bc}$₁ complex lacking the Rieske ironsulfur protein was used; von Jagow et al. (1977) Biochim. Biophys. Acta $\text{462}$, 549–558). The approximate stoichiometries and binding affinities were found to be very similar. Binding of cytochrome $\text{c}$ to isolated cytochrome $\text{b}$ which is another subunit of the reductase was not detectable by the gel-filtration method. Further, the same lysine residues of cytochrome $\text{c}$ were shielded towards chemical acetylation in the complexes $\text{c}\text{:}\text{c}$₁ and $\text{c}\text{:}\text{bc}$₁. From this we conclude that the same surface area of cytochrome $\text{c}$ is in direct contact with cytochrome $\text{bc}$₁ and with cytochrome $\text{c}$₁ in the respective complexes and that therefore cytochrome $\text{c}$ is most probably the structural ligand for cytochrome $\text{c}$ in mitochondrial cytochrome $\text{c}$ reductase.     

Cytochrome P-450 and NADPH cytochrome c reductase in rat brain: formation of catechols and reactive catechol metabolites.

Microsomes isolated from whole rat brain were found to contain cytochreme P-450 (0.025 to 0.051 nmoles/mg) and NADPH cytochrome $\text{c}$ reductase activity (26.0 to 55.0 nmoles/mg/min). The oxidation of estradiol to a reactive metabolite that became covalently bound to rat brain microsomal protein was inhibited 63% by an atmosphere of CO:O₂ (9:1), indicating the involvement of a cytochrome P-450 oxygenase. In contrast, this atmosphere had no effect on the binding of either the catechol estrogen, 2-hydroxyestradiol, or several catecholamines to rat brain microsomes. An antibody prepared against NADPH cytochrome $\text{c}$ reductase was found to decrease significantly both the formation of 2-hydroxyestradiol from estradiol by rat brain microsomes and the covalent binding of the catechol estrogen and catecholamines to rat brain microsomal protein.     

Liver microsomal electron transport systems. II. The involvement of cytochrome b5 in the NADH-dependent hydroxylation of 3,4-benzpyrene by a reconstituted cytochrome P-448-containing system

An enzyme system in $\text{rat liver microsomes}$ which catalyzes the NADH-dependent $\text{hydroxylation}$ of 3,4-benzpyrene has been reconstituted. The essential components of this NADH-mediated $\text{electron transport chain}$ are cytochrome b₅, NADH-cytochrome b₅ reductase, lipid, and cytochrome P-448.     

Preparation of chick brain synaptosomes and synaptosomal membranes

A method is described for the preparation of synaptosomes and synaptosomal membranes from chicken brain. Procedures for isolating rat synaptosomal membranes could not be used directly; several modifications of existing procedures are reported. Purity of the subcellular and subsynaptosomal fractions was monitored by electron microscopy and measurements of ferrocytochrome $\text{c}$: oxygen oxidoreductase (EC 1.9.3.1.), monoamine: oxygen oxidoreductase (deaminating) (EC 1.4.3.4), rotenoneinsensitive NADH: cytochrome $\text{c}$ oxidoreductase (EC 1.6.99.3), NADPH: cytochrome $\text{c}$ oxidoreductase (EC 1.6.99.1), orthophosphoric monoester phosphohydrolase (EC 3.1.3.2), ATP phosphohydrolase (EC 3.6.1.4), and levels of RNA. Microsomes are the main contaminant of the synaptosomal membrane fraction. Mitochondrial and lysosomal enzymes occur in lesser amounts. No myelin contamination was observed. Marker enzymes for contaminants suggest that these synaptosomal membranes are as pure as membranes described by others, and the specific activity of a neuronal membrane marker, $\text{(}\text{Na}{}^{\mathrm{+}}\text{?}\text{K}{}^{\mathrm{+}}\text{)-}\text{activated}$ ATPase, is as high as other preparations. Levels of this enzyme in the membrane fraction are enriched 13-fold over homogenate ATPase levels.     

Cobalt cytochrome c: enzymic assays.

An improved synthesis for cobalt-cytochrome $\text{c}$ has been developed; its half reduction potential is ?140 ± 20mV. Reduced ^Cocyt-$\text{c}$³ is oxidized by bovine heart cytochrome $\text{c}$ oxidase at a rate ～45% that of the native cytochrome $\text{c}$. It is not reduced by mitochondrial NADH or succinate cytochrome $\text{c}$ reductase nor by microsomal NADH or NADPH cytochrome $\text{c}$ reductase.     

Catabolite repression of the formation of precursors of electron transfer complexes III and IV in adapting bakers' yeast: dependence upon a mitochondrially translated repressor.

When bakers' yeast cells which had been grown anaerobically in galactose were aerated in the presence of 10% glucose, they showed a 40% decrease in $\text{in}\text{vivo}$ [¹⁴C]-leucine incorporation into a washed mitochondrial membrane fraction compared with cells which had been aerated in a low glucose medium. The observed catabolite repression of membrane protein synthesis was primarily due to a decrease in cytoplasmic translational activity, but this repression was entirely dependent upon concomitant mitochondrial translation. The inductions of reduced coenzyme Q cytochrome $\text{c}$ reductase (complex III) and of cytochrome $\text{c}$ oxidase (complex IV) activities were repressed 30 and 60%, respectively, by aeration of the cells for 8 hours in 10% glucose. The catabolite repression of the formation of these two inner membrane complexes was again shown to be dependent upon concomitant mitochondrial translation. Both the amino acid incorporation and enzyme induction data suggest that catabolite repression of both cytoplasmically and mitochondrially translated mitochondrial membrane proteins is mediated through a mitochondrially translated repressor.     

Changes in mitochondrial enzyme activity of rat lung during the development of silicosis.

Significant changes occurred in the activities of enzymes in silicotic rat lung at 30, 90 and 150 days after intratracheal injection of quartz dust. The pattern of changes indicated that the mitochondrial metabolism in silicosis is altered significantly indicating disturbances in bioenergetics. Increase in activity of cytochrome-$\text{c}$-oxidase and NADH-cytochrome-$\text{c}$-reductase at the early stage and a significant decline at the advanced stage of the disease suggest that metabolic changes in silicosis during the initial and the advanced stage of the disease are distinctly different. Besides, enhanced rate of glycolysis is also observed at the early stages of silicosis.     

2-Methylbutyryl CoA dehydrogenase from mitochondria of Ascarissuum and its relationship to NADH-dependent 2-methylcrotonyl CoA reduction

Acyl CoA dehydrogenase and electron-transfer flavoprotein have been isolated and partially purified from mitochondria of the anaerobic nematode, $\text{Ascaris}\text{suum}$. Dehydrogenase activity was greatest with 2-methylbutyryl CoA and the relative substrate specificities of the ascarid dehydrogenase(s) differ greatly from their mammalian counterparts. It appears that the ascarid dehydrogenase functions physiologically as a reductase, catalyzing the final step in the synthesis of branched-chain fatty acids. In fact, incubations of $\text{A}\text{.}\text{suum}$ mitochondrial membranes with electron-transfer flavoprotein, 2-methylbutyryl CoA dehydrogenase, 2-methylcrotonyl CoA and NADH resulted in a substantial, rotenone-sensitive, 2-methylbutyrate synthesis. These results suggest that the ascarid electron-transport chain and at least two soluble mitochondrial proteins are involved in the NADH-dependent reduction of 2-methylcrotonyl CoA.     

Protein synthesis in mitochondrial and microsomal fractions from rat brain and liver after acute or chronic ethanol administration

Incorporation of C¹⁴ Leucine was determined $\text{in vitro}$ or $\text{in vivo}$ in isolated mitochondria and microsomes of rat brain and liver after acute or chronic ethanol administration $\text{in vivo}$.The protein synthesis in mitochondrial and microsomal preparation was inhibited respectively by chloramphenicol and cycloeximide, specific inhibitors for the two systems tested. The experimental data demonstrate that the $\text{in vitro}$ protein synthesis in both systems, mitochondrial and microsomal, is strongly affected only after chronic treatment which produces significant activation at the mitochondrial and microsomal level in the liver and an inhibition on the same systems of the brain.The data for $\text{in vivo}$ protein synthesis instead show strong inhibition after acute administration, except for brain mitochondria, which are practically unaffected, while after chronic treatment no significant alterations are observed.     

Bound cytochrome C as proton donor and acceptor during enzymic oxidoreduction

When ferrocytochrome $\text{c}$ reacts with delipidated cytochrome oxidase under conditions which prevent oxidation, one proton is taken up per molecule of ferrocytochrome $\text{c}$ bound to cytochrome oxidase. When ferricytochrome $\text{c}$ reacts with delipidated Complex III, one proton is released per molecule of ferricytochrome $\text{c}$ bound to Complex III. From these data it can be concluded that the oxidation of ferrocytochrome $\text{c}$ by cytochrome oxidase leads to the release of a proton and an electron, whereas the reduction of ferricytochrome $\text{c}$ by Complex III leads to the uptake of a proton and an electron. Thus ferrocytochrome $\text{c}$ like QH₂ and NADH is both an electron and proton donor, and ferricytochrome $\text{c}$ like Q and O₂ is both an electron and proton acceptor. The pattern for the three mitochondrial electron transfer sequences NADH → Q, QH₂ → ferricytochrome $\text{c}$ and ferrocytochrome $\text{c}$ → O₂ involves separation of an electron and proton on the side of the membrane where electron transfer is initiated and recombination of an electron and a proton in the terminal acceptor on the side of the membrane where electron transfer terminates.     

A Novel function of cytochrome C (555, Chlorobium thiosulfatophilum) in oxidation of thiosulfate

Thiosulfate-cytochrome c-551 reductase derived from $\text{Chlorobium}\text{thiosulfatophilum}$ has been highly purified. The enzyme reduces cytochrome $\text{c}\text{-551 of}\text{C}\text{.}\text{thiosulfatophilum}$ in the presence of thiosulfate while cytochrome $\text{c}$-555 of the organism is not reduced by the enzyme. Cytochrome $\text{c}$-555 reacts with the enzyme at an appreciable rate only in the presence of cytochrome $\text{c}$-551. However, the reduction rate of cytochrome $\text{c}$-551 by the enzyme is greatly enhanced on addition of a catalytic amount of cytochrome $\text{c}$-555. Therefore, cytochrome $\text{c}$-555 seems to function as an effector on thiosulfate-cytochrome $\text{c}$-551 reductase as well as it acts as the electron donor to the light-excited chlorobium chlorophylls.     

Effect of thyroid hormone administration on synthesis and degradation of cytochrome c in rat liver.

Thyrotoxicosis can induce increases in the concentrations of the cytochromes of the inner mitochondrial membrane in rat liver. The purpose of this study was to determine whether the increase in hepatic cytochrome c concentration in thyrotoxic rats is maintained by an increase in the rate of synthesis, a decrease in the rate of degradation, or a combination of the two. The turnover of cytochrome c labeled with δ-amino [¹⁴C]levulinate was measured in the livers of thyrotoxic rats that were in steady state with respect to liver cytochrome c concentration, liver weight, and body weight. Cytochrome c concentration was increased 3.4-fold in the livers of the thyrotoxic animals. The $\text{t}\text{1}\text{2}$ of liver cytochrome c was 3.7 days in the thyrotoxic and 5.7 days in euthyroid animals. It was calculated that the 3.4-fold increase in cytochrome c concentration was maintained, in the face of a 63% increase in k_d, by a 5.5-fold increase in synthesis rate.     

Loose binding of testicular mitochondrial ATPase to the inner membrane

Rat testis mitochondrial ATPase was not inhibited by oligomycin at pH 7.5. It was inhibited only at higher alkaline pH's, and showed a lower sensitivity both to oligomycin and N,N′-dicyclohexylcarbodiimide and a higher one to efrapeptin. In submitochondrial particles, testis ATPase was only slightly inhibited by oligomycin, ossamycin, and efrapeptin. The possibility of a loose binding of F₁ to the membrane was supported by its recovery from the supernatant of the submitochondrial particles. Furthermore, by electron microscopy, after hypoosmotic shock and negative staining of the mitochondrial preparations, most of the inner mitochondrial membranes showed only a few “knobs” or none at all. The capacity of the testis mitochondrial preparation to produce ATP was tested and compared to that from liver. ATP synthetase/ATPase activity ratio was $\text{30}\text{1}$ in liver mitochondria, whereas in the testis it was $\text{3}\text{1}$. In spite of this large difference, at least part of the testis ATPase must be firmly bound to the membrane, since it is able to form ATP. The rest seems to be loosely bound and its functional significance is still unknown.     

Aryl hydrocarbon hydroxylase of rat brain mitochondria: Properties of,and effect of inhibitors and inducers on,enzyme activity

Aryl hydrocarbon hydroxylase (AHH), a typical example of mixed-function oxidase system, was studied in rat brain mitochondria. The enzyme was found to require oxygen and NADH for optimal expression of the activity. Coaddition of NADPH in the incubation system containing NADH resulted in an additive effect on the enzyme activity. NADH- and NADPH-dependent mitochondrial AHH activity was linear with respect to protein concentration and incubation time. The enzyme exhibited a sharp optima at pH 7.6. Specific activity of NADH-dependent mitochondrial AHH in rat brain was 3–4 and 8–11 times higher than that of NADPH-dependent mitochondrial and microsomal enzyme activity, respectively. Of the species investigated, NADH-dependent mitochondrial AHH followed the order: mice ? guinea pig > rat, while NADPH-supported mitochondrial AHH was in the order: rat > guinea pig ? mice. Specific activity of NADH-dependent mitochondrial AHH in various rat brain regions was similar with the exception of olfactory lobes which exhibited 60% higher activity than other region. When total region activities were added approximately whole brain activity was recovered. The apparent K_m value of NADH-dependent mitochondrial AHH was 1.18 μm with benzo(a)pyrene as a substrate. This K_m value was five to six times lower than that of NADPH-dependent microsomal AHH in rat brain (6.66 μm). NADH-dependent mitochondrial AHH was inhibited by KCN in a concentration-dependent manner while NADPH-supported mitochondrial AHH did not reveal any sensitivity to cyanide. Brain microsomal NADH as well as NADPH-supported AHH was also inhibited by KCN in a concentration-dependent manner. Carbon monoxide inhibited NADH-dependent mitochondrial AHH activity (48%) and had no effect on NADPH-dependent mitochondrial enzyme. Mitochondrial NADH and NADPH-dependent AHH activities were induced by 3-methylcholanthrene (64–73%) and benzo(a)pyrene (91–92%) pretreatments while no induction occurred with phenobarbital administration. 1-Benzylimidazole, SKF 525 A, metyrapone, and α-naphthoflavone inhibited both basal and 3-methylcholanthreneinduced NADH-dependent mitochondrial AHH activity. α-Naphthoflavone was more effective in inhibiting 3-methylcholanthrene-stimulated rat brain NADH-dependent mitochondrial AHH. Mitochondrial NADH-dependent AHH activity increased gradually with the onset of development and attained a steady state after 49–56 days of age. An increase of eight- to ninefold in the specific enzyme activity was observed between 7- and 56-day-old rats. No significant increase in brain mitochondrial AHH activity was observed between 56- and 91-day-old rats.