Reductive Titrations of Iron-Sulfur Proteins Containing Two to Four Iron Atoms

Cytochrome P-450scc-mediated oxidation of (20S)-22-thiacholesterol: characterization of mechanism-based inhibition

(20S)-22-thiacholesterol (1) is found to be a potent competitive inhibitor of pregnenolone biosynthesis from cholesterol by purified reconstituted bovine adrenal cytochrome P-450scc. The apparent dissociation constant Kd, determined from difference spectra, is 0.6 microM, close to the value from kinetic studies for the apparent inhibition constant, Ki, of 0.8 microM. Studies of the time course of pregnenolone production indicate that under turnover conditions the competitive inhibitor (1) is converted to a tighter binding inhibitor, shown to be (20S,22R)-22-thiacholesterol S-oxide (4), with high diastereoselectivity and in a time-dependent manner. Both the diastereomeric sulfoxides, (20S,22S)-22-thiacholesterol S-oxide (3) and (20S,22R)-22-thiacholesterol S-oxide (4), exhibit properties consistent with their being competitive versus cholesterol, but the (22R)-sulfoxide (4) binds approximately 10 times more tightly than the (22S) diastereomer (3). The apparent Kd values of sulfoxides 4 and 3 are 0.1 and 1.14 microM, respectively. EPR and absorption spectroscopic studies of enzyme-inhibitor complexes suggest direct coordination of the oxygen atom of the (22R)-sulfoxide (4) with the catalytic heme center. This implies that the inhibitor operates by directly blocking further reaction at the active site heme group, with a substantial lifetime of the enzyme-inhibitor complex.

Subcellular localization and partial purification of the 25-hydroxyvitamin D3 1-hydroxylation reaction in the chick myelomonocytic cell line HD-11

Hypercalcemia in human granuloma-forming diseases like sarcoidosis results from the endogenous overproduction of 1,25-dihydroxyvitamin D [1,25-(OH)2D] by disease-activated tissue macrophages. The recent identification of an immortalized chick myelomonocytic cell line, HD-11, that constitutively expresses a 25-hydroxyvitamin D (25-OHD) 1-hydroxylation reaction has alleviated dependence on studying primary macrophage cultures with no replicative potential in vitro. In these experiments we established conditions for the maximal expression of the HD-11 cell 25-OHD3-1-hydroxylation reaction and localized this activity to the mitochondrial fraction. On a per cell basis, the activity of HD-11 cell 25-OHD3 1-hydroxylation reaction was comparable to that in primary cultures of chick renal tubular epithelial cells, which express the authentic renal 25-OHD3 1-hydroxylase. Maximal product yield was achieved after incubation of HD-11 cells with 200 nM 25-OHD3 for 3 h. Although adherent monolayers possessed 3- to 4-fold more capacity for hormone production than cells in suspension, suspended cells exhibited easily detectable 25-OHD3 catalytic activity (0.58 +/- 0.08 pmol per 10(6) cells per h; +/- SEM), 50% of which remained solubilized in a sonicate of suspended cells cleared of nuclei and plasma membrane. Subcellular localization disclosed 91% of the residual activity to be concentrated in the mitochondrial subfraction. A detergent-solubilized extract of this mitochondrial subfraction contained 1.9 +/- 0.3 pmol 1,25-(OH)2D3 synthetic capacity per mg protein. The catalytic activity (1-hydroxylase activity) was concentrated 20.2-fold after chromatography on octyl-amino agarose and was associated with 0.054 nmol cytochrome P450 per mg protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of the NADH reductase component of alkene monooxygenase from Mycobacterium strain E3

Alkene monooxygenase, a multicomponent enzyme system which catalyzes the epoxidation of short-chain alkenes, is induced in Mycobacterium strain E3 when it is grown on ethene. We purified the NADH reductase component of this enzyme system to homogeneity. Recovery of the enzyme was 19%, with a purification factor of 920-fold. The enzyme is a monomer with a molecular mass of 56 kDa as determined by gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It is yellow-red with absorption maxima at 384, 410, and 460 nm. Flavin adenine dinucleotide (FAD) was identified as a prosthetic group at a FAD-protein ratio of 1:1. Tween 80 prevented irreversible dissociation of FAD from the enzyme during chromatographic purification steps. Colorimetric analysis revealed 2 mol each of iron and acid-labile sulfide, indicating the presence of a [2Fe-2S] cluster. The presence of this cluster was confirmed by electron paramagnetic resonance spectroscopy (g values at 2.011, 1.921, and 1.876). Anaerobic reduction of the reductase by NADH resulted in formation of a flavin semiquinone.

Kinetic properties of 25-hydroxyvitamin D- and 1,25-dihydroxyvitamin D-24-hydroxylase from chick kidney

1,25-Dihydroxyvitamin D3 induces both 25-hydroxyvitamin D3- and 1,25-dihydroxyvitamin D3- 24-hydroxylase activities. However, whether 24-hydroxylation of these substrates is catalyzed by a single enzyme is unknown. We have examined the substrate specificity of the enzyme using the solubilized and reconstituted chick renal mitochondrial 24-hydroxylase enzyme system. The soluble enzyme catalyzes 24-hydroxylation of both substrates. The apparent Km of the 24-hydroxylase for 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 were 1.47 and 0.14 microM, respectively. Kinetic studies demonstrated that 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 act as competitive inhibitors with respect to each other. 1,25-Dihydroxyvitamin D3 inhibited the production of 24,25-dihydroxyvitamin D3 with an apparent Ki of 0.09 microM and 25-hydroxyvitamin D3 inhibited the production of 1,24,25-trihydroxyvitamin D3 with an apparent Ki of 3.9 microM. These results indicate that chick 24-hydroxylase preferentially hydroxylates 1,25-dihydroxyvitamin D3 and support the idea that the 24-hydroxylation of these substrates is catalyzed by a single enzyme.

Characteristics of the 25-hydroxyvitamin D3- and 1,25-dihydroxyvitamin D3-24-hydroxylase(s) from HL-60 cells

1,25-Dihydroxyvitamin D3 induces the human promyelocyte leukemia cell line, HL-60, to differentiate into macrophages/monocytes via a steroid-receptor mechanism. This system is a relevant one for an investigation of the molecular mechanism of 1,25-dihydroxyvitamin D3. We have now examined the effect of 1,25-dihydroxyvitamin D3 on the induction of 1,25-dihydroxyvitamin D3- and 25-hydroxyvitamin D3-24-hydroxylase activities in HL-60 cells. The hydroxylase activities were measured by a periodate-based assay, which was validated by comparison with well-established HPLC analysis. HPLC analysis also suggested that 1,25-dihydroxyvitamin D3 induces a 23-hydroxylase in addition to the 24-hydroxylase. 1,25-Dihydroxyvitamin D3- and 25-hydroxyvitamin D3-24-hydroxylase activities were stimulated as early as 4 h after the addition of 10(-7) M 1,25-dihydroxyvitamin D3 and became maximal by 24 h. 1,25-Dihydroxyvitamin D3 stimulated both activities in a dose-dependent manner up to 10(-6) M. The Km of 24-hydroxylase for 1,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 were 2 x 10(-8) M and 5.2 x 10(-7) M, respectively. Cycloheximide (5 microM) inhibited 1,25-dihydroxyvitamin D3-mediated stimulation of 24-hydroxylase activity. Other differentiation inducers, such as retinoic acid and phorbol ester, did not induce either activity. 1,25-Dihydroxyvitamin D3-24-hydroxylase in HL-60 mitochondria was solubilized with 0.6% cholate and reconstituted with NADPH, beef adrenal ferredoxin, and beef adrenal ferredoxin reductase, each component being essential for 24-hydroxylase activity. These results strongly suggest that the 24-hydroxylase in HL-60 cells is a three-component cytochrome P450-dependent mixed-function oxidase.

A rapid assay for 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D 24-hydroxylase

A rapid method for the measurement of the 24-hydroxylated metabolites of 25-hydroxy[26,27-3H]vitamin D3 and 1,25-dihydroxy[26,27-3H]vitamin D3 has been developed. This measurement has, in turn, made possible a rapid assay for the 24-hydroxylases of the vitamin D system. The assay involves the use of 26,27-3H-labeled 1,25-dihydroxyvitamin D3 or 25-hydroxyvitamin D3 as the substrate and treatment of the enzyme reaction mixture with sodium periodate, which specifically cleaves the 24-hydroxylated products between carbons 24 and 25, releasing tritiated acetone. The acetone is measured after its separation from the labeled substrate by using a reversed-phase cartridge. The results obtained with this assay were validated by comparison with the results obtained with a well-established high-performance liquid chromatography assay. The activity of the enzyme determined by both methods was equal. This assay has been successfully used for the rapid screening of column fractions during purification of the enzyme and in the screening for monoclonal antibodies to the 24-hydroxylase.

Inhibition of electron transfer from adrenodoxin to cytochrome P-450scc by chemical modification with pyridoxal 5'-phosphate: identification of adrenodoxin-binding site of cytochrome P-450scc

Covalent modification of cytochrome P-450scc (purified from bovine adrenocortical mitochondria) with pyridoxal 5'-phosphate (PLP) was found to cause inhibition of the electron-accepting ability of this enzyme from its physiological electron donor, adrenodoxin, without conversion to the "P-420" form. Reaction conditions leading to the modification level of 0.82 and 2.85 PLP-Lys residues per cytochrome P-450scc molecule resulted in 60% and 98% inhibition, respectively, of electron-transfer rate from adrenodoxin to cytochrome P-450scc (with beta-NADPH as an electron donor via NADPH-adrenodoxin reductase and with phenyl isocyanide as the exogenous heme ligand of the cytochrome). It was found that covalent PLP modification caused a drastic decrease of cholesterol side-chain cleavage activity when the cholesterol side-chain cleavage enzyme system was reconstituted with native (or PLP-modified) cytochrome P-450scc, adrenodoxin, and NADPH-adrenodoxin reductase. Approximately 60% of the original enzymatic activity of cytochrome P-450scc was protected against inactivation by covalent PLP modification when 20% mole excess adrenodoxin was included during incubation with PLP. Binding affinity of substrate (cholesterol) to cytochrome P-450scc was found to be increased slightly upon covalent modification with PLP by analyzing a substrate-induced spectral change. The interaction of adrenodoxin with cytochrome P-450scc in the absence of substrate (cholesterol) was analyzed by difference absorption spectroscopy with a four-cuvette assembly, and the apparent dissociation constant (Ks) for adrenodoxin binding was found to be increased from 0.38 microM (native) to 33 microM (covalently PLP modified).(ABSTRACT TRUNCATED AT 250 WORDS)

Hydrogen bonding of sulfur ligands in blue copper and iron-sulfur proteins: detection by resonance Raman spectroscopy

The resonance Raman spectrum of the blue copper protein azurin from Alcaligenes denitrificans exhibits nine vibrational modes between 330 and 460 cm-1, seven of which shift 0.4-3.0 cm-1 to lower energy after incubation of the protein in D2O. These deuterium-dependent shifts have been previously ascribed to exchangeable protons on imidazole ligands [Nestor, L., Larrabee, J. A., Woolery, G., Reinhammar, B., & Spiro, T. G. (1984) Biochemistry 23, 1084] or to exchangeable protons on amide groups which are hydrogen bonded to the cysteine thiolate ligands (a feature common to all blue copper proteins of known structure). In order to distinguish between these two possibilities, a systematic investigation of Fe2S2(Cys)4-containing proteins was undertaken. Extensive hydrogen bonding between sulfur ligands and the polypeptide backbone had been observed in the crystal structure of ferredoxin from Spirulina platensis. The resonance Raman spectrum of this protein is typical of a chloroplast-type ferredoxin and exhibits deuterium-dependent shifts of -0.3 to -0.5 cm-1 in the Fe-S modes at 283, 367, and 394 cm-1 (assigned to the bridging sulfurs) and -0.6 to -0.8 cm-1 in the Fe-S modes at 328 and 341 cm-1 (assigned to the terminal cysteine thiolates). Considerably greater deuterium sensitivity is observed in the Raman spectra of spinach ferredoxin and bovine adrenodoxin, particularly for the symmetric stretching vibration of the Fe2S2 moiety at approximately 390 cm-1. This feature decreases by 0.8 and 1.1 cm-1, respectively, for the two oxidized proteins in D2O and by 1.8 cm-1 for reduced adrenodoxin in D2O.(ABSTRACT TRUNCATED AT 250 WORDS)

ACTH control of cholesterol side-chain cleavage at adrenal mitochondrial cytochrome P-450scc. Regulation of intramitochondrial cholesterol transfer

Rat adrenal mitochondria exhibit a linear 2-fold accumulation of cholesterol for 20 min following either in vivo ether stress or ACTH administration, providing cholesterol metabolism is inhibited by aminoglutethimide (AMG). Additional cycloheximide (CX) pretreatment only slightly decreases this increase, but the location of accumulation shifts from the inner membrane to the outer membrane, implying a decreased cholesterol transfer from outer to inner membrane. Although the capacity of outer mitochondrial membranes was saturated after a 10-min treatment with CX, a 20-min treatment resulted in further retention of cholesterol in intact mitochondria that was not recovered in the isolated membranes. An additional pool of loosely bound cholesterol is proposed for CX mitochondria. These studies provide evidence that the CX-sensitive step of adrenal steroidogenesis attributed to loss of a labile ACTH regulatory protein (Pedersen, R.C. and Brownie, A.C. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1882-1886) involves cholesterol transfer from the outer to the inner mitochondrial membrane. ACTH also enhances the PI and PE content of the outer membranes by a CX-sensitive mechanism that may contribute to intramitochondrial cholesterol transport. CX treatment does not affect cholesterol uptake by the inner membrane from phospholipid vesicles. The initial rate of endogenous metabolism in isolated inner membranes is insensitive to pretreatment (2 nmol/nmol P-450/min). The duration of this linear rate was increased 4-fold by AMG treatment while this increase was prevented by CX treatment. The kinetics indicate differences in inner membrane reactive cholesterol levels. Inner membranes also contained a fraction of unreactive cholesterol that is insensitive to pretreatment. Cholesterol-P-450scc complex formation for all pretreatments fits a single hyperbolic function of the reactive cholesterol content of the inner mitochondrial membrane (Kd = 0.025 mol cholesterol/mol phospholipid), and is activated over 5-fold upon mitochondrial disruption. All changes in inner membranes caused by CX can, therefore, be attributed solely to the restricted cholesterol access in vivo.

A new tritium-release assay for 25-hydroxyvitamin D-1 alpha-hydroxylase

A new, rapid assay for 1 alpha-hydroxylase has been developed using 25-hydroxy-[1 alpha-3H]vitamin D3 as the substrate. Using the solubilized and reconstituted chick 1 alpha-hydroxylase, conversion of this substrate to 1,25-dihydroxyvitamin D3 causes the release of tritium into the aqueous medium. This 3H2O can be easily separated from the labeled substrate by passing the reaction mixture through a reverse-phase silica cartridge. The release of tritium is stereospecific as evidenced by the lack of 3H2O formed when 25-hydroxy-[1 beta-3H]vitamin D3 is used as the substrate. In parallel reactions containing the 25-hydroxy-[26,27-3H]vitamin D3 substrate, production of labeled 1,25-dihydroxyvitamin D3 was assessed by extraction and high-performance liquid chromatography and found to agree very closely with the amount of 3H2O produced from 25-hydroxy-[1 alpha-3H]vitamin D3, validating the accuracy of the new assay. Finally, a major advantage of the tritium-release assay for 1 alpha-hydroxylase is that the results are not affected by further metabolism of the 1,25-dihydroxyvitamin D formed in the incubations.

Rapid spectral scan and stopped-flow studies of carbon monoxide binding to bovine adrenocortical cytochrome P-450scc

Carbon monoxide binding with both cholesterol-free (low-spin) and cholesterol-bound (high-spin) reduced forms of purified cytochrome P-450scc has been investigated by rapid-scan and stopped-flow spectrometry. CO binding occurs within 150 ms at 25 degrees C for both forms of P-450scc, with a typical absorption maximum at 450 nm. Isosbestic points occur at the following wavelengths: between reduced-CO and reduced cholesterol-free P-450scc at 434 and 471 nm; between reduced-CO and reduced cholesterol-bound P-450scc at 433 and 469 nm. Both the 'on' (k1) and 'off' rate constants (k-1) are found to be independent of pH between pH 5 and 9. The mean values of k1 for cholesterol-free (1.8 +/- 0.2) X 10(5) M-1 X s-1) and cholesterol-bound [1.9 +/- 0.1) X 10(5) M-1 X s-1) P-450scc are almost identical, while the mean value of k-1 for the former [2.3 +/- 0.3) X 10 s-1) is about double that of the latter [1.2 +/- 0.1) X 10 s-1). This suggests the instability of the reduced-CO complex in the absence of cholesterol.

Solubilization and reconstitution of chick renal mitochondrial 25-hydroxyvitamin D3 24-hydroxylase

Chick kidney mitochondrial 25-hydroxyvitamin D3 24-hydroxylase has been solubilized with sodium cholate and reconstituted with NADPH, beef adrenal ferredoxin, and beef adrenal ferredoxin reductase, each component being essential for maximal 24-hydroxylase activity. The product 24(R),25-dihydroxyvitamin D3 was identified by cochromatography with synthetic compound on straight-phase and reversed-phase high-performance liquid chromatography and by periodate oxidation. The enzyme has an apparent Km for 25-hydroxyvitamin D3 of 0.67 microM. At 1 microM 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3 production is linear with time for up to 15 min and with protein concentrations of up to 2 mg/mL. The antioxidant diphenyl-p-phenylenediamine (1.3 X 10(-4) M) has no effect on this reaction. Reconstituted 24-hydroxylase activity is enhanced by the addition of NaCl and KCl up to 100 mM, with higher concentrations having an inhibitory effect. 1 alpha-Hydroxylase is not present in this preparation from vitamin D replete chicks. The similarities of this reconstituted system to the 25-hydroxyvitamin D3 1 alpha-hydroxylase and the adrenal systems suggest that the 25-hydroxyvitamin D3 24-hydroxylase is also a cytochrome P-450 type mixed-function oxidase.

25-Hydroxylation of 1 alpha-hydroxyvitamin D-3 in rat and human liver

1 alpha-Hydroxyvitamin D-3 25-hydroxylase activity was measured in subcellular fractions of rat and human liver. The formation of 1,25-dihydroxyvitamin D-3 was determined by high pressure liquid chromatography. In rat liver 1 alpha-hydroxyvitamin D-3 25-hydroxylase activities were found in the purified nuclei, the heavy mitochondrial fraction and the microsomal fraction. The enrichment of 25-hydroxylase activity was highest in the heavy mitochondrial fraction. With this fraction a minimum amount (about 0.5 mg) of protein was required before formation of 1,25-dihydroxyvitamin D-3 could be detected. Above this amount the reaction was linear with amount of protein up to at least 2 mg/ml. The reaction was also linear with time up to 60 min. An apparent Km value of 2 X 10(-5) M was found. The mitochondrial 25-hydroxylase was stimulated by addition of cytosolic protein or bovine serum albumin. The degree of stimulation was dependent on the amount of mitochondrial protein present in the incubation mixture. Maximal stimulation was seen with 0.2 mg/ml of either protein in the presence of 0.5 mg mitochondrial protein. The stimulating effect remained after heating the protein for 5 min at 100 degrees C. The cytosolic protein did not stimulate a reconstituted mitochondrial 1 alpha-hydroxyvitamin D-3 25-hydroxylase. The mitochondrial vitamin D-3 25-hydroxylase was inhibited both by cytosolic protein and by bovine serum albumin. Human liver revealed only one 1 alpha-hydroxyvitamin D-3 25-hydroxylase activity located to the heavy mitochondrial fraction. The results are in agreement with previous studies on the localization of vitamin D-3 25-hydroxylase in rat and human liver. The difference in localization of the 25-hydroxylase between rat and human liver implies that studies on the regulation of the microsomal 25-hydroxylase in rat liver may not be relevant to the situation in human liver.

Electron transfer reactions in the soluble methane monooxygenase of Methylococcus capsulatus (Bath)

Aerobic stopped-flow experiments have confirmed that component C is the methane monooxygenase component responsible for interaction with NADH. Reduction of component C by NADH is not the rate-limiting step for component C in the methane monooxygenase reaction. Removal and reconstitution of the redox centres of component C suggest a correlation between the presence of the FAD and Fe2S2 redox centres and NADH: acceptor reductase activity and methane monooxygenase activity respectively, consistent with the order of electron flow: NADH----FAD----Fe2S2----component A. This order suggests that component C functions as a 2e-1/1e-1 transformase, splitting electron pairs from NADH for transfer to component A via the one-electron-carrying Fe2S2 centre. Electron transfer has been demonstrated between the reductase component, component C and the oxygenase component, component A, of the methane monooxygenase complex from Methylococcus capsulatus (Bath) by three separate methods. This intermolecular electron transfer step is not rate-determining for the methane monooxygenase reaction. Intermolecular electron transfer was independent of component B, the third component of the methane monooxygenase. Component B is required to switch the oxidase activity of component A to methane mono-oxygenase activity, suggesting that the role of component B is to couple substrate oxidation to electron transfer, via the methane monooxygenase components.

Further characterisation of the FAD and Fe2S2 redox centres of component C, the NADH:acceptor reductase of the soluble methane monooxygenase of Methylococcus capsulatus (Bath)

The absorbance contributions of the FAD and Fe2S2 redox centres of component C of the soluble methane monooxygenase complex have been resolved, using mersalyl to destroy the Fe2S2 centre. The Fe2S2 seems to be very similar to that of spinach ferredoxin, by its absorbance and electron paramagnetic resonance (EPR) spectra, and the FAD semiquinone is a neutral semiquinone. Spectrophotometry near room temperature and EPR spectroscopy near liquid-helium temperature allow the three redox couples of component C to be ordered. Component C can exist in Oe-1 (oxidised), 1e-1 (semiquinone), 2e-1 (mostly semiquinone and reduced Fe2S2), and 3e-1 forms (dihydroquinone and reduced Fe2S2), under equilibrium conditions. The ability of component C to support odd-electron forms is consistent with its proposed role as a 2e-1/1e-1 transformase, splitting electron pairs from NADH for passage to component A in one-electron steps. (The FAD appears to interact with NADH, and transfers single electrons to the Fe2S2, for donation to component A at a constant redox potential.) The mid-point potentials of component C were found using redox dyes and EPR spectroscopy and were: FAD/FAD., Em = -150 mV; Fe2S2/Fe2.S2,Em = -220 mV; FAD./FAD..,Em = -260 mV. the presence of NADH did not alter these mid-point potentials. These mid-point potentials are consistent with the role of component C as the NADH:component A reductase, passing electrons from NADH (Em = -320 mV) onto component A (Em = +150 mV and Em = -150 mV). The reducing power from NADH appears to be required by component A to activate one atom of oxygen, to insert into methane, and the reducing equivalents derived from NADH end up with the other oxygen atom, as water.

The catalytic cycle of cytochrome P-450scc and intermediates in the conversion of cholesterol to pregnenolone

Cytochrome P-450scc as isolated is a cholesterol-depleted low-spin haemoprotein; addition of cholesterol results in formation of a high-spin complex. Cytochrome P-450scc--cholesterol is a one-electron acceptor on titration with NADPH. Cytochrome P-450scc--cholesterol can be anaerobically reduced to the ferrous state which, on oxygenation, forms an oxygenated cytochrome P-450scc--cholesterol complex. This oxygenated complex in the absence of adrenodoxin autoxidises to ferric cytochrome P-450scc--cholesterol without oxidation of cholesterol. The decay of the oxygenated complex is first-order, k = 9.3 X 10(-3) S-1 at 4 degrees C. The rate of autoxidation is influenced by pH, ionic strength and the chemical nature of bound sterol. The activation energy of autoxidation is 75 kJ mol-1. Addition of equimolar amounts of adrenodoxin to cytochrome P-450scc--cholesterol followed by stoichiometric reduction under anaerobic conditions and subsequent oxygenation, allows single catalytic turnover cycles of cytochrome P-450scc to be observed. This has led to detection of intermediates in the conversion of cholesterol to pregnenolone and a precursor/product sequence of cholesterol----22-hydroxycholesterol----20,22-dihydroxy-cholesterol ----pregnenolone has been established. Addition of oxidised adrenodoxin to oxygenated cytochrome P-450scc--cholesterol results in formation of 22-hydroxycholesterol.

Oxidation state dependence of proton exchange near the iron-sulfur centers in ferredoxins and high-potential iron-sulfur proteins

For both the [2Fe-2S] and the [4Fe-4S] ferredoxins, dialysis against 2H2O prior to single electron reduction leads to the appearance of a deuterium modulation pattern in the electron spin echo decay envelope indicative of deuteron-proton exchange very near the paramagnetic center. In contrast, if the ferredoxin is exposed to 2H2O after its reduction in H2O, far less deuterium exchange near the metal center takes place. Thus, proton exchange with solvent is in part dependent on the redox state of the protein. For high potential iron-sulfur proteins, this type of proton-deuteron exchange near the metal center does not occur unless the protein is partially unfolded in dimethylsulfoxide in 2H2O.

Steroidogenic electron transport in adrenal cortex mitochondria

The flavoprotein NADPH-adrenodoxin reductase and the iron sulfur protein adrenodoxin function as a short electron transport chain which donates electrons one-at-a-time to adrenal cortex mitochondrial cytochromes P-450. The soluble adrenodoxin acts as a mobile one-electron shuttle, forming a complex first with NADPH-reduced adrenodoxin reductase from which it accepts an electron, then dissociating, and finally reassociating with and donating an electron to the membrane-bound cytochrome P-450 (Fig. 9). Dissociation and reassociation with flavoprotein then allows a second cycle of electron transfers. A complex set of factors govern the sequential protein-protein interactions which comprise this adrenodoxin shuttle mechanism; among these factors, reduction of the iron sulfur center by the flavin weakens the adrenodoxin-adrenodoxin reductase interaction, thus promoting dissociation of this complex to yield free reduced adrenodoxin. Substrate (cholesterol) binding to cytochrome P-450scc both promotes the binding of the free adrenodoxin to the cytochrome, and alters the oxidation-reduction potential of the heme so as to favor reduction by adrenodoxin. The cholesterol binding site on cytochrome P-450scc appears to be in direct communication with the hydrophobic phospholipid milieu in which this substrate is dissolved. Specific effects of both phospholipid headgroups and fatty acyl side-chains regulate the interaction of cholesterol with its binding side. Cardiolipin is an extremely potent positive effector for cholesterol binding, and evidence supports the existence of a specific effector lipid binding site on cytochrome P.450scc to which this phospholipid binds.

Exchange integral for a variety of tetranuclear ferredoxins

The temperature dependence of EPR spectra of oxidized [4Fe-4S](-1,-2) ferredoxins (previously designated HiPIP) and a reduced [4Fe-4S](-2,-3) ferredoxin have been analyzed so as to determine the energy of a low-lying excited electronic state. The values obtained were: Center S-3 from beef heart, 44 cm-1; Center S-3 from mung bean, 53 cm-1; the [4Fe-4S](-1,-2) ferredoxin from Thermus thermophilus, 78 cm-1; Center N-2 of NADH ubiquinone reductase, 83 cm-1. Increasing axial distortion in the EPR spectra of the [4Fe-4S](-1,-2), ferrodoxins was associated with higher energy differences. Center N-2, a [Fe-4S](-2,-3) iron-sulfur cluster does not fit this relationship.

Purification of NADPH-ferredoxin reductase from rat liver mitochondria

NADPH-ferredoxin reductase (NADPH:ferredoxin oxidoreductase, EC 1.6.7.1) has been identified in rat liver mitochondria and purified to homogeneity as judged by sodium dodecyl sulfate (SDS) gel electrophoresis. The protein was detected by its ability to reconstitute NADPH-cytochrome c reductase in the presence of adrenal ferredoxin. The purified protein had properties very similar to adrenal NADPH-ferredoxin reductase. The molecular weight was 52 000, as estimated by gel filtration. On SDS-polyacrylamide gels, mobility was identical to that of adrenal NADPH-ferredoxin reductase (Mr = 52 000). The enzyme exhibited a typical oxidized flavoprotein absorbance spectrum with maxima at 269, 377 and 450 nm and gave an absorbance ratio A450nm/A269nm of 0.138. The fluorescence excitation spectrum was identical to that of FAD. In the presence of NADPH and a ferredoxin, the reductase was found to be active in a reconstituted cytochrome P-450-dependent steroid 26-hydroxylase, which was recently isolated from rat liver mitochondria (Pedersen, J.I. (1978) FEBS Lett. 85, 35-39).

Soluble 25-hydroxyvitamin D3-1 alpha-hydroxylase from kidney mitochondria of rachitic pigs

1. Mitochondria isolated from the kidneys of rachitic pigs have been shown to contain an active 25-hydroxyvitamin D3-1 alpha-hydroxylase. From these mitochondria a cytochrome P-450 has been solubilized with a specific content of 0.02-0.04 nmol/mg protein. 2. In the presence of a bovine adrenal NADPH-ferredoxin reductase, bovine adrenal ferredoxin and NADPH, the cytochrome P-450 supported the formation of 1,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. 3. The hydroxylation reaction was linear with time up to 40 min, and with the amount of enzyme up to 0.03 nmol cytochrome P-450. The pH optimum for the reaction was 7.4, and the apparent Km was 3 x 10(-10) mol/mg protein. 4. The results show that 25-hydroxyvitamin D3 is metabolized in mammals by the same enzyme system as has been demonstrated in birds.

Purification and properties of paramagnetic protein from Clostridium pasteurianum W5

The purification to homogeneity of the non-heme iron protein, sometimes referred to as either "red protein" or "paramagnetic protein", from Clostridium pasteurianum W5 extracts is described and its physicochemical properties studied. This paramagnetic protein (g= 1.94) has a molecular weight of about 25000 and contains two iron and two acid-labile sulfur atoms per mol of protein. Its midpoint potential at pH 7.5, as determined by electron paramagnetic resonance titration, is -300 mV. Optical circular dichroism and electron paramagnetic resonance spectra of the paramagnetic protein are similar to those of two iron-two acid-labile sulfur ferredoxins. The biochemical reduction of the purified protein was also studied.

Magnetic studies of the four-iron high-potential, non-heme protein from Chromatium vinosum

Extensive EPR studies on high-potential, iron-sulfur protein from Chromatium vinosum indicate that the singular spectrum of this four-iron, non-heme protein consists of a superposition of three distinct signals; namely, two principal signals of equal weight, one reflecting axial and the other rhombic symmetry, and a third nearly isotropic minority component. In addition, magnetic susceptibility experiments on two oxidation states of the protein from 4.2 to approx. 260 degrees K indicate antiferromagnetic exchange coupling between iron atoms. Possible origins of the complex EPR signals are discussed, and a preferred model that is consistent with EPR, magnetic susceptibility, NMR, X-ray, and Mössbauer data is presented.

Thermodynamic and EPR characterization of iron-sulfur centers in the NADH-ubiquinone segment of the mitochondrial respiratory chain in pigeon heart

Several iron-sulfur centers in the NADH-ubiquinone segment of the respiratory chain in pigeon heart mitochondria and in submitochondrial particles were analyzed by the combined application of cryogenic EPR (between 30 and 4.2 degrees K) and potentiometric titration. Center N-1 (iron-sulfur centers associated with NADH dehydrogenase are designated with the prefix "N") resolves into two single electron titratins with EM7.2 values of minus 380 plus or minus 20 mV and minus 240 plus or minus 20 mV (Centers N-1a and N-1b, respectively). Center N-1a exhibits an EPR spectrum of nearly axial symmetry with g parellel = 2.03, g = 1.94, while that of Center N-1b shows more apparent rhombic symmetry with gz = 2.03, gy = 1.94 and gx = 1.91. Center N-2 also reveals EPR signals of axial symmetry at g parallel = 2.05 and g = 1.93 and its principal signal overlaps with those of Centers N-1a and N-1b. Center N-2 can be easily resolved from N-1a and N-1b because of its high EM7.2 value (minus 20 plus or minus 20 mV). Resolution of Centers N-3 and N-4 was achieved potentiometrically in submitochondrial particles. The component with EM7.2 = minus 240 plus or minus 20 mV is defined as Center N-3 (gz = 2.10, (gz = 2.10, (gy = 1.93?), GX = 1.87); the minus 405 plus or minus 20 mV component as Center N-4 (gz = 2.11, (gy = 1.93?), gx = 1.88). At temperatures close to 4.2 degrees K, EPR signals at g = 2.11, 2.06, 2.03, 1.93, 1.90 and 1.88 titrate with EM7.2 = minus 260 plus or minus 20 mV. The multiplicity of peaks suggests the presence of at least two different iron-sulfur centers having similar EM7.2 values (minus 260 plus or minus 20 mV); HENCE, tentatively assigned as N-5 and N-6. Consistent with the individual EM7.2 values obtained, addition of succinate results in the partial reduction of Center N-2, but does not reduce any other centers in the NADH-ubiquinone segment of the respiratory chain. Centers N-2, N-1b, N-3, N-5 and N-6 become almost completely reduced in the presence of NADH, while Centers N-1a and N-4 are only slightly reduced in pigeon heart submitochondrial particles. In pigeon heart mitochondria, the EM7.2 of Center N-4 lies much closer to that of Center N-3, so that resolution of the Center N-3 and N-4 spectra is not feasible in mitochondrial preparations. EM7.2 values and EPR lineshapes for the other iron-sulfur centers of the NADH-ubiquinone segment in the respiratory chain of intact mitochondria are similar to those obtained in submitochondrial particle preparations. Thus, it can be concluded that, in intact pigeon heart mitochondria, at least five iron-sulfur centers show EM7.2 values around minus 250 mV; Center N-2 exhibits a high EM7.2 (minus 20 plus or minus 20 mV), while Center N-1a shows a very low EM7.2 (minus 380 plus or minus 20 mV).

Studies on the conformational changes of metalloproteins induced by electrons in water-ethylene glycol solutions at low temperatures. III. Adrenal ferredoxin

The reduction of adrenal ferredoxin (adrenodoxin) at low temperatures was investigated in order to separate local modifications of the active centre of the protein on its reduction, from the conformational transition which seems to accompany the change of the redox state of the irons; The ESR spectra of the states of the protein, where the reduced active centre is to be found by the "oxidized" conformation of the apoprotein, were obtained. The transition from the states of the protein to the state which occurs on its chemical reduction at room temperature was also investigated. The results of the work support the view that conformational changes in proteins (enzymes) which take place while they are functioning proceed after modifications of the active centres (change of the redox state, adsorption of a substrate, etc.), and are essentially caused by them. Adrenal ferredoxin was the third subject in our studies of the intermediate states of proteins which appear after reduction of their active centres by means of electrons trapped in water-ethylene glycol mixtures at the temperature of liquid nitrogen [1, 2]. In the reduced state, the active centre of the protein has an ESR signal with a g-factor of 1.94 [3, 4] which is convenient for our purposes.