Cytochrome P-450 from mitochondria of bovine adrenal cortex: comparison of cholesterol side-chain cleavage P-450 with steroid 11beta-hydroxylation P-450 and immunochemical cross-reactivity between adrenal mitochondrial and liver microsomal cytochromes P-450.

Adrenocortical mitochondrial cytochrome P-450 specific to the cholesterol side-chain cleavage (desmolase) reaction differs from that for the 11beta-hydroxylation reaction of deoxycorticosterone. The former cytochrome appears to be more loosely bound to the inner membrane than the latter. Upon ageing at 0 degrees C or by aerobic treatment with ferrous ions, the desmolase P-450 was more stable than the 11beta-hydroxylase P-450. By utilizing artificial hydroxylating agents such as cumene hydroperoxide, H2O2, and sodium periodate, the hydroxylation reaction of deoxycorticosterone to corticosterone in the absence of NADPH was observed to a comparable extent with the reaction in the presence of adrenodoxin reductase, adrenodoxin and NADPH. However, the hydroxylation reaction of cholesterol to pregnenolone was not supported by these artificial agents. Immunochemical cross-reactivity of bovine adrenal desmolase P-450 with rabbit liver microsomal P-450LM4 was also investigated. We found a weak but significant cross-reactivity between the adrenal mitochondrial P-450 and liver microsomal P-450LM4, indicating to some extent a homology between adrenal and liver cytochromes P-450.     

Common characteristics of the cytochrome P-450 system involved in 18- and 11 beta-hydroxylation of deoxycorticosterone in rat adrenals.

18- and 11beta-Hydroxylation of deoxycorticosterone and side chain cleavage of cholesterol were studied in mitochondria and submitochondrial reconstituted systems prepared from rat and bovine adrenals. A mass fragmentographic technique was used that allows determination of hydroxylation of both exogenous and endogenous cholesterol. The following results were obtained. (1) Treatment of rats with excess potassium chloride in drinking fluid increased mitochondrial cytochrome P-450 as well as 18- and 11beta-hydroxylase activity in the adrenals. Cholesterol side chain cleavage was not affected. In the presence of excess adrenodoxin and adrenodoxin reductase, cytochrome P-450 isolated from potassium chloride-treated rats had higher 18- and 11beta-hydroxylase activity per nmol than cytochrome P-450 isolated from control rats. The stimulatory effects on 18- and 11beta-hydroxylation were of similar magnitude. (2) Long-term treatment with ACTH increased cholesterol side chain cleavage in the adrenals but had no effect on 18- and 11beta-hydroxylase activity. The amount of cytochrome P-450 in the adrenals was not affected by the treatment. It was shown with isolated mitochondrial cytochrome P-450 in the presence of excess adrenodoxin and adrenodoxin reductase that the effect of ACTH was due to increase of side chain cleavage activity per nmol cytochrome P-450. Side chain cleavage of exogenous cholesterol was affected more than that of endogenous cholesterol. (3) Gel chromatography of soluble cytochrome P-450 prepared from rat and bovine adrenal mitochondria yielded chromatographic fractions having either a high 18- and 11beta-hydroxylase activity and a low cholesterol side chain cleavage activity or the reverse. The ratio between 18- and 11beta-hydroxylase activity was approximately constant, provided the origin of cytochrome P-450 was the same. (4) Addition of progesterone to incubations of deoxycorticosterone with soluble or insoluble rat adrenal cytochrome P-450 competitively inhibited 18- and 11beta-hydroxylation of deoxycorticosterone to the same degree. Addition of deoxycorticosterone competitively inhibited 11beta-hydroxylation of progesterone with the same system. Progesterone was not 18-hydroxylated by the system. From the results obtained, it is concluded that 18- and 11beta-hydroxylation have similar properties and that the binding site for deoxycorticosterone is similar or identical in the two hydroxylations. The possibility that the same specific type of cytochrome P-450 is responsible for both 18- and 11beta-hydroxylation of deoxycorticosterone is discussed.     

P-45011 beta-dependent conversion of cortisol to cortisone, and 19-hydroxyandrostenedione to 19-oxoandrostenedione

Purified bovine adrenal P-45011 beta has been shown to catalyze conversions of cortisol to cortisone (11-oxidase activity), and 19-hydroxyandrostenedione to 19-oxoandrostenedione (19-oxidase activity), in the reconstituted system consisting of NADPH, NADPH:adrenodoxin reductase, and adrenodoxin. The turnover numbers (mol of product formed/min/mol of P-450) were 1.2 for the 11-oxidase activity and 1.4 for the 19-oxidase activity. No reactions took place when any one of the electron-donating components were omitted either in the presence or in the absence of added NADP+. Likewise, rabbit antibody prepared against P-45011 beta immunoprecipitated the 11-oxidase activity with concomitant loss of deoxycorticosterone 11 beta-hydroxylase activity.     

The participation of a second molecule of adrenodoxin in cytochrome P-450-catalyzed 11beta hydroxylation.

We have utilized 11beta-hydroxylase activity and visible absorption spectrophotometry to detect possible complex formation among adrenodoxin reductase, adrenodoxin, and cytochrome P-450(11)beta. At low ionic strength, a 1:1 complex between adrenodoxin reductase and adrenodoxin occurs but does not support maximal rates of 11beta hydroxylation; at least 1 additional molecule of adrenodoxin in excess of the 1:1 complex is required for full hydroxylase activity. Spectrophotometric titration of a mixture of adrenodoxin reductase and cytochrome P-450(11)beta with adrenodoxin indicates sequential formation of 1:1 complexes between adrenodoxin reductase and adrenodoxin and then between a second adrenodoxin and cytochrome P-450(11beta; the adrenodoxin-cytochrome P-450(11)beta complex is only detectable when the concentration of adrenodoxin exceeds that of adrenodoxin reductase.     

Adrenal P-450scc catalyzes deoxycorticosterone 6 beta-hydroxylase reaction

Purified bovine P-450scc, the cholesterol side-chain cleaving P-450 in adrenal cortex mitochondria, was found to catalyze a deoxycorticosterone 6 beta-hydroxylase reaction. A turnover number (moles of product formed/min/mol of P-450) of 12 was found similar to that for cholesterol side chain cleavage activity. Conversion was dose-dependent in terms of P-450scc and no reaction took place when any one of the required electron donating components such as NADPH, NADPH-adrenodoxin reductase, or adrenodoxin was omitted. These results confirm and extend earlier observations that 21-hydroxypregnenolone is transformed into both deoxycorticosterone and 6 beta-hydroxydeoxycorticosterone by incubation of adrenal gland slices.     

Adrenodoxin with a COOH-terminal deletion (des 116-128) exhibits enhanced activity

Adrenodoxin, purified from bovine adrenal cortex, was subjected to trypsin cleavage to yield a trypsin-resistant form, designated TT-adrenodoxin. Sequencing with carboxypeptidase Y identified the trypsin cleavage site as Arg-115, while Edman degradation indicated no NH2-terminal cleavage. Native adrenodoxin and TT-adrenodoxin exhibited similar affinity for adrenodoxin reductase as determined in cytochrome c reductase assays. In side chain cleavage assays using cytochrome P-450scc, however, TT-adrenodoxin demonstrated greater activity than adrenodoxin with cholesterol, (22R)-22-hydroxycholesterol, or (20R,22R)-20,22-dihydroxycholesterol as substrate. This enhanced activity is due to increased affinity of TT-adrenodoxin for cytochrome P-450scc; TT-adrenodoxin exhibits a 3.8-fold lower apparent Km for the conversion of cholesterol to pregnenolone. TT-Adrenodoxin was also more effective in coupling with cytochrome P-450(11) beta, exhibiting a 3.5-fold lower apparent Km for the 11 beta-hydroxylation of deoxycorticosterone. In the presence of partially saturating cholesterol, TT-adrenodoxin elicited a type I spectral shift with cytochrome P-450scc similar to that induced by adrenodoxin, and spectral titrations showed that oxidized TT-adrenodoxin exhibited a 1.5-fold higher affinity for cytochrome P-450scc. These results establish that COOH-terminal residues 116-128 are not essential for the electron transfer activity of bovine adrenodoxin, and the differential effects of truncation at Arg-115 on interactions with adrenodoxin reductase and cytochromes P-450 suggest that the residues involved in the interactions are not identical.     

Ionic effects on adrenal steroidogenic electron transport. The role of adrenodoxin as an electron shuttle.

We have shown (Seybert, D., Lambeth, D., and Kamin, H. (1978), J. Biol. Chem. 253, 8355-8358) that, whereas the 1:1 complex between adrenodoxin reductase and adrenodoxin is the active species for cytochrome c reduction, the complex is not sufficient to allow cytochrome P-45011 beta-mediated hydroxylations;adrenodoxin in excess of reductase is required. In the present studies, reduction by NADPH of excess adrenodoxin is shown to occur at a rate sufficient to support both cytochrome P-450 11 beta-mediated hydroxylation of deoxycorticosterone, and cytochrome P-450sec-mediated side chain cleavage of cholesterol. Oxidation-reduction potential and ion effect studies indicate that the mechanism of steroidogenic electron transport involves an adrenodoxin electron "shuttle" rather than a macromolecular complex of reductase, adrenodoxin, and cytochrome. The oxidation-reduction potential of adrenodoxin is shifted about -100 mV when bound to reductase, and reduction of the iron-sulfur protein thus promotes dissociation of the complex. The rate of adrenodoxin reduction is first stimulated, then inhibited by increasing salt; the effect is ion-specific, with Ca2+ approximately Mg2+ greater than Na+ greater than NH/+. Similar ion-specific rate effects are observed for both of the cytochrome P-450-mediated hydroxylations, indicating that the same reduction mechanism is required for these reactions. Increasing salt concentrations caused dissociation of the complex; dissociation of the form of the complex containing reduced adrenodoxin occurred at lower salt concentrations than that containing oxidized adrenodoxin. The order of effectiveness of ions in causing dissociation is the same as the order for stimulation of adrenodoxin reduction, suggesting a dissociation step in the mechanism. This proposed model, together with dissociation constants for the form of the complex containing either oxidized or reduced adrenodoxin, allows accurate prediction of the salt rate effects curve. For all ions, an activity maximum is seen at the ion concentration which produces the largest molar difference between associated-oxidized and dissociated-reduced states, and the model predicts the positions of the maxima for adrenodoxin reduction, 11 beta-hydroxylation, and side chain cleavage. Thus reduction-induced dissociation of adrenodoxin from adrenodoxin reductase appears to be a required step in steroidogenic electron transport by this system, and a role for adrenodoxin as a mobile electron shuttle is proposed.     

The active form of cytochrome P-45011beta from adrenal cortex mitochondria.

Cytochrome P-45011beta has been solubilized and partially purified from bovine adrenal cortex mitochondria by means of chromatography on Octyl-Sepharose CL-4B or DEAE-Sepharose CL-6B. The partially purified P-450 preparations were about 90% pure as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but had a low specific content of P-450 (between 1 and 2 nmol of P-450 per mg of protein). In the presence of purified preparations of adrenodoxin reductase and adrenodoxin, the partially purified P-450 preparations catalyzed NADPH-supported 11beta-hydroxylation of unconjugated and sulfoconjugated deoxycorticosterone. In the reconstituted system the hydroxylation of deoxycorticosterone sulfate proceeded at a much higher rate than in intact mitochondria, indicating that in the former case interactions between the hydrophilic substrate and P-450 were facilitated. In the presence of Triton X-100 the partially purified cytochrome P-45011beta had a Stokes radius of 4.5 nm, a sedimentation coefficient of 3.1 S, and a partial specific volume of about 0.85 cm3/g. These results indicate that the cytochrome P-45011beta . Triton X-100 complex had a molecular weight of about 100,000 and that P-45011beta bound about 1.1 g of Triton X-100 per g of protein. The P-45011beta . Triton X-100 complex was catalytically active in hydroxylation reactions supported by NADPH or the hydroxylating agent ortho-nitroiodosobenzene, suggesting that the monomer of cytochrome P-45011beta is the active form of the protein.     

Electrochemical investigations on the oxygen activation by cytochrome P-450.

The application of cytochrome P-450 in substrate conversion is complicated both due to the limited stability and the cofactor regeneration problems. To overcome the disadvantages of NADPH consumption the transfer of the reduction equivalents from an electrode into the cytochrome P-450-system was studied: 1. NADPH was cathodically reduced at a mercury pool electrode. By immobilization of NADP on dialdehyde Sephadex the reductive recycling was possible. 2. Different forms of reduced oxygen were produced by the cathode: a) The reaction of O2- with deoxycorticosterone yields a carboxylic acid derivative. In contrast the cytochrome P-450 catalyzed NADPH-dependent reaction with the same substrate gives corticosterone, O2- represents only an intermediate in the activation of oxygen and is not the "activated oxygen" species. b) Molecular oxygen was reduced to HO2- and H2O2, respectively. The interaction of adsorbed cytochrome P-450 on the electrode surface with the reduced oxygen species in the absence of NADPH was studied. The electrochemically generated peroxide seems to be more active than added H2O2. 3. In a model of electro-enzyme-reactor several substrates were hydroxylated by microsomal cytochrome P-450 with cathodically reduced oxygen which substitutes NADPH.     

Reconstruction and study of a multi-enzyme system by 11 beta-hydroxylase steroids

The reconstitution of the steroid 11 beta-hydroxylase system based on the homogeneous proteins isolated from bovine adrenocortical mitochondria, cytochrome P-450 (P-450 (11 beta), 19-20.5 nmol of heme P-450 per 1 mg of protein), adrenodoxin (Adx) and adrenodoxin reductase (AR) was carried out. The reconstitution of the multienzyme system requires the presence of a non-ionic detergent due to the high hydrophobicity of P-450 (11 beta). Low concentrations of Tween 20 (below 0.015% or 115 microM) stimulate the reaction of steroid 11 beta-hydroxylation by improving the hemoprotein solubility. With a further increase in the detergent concentration, the reaction is inhibited due to the inactivation of the cytochrome and its impaired interaction with Adx. The electron transfer activity of adrenodoxin reductase and the dienzyme AR-Adx complex does not change within the Tween 20 concentration range of 0-0.4%. In solutions with the optimal concentration of Tween 20 (0.010-0.015%), the concentrations of AR and Adx providing for the half-maximum hydroxylation activity are 9 nM for AR and 280 nM for Adx. It was shown that in a reconstituted 11 beta-hydroxylase system, 75% of the reducing equivalents are involved in the formation of oxygen radicals, whereas 25%--in hydroxylation. 74% of the radical species are, in their turn, formed in the active site of the hemoprotein, while 26%--in the Fe2S2 center of adrenodoxin. The radical formation process predominates over the 11 beta-hydroxylation within a wide range of Adx/cytochrome ratios, i.e., 1.0-100. The hydroxylation substrate induces a 4-fold increase in the electron transfer rate by stimulating the enzymatic reduction of P-450 (11 beta), but only 35% of the additional reduced equivalents are consumed by the 11 beta-hydroxylation and 65%--by the oxygen radical formation.     

Characterization of a cytochrome P-450-dependent steroid hydroxylase system present in Bacillus megaterium.

Cell-free extracts from sonically disrupted Bacillus megaterium ATCC 13368 hydroxylated a variety of 3-oxo-delta4-steroids in position 15beta in the presence of NADPH and O2. Ring A-reduced, aromatic and 3beta-hydroxy-delta5-steroids did not serve as substrates for the 15beta-hydroxylase system. Using ion exchange chromatography on DEAE-cellulose and gel filtration on Ultrogel ACA-54 it was possible to resolve the hydroxylase system into three proteins: a strictly NADPH-dependent FMN-containing (megaredoxin reductase), an iron-sulfur protein (megaredoxin), and cytochrome P-450 (P-450meg). The activity of the 15beta-hydroxylase system was fully reconstituted upon combination of these three proteins and addition of NADPH. Megaredoxin had an apparent sulfur to iron ration of 0.98 and showed g-signals at 1.90, 1.93, and 2.06 when analyzed by electron paramagnetic reso0 times and the preparation contained 1 to 2 nmol of cytochrome P-450 per mg of protein. This preparation of cytochrome P-450meg sedimented as a homogeneous zone on sucrose gradients with a sedimentation coefficient of 3.3 S and contained 0.94 nmol of heme per nmol of cytochrome P-450. The oxidized form of cytochrome P-450meg showed absolute absorption maxima at 416, 528, and 565 nm whereas the reduced form showed maxima at 411 and 542 nm. The following scheme is suggested for the electron transport in the 15beta-hydroxylase system in B. megaterium: NADPH leads to megaredoxin reductase leads to megaredoxin leads to cytochrome P-450meg.     

A covalently linked complex of cytochrome P-450 with adrenodoxin. Localization of the adrenodoxin binding site of cytochrome P-450

Cytochrome P-450scc and adrenodoxin were cross-linked with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The sample containing 94% of a cross-linked complex and 6% of free cytochrome P-450scc was obtained after purification on cholate-Sepharose. Cytochrome P-450scc in the cross-linked complex is not reduced in the presence of NADPH and adrenodoxin reductase, but completely preserves its high spin form in the presence of Tween-20 or pregnenolone. The use of radioactive labelled adrenodoxin, chemical cleavage of cytochrome P-450scc from the cross-linked complex by o-iodosobenzoic acid and HPLC for separation of peptides demonstrated that the cytochrome P-450scc complex with adrenodoxin was cross-linked through two amino acid sequences of cytochrome P-450scc, i.e., Leu 88-Trp108 and Leu368-Trp417.     

Conversion of 11-deoxycorticosterone and corticosterone to aldosterone by cytochrome P-450 11 beta-/18-hydroxylase from porcine adrenal

Highly purified cytochrome P-450 11 beta-/18-hydroxylase and the electron carriers adrenodoxin and adrenodoxin reductase were prepared from porcine adrenal. When the enzyme was incubated with the electron carriers, 11-deoxycorticosterone (DOC) and NADPH, the following products were isolated and measured by HPLC: corticosterone, 18-hydroxy-11-deoxycorticosterone (18-hydroxyDOC), 18-hydroxycorticosterone and aldosterone. All of the DOC consumed by the enzyme can be accounted for by the formation of these four steroids. Aldosterone was identified by mass spectroscopy and by preparing [3H]aldosterone from [3H]corticosterone followed by recrystallization at constant specific activity after addition of authentic aldosterone. Corticosterone and 18-hydroxycorticosterone were also converted to aldosterone. Conversion of corticosterone and 18-hydroxycorticosterone to aldosterone required P-450, both electron carriers, NADPH and substrate. The reaction is inhibited by CO and metyrapone. Moreover, all three activities of the purified enzyme decline at the same rate when the enzyme is kept at room temperature for various periods of time and when the enzyme is treated with increasing concentrations of anti-11 beta-hydroxylase (IgG) before assay. It is concluded that cytochrome P-450 11 beta-/18-hydroxylase can convert DOC to aldosterone via corticosterone and 18-hydroxycorticosterone. The stoichiometry of this conversion was found to be 3 moles of NADPH, 3 moles of H+ and 3 moles of oxygen per mole of aldosterone produced.     

Reconstitution of cytochrome P-450-dependent digitoxin 12 beta-hydroxylase from cell cultures of foxglove (Digitalis lanata EHRH.).

Cytochrome P-450-dependent digitoxin 12 beta-hydroxylase from cell cultures of foxglove (Digitalis lanata) was solubilized from microsomal membranes with CHAPS (3-[(3-cholamidopropyl)dimethylammonio]propane-1-sulphonic acid). Cytochrome P-450 was separated from NADPH: cytochrome c (P-450) reductase by ion-exchange chromatography on DEAE-Sephacel. NADPH:cytochrome c (P-450) reductase was further purified by affinity chromatography on 2',5'-ADP-Sepharose 4B. This procedure resulted in a 248-fold purification of the enzyme; on SDS/polyacrylamide-gel electrophoresis after silver staining, only one band, corresponding to a molecular mass of 80 kDa, was present. The digitoxin 12 beta-hydroxylase activity could be reconstituted by incubating partially purified cytochrome P-450 and NADPH:cytochrome c (P-450) reductase together with naturally occurring microsomal lipids and flavin nucleotides. This procedure yielded about 10% of the original amount of digitoxin 12 beta-hydroxylase.     

18-Hydroxylation of deoxycorticosterone by reconstituted systems from rat and bovine adrenals.

18-Hydroxylation of deoxycorticosterone was studies with rat or bovine adrenal mitochondria or with reconstituted systems obtained from these fractions. The reconstituted systems consisted of a partially purified preparation of cytochrome P-450 from rat adrenals and a partially purified NADPH-cytochrome P450 reductase preparation from bovine adrenals. In some experimenta a soluble cytochrome P-450 fraction from bovine adrenals was used. Adrenodoxine and adrenodoxine reductase were shown to be the active components of the NADPH-cytochrome P-450 reductase preparation. Optimal assay conditions were determined for 18-hydroxylation by the crude mitochondrial fraction as well as by the reconstituted systems. In the presence of excess NADPH-cytochrome P-450 reductase fraction, the rate of 18-hydroxylation was linear with time and with the amount of cytochrome P-450. In incubations with intact rat adrenal mitochondria to which Ca2+ and an excess NADPH had been added, NADPH-cytochrome P-450 reductase increased the rate of 18-hydroxylation about 100%, indicating that NADPH-cytochrome P-45o reductase was to some extent rate-limiting. The rate of 18-hydroxylation of deoxycorticosterone by the reconstituted system as well as by intact mitochondrial fraction was much higher than the rat of 18-hydroxylation of corticosterone and progesterone. When the cytochrome P-450 preparation from rat adrenals in the reconstituted system was substituted for cytochrome P-450 from bovine adrenals, the rate of 18-hydroxylation decreased considerably. Under all experimental conditions, the 18-hydroxylation of deoxycorticosterone occurred with a concomitant and efficient 11beta-hydroxylation. Provided the source of cytochrome P-450 was the same, the ratio between 11beta- and 18hydroxylation was constant under all conditions and was not significantly different in the presence of metopirone, carbon monoxide, cytochrome c or different steroids. It is suggested that identical or at least very similar types of cytochrome P-450 are involved in 11beta- and 18-hydroxylation of deoxycorticosterone.     

The formation of binary and ternary complexes of cytochrome P-450scc with adrenodoxin and adrenodoxin reductase.adrenodoxin complex. The implication in ACTH function.

Binary and ternary complexes of bovine adrenocortical mitochondrial cytochrome P-450scc with adrenodoxin and adrenodoxin reductase.adrenodoxin complex are formed in the presence of cholesterol and Emulgen 913. Both cholesterol and Emulgen 913 are required for the binding of cytochrome P-450scc with adrenodoxin. Since phospholipids are able to replace Emulgen 913 in this reaction, in vivo phospholipids of the mitochondrial inner membrane appear to play the function of the detergent. The dissociation constants of the cytochrome.adrenodoxin complex are 0.3 to 0.4 microM at 130 microM dimyristoylphosphatidylcholine and 0.9 microM at 120 microM Emulgen 913, whereas the dissociation constant for the ternary complex of cytochrome P-450scc with adrenodoxin reductase and adrenodoxin is 4.0 microM at 150 microM Emulgen 913. The stoichiometry of binary and ternary complexes reveals the 1:1 and 1:1:1 molar ratios, respectively, judging from chemical analyses after the fractionation of the complexes by gel filtration. Emulgen 913, Tween 20, ethylene glycol, myristoyllysophosphatidylcholine, dimyristoylphosphatidylcholine, and phosphatidylethanolamine show the enhanced activity of cholesterol side chain cleavage reaction with cytochrome P-450scc, adrenodoxin, adrenodoxin reductase, and NADPH. These results, in conjunction with earlier experiments, lead us to the proposal on the structure of the hydroxylase complex in the membrane and to the hypothesis on the regulation of the enzymatic activity by the availability of substrate cholesterol to the cytochrome. Hence, we propose a mobile P-450scc hypothesis for the response of the mitochondrion to adrenocorticotropic hormone stimuli.     

NADPH-cytochrome P-450 reductase from rat liver: purification by affinity chromatography and characterization.

(NADPH)-cytochrome P-450 reductase was purified to apparent homogeneity by a procedure utilizing nicotinamide adenine dinucleotide phosphate (NADP)-Sepharose affinity column chromatography. The purified flavoprotein has a molecular weight of 79 700 and catalyzes cytochrome P-450 dependent drug metabolism, as well as reduction of exogenous electron acceptors. Aerobic titration of cytochrome P-450 reductase with NADPH indicates that an air-stable reduced form of the enzyme is generated by the addition of 0.5 mol of NADPH per mole of flavin, as judged by spectral characteristics. Further addition of NADPH causes no other changes in the absorbance spectrum. A Km value for NADPH of 5 micron was observed when either cytochrome P-450 or cytochrome c was employed as electron acceptor. A Km value of 8 +/- 2 micron was determined for cytochrome c and a Km of 0.09 +/- 0.01 micron was estimated for cytochrome P-450.     

Cross-linking studies of the cholesterol hydroxylation system from bovine adrenocortical mitochondria

Cytochrome P-450SCC and adrenodoxin were cross-linked with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The sample containing 94% of cross-linked complex and 6% of free cytochrome P-450SCC was obtained after purification on cholate-Sepharose. Cytochrome P-450SCC in cross-linked complex completely preserves its high-spin form in the presence of Tween 20 or pregnenolone. Utilization of radioactively labelled adrenodoxin, chemical cleavage of cytochrome P-450SCC from cross-linked complex with o-iodosobenzoic acid and HPLC for separation of peptides allow us to conclude that the complex of cytochrome P-450SCC with adrenodoxin was cross-linked through two amino acid sequences of cytochrome P-450SCC-Leu-88-Thr-107 and Leu-368-Gly-416. The cross-linked complex of adrenodoxin reductase, adrenodoxin and cytochrome P-450SCC with an apparent molecular mass of 114 kDa was obtained with N-succinimidyl-6-(4'-azido-2'-nitrophenylamino)hexanoate. The composition of cross-linked complex was determined by immunoblotting and by evaluation of radioactivity using preliminary N-ethyl[2,3-14C]maleimide-modified adrenodoxin. From this data it appears that the ternary complex may exist in solution.     

Ecdysterone biosynthesis: a microsomal cytochrome-P-450-linked ecdysone 20-monooxygenase from tissues of the African migratory locust.

Ecdysone 20-monooxygenase, an enzyme which converts ecdysone to ecdysterone (the major moulting hormone of insects) has been characterized in cell-free preparations of tissues from African migratory locust. The product of the reaction has been identified as ecdysterone on the basis of several microchemical derivatization and chromatographic methods. Ecdysone 20-monooxygenase activity is located primarily in the microsomal fraction which also carries NADPH cytochrome c reductase and cytochrome P-450, as shown by sucrose density gradient centrifugation. Optimal conditions for the ecdysone 20-monooxygenase assay have been determined. The enzyme has a Km for ecdysone of 2.7 x 10(-7) M and is competitvely inhibited by ecdysterone (Ki = 7.5 x 10(-7) M). Ecdysone 20-monooxygenase is a typical cytochrome P-450 linked monooxygenase: the reaction requires O2 and is inhibited by CO, an effect partially reversed by white light. The enzyme is effectively inhibited by several specific monooxygenase inhibitors and by sulfhydryl reagents, but not by cyanide ions. Ecdysone elicits a type I difference spectrum when added to oxidized microsomes. NADPH acts as preferential electron donor. The transfer of reducing equivalents proceeds through NADPH cytochrome c (P-450) reductase: ecdysone 20-monooxygenase is inhibited by cytochrome c. Both NADPH cytochrome c reductase and ecdysone 20-monooxygenase are inhibited by NADP+ and show a similar Km for NADPH. The Malpighian tubules have the highest specific activity of ecdysone 20-monooxygenase, while fat body contain most of the cytochrome P-450 and NADPH cytochrome c reductase.     

A shunted system of electron transport from NAD(P)H to cholesterol-hydroxylating cytochrome P-450 in adrenal cortex mitochondria

The two main approaches presently used for cytochrome P-450scc modelling are as follows: i) the use of chemical compounds carrying activated oxygen species, e. g., peracids, organic hydroperoxides, iodosobenzene, etc., ii) the use of electrochemical reduction in the presence of redox-active compounds. In the present work, a new model system for simulation of steroidogenic electron transfer is proposed, which reduces cytochrome P-450 scc by NADPH in the absence of adrenodoxin reductase and adrenodoxin. Phenazine methosulfate is used as an electron carrier. More than 95% of cytochrome P-450scc is reduced in a model system. The reduction kinetics is characterized by a lag phase, thus indicating complex formation between cytochrome P-450scc and phenazine methosulfate or formation of intermediate reducing equivalents. NADH may also serve as an electron donor for cytochrome P-450scc. Phenazine methosulfate can reduce microsomal cytochrome P-450 LM2 and b5, but not cytochrome P-450 LM4. Superoxide dismutase does not affect the reduction, thus indicating that O9.- is not involved in the reduction process. The mechanism of hemoprotein reduction and the nature of intermediates which can be formed in the model system is proposed.