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

Functional interactions of adrenodoxin with several human mitochondrial cytochrome P450 enzymes

Seven of the 57 human cytochrome P450 (P450) enzymes are mitochondrial and carry out important reactions with steroids and vitamins A and D. These seven P450s utilize an electron transport chain that includes NADPH, NADPH-adrenodoxin reductase (AdR), and adrenodoxin (Adx) instead of the diflavin NADPH-P450 reductase (POR) used by the other P450s in the endoplasmic reticulum. Although numerous studies have been published involving mitochondrial P450 systems, the experimental conditions vary considerably. We compared human Adx and bovine Adx, a commonly used component, and found very similar catalytic activities in reactions catalyzed by human P450s 11B2, 27A1, and 27C1. Binding constants of 6-200 nM were estimated for Adx binding to these P450s using microscale thermophoresis. All P450 catalytic reactions were saturated at 10 μM Adx, and higher concentrations were not inhibitory up to at least 50 μM. Collectively these studies demonstrate the tight binding of Adx (both human and bovine) to AdR and to several mitochondrial P450s and provide guidance for optimization of Adx-dependent P450 reactions.

Human mitochondrial cytochrome P450 27C1 is localized in skin and preferentially desaturates trans-retinol to 3,4-dehydroretinol

Recently, zebrafish and human cytochrome P450 (P450) 27C1 enzymes have been shown to be retinoid 3,4-desaturases. The enzyme is unusual among mammalian P450s in that the predominant oxidation is a desaturation and in that hydroxylation represents only a minor pathway. We show by proteomic analysis that P450 27C1 is localized to human skin, with two proteins of different sizes present, one being a cleavage product of the full-length form. P450 27C1 oxidized all-trans-retinol to 3,4-dehydroretinol, 4-hydroxy (OH) retinol, and 3-OH retinol in a 100:3:2 ratio. Neither 3-OH nor 4-OH retinol was an intermediate in desaturation. No kinetic burst was observed in the steady state; neither the rate of substrate binding nor product release was rate-limiting. Ferric P450 27C1 reduction by adrenodoxin was 3-fold faster in the presence of the substrate and was ∼5-fold faster than the overall turnover. Kinetic isotope effects of 1.5-2.3 (on k_cat/K_m ) were observed with 3,3-, 4,4-, and 3,3,4,4-deuterated retinol. Deuteration at C-4 produced a 4-fold increase in 3-hydroxylation due to metabolic switching, with no observable effect on 4-hydroxylation. Deuteration at C-3 produced a strong kinetic isotope effect for 3-hydroxylation but not 4-hydroxylation. Analysis of the products of deuterated retinol showed a lack of scrambling of a putative allylic radical at C-3 and C-4. We conclude that the most likely catalytic mechanism begins with abstraction of a hydrogen atom from C-4 (or possibly C-3) initiating the desaturation pathway, followed by a sequential abstraction of a hydrogen atom or proton-coupled electron transfer. Adrenodoxin reduction and hydrogen abstraction both contribute to rate limitation.

Partial fusion of a cytochrome P450 system by carboxy-terminal attachment of putidaredoxin reductase to P450cam (CYP101A1)

Cytochrome P450 (CYP) enzymes catalyze the insertion of oxygen into carbon-hydrogen bonds and have great potential for enzymatic synthesis. Application development of class I CYPs is hampered by their dependence on two redox partners (a ferredoxin and ferredoxin reductase), slowing catalysis compared to self-sufficient CYPs such as CYP102A1 (P450BM3). Previous attempts to address this have fused all three components in several permutations and geometries, with much reduced activity compared to the native system. We report here the new approach of fusing putidaredoxin reductase (PdR) to the carboxy-terminus of CYP101A1 (P450cam) via a linker peptide and reconstituting camphor hydroxylase activity with free putidaredoxin (Pdx). Initial purification of a P450cam-PdR fusion yielded 2.0% heme incorporation. Co-expression of E. coli ferrochelatase, lengthening the linker from 5 to 20 residues, and altering culture conditions for enzyme production furnished 85% heme content. Fusion co-expression with Pdx gave a functional system with comparable in vivo camphor oxidation activity as the native system. In vitro, the fused system's steady state NADH oxidation rate was two-fold faster than that of the native system. In contrast to the native system, NADH oxidation rates for the fusion enzyme showed non-hyperbolic dependence on Pdx concentration, suggesting a role for the PdR domain; these data were consistent with a kinetic model based on two-site binding of Pdx by P450cam-PdR and inactive dimer formation of the fusion. P450cam-PdR is the first example of a class I P450 fusion that exhibits significantly more favorable behavior than that of the native system.

Biotransformation of the mineralocorticoid receptor antagonists spironolactone and canrenone by human CYP11B1 and CYP11B2: Characterization of the products and their influence on mineralocorticoid receptor transactivation

Spironolactone and its major metabolite canrenone are potent mineralocorticoid receptor antagonists and are, therefore, applied as drugs for the treatment of primary aldosteronism and essential hypertension. We report that both compounds can be converted by the purified adrenocortical cytochromes P450 CYP11B1 and CYP11B2, while no conversion of the selective mineralocorticoid receptor antagonist eplerenone was observed. As their natural function, CYP11B1 and CYP11B2 carry out the final steps in the biosynthesis of gluco- and mineralocorticoids. Dissociation constants for the new exogenous substrates were determined by a spectroscopic binding assay and demonstrated to be comparable to those of the natural substrates, 11-deoxycortisol and 11-deoxycorticosterone. Metabolites were produced at preparative scale with a CYP11B2-dependent Escherichia coli whole-cell system and purified by HPLC. Using NMR spectroscopy, the metabolites of spironolactone were identified as 11β-OH-spironolactone, 18-OH-spironolactone and 19-OH-spironolactone. Canrenone was converted to 11β-OH-canrenone, 18-OH-canrenone as well as to the CYP11B2-specific product 11β,18-diOH-canrenone. Therefore, a contribution of CYP11B1 and CYP11B2 to the biotransformation of drugs should be taken into account and the metabolites should be tested for their potential toxic and pharmacological effects. A mineralocorticoid receptor transactivation assay in antagonist mode revealed 11β-OH-spironolactone as pharmaceutically active metabolite, whereas all other hydroxylation products negate the antagonist properties of spironolactone and canrenone. Thus, human CYP11B1 and CYP11B2 turned out to metabolize steroid-based drugs additionally to the liver-dependent biotransformation of drugs. Compared with the action of the parental drug, changed properties of the metabolites at the target site have been observed.

A recombinant CYP11B1 dependent Escherichia coli biocatalyst for selective cortisol production and optimization towards a preparative scale

BACKGROUND

Human mitochondrial CYP11B1 catalyzes a one-step regio- and stereoselective 11β-hydroxylation of 11-deoxycortisol yielding cortisol which constitutes not only the major human stress hormone but also represents a commercially relevant therapeutic drug due to its anti-inflammatory and immunosuppressive properties. Moreover, it is an important intermediate in the industrial production of synthetic pharmaceutical glucocorticoids. CYP11B1 thus offers a great potential for biotechnological application in large-scale synthesis of cortisol. Because of its nature as external monooxygenase, CYP11B1-dependent steroid hydroxylation requires reducing equivalents which are provided from NADPH via a redox chain, consisting of adrenodoxin reductase (AdR) and adrenodoxin (Adx).

RESULTS

We established an Escherichia coli based whole-cell system for selective cortisol production from 11-deoxycortisol by recombinant co-expression of the demanded 3 proteins. For the subsequent optimization of the whole-cell activity 3 different approaches were pursued: Firstly, CYP11B1 expression was enhanced 3.3-fold to 257 nmol∗L(-1) by site-directed mutagenesis of position 23 from glycine to arginine, which was accompanied by a 2.6-fold increase in cortisol yield. Secondly, the electron transfer chain was engineered in a quantitative manner by introducing additional copies of the Adx cDNA in order to enhance Adx expression on transcriptional level. In the presence of 2 and 3 copies the initial linear conversion rate was greatly accelerated and the final product concentration was improved 1.4-fold. Thirdly, we developed a screening system for directed evolution of CYP11B1 towards higher hydroxylation activity. A culture down-scale to microtiter plates was performed and a robot-assisted, fluorescence-based conversion assay was applied for the selection of more efficient mutants from a random library.

CONCLUSIONS

Under optimized conditions a maximum productivity of 0.84 g cortisol∗L(-1)∗d(-1) was achieved, which clearly shows the potential of the developed system for application in the pharmaceutical industry.

Cytochrome P450 systems—biological variations of electron transport chains

Cytochromes P450 (P450) are hemoproteins encoded by a superfamily of genes nearly ubiquitously distributed in different organisms from all biological kingdoms. The reactions carried out by P450s are extremely diverse and contribute to the biotransformation of drugs, the bioconversion of xenobiotics, the bioactivation of chemical carcinogens, the biosynthesis of physiologically important compounds such as steroids, fatty acids, eicosanoids, fat-soluble vitamins and bile acids, the conversion of alkanes, terpenes and aromatic compounds as well as the degradation of herbicides and insecticides. Cytochromes P450 belong to the group of external monooxygenases and thus receive the necessary electrons for oxygen cleavage and substrate hydroxylation from different redox partners. The classical as well as the recently discovered P450 redox systems are compiled in this paper and classified according to their composition.

Adrenodoxin: structure, stability, and electron transfer properties

Adrenodoxin is an iron-sulfur protein that belongs to the broad family of the [2Fe-2S]-type ferredoxins found in plants, animals and bacteria. Its primary function as a soluble electron carrier between the NADPH-dependent adrenodoxin reductase and several cytochromes P450 makes it an irreplaceable component of the steroid hormones biosynthesis in the adrenal mitochondria of vertebrates. This review intends to summarize current knowledge about structure, function, and biochemical behavior of this electron transferring protein. We discuss the recently solved first crystal structure of the vertebrate-type ferredoxin, the truncated adrenodoxin Adx(4-108), that offers the unique opportunity for better understanding of the structure-function relationships and stabilization of this protein, as well as of the molecular architecture of [2Fe-2S] ferredoxins in general. The aim of this review is also to discuss molecular requirements for the formation of the electron transfer complex. Essential comparison between bacterial putidaredoxin and mammalian adrenodoxin will be provided. These proteins have similar tertiary structure, but show remarkable specificity for interactions only with their own cognate cytochrome P450. The discussion will be largely centered on the protein-protein recognition and kinetics of adrenodoxin dependent reactions.

The role of threonine 54 in adrenodoxin for the properties of its iron-sulfur cluster and its electron transfer function

The amino acid in position 54 of adrenodoxin is strongly conserved among ferredoxins, consisting of a threonine or serine. Its role was studied by analyzing mutants T54S and T54A of bovine adrenodoxin. Absorption, circular dichroism, fluorescence, and electron paramagnetic resonance spectra of mutant T54S show that this substitution has no influence on the formation and stability of the ferredoxin. The redox potential of this mutant, however, was lowered by 55 mV as compared with native adrenodoxin, indicating a role for this residue in redox potential modulation. Incorporation of the iron-sulfur cluster was not impaired in the T54A mutant, although structural features of the oxidized protein were considerably changed. The decreased stability of the T54A mutant as compared with the wild type and mutant T54S indicates that a hydrogen bond donor at this position stabilizes the protein. Both mutants have been shown to be functionally active. Replacement of threonine 54 by serine or alanine, however, leads to rearrangements at the recognition sites for its redox partners. This is reflected by decreased Km and Kd values of both mutants for the cytochromes P450, whereas only T54A displayed a decreased Km value in cytochrome c reduction. Substrate conversion was accelerated (2.2- and 2.4-fold for mutants T54A and T54S, respectively) in the CYP11B1-, but not in the CYP11A1-dependent reaction.

The role of endozepine in the regulation of steroid synthesis

Endozepine has recently been isolated from various steroid-forming organs. The following article explores the role of endozepine in the regulation of steroid synthesis. Steroid hormone synthesis from cholesterol begins in the inner mitochondrial membrane, where cytochrome P450 converts cholesterol to pregnenolone. Scientists thought that ACTH would stimulate this conversion, but experiments showed no such stimulation. However, addition of aminoglutethimide to block side-chain cleavage caused the expected reaction of ACTH to take place. Next the role of protein synthesis on the actions of ACTH was explored. Then endozepine was isolated from bovine fasciculata based on stimulation of pregnenolone production by freshly prepared mitochondria. After further experimentation it was concluded that endozepine is a peptide with at least two groups of actions: It binds GABAA receptors in the central nervous system, and it increases the mitochondrial synthesis of pregnenolone.

Absorbance spectral change of the cytochrome P-450S21-phenylisocyanide complex upon binding of reduced NADPH-cytochrome-P-450 reductase

Reduction of cytochrome P-450S21 (SF) (SF, substrate-free; purified from bovine adrenocortical microsomes) with sodium dithionite (Na2S2O4) in the presence of phenylisocyanide produced a ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex with Soret absorbance maxima at 429 and 456 nm. On the other hand, when a preformed ferric cytochrome P-450S21 (SF)-NADPH-cytochrome-P-450 reductase (Fp2) complex was reduced chemically or enzymatically under the same conditions, the absorbance spectrum of the ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex changed drastically, as characterized by an increase in absorbance intensity at 429 nm and a decrease at 456 nm. Similar spectral changes were observed by addition of reduced Fp2 to the preformed ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex. Experiments to reduce a ferric cytochrome P-450S21 (SF)-phenylisocyanide complex with sodium dithionite in the presence of various amounts of Fp2 showed that; (1), the spectral change reached maxima for both absorption increase at 429 nm and decrease at 456 nm when cytochrome P-450S21 and Fp2 were previously mixed at the cytochrome P-450S21:Fp2 ratio of 1:5; (2), the spectral change was suppressed in 300 mM potassium phosphate buffer (pH 7.4). These results suggest that the absorbance spectral change is due to a conformational change around the heme moiety induced by association with reduced Fp2.

Distinctive properties of adrenal cortex mitochondria

The mitochondria in cells that synthesize steroid hormones not only have enzymes not present in mitochondria of non-steroidogenic cells but also have unique mechanisms for regulating the steroid substrate availability for certain of these enzymes. We have considered in detail the cytochrome P-450scc system that is located in the inner mitochondrial membrane and that catalyzes the initial and rate-determining step in the steroid hormone biosynthetic pathway. The flux through this pathway is regulated both by the levels of these catalysts themselves and by the availability of the substrate cholesterol for conversion to pregnenolone. These two levels of regulation occur in different time frames but are both controlled externally by the action of tissue-specific peptide hormone. We have used the adrenal cortex fasciculata cells as our paradigmatic cell type. The overall picture seems closely similar for mitochondria in other such steroidogenic cells when analogous data are available. Thus, in adrenal cortex fasciculata cells ACTH triggers several long-term (trophic) and short-term (acute) effects upon and within mitochondria that influence the initial and rate-determining step in the steroid hormone biosynthetic pathway. The only second messenger for both effects characterized thus far is cAMP. An increase in membrane-associated cAMP rapidly activates cAMP-dependent protein kinase, which in turn phosphorylates several cellular proteins, e.g., cholesterol ester hydrolase (vide supra). The trophic action, i.e., that produced by exposure of the cells to increased levels of ACTH or cAMP for a prolonged period (minutes to hours), increases the amounts of the steroid hormone synthesizing proteins in the mitochondria by increasing the transcription of the relevant nuclear genes. This latter process is not needed for the acute increase in the rate of steroid hormone biosynthesis. Whether induction of steroidogenic enzymes requires activation of a kinase has not been determined. However, the postulated SHIP proteins provide a mechanism by which cAMP levels and protein synthesis itself may regulate this induction. Mitochondria in steroidogenic tissues exert control over this process by their ability to recognize, import and process correctly the nuclear encoded precursors of the steroidogenic enzymes. Whether control at this level is ultimately dictated by nuclear or mitochondrial gene products or by an interplay between them is still unknown.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

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.

The oxidation of reduced putidaredoxin reductase by oxidized putidaredoxin

NAD+ was shown to be essential for the kinetically favored oxidation of putidaredoxin reductase from the two electron reduced form to the semiquinone form by oxidized putidaredoxin. The rate of oxidation of putidaredoxin reductase from the semiquinone to the oxidized form was not commensurate with the overall rate of catalysis seen in the cytochrome c reductase assay. The presence of excess NADH was shown to result in a substantial enhancement of the rate of the catalytic cycle. Thus, pyridine nucleotides play an important role in the reoxidation of putidaredoxin reductase by putidaredoxin. The initial velocity patterns for the addition of varying concentrations of substrates indicate a ping-pong mechanism for substrate addition while product inhibition patterns indicate otherwise. A model of putidaredoxin reductase catalysis is described which accounts for the product inhibition patterns and the observed role of pyridine nucleotides in the reaction cycle. The two site mechanism described in this manuscript has both sequential and ping-pong features which are accounted for by the two addition steps of oxidized putidaredoxin during catalytic turnover.

Stoichiometry of mitochondrial cytochromes P-450, adrenodoxin and adrenodoxin reductase in adrenal cortex and corpus luteum. Implications for membrane organization and gene regulation

We have estimated the concentrations of cytochromes P-450scc and P-45011 beta and the electron-transfer proteins adrenodoxin reductase and adrenodoxin in the adrenal cortex and corpus luteum using specific antibodies against these enzymes. While in the adrenal cortex the concentrations of these enzymes are relatively constant in different animals and show no significant sex differences, in corpora lutea they vary considerably and can increase at least up to fifty-fold over the levels found in the ovary. The average relative concentrations of adrenodoxin reductase, adrenodoxin and P-450 are 1:3:8 in the adrenal cortex (which has two cytochromes P-450, P-450scc and P-450(11) beta, in equal concentrations) and 1:2.5:3 in the corpus luteum (which has only P-450scc). We further present evidence that the levels of cytochrome c oxidase also show a degree of correlation with the levels of the mitochondrial steroidogenic enzymes.

Cross-linking studies of adrenocortical cytochrome P-450scc. Evidence for a covalent complex with adrenodoxin and localization of the adrenodoxin-binding domain

A cleavable cross-linking reagent, dimethyl-3,3'-dithiobispropionimidate, was used to study the molecular organization of adrenocortical cytochrome P-450scc. Extensive cross-linking was found to occur, resulting in the formation of heterologous oligomers up to octamer. The covalently cross-linked complex of adrenocortical cytochrome P-450scc with adrenodoxin has been obtained by using dimethyl-3,3'-dithiobispropionimidate. In the presence of NADPH and adrenodoxin reductase, electron transfer to cytochrome P-450scc occurs in the complex, and, in the presence of cholesterol, the latter effectively oxidizes to pregnenolone. By using covalently immobilized adrenodoxin and heterobifunctional reagent, N-succinimidyl-3-(2-pyridyldithio)propionate, the adrenodoxin-binding site was shown to be located in the heme-containing, catalytic domain of cytochrome P-450scc. The data obtained indicate the existence of two different sites on the adrenodoxin molecule that are responsible for the interaction with adrenodoxin reductase and cytochrome P-450scc. This is consistent with the model mechanism of electron transfer in the organized complex.

Modification of a lysine residue of adrenodoxin reductase, essential for complex formation with adrenodoxin

The NADPH-cytochrome c reductase activity of NADPH-adrenodoxin reductase from NADPH to cytochrome c via adrenodoxin was inhibited by pyridoxal 5'-phosphate and other reagents that modified the lysine residues. However, the NADPH-ferricyanide reductase activity was not affected. Loss of the cytochrome c reductase activity could be prevented by adrenodoxin, but not by NADP+. One lysine residue of the adrenodoxin reductase could be protected from the modification with pyridoxal 5'-phosphate by complex formation with adrenodoxin. Loss of the NADPH-cytochrome c reductase activity was not due to the conformational change of the modified adrenodoxin reductase, judging from circular dichroism spectrometric studies.

Properties of ferredoxin reductase and ferredoxin from the bovine corpus luteum

Ferredoxin reductase and ferredoxin were purified from the bovine corpus luteum and their properties compared to the corresponding adrenal proteins. The luteal and adrenal proteins had similar absorbance spectra and molecular weights. Evidence was obtained from spectrophotometric titrations for formation of 1:1 complexes between luteal ferredoxin reductase and ferredoxin and between ferredoxin and cytochrome P-450scc. Adrenal ferredoxin reductase and ferredoxin were equally as effective as luteal ferredoxin reductase and ferredoxin in supporting cholesterol side-chain cleavage by luteal cytochrome P-450scc.

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.

Magnetic and natural circular dichroism spectra of cytochgromes P-450(11) beta and P-450scc purified from bovine adrenal cortex

Magnetic (MCD) and natural circular dichroism (CD) spectra various complexes of cytochrome P-450(11) beta (P-450(11) beta) and cytochrome P-450scc (P-450scc) from bovine adrenal cortex were measured from 250 nm to 700 nm. MCD and CD spectral contours of cytochromes P-450(11) beta and P-450scc in the Soret and visible regions were, as a whole, analogous to those of cytochromes P-450 from rabbit liver microsomes and also from Pseudomonas putida in their high-spin ferric, high-spin ferrous and ferrous-CO complexes. MCD spectrum of the low-spin ferric P-450scc free from the substrate, cholesterol, was very similar to that caused by addition of 20 alpha-hydroxycholesterol, a reaction intermediate. However, it was distinct from those of the low-spin ferric P-450(11) beta and P-450scc complexes caused by addition of external nitrogenous ligands. The electronic states of the heme in the low-spin ferric P-450 free from substrates seemed to be subtly different from those of low-spin complexes coordinated with external nitrogenous ligands. Soret CD spectra of ferric low-spin complexes were not so different from each other. Upon reduction of high-spin ferric P-450(11) beta or P-450scc, the Soret CD magnitudes increased significantly in contrast with those of other P-450s, the Soret CD magnitudes of which decrease upon reduction. This may reflect an increased proximity of the neighbouring aromatic groups upon reduction of high-spin P-450(11) beta or P-450scc. High substrate specificity of adrenal P-450s compared with liver P-450s can be explained in view of the above findings. THe CD spectra in the near ultraviolet region (250-350 nm) were found to be quite sensitive to the spin change for ferric P-450scc, while the MCD spectra in this region did not reflect substantially the spin state of the enzyme. MCD parameters of cytochrome P-450s were compared to those of other hemoproteins in diagrams describing selected MCD spectral values of hemoproteins so far available and were discussed in connection with the structures of the heme environment of P-450.

ACTH stimulation on cholesterol side chain cleavage activity of adrenocortical mitochondria. Transfer of the stimulus from plasma membrane to mitochondria

Adrenocortical mitochondrial cholesterol side chain cleavage reactions are regulated by the influence of pituitary ACTH. The mechanism of the stimulation involves adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase, and ribosomal labile protein synthesis. Through these reactions the stimulus reaches the mitochondrial side chain cleavage enzyme system. In this review article, the current implications on the stimulus transfer from the plasma membrane to the mitochondrial inner membrane are summarized. In particular the availability of cholesterol to P-450scc was discussed in terms of the distribution of cholesterol molecules in the membranes.