Properties of an adrenal cytochrome P-450 (P-450SCC) for the side chain cleavage of cholesterol

Inhibition and stimulation of activity of purified recombinant CYP11A1 by therapeutic agents

In vertebrates, the biosynthesis of steroid hormones is initiated by cytochrome P450 CYP11A1 which converts cholesterol to pregnenolone. We investigated whether some of the experimental and FDA-approved therapeutic agents alter the activity of CYP11A1 in the reconstituted system in vitro. We found that under the experimental conditions used and when phospholipids are included, ketoconazole, posaconazole, carbenoxolone, and selegiline inhibit CYP11A1-mediated production of pregnenolone by at least 67%. Conversely, pemirolast, clobenpropit, desogestrel, dexmedetomidine, and tizanidine stimulate the enzyme activity by up to 70%. We then evaluated the identified inhibitors and activators for spectral binding to CYP11A1 and their effect on enzyme activity in the absence of phospholipids. The data obtained provide insight into how different drugs interact with CYP11A1 and demonstrate that P450 association with the lipid bilayer determines, in many cases, a drug's effect on enzyme activity.

Early years of oxygenase research in Bethesda, Osaka, Urbana, and Kanazawa

In this brief review, I recollect my experiences of how the studies of pyrocatechase and salicylate hydroxylase led to the isolation and revelation of P450cam. Those experiences were instrumental in the separation, purification, and characterization of the two forms of adrenal cortex mitochondrial P450.

Stripping down the mitochondrial cholesterol hydroxylase system, a kinetics study

The origin of steroid hormones in mammals is cholesterol that is metabolized by the mitochondrial CYP11A1 system. The cytochrome P450 is fed with reduction equivalents via a small electron transfer chain consisting of NADPH, adrenodoxin reductase, and adrenodoxin. Though the redox behavior of the individual protein components has been studied previously, the kinetics of the system in its entirety has not yet been analyzed. In this study we combine surface plasmon resonance experiments to determine the binding constants for the different pairs of redox partners with measurements of the pre-steady-state kinetics of the different reaction steps of this system and steady-state kinetics. We could correlate the individual protein-protein interactions with the effect of distinct reduction-oxidation steps on the overall catalytic activity of the CYP11A1 system. For the first time, we were able to follow the reduction of each of the protein components of this system within one measurement when we mixed all oxidized protein components with NADPH. These measurements allowed the determination of the individual apparent rate constants for the reduction of all three proteins involved. In addition, variation of the ionic strength in these experiments revealed different optimum salt concentrations for the reduction of adrenodoxin reductase and adrenodoxin, respectively, and unraveled dramatically changing reduction rates of CYP11A1 by adrenodoxin.

The interaction of bovine adrenodoxin with CYP11A1 (cytochrome P450scc) and CYP11B1 (cytochrome P45011beta ). Acceleration of reduction and substrate conversion by site-directed mutagenesis of adrenodoxin

The kinetics of protein-protein interaction and heme reduction between adrenodoxin wild type as well as eight mutants and the cytochromes P450 CYP11A1 and CYP11B1 was studied in detail. Rate constants for the formation of the reduced CYP11A1.CO and CYP11B1.CO complexes by wild type adrenodoxin, the adrenodoxin mutants Adx-(4-108), Adx-(4-114), T54S, T54A, and S112W, and the double mutants Y82F/S112W, Y82L/S112W, and Y82S/S112W (the last four mutants are Delta113-128) are presented. The rate constants observed differ by a factor of up to 10 among the respective adrenodoxin mutants for CYP11A1 but not for CYP11B1. According to their apparent rate constants for CYP11A1, the adrenodoxin mutants can be grouped into a slow (wild type, T54A, and T54S) and a fast group (all the other mutants). The adrenodoxin mutants forming the most stable complexes with CYP11A1 show the fastest rates of reduction and the highest rate constants for cholesterol to pregnenolone conversion. This strong correlation suggests that C-terminal truncation of adrenodoxin in combination with the introduction of a C-terminal tryptophan residue enables a modified protein-protein interaction rendering the system almost as effective as the bacterial putidaredoxin/CYP101 system. Such a variation of the adrenodoxin structure resulted in a mutant protein (S112W) showing a 100-fold increased efficiency in conversion of cholesterol to pregnenolone.

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.

Branched phosphatidylcholines stimulate activity of cytochrome P450SCC (CYP11A1) in phospholipid vesicles by enhancing cholesterol binding, membrane incorporation, and protein exchange

Phosphatidylcholines (PCs) with branched fatty acyl chains substituted in the two positions of the main chains (branched PCs) have been shown to be potent activators of the side chain cleavage activity of cytochrome P450SCC (CYP11A1) (Schwarz, D., Kisselev, P., Wessel, R., Jueptner, O., and Schmid, R. D. (1996) J. Biol. Chem. 271, 12840-12846). The present study reports on the effect of a series of branched PC on cholesterol binding, membrane integration, and protein exchange in large unilamellar vesicles prepared by an extrusion technique. Enzyme kinetics using vesicles as well as optical titration using a micelle system with the detergent Tween 20 demonstrate that activation is correlated with the fraction of P450SCC in the high spin form. The potency of branched PCs both to activate the enzyme and to induce spin state changes increases with increasing lengths of both the branched and main fatty acyl chains. We found that the extent as well as the rate of integration of P450SCC into vesicle membranes studied by gel chromatography and stopped flow kinetics were increased by branched PC. Finally, it is demonstrated by measurement of the enzymatic activity in primary and secondary vesicles that branched PCs are potent in retaining a very rapid exchange of P450SCC between vesicles, in contrast to cardiolipin, that partially inhibits this exchange process. The data suggest that different properties of P450SCC in membrane systems including cholesterol binding, membrane integration, and protein exchange are affected by branched PCs and probably by other phospholipids, too, and therefore must be considered in an explanation of the observed high stimulation of activity.

Localization of multiple forms of inducible cytochromes P450 in rat liver mitochondria: immunological characteristics and patterns of xenobiotic substrate metabolism

Hepatic mitochondria contain inducible cytochromes P450 that cross-react with antibodies to P4501A1/2 and 2B1/2. In the present study, we present evidence for the occurrence of additional P450 forms in rat liver mitochondria that cross-react with antibodies to microsomal P4503A1/2 and 2E1. Protease protection and also immunoelectron microscopy studies were carried out to support the mitochondrial location of the immunoreactive P450s. The solubility of immunoreactive proteins in 0.1 M Na2CO3 suggests that the mitochondrial P450 forms tested are not membrane-integral proteins. The mitochondrial-associated P450 forms are capable of metabolizing resorufin derivatives, erythromycin, and p-nitrophenol in an adrenodoxin- and adrenodoxin reductase-supported system. Treatment of rats with phenobarbital (PB) resulted in the induction of mitochondrial pentoxyresorufin O-deethylase (PROD), benzoxyresorufin O-deethylase (BROD), and erythromycin N-demethylase (ERND) activities by 17-, 23-, and 2-fold, respectively. These activities were inhibited by 33 to 64% by antibodies to P4502B1/2 and P4503A1/2. The induction of the above monooxygenase activities correlated with the levels of mitochondrial proteins cross-reacting with antibodies to P4502B1/2 and P4503A1/2 in PB-treated livers. Similarly, administration of beta-naphthoflavone (BNF) resulted in a marked elevation of O-deethylation of ethoxy-, benzoxy-, and methoxyresorufins and a 2-fold increase in ERND activity. Immunoblot and immunoinhibition experiments using P4501A1/2, P4502B1/2, P4503A1/2, and P4502E1 antibodies revealed the presence of P450 forms closely related to the microsomal inducible forms. Results of immunoinhibition studies, using antibodies to adrenodoxin and reconstitution of enzyme activity with purified P450 forms, suggested a role for the mitochondrial P450 in the metabolism of xenobiotic substrates. The purified mitochondrial P450s also exhibited overlapping substrate specificities for resorufin derivatives and erythromycin.

Incorporation of bovine adrenal 3 beta-hydroxy-5-ene steroid dehydrogenase/delta 5-delta 4 isomerase into phospholipid vesicles

3 beta-Hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase (3 beta-HSD/I) and cytochrome P-450C21 were co-purified from bovine adrenocortical microsomes by an improved method. The 3 beta-HSD/I was successfully incorporated into liposomal membranes in which the enzyme activity was greatly stabilized. Enzymatic activities and kinetic parameters of the 3 beta-HSD/I proteoliposomes were almost the same as those of the solubilized form.

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.

Two different cytochrome P450 enzymes are the adrenal antigens in autoimmune polyendocrine syndrome type I and Addison's disease

Autoimmune polyendocrine syndrome type I (APS I) and idiopathic Addison's disease are both disorders with adrenal insufficiency but with differences in genetic background, clinical presentation, and extent of extraadrenal manifestations. In this study the major adrenal autoantigen identified with sera from patients with APS I was characterized by analyses using indirect immunofluorescence, Western blots of adrenal subcellular fractions and of recombinant proteins, immunoprecipitations of [35S]methionine-labeled lysates of a human steroid-producing cell line, and studies of enzymatic activity. Sera from patients with APS I, identifying cells in adrenal glands and testes involved in steroid synthesis, reacted in Western blots with a 53-kD antigen, which comigrated with the cytochrome P450 cholesterol side chain cleavage enzyme (SCC). The sera also immunoprecipitated this protein from lysates of radiolabeled adrenal cells. The enzymatic activity of SCC was inhibited by the APS I sera but not by control sera. Sera from patients with idiopathic Addison's disease did not react with the SCC. The results show that the autoimmune responses towards adrenal tissue in patients suffering from APS I and Addison's disease are remarkably selective and suggest that a determination of the antigen involved in a patient with autoimmune adrenal insufficiency will have diagnostic as well as prognostic implications.

Substrate-binding region of cytochrome P-450SCC (P-450 XIA1). Identification and primary structure of the cholesterol binding region in cytochrome P-450SCC

Cytochrome P-450SCC (P-450 XIA1) from bovine adrenocortical mitochondria was investigated using a suicide substrate: [14C]methoxychlor. [14C]Methoxychlor irreversibly abolished the activity of the side-chain cleavage enzyme for cholesterol (P-450SCC) and the inactivation was prevented in the presence of cholesterol. The binding of [14C]methoxychlor and cytochrome P-450SCC occurred in a molar ratio of 1:1 and the cholesterol-induced difference spectrum of cytochrome P-450SCC was similar with the methoxychlor-induced difference spectrum. [14C]Methoxychlor-binding peptides were purified from tryptic-digested cytochrome P-450SCC modified with [14C]methoxychlor. Determination of the sequence of the amino-acid residues of a [14C]methoxychlor-binding peptide allowed identification of the peptide comprising the amino-terminal amino-acid residues 8 to 28.

Direct expression of mature bovine adrenodoxin in Escherichia coli

Site-directed mutagenesis was utilized to enable direct expression of the mature form of bovine adrenodoxin cDNA using the pKK223-3 expression vector in Escherichia coli. Expression was under control of the "tac" promoter and resulted in a direct expression of soluble mature bovine adrenodoxin (greater than 15 mg per liter). Chromatographic behavior of recombinant adrenodoxin did not differ from that reported for mature native adrenodoxin. The purified recombinant protein was identical to native mitochondrial adrenodoxin on the basis of molecular weight, NH2 terminal sequencing and immunoreactivity. E. coli lysates were brown in color, and the purified protein possessed a visible absorbance spectra identical to native bovine adrenodoxin consistent with incorporation of a [2Fe-2S] cluster in vivo. Recombinant bovine adrenodoxin was active in cholesterol side-chain cleavage when reconstituted with adrenodoxin reductase and cytochrome P450scc and exhibited kinetics reported for native bovine adrenodoxin. The presence of the adrenodoxin amino terminal presequence does not appear to be essential for correct folding of mature recombinant adrenodoxin in E. coli. This expression system should prove useful for overexpression of adrenodoxin mutants in future structure/function studies. The approach described herein can potentially be used to directly express the mature form of any protein in bacteria.

Expression of human ferredoxin in Saccharomyces cerevisiae: mitochondrial import of the protein and assembly of the [2Fe-2S] center

Vertebrate ferredoxins function in the transfer of reducing equivalents from NADPH:ferredoxin oxidoreductase to cytochrome P450 enzymes involved in steroid metabolism. We report here the expression of human mitochondrial ferredoxin in the yeast Saccharomyces cerevisiae. The full-length ferredoxin protein containing the ferredoxin mitochondrial leader sequence could not be stably expressed in S. cerevisiae, but a fusion protein consisting of the mature portion of ferredoxin linked to the mitochondrial leader sequence of the S. cerevisiae cytochrome c oxidase subunit Va protein (COX5a) could be stably expressed. The COX5a:ferredoxin fusion protein was targeted to the mitochondria as a preprotein and was cleaved at the normal processing site of the COX5a presequence during import into the matrix. Absorption spectra and electron transfer activity of the isolated fusion protein established that the [2Fe-2S] center was correctly assembled and incorporated into the recombinant ferredoxin in this heterologous system.

Steroid 6 beta-hydroxylase and 6-desaturase reactions catalyzed by adrenocortical mitochondrial P-450

The minor steroid hydroxylase activity of purified bovine adrenocortical mitochondrial P-450 is described. The results indicate that both P-450scc and P-450(11 beta) act on deoxycorticosterone and androstenedione to form 6 beta-hydroxydeoxycorticosterone and 6 beta-hydroxyandrostenedione (6 beta-hydroxylase), respectively. Both forms of P-450 also catalyze 6-desaturation of androstenedione to form 4,6-androstadiene-3,17-dione (6-desaturase).

Expression of functional bovine cholesterol side chain cleavage cytochrome P450 (P450scc) in Escherichia coli

Escherichia coli expression vectors containing the trc promoter and the complete DNA sequence of either the precursor or the mature form of bovine adrenocortical cholesterol side chain cleavage cytochrome P450 (P450scc) were transformed into E. coli strain JM109 and transcription induced with isopropyl-beta-D-thiogalactopyranoside (IPTG). Immunoreactive cytochrome P450scc was produced using the plasmid containing the mature P450scc sequence but not with the plasmid containing the sequence of the precursor form of P450scc, even though P450scc RNA was detectable in both cases. The mature form of P450scc was detected spectrophotometrically in a reduced CO-difference spectrum in E. coli (40-60 nmol/liter culture). Cholesterol and hydroxylated derivatives (22-hydroxycholesterol and 25-hydroxycholesterol) produce a type 1 substrate-binding spectrum in IPTG-induced, transformed E. coli. The P450scc was found to be associated with the E. coli membrane fraction and the enzymatic activity of side chain cleavage of 25-hydroxycholesterol was reconstituted using solubilized membranes, in the presence of purified bovine adrenocortical adrenodoxin and NADPH-adrenodoxin reductase (turnover number; 15.4 nmol/min/nmol P450). This bacterial expression system provides functional P450scc, in the absence of other forms of P450, which can be used for evaluation of enzymatic and spectral properties of this mitochondrial P450 by site-directed mutagenesis.

Catalytic properties of cytochrome P-450scc from bovine and porcine adrenocortical mitochondria: effect of Tween20 concentration

Cholesterol side-chain cleavage activities of cytochrome P-450ssc purified from bovine adrenocortical mitochondria were measured for various substrates, including cholesterol, 20[S]-hydroxycholesterol, 22[R]-hydroxycholesterol and 20[R], 22[R]-dihydroxycholesterol, in the reconstituted enzyme system at various Tween20 concentrations. The side-chain cleavage activity for cholesterol showed more than 10-fold enhancement upon addition of 0.1% Tween20, compared with that without the detergent. Addition of Tween20 did not cause any enhancement of the side-chain cleavage activities for 20[S]-hydroxycholesterol and 22[R]-hydroxycholesterol; rather, it resulted in an inhibition of the activities. The side-chain cleavage activity for 20[R],22[R]-dihydroxycholesterol showed a very high value even without the detergent. As the stimulatory effect of Tween20 was only specific for cholesterol, Tween20 seemed to enhance the rate of access of cholesterol to cytochrome P-450scc. These results are consistent with the suggestion that a transfer of substrate, cholesterol, in mitochondrial inner membrane, to the substrate-binding site of cytochrome P-450scc is the rate-limiting step in the cholesterol side-chain cleavage reaction.

Purification and properties of cytochrome P-450 (SCC) from pig testis mitochondria

Cytochrome P-450 was purified from pig testis mitochondria to a specific content of 13.1 n mol/mg of protein. The purified preparation was found to contain a single species of P-450, on sodium dodecyl sulfate polyacrylamide gel electrophoresis, with an apparent molecular weight of about 53000 +/- 2000. The cholesterol side chain-cleavage system could be reconstituted by mixing the purified cytochrome P-450, adrenodoxin reductase, adrenodoxin, cholesterol and NADPH. The rate of conversion of cholesterol to pregnenolone was 6.2 n mol/min/n mol of P-450 under the conditions employed. The absorption spectrum of the oxidized cytochrome P-450 had maxima at 416, 530 and 568 nm. The reduced CO-complex of the cytochrome P-450 exhibited an absorption maximum at 448 nm. The purified P-450 was subjected to microsequence analysis and its NH2-terminal amino acid sequence was found to show considerable homology with that of bovine adrenal P-450 (SCC).

Cytochrome P-45011 beta in rat brain

The presence of cytochrome P-45011 beta in rat brain was studied by immunohistochemistry using polyclonal rabbit antibodies raised against purified bovine adrenocortical P-45011 beta, which is involved in the steroid 11 beta-hydroxylation and glucocorticoid formation. The results showed that cytochrome P-45011 beta immunoreactivity is selectively localized to the tracts of myelinated fibers throughout the brain. The specificity of immunohistochemical stainings with P-45011 beta antibodies was established by control tests including nonimmune rabbit immunoglobulin Gs and P-45011 beta antibodies absorbed with purified antigen. Western immunoblots of homogenates from different brain areas with P-45011 beta antibodies, together with biochemical enzymatic assays for cytochrome P-45011 beta monooxygenase activity in these homogenates, confirmed the selective localization of this enzyme observed with immunohistochemistry. Cytochrome P-45011 beta and 11 beta-hydroxylase activity were detected in a homogenate from the cortical white matter (brain area rich in myelinated fibers) as in that from the rat adrenal, but were not detectable in a homogenate from the cerebral cortex (brain area poor in myelinated fibers). Furthermore, quantitation of the P-45011 beta bands on the immunoblots by the areal density revealed that the cortical white matter contains approximately 1.4 pmol of cytochrome P-45011 beta/mg of tissue protein, the value of which was about one sixth of the corresponding value estimated in the rat adrenal. This relatively high content of cytochrome P-45011 beta was also reflected in a relatively high level of 11 beta-hydroxylase activity measured in a homogenate of this brain area by biochemical enzymatic assays using [4-14C]-11-deoxycorticosterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and comparative characterization of cytochrome P-450scc from porcine adrenocortical mitochondria

Cytochrome P-450scc (cholesterol side-chain cleavage enzyme) was purified from porcine adrenocortical mitochondria. 2. The purified cytochrome P-450scc was found to be homogeneous on SDS-polyacrylamide gel electrophoresis. 3. The heme content of the purified enzyme was 20.6 nmol/mg protein. 4. The enzymatic activity of the reconstituted cytochrome P-450scc-linked monooxygenase system amounted to 7.8 nmol of pregnenolone formed per nmole of P-450 per minute, with cholesterol as a substrate. 5. The amino acid sequence of the amino-terminal region of the cytochrome P-450scc and the amino acid residue at the carboxyl terminal were determined and compared with those of other mammalian cytochromes P-450scc.

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.

Studies of the immunohistochemical and biochemical localization of the cytochrome P-450scc-linked monooxygenase system in the adult rat brain

Immunohistochemical and biochemical studies were performed on the brains of adult female and male rats using a specific antibody against bovine adrenocortical cytochrome P-450scc. The results showed that in both male and female rats, the myelinated regions of the white matter are selectively immunostained throughout the brain and that even in rats pretreated with colchicine, there is never positive staining of neuronal cell bodies and their dendrites in any brain region. Western immunoblotting with the P-450scc antibody and enzymatic assays revealed that P-450scc and cholesterol side-chain cleavage activity were present in a homogenate derived from the cortical white matter, but not detectable in that from the cerebral cortex. Furthermore, quantitation of the P-450scc protein in the immunoblots indicated that the concentration of P-450scc in the cortical white matter of both female and male rat brains is approx. 3-4 pmol per mg tissue protein. Thus it could be concluded that in the adult rat brain, P-450scc and cholesterol side-chain cleavage activity are selectively localized only in the myelinated region of the white matter.

Modified acetylenic steroids as potent mechanism-based inhibitors of cytochrome P-450SCC

Synthesized 20-(4-tetrahydropyranyl-1-butynyloxy)-5-pregnen-3 alpha,20 beta- diol [steroid I] and 20-(3-tetrahydropyranyl-1-propargyloxy)-5-pregnen- 3 alpha,20 beta-diol [steroid III] have been found to inactivate purified adrenocortical cytochrome P-450SCC. When incubated with the enzyme under turnover conditions, steroid I inactivated cytochrome P-450SCC by about 85% in 40 min. This is in contrast to the free triol analog, steroid II which inactivated the enzyme by only 45% within the same incubation period. A comparison of steroid III with its free triol analog, steroid IV, also showed that the diol is a more effective inactivator of the enzyme than the triol. The partition ratio was calculated by two different methods. Each of the steroids I-IV bound to the enzyme with spectrophotometric dissociation constant (Ks) in the micromolar range, producing Type II low spin spectra changes during titration of the enzyme. In addition, it was found that the binding of each of the compounds to the enzyme occurred without inactivation of the enzyme and that the inactivation under turnover condition, is not as a result of conversion to the denatured P-420 species. This demonstrated that steroids I and III could correctly be designated as mechanism-based (suicide) inhibitors. The kinetic studies demonstrated that steroids with the tetrahydropyranyl substituent are more potent inhibitors of cytochrome P-450SCC as shown by an initial turnover rate of 0.06 min-1, an inactivation rate constant of 0.05 min-1, and a partition ratio of about 1.0 for steroid I. Based on our finding, possible mechanisms of inactivation of cytochrome P-450SCC by these acetylenic steroids are proposed.

Cytochrome P-450-linked monooxygenase systems of bovine brain mitochondria and microsomes

1. Cytochrome P-450-linked monooxygenase of mitochondria and microsomes of the cerebrum, cerebellum and pituitary gland of bovine brain were investigated biochemically and immunochemically. 2. The cytochromes P-450scc and P-450(11) beta, NADPH-ferredoxin reductase and adreno-ferredoxin were detected and their enzymatic activities of steroid hydroxylations were found in the bovine brain.

Sensitive assay of cytochrome P450scc activity by high-performance liquid chromatography

We have developed a simple procedure for analyzing the reaction intermediates and product of the cholesterol side-chain cleavage system by high-performance liquid chromatography with uv absorption monitoring. After the cholesterol side-chain cleavage system had been incubated and the reaction then halted by heat treatment, the product was converted into 3-one-4-en steroid showing intense absorption at 240 nm upon reaction with cholesterol oxidase. The converted steroids were then analyzed by normal-phase HPLC. In consequence, the catalytic activity of the reconstituted adrenocortical cytochrome P450scc system was readily assayed with a sensitivity more than 10-fold higher by this conversion. Also, it was shown that 22R-hydroxy-cholest-4-en-3-one could serve as a good substrate for cytochrome P450scc and that the 20R,22R-dihydroxy derivative could be clearly detected as a reaction intermediate in the reconstituted system.

Characterization and N-terminal amino acid sequence of multiple ferredoxins in kidney and adrenal mitochondria

Two separate ferredoxins that differ in molecular mass by about 1.5 kDa were isolated from both pig kidney and bovine adrenal mitochondria. The proteins had different biochemical and immunological properties and appeared to be distinct gene products. The smaller ferredoxin from pig kidney (renodoxin, Mr approximately equal to 13,500) was very similar to bovine adrenodoxin (Mr = 14,048). Both proteins had nearly identical N-terminal amino acid sequences and electron-transfer activities. However, renodoxin and adrenodoxin expressed distinct antigenic determinants, although they were immunologically cross-reactive. The larger kidney (approximately equal to 15-kDa) and adrenal (approximately equal to 15.3-kDa) ferredoxins were biochemically similar to each other but they had lower specific activities and their N-terminal sequences were different when compared to renodoxin and adrenodoxin. Each of the four ferredoxins had a visible absorption spectrum characteristic of a [2Fe-2S] chromophore. But in addition, the larger ferredoxins displayed a prominent A276 peak due to their higher tyrosine content and the presence of tryptophan, which is absent in adrenodoxin and renodoxin. Consequently, the larger ferredoxins were termed Trp-ferredoxin. Using antibody to pig kidney Trp-ferredoxin, the larger adrenal and kidney ferredoxins were found to be very similar immunologically; however, the Trp-containing and adrenodoxin-type ferredoxins did not cross-react in immunoblot analysis. Nevertheless, it was shown from competition ELISA and activity-inhibition analysis that the two ferredoxin types had limited common antigenic determinants. Trp-ferredoxin was the major iron-sulfur protein in kidney whereas adrenodoxin was the dominant molecular form in adrenal gland.

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)

Immunohistochemical studies of steroidogenic enzymes (aromatase, 17 alpha-hydroxylase and cholesterol side-chain cleavage cytochromes P-450) in sex cord-stromal tumors of the ovary

Aromatase, 17 alpha-hydroxylase, and cholesterol side-chain cleavage P-450 cytochromes (P-450AROM, P-450(17 alpha,) and P-450SCC, respectively) were immunohistochemically localized in nine granulosa cell tumors, 15 thecomas, ten Sertoli-Leydig cell tumors, two steroid cell tumors, five fibromas, and five sclerosing stromal tumors. In the thecomas, P-450SCC and P-450(17 alpha) were positive in luteinized theca cells and in cells with vacuolated cytoplasm, while P-450AROM was not observed. In the steroid cell tumors, all the P-450 cytochromes were intensely stained. P-450SCC and P-450(17 alpha) were present in cells with vacuolated cytoplasm in two cases of sclerosing stromal tumor. P-450AROM was weakly demonstrated in one of the granulosa cell tumors. P-450(17 alpha,) P-450SCC, and P-450AROM were all faintly stained in the Sertoli-Leydig cell tumors. No P-450 cytochrome immunoreactivity was observed in any fibroma.

Immunohistochemical demonstration of cholesterol side-chain cleavage cytochrome P-450 in bovine and human adrenals

Cytochrome P-450 specific for cholesterol side-chain cleavage (P-450SCC) was purified from the bovine adrenal and a specific antibody was raised in rabbits. The antiserum was applied for immunohistochemical visualization of the P-450SCC in the bovine and human adrenal cortex. The immunoreactivity was intense in the zona fasciculata (ZF) and reticularis (ZR) while weak in the zona glomerulosa (ZG) in the normal adrenals. In adrenocortical hyperplasia, a marked immunoreactivity was observed in the ZG in idiopathic hyperaldosteronism and the inner ZF and reticularis particularly in cells of micronodules in Cushing's disease, corresponding to cells with active steroidogenesis. In aldosteronoma and adenoma with Cushing's syndrome, P-450SCC was generally present in compact cells of adenomas.

Multiple molecular forms of cytochrome P-450SCC purified from bovine corpus luteum mitochondria

Cytochrome P-450 related to side-chain cleavage of cholesterol (P-450SCC) was isolated from bovine corpus luteum mitochondria in the form of its stable cholesterol complex. The isolation procedure included ammonium sulfate fractionation and chromatography on omega-aminohexyl-Sepharose (AH-Sepharose). Corpus luteum P-450SCC was resolved into one minor (AH-I) and two major (AH-II and AH-III) fractions by the chromatography. Results of re-chromatography suggested the possibility that AH-III Fraction was originally complexed with lipidic material. The two major fractions purified by the re-chromatography (AH-IIR and AH-IIIR Fractions) showed essentially a single band on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and their absorption spectra were indistinguishable from each other. Both fractions were further resolved into two major and some minor bands of P-450SCC by isoelectric focusing on polyacrylamide gel in the presence of a non-ionic detergent, as detected by protein staining, heme staining and immunoblot analysis with anti-bovine P-450SCC monoclonal antibody. Both AH-IIR and AH-IIIR Fractions were further resolved by high-performance liquid chromatography (HPLC) on SP-TSK gel column into two fractions, SP-I and SP-II. These fractions had the same N-terminal amino acid sequence, showed similar catalytic activity and resolved into one major and a few minor bands on isoelectric focusing on polyacrylamide gel. Much more heterogeneity was observed in purified P-450SCC preparations from bovine adrenal cortex mitochondria. These results indicated the presence of multiple molecular forms of corpus luteum P-450SCC as well as adrenal cortex P-450SCC. Computer simulation studies were carried out in order to analyze the mechanism of formation of multiple bands on isoelectric focusing. The multiple bands of corpus luteum P-450SCC could be explained by postulating the presence of two isozymes (or molecular forms) having a pair of sites each with or without a charged group.

Molecular genetics of congenital adrenal hyperplasia

Congenital adrenal hyperplasia results from a deficiency in any of the five enzymes necessary to synthesize cortisol from cholesterol: cholesterol desmolase (P450scc), 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), 17-hydroxylase (P450c17), 21-hydroxylase (P450c21) and 11-hydroxylase (P450c11). P450scc and P450c11 are structurally-related mitochondrial cytochrome P450 enzymes, whereas P450c17 and P450c21 are microsomal enzymes. The P450scc gene, CYP11A, is located on chromosome 15, and the P450c17 gene, CYP17, is on chromosome 10. The P450c21 gene, CYP21B, and a pseudogene, CYP21A, are located in the HLA major histocompatibility complex on chromosome 6p, while the P450c11 gene, CYP11B, is on chromosome 8q along with a second related gene of unknown function. Thus, despite common regulation by ACTH, there is no clustering of the genes for steroidogenic enzymes. CYP11A and CYP11B have an identical intron-exon organization, and CYP17 and CYP21B have similar gene structures, but the two pairs of genes encoding mitochondrial and microsomal P450 enzymes resemble each other poorly. More than 90% of cases of congenital adrenal hyperplasia result from 21-hydroxylase deficiency, and most of the remainder are caused by 11-hydroxylase deficiency. About one-quarter of 21-hydroxylase deficiency alleles are associated with a deletion of all or part of CYP21B. Most of the remaining mutant alleles result from transfers of deleterious mutations from the CYP21A pseudogene to CYP21B, a process termed gene conversion. These mechanisms provide an explanation for the relatively high frequency of 21-hydroxylase deficiency. The clinical severity of various forms of 21-hydroxylase deficiency may be roughly correlated with particular mutations.

Photoreactive cardiolipin analogues

New photoreactive analogues of cardiolipin have been chemically synthesized. Photoreactive aryl azido acyl groups were placed at two different locations within the cardiolipin molecule: at the 2-sn position of the 2-sn glycerol of cardiolipin; at the 2-sn position of the 3-sn-phosphatidyl group; or at both locations to provide a dual labeled analogue. Thus three different cardiolipin analogues distinguished by the positions of the aryl azido acyl groups were prepared. Two different aryl azido acyl groups were employed in the above syntheses and the site of acylation was stereospecifically identified using several phospholipids of known specificity for cardiolipin. Acylation of cardiolipin with the symmetrical anhydride of either acyl aryl azido fatty acid analogue, 2-(N-4-azido-2-nitrophenyl)beta-alanine or 12-(N-4-azido-2-nitrophenyl)aminododecanoic acid provided 1-(3-sn-phosphatidyl)-2-(acyl aryl azido)-3-(3-sn-phosphatidyl)-sn-glycerol. Acylation of monolysocardiolipin (1-(3-sn-phosphatidyl)-3-(1-acyl-2-lyso-glycero(3)phospho)-sn-glyce++ + rol provided two products. 1-(3-sn-phosphatidyl)-3-(1-acyl-2-(acyl aryl azido)-glycero(3)phospho)-sn-glycerol and the doubly labeled 1-(3-sn-phosphatidyl)-2-(acyl aryl azido)-3-(1-acyl-2-(acyl aryl azido)glycero(3)phospho)-sn-glycerol. These are the first reported photoreactive analogues for cardiolipin. The analogues were positive effectors for cytochrome P-450sec, and as shown by SDS-PAGE, they labeled the single subunit of cytochrome P-450sec and the smallest subunits of cytochrome c oxidase from beef heart.

Phosphorylation of bovine adrenodoxin. Structural study and enzymatic activity

Adrenodoxin is an iron-sulfur protein which functions as a carrier of reducing equivalents in steroid hydroxylation reactions catalyzed by specific cytochromes P-450 in steroidogenic tissues such as adrenal cortex. Purified bovine adrenocortical adrenodoxin was shown to be selectively phosphorylated upon incubation with purified cAMP-dependent protein kinase, whereas other protein kinases were ineffective. The phosphorylation reaction was completed within 45 min at 30 degrees C and resulted in the optimal incorporation of 1 mol phosphate/mol adrenodoxin. Apoadrenodoxin, lacking the iron-sulfur cluster, was also phosphorylated under similar conditions. An apparent Km of 55 microM with a Vmax of 0.3 pmol 32P incorporated min-1 mg adrenodoxin-1 was calculated. Phosphorylation resulted in a striking change in several molecular properties of adrenodoxin, such as electrophoretic behavior and hydroxyapatite affinity, thus providing the possibility of clearly separating phosphorylated from unphosphorylated adrenodoxin. In addition, phosphoadrenodoxin became refractory to mild trypsin degradation, whereas this was not the case with apoadrenodoxin. The phosphorylated site of adrenodoxin was identified as a serine residue; study of peptide products resulting from CNBr and proteolytic cleavages of phosphoadrenodoxin suggested that Ser-88 was the target of the phosphorylation reaction. The influence of phosphorylation upon adrenodoxin activity was examined using cholesterol side-chain cleavage and 11 beta-hydroxylase (11 beta) systems, reconstituted from purified components. Phosphorylation of adrenodoxin resulted in an average twofold decrease in its Km values for the two specific cytochromes P-450 involved. This effect was paralleled by a positive relationship between the degree of adrenodoxin phosphorylation and its ability to support the overall activity of reconstituted side-chain cleavage and 11 beta-hydroxylase systems. Although it remains to be examined whether adrenodoxin is phosphorylated in the intact cell, the present observations suggest that it represents a potential target in the hormonal regulation of the adrenocortical differentiated functions, especially by stimulatory agents acting through a cyclic-AMP-dependent mechanism, such as adrenocorticotropin.

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.

A sensitive method for quantitation of steroid hydroxylase activities of individual P-450 in tissue homogenates

Assay methods which allow measurements of the level of individual P-450's in a homogenate of steroidogenic tissues have been developed. The assay is based simply on the determination of the specific products of steroid monooxygenase reactions under conditions in which sufficient amounts of the purified electron-donating components as well as lipids and detergents are supplemented so that the membrane-associated P-450 is able to exhibit its maximum activity. Under these conditions, an at least 10-fold increase in the NADPH-dependent monooxygenase activity of P-450 occurred as compared to that measured in the unenriched system. In addition, the present assay used ascorbate as an O2- -scavenger which effectively prevented possible inhibition caused by initiation of O2- -formation. Under the present assay conditions, nearly quantitative recovery of activities was accomplished of each purified P-450 that had been added to the homogenates as an internal standard. Furthermore, the high sensitivity of this assay allows the measurement of P-450 in small specimens (at least 100 mg of tissues), and could be used for measurements in autopsied cases or the glands of small animals.

The side-chain cleavage of cholesterol sulfate--III. The effect of adrenodoxin, membrane phospholipids and Tween 80 on the kinetics of oxidation of the sterol sulfate by a reconstituted cholesterol desmolase system

This paper reports the Km values of a reconstituted cholesterol side-chain cleavage system for cholesterol sulfate, cholesterol, and adrenodoxin, determined under several experimental conditions. The Km values for adrenodoxin change depending on whether cholesterol or its sulfate is used as the substrate. Moreover, the Km values for both of the substrates and for adrenodoxin are greatly modulated by both membrane phospholipids, isolated from adrenal mitochondria, and Tween 80, 0.002%. In the absence of detergents or phospholipids, the enzyme system shows a high affinity for cholesterol sulfate, but is inhibited when high concentrations of the sterol sulfate are added to the incubation mixture. Raising the concentration of adrenodoxin in the assay mixture prevents the substrate inhibition. When cholesterol sulfate is incorporated into micelles containing the phospholipids, the enzyme system does not display substrate inhibition, and the kinetics of cleavage of the sterol sulfate are relatively independent of the concentration of adrenodoxin in the assay mixture. In the absence of phospholipids, the apparent kinetics of cleavage of cholesterol and its sulfate are quite different from each other, but when incorporated into micelles containing phospholipids, the kinetics of cleavage of the two substrates are similar to each other.

The side-chain cleavage of cholesterol sulfate--I. The effect of adrenodoxin on the binding of cholesterol sulfate to cytochrome P-450scc

Difference spectroscopy was used to measure the binding of cholesterol sulfate (CS) to cytochrome P-450scc. The uncomplexed cytochrome and the complex of the cytochrome with adrenodoxin (ADX) were both titrated with CS in order to test whether ADX increased the affinity of the cytochrome for the sterol sulfate. The addition of ADX to the cytochrome had different effects on the binding of the sterol sulfate depending on several factors including: (1) The method of preparation of the cytochrome P-450scc, (2) The concentration of cytochrome P-450scc, (3) The method by which CS was suspended in aqueous solution, and (4) Whether or not the solutions of cytochrome contained non-ionic detergents. The results of this study suggest that the method of isolation of cytochrome P-450scc, and non-ionic detergents, greatly modulate the apparent affinity of cytochrome P-450scc for CS. In the absence of detergents the addition of adrenodoxin to dilute solutions of cytochrome P-450scc appears to enhance only slightly (1- to 2-fold) the affinity of the cytochrome for the sterol sulfate.

Adrenal cortical 11 beta-hydroxylase and side-chain cleavage enzymes. Requirement for the A- or B-pyridyl ring in metyrapone for inhibition

The adrenal cortical enzyme systems, 11 beta-hydroxylase, P-450 11 beta, and the side-chain cleavage complex, P-450 scc, differ only in their cytochrome P-450s. Structural modifications of metyrapone, an inhibitor of cytochrome P-450 enzyme systems, have been made to determine the requirement for the A- or B-pyridyl ring for inhibition of P-45011 beta and P-450 scc activities. Three new analogs of metyrapone (A-phenylmetyrapone, B-phenylmetyrapone and diphenylmetyrapone) were synthesized and evaluated as inhibitors using a crude, defatted bovine adrenal cortical mitochondrial preparation. Characterization of the mitochondrial preparation demonstrated: enhancement of both activities by the addition of 15.0 microM adrenodoxin, the addition of 1% ethanol decreased both activities less than 10%, and the apparent Km of deoxycorticosterone for P-45011 beta was 6.8 microM and the apparent Km of cholesterol for P-450 scc was 21.6 microM. Inhibition of P-45011 beta and P-450 scc activities with these compounds demonstrated: the B-pyridyl ring of metyrapone is required for inhibition of both activities whereas requirement for the A-ring is less stringent, and the four metyrapone analogs were more selective inhibitors of P-45011 beta activity. These studies suggest that the A-phenyl metyrapone analog is a good candidate for further development of a selective adrenocortical radiopharmaceutical.

Ecdysone 20-mono-oxygenase in the desert locust, Schistocerca gregaria

The enzyme catalysing the hydroxylation of ecdysone to 20-hydroxyecdysone, ecdysone 20-mono-oxygenase (EC 1.14.99.22), was investigated in the Malpighian tubules of fifth-instar locusts, Schistocerca gregaria. Enzyme activity was optimal at 35 degrees C and pH 6.8-8.0. Under these conditions the mono-oxygenase exhibited an apparent Km for ecdysone of 7.1 X 10(-7) M, a maximal specific activity of 1.1 nmol/h per mg of protein and was competitively inhibited by 20-hydroxyecdysone with an apparent Ki of 6.3 X 10(-7) M. Enzyme activity was decreased in the presence of Ca2+, Mg2+, EDTA and non-ionic detergents. The Malpighian tubule ecdysone 20-mono-oxygenase was localized primarily in the subcellular fraction sedimenting at 7500 g and, on the basis of marker enzyme profiles, was assigned mainly to the mitochondria. NADPH was required for activity, although addition of NADH together with NADPH had a synergistic effect. NADP+-dependent isocitrate dehydrogenase (EC 1.1.1.42) and an energy-dependent NAD(P) transhydrogenase (EC 1.6.1.1.) appeared to be the major sources of reducing equivalents, with the contribution from the 'malic enzyme' (EC 1.1.1.40) being less important. The monooxygenase was characterized as a cytochrome P-450-containing mixed-function oxidase from the inhibition patterns with metyrapone, CO and cyanide; CO inhibition was reversible with monochromatic light at 450 nm. However, the ecdysone 20-mono-oxygenase shows much lower sensitivity to CO inhibition and to photodissociation of the CO-inhibited complex than do vertebrate cytochrome P-450-dependent hydroxylation systems. The concentration of cytochrome P-450 in the Malpighian tubule mitochondria was 30 pmol/mg of protein. The properties of the mono-oxygenase are discussed in relation to hydroxylation enzymes from other sources.

Effects of aminoglutethiumide and its metabolite, N-acetylaminoglutethimide, on bovine adrenocortical and human placental cytochromes P-450scc

The ability of aminoglutethimide to inhibit cholesterol conversion to pregnenolone was lost upon acetylation of the arylamine nitrogen. This appears to be due to failure of N-acetyl-d-aminoglutethimide to bind to cytochrome P-450scc, since it does not produce the altered low spin form of the enzyme formed upon binding of d-aminoglutethimide. These findings provide further evidence for a role of the free arenamine function in aminoglutethimide and related inhibitors.