Import and processing of the precursor of cytochrome P-450(SCC) by bovine adrenal cortex mitochondria

Isolated bovine adrenal cortex mitochondria imported in vitro synthesized pre-P-450(SCC) and processed it to the mature form. Partial radio-sequencing of the processed P-450(SCC) gave a result identical with that for authentic P-450(SCC). Rat liver mitochondria also imported pre-P-450(SCC) and processed it to the mature form, whereas bovine heart mitochondria were unable to import and process pre-P-450(SCC) although both mitochondrial preparations imported and processed pre-adrenodoxin. The pre-P-450(SCC) processing activity of bovine adrenal cortex mitochondria was associated with the matrix side surface of the inner membrane. The processing protease could be solubilized by sodium cholate and partially purified by ammonium sulfate fractionation. The partially purified processing protease cleaved pre-P-450(SCC) at the correct position. It was also active in processing pre-P-450(11 beta) but inactive toward pre-adrenodoxin. Bovine heart mitochondria lacked the processing activity to pre-P-450(SCC). The localization of pre-P-450(SCC) and mature P-450(SCC) in bovine adrenal cortex mitochondria was examined. Mature P-450(SCC) processed by the mitochondria was found associated with the matrix-side surface of the inner membrane, which is the correct location of P-450(SCC) in the cell. In the presence of o-phenanthroline, pre-P-450(SCC) was imported into the organelles without being processed and remained soluble in the matrix. The incorporation of newly processed mature P-450(SCC) into the inner membrane was also observed when pre-P-450(SCC) was incubated with inner membrane vesicles. Mature P-450(SCC) generated in vitro from pre-P-450(SCC) by the partially purified processing protease was incorporated not only into the inner membrane vesicles but also into bovine adrenal cortex microsomes. These findings suggested that the processing of pre-P-450(SCC) occurred prior to the incorporation of mature-P-450(SCC) into the inner membrane.     

Purification and characterization of a processing protease from rat liver mitochondria.

A processing protease has been purified from the matrix fraction of rat liver mitochondria. The purified protease contained two protein subunits of 55 kd (P-55) and 52 kd (P-52) as determined by SDS-PAGE. The processing protease was estimated to be 105 kd in gel filtration, indicating that the two protein subunits form a heterodimeric complex. At high ionic conditions, the two subunits dissociated. The purified processing protease cleaved several mitochondrial protein precursors destined to different mitochondrial compartments, including adrenodoxin, malate dehydrogenase, P-450(SCC) and P-450(11 beta), but the processing efficiencies were different each other. The endoprotease nature of the processing protease was confirmed with the purified enzyme using adrenodoxin precursor as the substrate; both the mature form and the extension peptide were detected after the processing. The processing activity of the protease was inhibited by metal chelators, and reactivated by Mn2+, indicating that the protease is a metalloprotease.     

Import and processing of P-450SCC and P-450(11) beta precursors by corpus luteal mitochondria: a processing pathway recognizing homologous and heterologous precursors

Maturation of the precursor forms of bovine cholesterol side-chain cleavage cytochrome P-450 (P-450SCC) and 11 beta-hydroxylase cytochrome P-450 (P-450(11)beta) was investigated using mitochondria from bovine corpus luteum. The results show that both precursors, whose synthesis was directed by bovine adrenocortical RNA, can be imported and proteolytically processed to their corresponding mature forms by bovine corpus luteal mitochondria, even though P-450(11)beta is not expressed in this tissue. Furthermore, the efficiency of processing of pre-P-450(11)beta by corpus luteal mitochondria is similar to that of pre-P-450SCC, an endogenous enzyme of these mitochondria. However, the P-450(11)beta precursor is not processed by mitochondria from a nonsteroidogenic tissue (heart), a result observed previously for the P-450SCC precursor (M. F. Matocha and M. R. Waterman (1984) J. Biol. Chem. 259, 8672-8678). This discriminatory processing of pre-P-450(11)beta by heterologous mitochondria suggests that the precursor forms of P-450SCC and P-450(11)beta are processed via a common pathway in steroidogenic mitochondria and that this pathway is absent in nonsteroidogenic mitochondria.     

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.     

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.     

Effects of synthetic model peptides resembling the extension peptides of mitochondrial enzyme precursors on import of the precursors into mitochondria

One common and characteristic feature of the extension peptides of mitochondrial enzyme precursors is the presence of repeating short stretches of uncharged amino acids linked by basic amino acids. We synthesized several model peptides having this particular feature of the extension peptides. The peptides contained arginine or lysine as a basic amino acid residue linking sequences of two to four residues of leucine and alanine. We examined the effects of the peptides on the import of the precursors of two mitochondrial enzymes, cytochrome P-450(SCC) and adrenodoxin, and found that the peptides were generally inhibitory to the import of the precursors into mitochondria. The effective concentrations of some of the inhibitory peptides were as low as a few microM. The peptides containing lysine instead of arginine had an essentially similar inhibitory effect on the import. The peptides did not inhibit the binding of pre-P450(SCC) to the surface of mitochondria. The synthetic model peptides uncoupled oxidative phosphorylation of mitochondria prepared from either rat liver or bovine adrenal cortex, and induced leakage of enzymes from the inner compartments of mitochondria. However, the synthetic model peptides did not solubilize membrane-bound enzymes from mitochondria, suggesting that their effect on the membranes is different from that of detergents. The synthetic model peptides seem to bind to the membranes causing significant perturbation in the membrane structure, which is possibly related to the functions of the particular common sequence found in the extension peptides of mitochondrial enzyme precursors.     

Processing-independent in vitro translocation of cytochrome P-450(SCC) precursor across mitochondrial membranes

In the presence of a membrane-permeable metal chelator, bovine adrenal cortex mitochondria imported P-450(SCC) precursor without processing of the amino-terminal extension peptide. The imported precursor was bound to the matrix side surface of the inner membrane. When the inhibition due to the metal chelator was removed, the imported precursor was processed to the mature form. Unprocessed precursor was also detected in mitochondria when the import reaction was carried out at relatively low temperature. These results suggest that the translocation of P-450(SCC) precursor across mitochondrial membranes is independent of its processing to the mature form. Both membrane-bound and solubilized P-450(SCC) could be cleaved by trypsin into two fragments with molecular weights of 29 kDa and 26 kDa, respectively, suggesting a two-domain structure of the molecule. The in vitro-imported and processed P-450(SCC) was also cleaved by trypsin in the same way. This finding indicated that the in vitro-imported and processed P-450(SCC) has the same conformation as the native form.     

Contributions of cytoplasmic free and membrane-bound ribosomes to the synthesis of mitochondrial cytochrome P-450(SCC) and P-450(11 beta) and microsomal cytochrome P-450(C-21) in bovine adrenal cortex

Rabbit antibodies against cytochrome P-450 (SCC), P-450 (11 beta), and P-450 (C-21) from bovine adrenal cortex were prepared, and it was confirmed that these three cytochrome P-450 species are immunologically distinct from one another. Cytoplasmic sites of synthesis of P-450 (SCC), P-450 (11 beta), and P-450 (C-21) in bovine adrenal cortex were determined by examining the presence of their nascent peptides on isolated free and bound ribosomes. Nascent peptides were released in vitro from ribosomes by [3H]puromycin in a high salt buffer in the presence of a detergent, and the nascent peptides of P-450 (SCC), P-450 (11 beta), and P-450 (C-21) were isolated by immunoprecipitation. The nascent peptides of these three cytochrome P-450 species were found in both free and bound ribosomal fractions, suggesting that they share common sites of synthesis in the cytoplasm. However, the nascent peptides of mitochondrial P-450 (SCC) and P-450 (11 beta) were more concentrated in the free ribosomal fraction, whereas those of microsomal P-450 (C-21) were more abundant in the bound ribosomal fraction. The nascent peptides of the three cytochrome P-450 species were released from the membrane-bound ribosomes of rough microsomes into the cytoplasmic surface of microsomal vesicles by puromycin treatment.     

Molecular properties of cytochrome P-45011 beta from adrenal cortex mitochondria.

Cytochrome P-45011 beta has been solubilized and partially purified from bovine adrenal cortex mitochondria using chromatography on Octyl-Sepharose CL-4B or DEAE-Sepharose CL-6B. The partially purified P-450 preparations were about 90% pure as judged by SDS-polyacrylamide gel electrophoresis. In the presence of purified preparations of adrenodoxin reductase and adrenodoxin, the partially purified P-450 preparations catalyzed NADPH-supported 11 beta-hydroxylation of unconjugated and sulphoconjugated deoxycorticosterone. In presence of Triton X-100 the partially purified cytochrome P-45011 beta had a Stoke's 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-45011 beta-Triton X-100 complex has a molecular weight of about 100 000 and that P-45011 beta bound about 1.1 g of Triton X-100 per g of protein. The P-45011 beta-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-45011 beta is an active form of the protein.     

Aldosterone biosynthesis and cytochrome P-45011 beta: evidence for two different forms of the enzyme in rats

Whereas cytochrome P-45011 beta has been recently shown to catalyze the two-step conversion of corticosterone to aldosterone in the bovine and porcine adrenal cortex, the identity of the enzyme involved in the two final steps of aldosterone biosynthesis in the rat adrenal cortex is as yet unknown. Mitochondria from capsular adrenals of sodium-deficient, potassium-replete rats converted corticosterone to 18-hydroxycorticosterone and aldosterone at markedly higher rates than mitochondria from capsular adrenals of sodium-replete, potassium-deficient rats. However, the same preparations exhibited no difference in the 11 beta-hydroxylase activity, i.e. the conversion of deoxycorticosterone to corticosterone. Only mitochondria of zona glomerulosa from rats with stimulated aldosterone biosynthesis contained a 49K protein which showed a strong cross-reactivity with a monoclonal antibody raised against purified bovine cytochrome P-45011 beta. By contrast, a crossreactive protein with a molecular weight of 51K was found in mitochondria of zona fasciculata and in mitochondria of zona glomerulosa from rats with a suppressed aldosterone biosynthesis. These findings indicate the existence of two different forms of cytochrome P-45011 beta in the rat adrenal cortex, with only one of them, i.e. the 49K form, being capable of catalyzing the two final steps of aldosterone biosynthesis in situ.     

ATP-dependent protease in bovine adrenal cortex. Tissue specificity, subcellular localization, and partial characterization

Proteolytic activities in bovine adrenocortical mitochondria were investigated using [14C-methyl]casein as a substrate. Washed mitochondria showed a low proteolytic activity at pH 7.5 or 8.2. ATP (5 mM) plus MgCl2 (7.5 mM) stimulated the proteolysis 9 times at pH 8.2. It was further demonstrated unequivocally by various approaches that the ATP-dependent proteolytic activity localizes in mitochondrial matrix. The activity of the solubilized protease was sensitive to N-ethylmaleimide, mersalyl acid, phenylmethylsulfonyl fluoride, o-vanadate, m-vanadate, vanadyl sulfate, and quercetin but not by oligomycin and ouabain. The ATP-dependent proteolytic activity was eluted at the position of 650,000 daltons on an Ultrogel AcA 22 column as a single symmetrical peak. The gel-filtered enzyme showed high specificity to ATP. GTP and UTP partially substituted ATP. ADP, AMP, tripolyphosphate, alpha, beta-methylene ATP, and beta, gamma-methylene ATP had little or no stimulating activity. ATP did not stimulate the activity in the absence of MgCl2. We measured ATP-dependent proteolytic activities in mitochondrial fractions from several tissues in rat and bovine. Adrenal cortex was one of the tissues of highest activity. In addition, we investigated the effect of adrenal atrophy on the ATP-dependent protease activity in rat adrenal. The ATP-dependent protease activity/adrenal decreased by dexamethasone treatment. The extent of the decrease was similar to that of cytochrome oxidase and succinate dehydrogenase, but smaller than that of cytochrome P-450.     

Partial purification of a metalloprotease catalyzing the processing of adrenodoxin precursor in bovine adrenal cortex mitochondria

In vitro translation of bovine adrenal cortex RNA in rabbit reticulocyte lysate cell-free system produced the precursor form of adrenodoxin having a molecular weight of approximately 22,000 daltons, which was about 10,000 daltons larger than mature adrenodoxin. The precursor of adrenodoxin was efficiently imported into adrenal cortex mitochondria in vitro. The precursor was also imported into rat liver mitochondria, suggesting the lack of tissue specificity and species specificity of the import process. The enzyme which processed the precursor of adrenodoxin to the mature form was in the matrix fraction from bovine adrenal cortex mitochondria, and the processing protease was partially purified from the matrix fraction. The apparent molecular weight of the processing protease was about 60,000 daltons as determined by Sephadex G-150 gel filtration, and its activity was optimal at pH 8.5. The processing protease was not inhibited by various bacterial protease inhibitors examined. Metal chelators (EGTA, GTP, 8-hydroxyquinoline, and Zincon) inhibited the processing, and EDTA and o-phenanthroline were more strongly inhibitory than other chelators. The processing protease was completely inactivated by incubation with 10 microM EDTA, and its activity was restored by addition of excess amounts of Mn2+, Fe2+, or Co2+. These results indicate that the maturation of the precursor of adrenodoxin is catalyzed by a soluble metalloprotease in the matrix.     

Isolation of aldosterone synthase cytochrome P-450 from zona glomerulosa mitochondria of rat adrenal cortex

A cytochrome P-450 capable of producing aldosterone from 11-deoxycorticosterone was purified from the zona glomerulosa of rat adrenal cortex. The enzyme was present in the mitochondria of the zona glomerulosa obtained from sodium-depleted and potassium-repleted rats but scarcely detected in those from untreated rats. It was undetectable in the mitochondria of other zones of the adrenal cortex from both the treated and untreated rats. The cytochrome P-450 was distinguishable from cytochrome P-45011 beta purified from the zonae fasciculata-reticularis mitochondria of the same rats. Molecular weights of the former and the latter cytochromes P-450, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, were 49,500 and 51,500, respectively, and their amino acid sequences up to the 20th residue from the N terminus were different from each other at least in one position. The former catalyzed the multihydroxylation reactions of 11-deoxycorticosterone giving corticosterone, 18-hydroxydeoxycorticosterone, 18-hydroxycorticosterone, and a significant amount of aldosterone as products. On the other hand, the latter catalyzed only 11 beta- and 18-hydroxylation reactions of the same substrate to yield either corticosterone or 18-hydroxydeoxycorticosterone. Thus, at least two forms of cytochrome P-450, which catalyze the 11 beta- and 18-hydroxylations of deoxycorticosterone, exist in rat adrenal cortex, but aldosterone synthesis is catalyzed only by the one present in the zona glomerulosa mitochondria.     

Aldosterone biosynthesis in mitochondria of isolated zones of adrenal cortex

An assumption that the aldosterone-synthesizing enzyme exists only in zona glomerulosa cells apparently contradicts our recent findings that a purified bovine adrenocortical cytochrome P-45011 beta catalyzes the aldosterone formation and the enzyme exists in both zones of the adrenal cortex. To gain more insight into the zone specificity of aldosterone production, the aldosterone-synthesizing activity of mitochondria prepared from the isolated zones of adrenal cortex of various animal species was investigated. The intact mitochondria from the bovine or porcine zonae fasciculata-reticularis could not produce aldosterone whereas those from the zona glomerulosa produced it at a significant rate. When the mitochondria from the zonae fasciculata-reticularis were solubilized by the addition of cholate, they produced aldosterone from corticosterone at a rate comparable to that of those from the zona glomerulosa. The presence of specific factor(s) in the zonae fasciculata-reticularis mitochondria inhibiting expression of the aldosterone synthetic activity is discussed. The mitochondria of the rat zonae fasciculata-reticularis could hardly catalyze aldosterone synthesis under the detergent-solubilized conditions, whereas those of the zona glomerulosa could. Immunoblot analysis revealed that the mitochondria of the zonae fasciculata-reticularis contained a protein of Mr 51,000 which was immunocrossreactive with a monoclonal antibody directed against P-45011 beta, whereas those of the zona glomerulosa contained two immunocrossreactive proteins of Mr 51,000 and 49,000. These results suggest that in the case of rat adrenal cortex, a specific aldosterone-synthesizing enzyme exists in the zona glomerulosa.     

The synthesis of aldosterone by the adrenal cortex. Two zones (fasciculata and glomerulosa) possess one enzyme for 11 beta-, 18-hydroxylation, and aldehyde synthesis

In order to establish the nature of the aldosterone synthetase activity in the adrenal cortex, we have used porcine adrenal, bovine adrenal cortex, highly purified bovine and porcine 11 beta-/18-hydroxylase, and antibodies raised against the latter enzyme. Mitochondria from two zones (glomerulosa and fasciculata) of the bovine cortex synthesize aldosterone, but those from glomerulosa are much more active than those from fasciculata. Partially purified (cholate-extracted plus ammonium sulfate-precipitated) extracts of mitochondria from the two zones are equally active in catalyzing all three steps in the conversion of 11-deoxycorticosterone to aldosterone. 18-Hydroxylase and aldehyde synthetase activities (18-hydroxycorticosterone----aldosterone) were completely precipitated from cholate extracts of mitochondria from bovine adrenal by antibodies to the pure porcine enzyme. No activity corresponding to any of the three steps in the conversion of 11-deoxycorticosterone to aldosterone was found in extramitochondrial fractions of the bovine cortex. Synthesis of aldosterone by the pure porcine enzyme was inhibited by antibodies to this enzyme and by metyrapone (an inhibitor of 11 beta-/18-hydroxylase). When fractions of porcine adrenal, resulting from purification of the enzyme from mitochondria, were exhaustively tested for any of the enzyme activities required for the synthesis of aldosterone, activity was found only in those fractions containing the 11 beta-/18-hydroxylase, i.e. no additional enzyme was discarded during the purification procedure. It is concluded that the only adrenocortical enzyme capable of synthesizing aldosterone in bovine and porcine adrenal is the well known 11 beta-hydroxylase, that the conversion of 18-hydroxycorticosterone to aldosterone is catalyzed by this cytochrome P-450, and that this step (aldehyde synthetase) requires the heme of the P-450 as demonstrated by the photochemical action spectrum.     

Expression and transport into mitochondria of bovine cytochrome P-450(SCC) in insect cells using the baculovirus expression system.

Bovine cytochrome P-450(SCC) introduced with the baculovirus host vector system was found to be expressed in Spodoptera frugiperda cells. Cell fractionation analysis indicated that the P-450(SCC) expressed as the precursor form was transported into mitochondria and converted to a mature form. However, this form did not exhibit definite activity for cholesterol side chain cleavage. These findings suggest that most of the P-450(SCC) expressed by this system is an inactive protein within mitochondria that is not folded to the conformation of the active enzyme and/or does not incorporate heme appropriately.     

Hydroxylations in biosynthesis of bile acids. Isolation of a cytochrome P-450 from rabbit liver mitochondria catalyzing 26-hydroxylation of C27-steroids

A cytochrome P-450 catalyzing 26-hydroxylation of C27-steroids was purified from liver mitochondria of untreated rabbits. The enzyme fraction contained 10 nmol of cytochrome P-450/mg of protein and showed only one protein band with a minimum Mr = 53,000 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified mitochondrial cytochrome P-450 showed apparent molecular weight similar to microsomal cytochromes P-450LM4 but differed in spectral and catalytic properties from these microsomal isozymes. The purified cytochrome P-450 catalyzed 26-hydroxylation of cholesterol, 5-cholestene-3 beta,7 alpha-diol, 7 alpha-hydroxy-4-cholesten-3-one, 5 beta-cholestane-3 alpha,7 alpha-diol, and 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol up to 1000 times more efficiently than the mitochondria. The cytochrome P-450 required both ferredoxin and ferredoxin reductase for catalytic activity. Microsomal NADPH-cytochrome P-450 reductase could not replace ferredoxin and ferredoxin reductase. The cytochrome P-450 was inactive in 7 alpha-, 12 alpha- and 25-hydroxylations of C27-steroids. The results suggest that mitochondrial 26-hydroxylation of various C27-steroids is catalyzed by the same species of cytochrome P-450.     

Protein kinase stimulation of a reconstituted cholesterol side chain cleavage enzyme system in the bovine corpus luteum.

A solubilized preparation of cytochrome P-450, obtained by treatment of mitochondria from bovine corpora lutea with phospholipase A, contained all of the necessary components for the cholesterol side chain cleavage activity. The solubilized cytochrome -450 preparation could be isolated essentially free of endogenous cholesterol side chain cleavage activity by various fractionation techniques. A cholesterol side chain cleavage enzyme system was reconstituted using the isolated cytochrome P-450 preparation and purified adrenodoxin and adrenodoxin reductase (components of the enzyme system purified from the adrenal cortex). Protein kinase was partially purified from the cytosol fraction of bovine corpora lutea. It was purified 43-fold and the activity was highly dependent on cyclic adenosine 3:5-monophosphate (cyclic AMP). When ATP and this partially purified cyclic AMP-dependent protein kinase were added to the reconstituted cholesterol side chain cleavage enzyme assay in which cytochrome P-450 was limiting, a stimulation (20 to 74%) of the conversion of cholesterol into pregnenolone was observed. This stimulation was statistically significant with p value less than 0.001. The stimulatory effect of the protein kinase appeared to be dependent on ATP and was not mimicked by bovine serum albumin, indicating that the effect was specific for protein kinase. Protein kinase caused a phosphorylation of the cytochrome P-450 preparation when large amounts of this preparation were used in the assay. It is concluded from these results that the direct activation of the cytochrome P-450 component of the cholesterol side chain cleavage by protein kinase may be one of the ways by which cyclic AMP mediates the effect of luteinizine.     

Effects of phospholipid and detergent on the substrate specificity of adrenal cytochrome P-450scc. Substrate binding and kinetics of cholesterol side chain oxidation

Cytochrome P-450scc was isolated from mitochondria of bovine adrenal cortex by hydrophobic chromatography on octyl Sepharose followed by affinity chromatography on cholesterol-7-(thiomethyl)carboxy-3 beta-acetate-Sepharose. The partially purified eluate from the octyl Sepharose resin was free of adrenodoxin and adrenodoxin reductase and displayed biphasic binding characteristics for cholesterol, cholesterol sulfate, and cholesterol acetate (CA). Chromatography of the octyl Sepharose eluate on CA-Sepharose removed extraneous proteins and resolved the cytochrome P-450scc into two fractions, each of which displayed monophasic binding with all three substrates. These fractions behaved identically with respect to their ability to bind substrates, their kinetic properties, and their rate of migration during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The dissociation constants of the cytochrome P-450scc.substrate complexes are 1.1, 2.6, and 1.3 microM for cholesterol, cholesterol sulfate, and cholesterol acetate, respectively. Addition of phospholipids isolated from adrenal cortex mitochondria or adrenodoxin had no effect on the equilibrium binding constants. Addition of Emulgen 913, however, decreased the binding affinities 10-20-fold. Emulgen 913 also inhibited the interaction of adrenodoxin with the cytochrome. An active side chain cleavage system was reconstituted with purified P-450 by addition of saturating amounts of adrenodoxin, adrenodoxin reductase, and NADPH-generating system. The apparent Km values for this reconstituted system of cholesterol, cholesterol sulfate, and cholesterol acetate are 1.8, 1.9, and 0.6 microM, respectively. Since the Km values of substrate oxidation are similar to the Kd values of the cytochrome P-450.substrate complexes, it seems likely that the binding of substrates, particularly when the side chain cleavage system is free of mitochondrial membranes, is not rate-limiting. Based on these results and electrophoretic data, it appears that one cytochrome P-450 present in adrenal mitochondria can oxidize cholesterol, its sulfate, and its acetate. This enzyme represented about 60% of the cytochrome P-450 present in the octyl Sepharose eluate. The factors responsible for the biphasic kinetics of oxidation by intact mitochondria and biphasic binding of sterol substrates by partially purified preparations of cytochrome P-450scc are still unknown.