Inhibition of hCG- and cAMP-stimulated progesterone production in MA-10 mouse Leydig tumor cells by ketoconazole

In this study we attempted to examine the effects of ketoconazole on steroid biosynthesis and to determine which steps in the steroidogenic pathway were blocked using MA-10 Mouse Leydig tumor cells. This cloned cell line produces progesterone as the major steroid following stimulation by hCG or dbcAMP. At a concentration of 1 microM ketoconazole completely inhibited the hCG- and dbcAMP-stimulated progesterone synthesis in MA-10 Leydig cells. The conversion of 25-hydroxycholesterol and 22R-hydroxycholesterol into progesterone was also suppressed by this drug. The presence of ketoconazole inhibited mitochondrial steroid synthesis but required high concentrations of the drug as compared to inhibition in intact cells. No accumulation of pregnenolone was observed in the presence of ketoconazole indicating that the activity of 3 beta-hydroxysteroid dehydrogenase was not affected. We conclude that ketoconazole directly inhibits the activity of cholesterol side-chain cleavage enzyme (CSCC), a rate-determining enzymatic step in steroidogenesis, by interacting with cytochrome P-450scc.     

Purification and immunological characterization of human placental mitochondrial cytochrome P-450

Human placental mitochondrial cytochrome P-450 was purified to electrophoretic homogeneity by hydrophobic, anion exchange and cation exchange column chromatography. The specific content of the purified protein was 15.7 nmol/mg protein and it showed a single band mol. wt 48,000 D in SDS-gel electrophoresis. When reconstituted with bovine adrenal adrenodoxin reductase and adrenodoxin it converted cholesterol to pregnenolone (cholesterol side-chain cleavage activity, CSCC) at the rate of 1 pmol/min/pmol P-450. Antibodies against the purified protein were raised in rabbits. Inhibition studies demonstrated 85% inhibition of placental CSCC activity at an antibody/protein ratio of 10:1. Placental microsomal aromatase activity was inhibited by 47% at the same antibody/protein ratio. The antibody inhibited bovine mitochondrial CSCC activity by 87% at the same antibody/protein ratio. Placental microsomal 7-ethoxycoumarin O-deethylase, aryl hydrocarbon hydroxylase and 7-ethoxyresorufin O-deethylase activities were not significantly inhibited by the antibody. The results indicate that the purified protein catalyzes cholesterol side-chain cleavage reaction, human placental microsomal aromatase and bovine adrenal mitochondrial P-450scc may share common antigenic determinants with placental P-450scc, but the placental microsomal xenobiotic-metabolizing cytochrome(s) is (are) distinctly different.     

Production of sex steroid hormones from DHEA in articular chondrocyte of rats

Dehydroepiandrosterone (DHEA), a precursor of sex steroid hormones, is synthesized by cholesterol side-chain cleavage cytochrome P-450 and 17alpha-hydroxylase cytochrome P-450 mainly from cholesterol and converted to testosterone and estrogen by 3beta-hydroxysteroid dehydrogenase (3beta-HSD), 17beta-HSD, and aromatase cytochrome P-450. Although sex steroid hormones have important effects in the protection of articular cartilage, it is unclear whether articular cartilage has a local steroidogenic enzymatic machinery capable of metabolizing DHEA. This study was aimed to clarify whether steroidogenesis-related enzymes are expressed in articular chondrocytes, whether expression levels are changed by DHEA, and whether articular chondrocytes are capable of synthesizing sex steroid hormones from DHEA. Articular chondrocytes isolated from adult rats were cultured with DHEA for 3 days. All of the mRNA expressions of steroidogenesis-related enzymes were detected in cultured articular chondrocytes of rats, but the mRNA expression levels of testosterone and estradiol in cultured media increased after the addition of DHEA. These findings provided the first evidence that articular chondrocytes expressed steroidogenesis-related enzyme genes and that they are capable of locally synthesizing sex steroid hormones locally from DHEA.     

The side-chain cleavage of cholesterol sulfate--II. The effect of phospholipids on the oxidation of the sterol sulfate by inner mitochondrial membranes and by a reconstituted cholesterol desmolase system

This study compares the side-chain cleavage of aqueous suspensions of cholesterol sulfate with the side-chain cleavage of cholesterol sulfate which is incorporated into phospholipid vesicles. Three different cholesterol desmolase systems are examined: the membrane-bound cholesterol side-chain cleavage system present in inner mitochondrial membranes isolated from bovine adrenal mitochondria; a soluble, lipid-depleted, reconstituted side-chain cleavage system prepared from cytochrome P-450scc, adrenodoxin and adrenodoxin reductase; a membrane associated side-chain cleavage system prepared by adding phospholipid vesicles, prepared from adrenal mitochondrial, to the reconstituted system. Soluble cholesterol sulfate, in low concentration, is a good substrate for the lipid-depleted reconstituted side chain cleavage system. However, at concentrations above 2 microM, in the absence of phospholipids, the sterol sulfate appears to bind at a non-productive site on cytochrome P-450scc which leads to substrate inhibition. Phospholipids, while inhibiting the binding of cholesterol sulfate to the cytochrome, also appear to prevent non-productive binding of the sterol sulfate to the cytochrome. Thus the addition of phospholipids to the lipid-depleted enzyme system leads to an activation of side-chain cleavage of high concentrations of the sterol sulfate. Soluble cholesterol sulfate is a good substrate for both the native and reconstituted membrane-bound systems and no substrate inhibition is observed when the membrane bound enzyme systems are employed in the assay of side-chain activity. However, the cleavage of cholesterol sulfate, which is incorporated into phospholipid vesicles, by both membrane bound enzyme systems appears to be competitively inhibited by the phospholipids of the vesicles. The results of this study suggest that the regulation of the side-chain cleavage of cholesterol sulfate may be entirely different than the regulation of the side-chain cleavage of cholesterol, if cholesterol sulfate exists intracellularly as a soluble non-complexed substrate. If, on the other hand, cholesterol sulfate is present in the cell in lipid droplets as a complex with phospholipids, its metabolism may be under the same constraints as the side-chain cleavage of cholesterol.     

Cholesterol sulfate is a naturally occurring inhibitor of steroidogenesis in isolated rat adrenal mitochondria

We previously reported (Lambeth, J. D., Xu, X. X., and Glover, M. (1987) J. Biol. Chem. 262, 9181-9188) that exogenously added cholesterol sulfate inhibits the conversion of cholesterol to pregnenolone in isolated adrenal mitochondria, and does so by affecting intramitochondrial cholesterol movement but not its subsequent metabolism to pregnenolone by cytochrome P-450scc. We now report that a major kinetic component of the inhibition is noncompetitive with respect to cholesterol, consistent with an allosteric effect at a site other than the substrate binding site of cytochrome P-450scc. We now also report that cholesterol sulfate is present as an endogenous compound in preparations of adrenal mitochondria. Its content varied from 0.05 to 0.8 nmol/mg protein. Cholesterol sulfate level correlated inversely with the mitochondrial cholesterol side-chain cleavage activity. Endogenous cholesterol sulfate thus appeared to account for the variable rates of pregnenolone synthesis which were seen in different mitochondrial preparations. Cholesterol sulfate was metabolized to pregnenolone sulfate by a mitochondrial side-chain cleavage system, but proved to be a relatively poor substrate for an extramitochondrial steroid sulfatase activity present in adrenal cortex. Confirming a role as a naturally occurring inhibitor, removal of endogenous mitochondrial cholesterol sulfate by metabolism to pregnenolone sulfate correlated with a 3-fold activation of cholesterol side-chain cleavage. We suggest that cholesterol sulfate functions in steroidogenic tissues to regulate the magnitude of the steroidogenic response.     

25-Hydroxylation of vitamin D3 by a cytochrome P-450 from rabbit liver mitochondria.

A cytochrome P-450 catalysing 25-hydroxylation of vitamin D3 was purified from liver mitochondria of untreated rabbits. The enzyme fraction contained 9 nmol of cytochrome P-450/mg of protein and showed only one protein band with an apparent Mr of 52,000 upon SDS/polyacrylamide-gel electrophoresis. The preparation showed a single protein spot with an apparent isoelectric point of 7.8 and an Mr of approx. 52,000 upon two-dimensional isoelectric-focusing-polyacrylamide-gel electrophoresis. The purified cytochrome P-450 catalysed 25-hydroxylation of vitamin D3 up to 5000 times more efficiently than did 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 catalysed, in addition to 25-hydroxylation of vitamin D3, the 25-hydroxylation of 1 alpha-hydroxyvitamin D3 and the 26-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol. The enzyme did not catalyse side-chain cleavage of cholesterol, 11 beta-hydroxylation of deoxycorticosterone, 1 alpha-hydroxylation of 25-hydroxyvitamin D3, hydroxylations of lauric acid and testosterone or demethylation of benzphetamine. The results raise the possibility that the 25-hydroxylation of vitamin D3 and the 26-hydroxylation of C27 steroids are catalysed by the same species of cytochrome P-450 in liver mitochondria. The possible role of the liver mitochondrial cytochrome P-450 in the metabolism of vitamin D3 is discussed.     

Side-chain cleavage of cholesterol esters by human cytochrome P-450scc

In order to define the substrate binding site of human cytochrome P-450_scc in the vicinity of the 3β-hydroxyl group of cholesterol, we have tested the ability of the cytochrome to cleave the side chain of a range of cholesterol esters and cholesterol methyl ether. Using a Tween-20 detergent reconstituted system we found that cholesterol sulphate could undergo side-chain cleavage with the same turnover number (k_cat) as that for cholesterol, but with a higher K_m. Cholesterol methyl ether underwent side-chain cleavage to pregnenolone methyl ether with k_cat and K_m values 30% of those for cholesterol. Cholesterol fatty acid esters with acyl chain lengths of up to four carbons were able to undergo side-chain cleavage with K_m values similar to those for cholesterol, but k_cat values only 12–23% of those for cholesterol. Turnover numbers decreased as the acyl group length increased beyond four carbons, although some activity was still detected with cholesterol palmitate as substrate. Analysis of bovine cytochrome P-450_scc revealed that it could also cleave the side chain of acyl and sulphate esters of cholesterol. This study indicates that the substrate binding site of cytochrome P-450_scc in the vicinity of the 3β-hydroxyl group is larger than previously believed.     

The insulin-like growth factor, somatomedin C, induces the synthesis of cholesterol side-chain cleavage cytochrome P-450 and adrenodoxin in ovarian cells

The actions of insulin and somatomedin C (insulin-like growth factor I) on cholesterol side-chain cleavage activity and the synthesis of cytochrome P-450scc and adrenodoxin were investigated in primary cultures of swine ovarian (granulosa) cells. Nanomolar concentrations of pure human somatomedin C stimulated biosynthesis of progesterone and 20 alpha-hydroxypregn-4-en-3-one. Moreover, in the presence of exogenous sterol substrate for cholesterol side-chain cleavage, somatomedin C significantly enhanced pregnenolone biosynthesis in a time- and dose-dependent manner. This augmentation of functional cholesterol side-chain cleavage activity was accompanied by a dose-dependent (2-16-fold) increase in [35S]methionine incorporation into specific immunoprecipitable cytochrome P-450scc and adrenodoxin. Micromolar concentrations of insulin (but not proinsulin or desoctapeptide) also induced synthesis of cholesterol side-chain cleavage constituents by 4-7-fold. These results demonstrate that an insulin-like growth factor, somatomedin C, exerts discrete differentiating effects on ovarian cells characterized by increased synthesis of immunospecific cytochrome P-450scc and adrenodoxin. Thus, we infer that somatomedin C may serve a critical role in the differentiation of steroidogenic cells in the mammalian ovary.     

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.     

Side-chain cleavage of cholesterol sulfate by ovarian mitochondria

Mitochondria isolated from porcine corpora lutea and from the luteinized ovaries of gonadotropin-treated immature rats were found to efficiently cleave the side-chain of cholesterol sulfate to produce 3 beta-hydroxy-5-pregnen-20-one sulfate (pregnenolone sulfate). When mitochondria were preincubated with cholesterol sulfate, the time-course for the side-chain cleavage of cholesterol sulfate was biphasic. With 200 microM cholesterol sulphate, the initial rate of the reaction was the same as that observed for 25-hydroxycholesterol. This rate was not increased when both cholesterol sulfate and 25-hydroxycholesterol were incubated together. The rate of side-chain cleavage by isolated mitochondria supplied with 75 microM cholesterol sulfate as substrate was inhibited by 97% by aminoglutethimide, a specific inhibitor of cytochrome P-450scc. The slow phase of side-chain cleavage of cholesterol sulfate appeared to be limited by the rate of substrate movement to the mitochondrial site of the reaction. Cholesterol sulfate translocation rates were however up to 8 times greater than those observed for cholesterol when equivalent concentrations of the two substrates were added to the mitochondria. We conclude that cholesterol sulfate is a better substrate than cholesterol for side-chain cleavage by isolated mitochondria and that both reactions are catalysed by the same cytochrome P-450scc enzyme.     

Thermodynamic properties of oxidation-reduction reactions of bacterial, microsomal, and mitochondrial cytochromes P-450: an entropy-enthalpy compensation effect

An optically transparent thin-layer electrode cell with a very small volume was used for determination of the formal reduction potentials of bacterial, microsomal, and mitochondrial cytochromes P-450. At an extrapolated zero concentration of dye, the bacterial cytochrome from Pseudomonas putida catalyzing the hydroxylation of camphor and the adrenal mitochondrial cytochrome catalyzing the cholesterol side-chain cleavage reaction had formal reduction potentials of -168 and -285 mV (pH 7.5 and 25 degrees C), respectively. The oxidation-reduction potentials for the rabbit liver microsomal cytochrome P-450 induced by 3-methylcholanthrene and the mitochondrial cytochrome for steroid 11 beta-hydroxylation were found as -360 and -286 mV, respectively. Potential measurements at different temperatures allowed documentation of the standard thermodynamic parameters for cytochrome P-450 reduction for the first time. All cytochromes tested were found to have a relatively large negative entropy change upon reduction. The extent of these changes is comparable to that observed for the ferric-ferrous couple of cytochrome c. An entropy-enthalpy compensation effect was observed among the four cytochromes P-450 examined although the correlation is weaker than that observed with cytochrome c isolated from various sources.     

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

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

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.     

Molecular modeling of the 3-D structure of cytochrome P-450scc.

Sequence-alignment studies of the bovine mitochondrial cholesterol side-chain cleavage enzyme cytochrome P-450scc with the bacterial cytochrome P-450cam (camphor hydroxylating enzyme) have been undertaken. Our novel alignment of the sequences revealed 69 identical residues and many highly conserved regions. The results of the sequence alignment studies were used to model the 3-D structure of P-450scc based on the available crystal structure of P-450cam. The major insertions in the sequence are found mainly on four external-loop regions of the molecule, while the core structure of P-450cam is retained with subtle internal modifications. The most hydrophobic of these four external loops is proposed as a candidate for membrane attachment.     

Dominant bovine ovarian follicular cysts express increased levels of messenger RNAs for luteinizing hormone receptor and 3 beta-hydroxysteroid dehydrogenase delta(4),delta(5) isomerase compared to normal dominant follicles

The objective was to compare ovarian steroids and expression of mRNAs encoding cytochrome P450 side-chain cleavage, cytochrome P450 17 alpha-hydroxylase, cytochrome P450 aromatase, 3 beta-hydroxysteroid dehydrogenase Delta(4),Delta(5) isomerase, LH, and FSH receptors and estrogen receptor-beta in ovaries of cows with dominant and nondominant ovarian follicular cysts and in normal dominant follicles. Estradiol-17 beta, progesterone, and androstenedione concentrations were determined in follicular fluid using specific RIAs. Dominant cysts were larger than young cysts or dominant follicles, whereas nondominant cysts were intermediate. Estradiol-17 beta (ng/ml) and total steroids (ng/follicle) were higher in dominant cysts than in dominant follicles. Expression of LH receptor and 3 beta-hydroxysteroid dehydrogenase mRNAs was higher in granulosa cells of dominant cysts than in dominant follicles. Nondominant cysts had higher follicular concentrations of progesterone, lower estradiol-17 beta concentrations, and lower expression of steroidogenic enzyme, gonadotropin receptor, and estrogen receptor-beta mRNAs than other groups. In summary, increased expression of LH receptor and 3 beta-hydroxysteroid dehydrogenase mRNAs in granulosa and increased follicular estradiol-17 beta concentrations were associated with dominant cysts compared to dominant follicles. Study of cysts at known developmental stages is useful in identifying alterations in follicular steroidogenesis.     

Ultrastructural localization of cytochrome P-450scc in the bovine placentome using protein A-gold technique

We have previously reported that the steroidogenic activity of the bovine placentome is stimulated by a calcium-mediated, cyclic nucleotide-independent mechanism and that this steroidogenesis is limited by the availability of sterol substrate to the side-chain cleavage enzyme. We have recently established that the antibody against bovine adrenal cytochrome P-450 cholesterol side-chain cleavage enzyme (P-450scc) can be used to specifically detect P-450scc in both bovine placentome and corpus luteum. In the present study, we used an immunogold technique to localize the P-450scc in the bovine placentome by electron microscopy. The mononucleate cell of the cotyledon showed both giant and normal-sized mitochondria, with the latter, predominating. Both mitochondrial types found in the mononucleate cells clearly displayed gold particles located on the cristae; in contrast, these particles were absent in the binucleate cells. It is worth noting that giant mitochondria were found exclusively in the placental mononucleate cells in both the fetal and maternal sites but not in the binucleate cells. These findings suggest that the cholesterol side-chain cleavage enzyme is present in bovine cotyledon cells, primarily in mononucleate cells. The variations in P-450scc immunoreactivity among different cells of the placenta are suggestive of different steroidogenetic capacities of the cells.     

Comparison of the immunochemical properties of human placental and bovine adrenal cholesterol side-chain cleavage enzyme complex

The immunochemical relatedness between human and bovine proteins catalyzing the cholesterol side-chain cleavage reaction was investigated. In dot-immunobinding analysis, antibodies against bovine adrenocortical cytochrome P-450SCC, adrenodoxin, and adrenodoxin reductase recognized the corresponding proteins in a dose-dependent manner in mitochondrial preparations from human placenta. Limited proteolysis with trypsin cleaved bovine P-450SCC into fragments F1 and F2, which represent the NH2- and C-terminal parts of P-450SCC, respectively. Identical trypsin treatment yielded similar-size fragments from human placental P-450SCC. In Western immunoblots, anti-F1 and anti-F2 antibodies recognized the corresponding fragments in both trypsin-digested bovine and human P-450SCC. Antibodies against bovine P-450SCC, fragments F1 and F2, adrenodoxin and adrenodoxin reductase inhibited cholesterol side-chain cleavage activity in bovine adrenocortical mitochondria by 24-51%, but failed to affect the activity in human placental mitochondria. These data indicate that human and bovine P-450SCC share common antigenic determinants located outside the enzyme active site. The immunological similarity between bovine adrenodoxin and human ferredoxin allowed for a simple purification protocol of human placental P-450SCC by adrenodoxin affinity chromatography. The P-450SCC obtained by this method was electrophoretically homogeneous and showed characteristics typical to P-450SCC.     

A -glucuronidase deficiency mucopolysaccharidosis: studies in cultured fibroblasts

Cytochrome P-450 cannot be detected spectrophotometrically in testis mitochondria of untreated rats because of the high cytochrome a₃ to Cytochrome P-450 ratio. Injection of human chorionic gonadotrophin (HCG) causes a large increase in mitochondrial cytochrome P-450. After 14 days injection, mitochondrial cytochrome P-450 levels are increased 15- to 30-fold (from 0.007 to 0.134 nmoles/mg protein) over control levels. Levels of cytochrome a + a₃ are not altered by this treatment. Mitochondrial cytochrome P-450 can also be demonstrated by injection of HCG into rats which were hypophysectomized 24 days previously. During hypophysectorny the mitochondrial cytochromes c + c_i, a + a₃ and mitochondrial protein decay with halflives of 14, 16, and 15.5 days, respectively. HCG treatment for 8 days increases mitochondrial cytochrome P-450 (from < 0.003 to 0.24 nmoles/mg protein) without altering the levels of the other mitochondrial cytochromes. The control of cytochrome P-450 levels in the mitochondria by HCG suggests that the level of this key component of cholesterol side-chain cleavage enzyme may be of importance in the regulation of steroidogenesis in the testis.     

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

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

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)