Steroidogenesis in liposomal system containing adrenal microsomal cytochrome P-450 electron transfer components

Tributyltin disturbs bovine adrenal steroidogenesis by two modes of action

Tributyltin, an environmental pollutant, affected adrenal steroid hormone biosynthesis by two modes of action. Treatment of bovine adrenal cultured cells with 10-100 nM tributyltin for 48 h suppressed cortisol and androstenedione secretion, but induced the accumulation of 17alpha-hydroxyprogesterone and deoxycortisol, indicating that the P450(C21) and P450(11beta) activities were specifically suppressed. Direct inhibition of the enzymatic activities due to tributyltin was not observed in isolated organelles of untreated cells at concentrations less than 10 microM. Western blotting experiments using specific antibodies against steroidogenic enzymes showed that treatment with 1-100 nM tributyltin caused a decrease in cellular P450(C21) and P450(11beta) protein levels, and real-time PCR experiments showed that the decrease in protein content was attributable to decreases in mRNA of the enzymes. Tributyltin at concentrations higher than 100 nM suppressed all steroid biosynthesis in the adrenal cells. This suppression was closely correlated to the decrease in steroidogenic acute regulatory protein. Since nanomolar concentrations of tributyltin disturbed steroidogenesis in mammalian cells, there is the possibility that steroid hormone synthesis in polluted wild animals is affected by this compound.

Corticosterone enhances adrenocorticotropin-induced calcium signals in bovine adrenocortical cells

The rapid effects of steroid hormones on Ca(2+) signals have been examined in bovine adrenocortical cells. Among the steroid molecules tested, only corticosterone rapidly stimulated Ca(2+) signals upon addition of ACTH, although corticosterone alone did not induce Ca(2+) signals. Corticosterone also enhanced steroidogenesis induced by ACTH. The enhancement of ACTH-induced Ca(2+) signals was also observed with membrane-impermeable corticosterone conjugated to BSA and was not inhibited by cycloheximide. In addition, corticosterone did not enhance Ca(2+) signals induced by ATP or angiotensin II. These results suggest that corticosterone selectively stimulates ACTH-induced Ca(2+) signals in a nongenomic way by acting on a target in the plasma membrane. Furthermore, the supernatants of cells incubated with ACTH or ATP enhanced Ca(2+) signals, suggesting that steroids produced by such treatment act in an autocrine fashion. Consistent with this idea, these effects were inhibited by inhibitors of steroidogenesis (aminoglutethimide or metyrapone). These results show that steroid molecules synthesized in adrenocortical cells facilitate ACTH-induced Ca(2+) signals. Taken together, corticosterone secreted from adrenocortical cells activates ACTH-induced Ca(2+) signals and steroidogenesis by nongenomic means.

Identification of outer mitochondrial membrane cytochrome b5 as a modulator for androgen synthesis in Leydig cells

Outer mitochondrial membrane cytochrome b5 is an isoform of microsomal membrane cytochrome b5. In rat testes the outer mitochondrial membrane cytochrome b5 is present in both mitochondria and microsomes, whereas microsomal membrane cytochrome b5 is undetectable. Outer mitochondrial membrane cytochrome b5 present in the testis was localized in Leydig cells with cytochrome P-45017alpha, which catalyzes androgenesis therein. We therefore analyzed the functions of outer mitochondrial membrane cytochrome b5 in rat testis microsomes by using a proteoliposome system. In a low but physiological concentration of NADPH-cytochrome P-450 reductase and excess amount of progesterone, outer mitochondrial membrane cytochrome b5 stimulated the cytochrome P-45017alpha-catalyzed reactions, 17alpha-hydroxylation and C17-C20 bond cleavage. The effects were different from those by microsomal membrane cytochrome b5 as follows: preferential elevation of the 17alpha-hydroxylase activity by outer mitochondrial membrane cytochrome b5 in an amount-dependent manner versus that of the lyase activity by microsomal membrane cytochrome b5 at the low concentration, and the inhibition of both activities at the high concentration. At a low concentration of progesterone reflecting a physiological cholesterol supply, outer mitochondrial membrane cytochrome b5 elevated primarily the production of 17alpha-hydroxyprogesterone and then facilitated the conversion of the released intermediate to androstenedione. Thus, we demonstrated that outer mitochondrial membrane cytochrome b5 and not microsomal membrane cytochrome b5 functions as an activator for androgenesis in rat Leydig cells.

Membrane reconstitution of recombinant guinea pig cytochrome P45017alpha and the effects of site-directed mutagenesis on androgen formation

Although cytochrome P45017alpha catalyzes the formation of androgen from both pregnenolone and progesterone, the production of androstenedione from progesterone is a major pathway in the guinea pig, rat, mouse, and hamster. In contrast, human, bovine and sheep P45017alpha produce dehydroepiandrosterone from pregnenolone. Cytochrome P45017alphas from all of these animals have high homology in the amino acid sequence around the 340-370 region. To investigate the substrate preferences for androgen production, we replaced a few amino acids in the 340-370 region of guinea pig P45017alpha with those found in the other animals. The recombinant P45017alphas were expressed in E. coli DH5alpha, purified by column chromatography and incorporated into liposome membranes. The (His)(4) tag in the recombinant P45017alphas had little effect on the interaction with NADPH-P450 reductase in the membranes. The recombinant P45017alphas with a single-position mutation of F344I, H349R or M352L and with double-position mutations of F344I and H349R and triple-position mutations showed decreases in the production of 17alpha-hydroxypregnenolone, androstenedione and dehydroepiandrosterone. The activity for 17alpha-hydroxyprogeterone production was increased significantly by the F344I mutation. The addition of cytochrome b5 did not have much of an effect on the 17alpha-hydroxylation but had a significant effect on androgen production in both the nonmutated and mutated P45017alphas.

Expression profiles of COUP-TF, DAX-1, and SF-1 in the human adrenal gland and adrenocortical tumors: possible implications in steroidogenesis

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF), DAX-1, and steroidogenic factor-1 (SF-1) are orphan members of the nuclear hormone receptor superfamily. COUP-TF and DAX-1 have been shown to negatively regulate the transcriptional activity of SF-1, a steroidogenic cell-specific activator of various steroidogenic cytochrome P450 genes. We therefore examined the expression levels and immunolocalization of COUP-TF, DAX-1, and SF-1 in human adrenal gland (NL) and adrenocortical adenomas, and compared the results with CYP17 expression levels and its enzyme activities to study their potential correlation with adrenocortical steroidogenesis. In NL (n = 10), expressions of COUP-TF, DAX-1, and SF-1 were detected in the nuclei of adrenocortical cells, but not in the medulla. In cortisol-producing adenomas causing Cushing syndrome (CS, n = 20), CYP17 expression was upregulated (298 +/- 2% vs NL 98 +/- 4%), whereas expression levels of both COUP-TFs (COUP-TFI, 52 +/- 5% vs NL 98 +/- 4%; COUP-TFII, 18 +/- 4% vs NL 98 +/- 4%) and DAX-1 (42 +/- 4% vs NL 100 +/- 4%) were reduced. In deoxycorticosterone-producing adenomas (DOC, n = 2), on the other hand, CYP17 expression was extremely reduced (8 and 12% vs NL 98 +/- 4%), whereas DAX-1 expression increased markedly (350 and 360% vs NL 100 +/- 4%). Expression levels of SF-1 did not differ between NL (100 +/- 8%) and CS (106 +/- 10%), but its expression appeared to be decreased in DOC (25 and 20%). These results showed CYP17 expression to be upregulated and downregulated in CS and DOC, respectively, in a manner reciprocal to that of its repressors, COUP-TF and/or DAX-1. In summary, the results indicate that co-localization of COUP-TF, DAX-1, and SF-1 in NL was lost in adrenocortical tumors and that these orphan receptors play an important role in the regulation of steroidogenesis in human adrenals.

The rate-determining step in P450 C21-catalyzing reactions in a membrane-reconstituted system

Adrenal cytochrome P450 C21 in a membrane-reconstituted system catalyzed 21-hydroxylation of 17alpha-hydroxyprogesterone at a rate higher than that for progesterone in the steady state at 37 degrees C. The rate of product formation in the steady state increased with the concentration of the complex between P450 C21 and the reductase in the membranes. The complex formation was independent of the volume of the reaction, showing that the effective concentrations of the membrane proteins should be defined with the volume of the lipid phase. The rates of conversion of progesterone and 17alpha-hydroxyprogesterone to the product in a single cycle of the P450 C21 reaction were measured with a reaction rapid quenching device. The first-order rate constant for the conversion of progesterone by P450 C21 was 4.3 +/- 0.7 s(-)1, and that for 17alpha-hydroxyprogesterone was 1.8 +/- 0.5 s(-)1 at 37 degrees C. It was found from the analysis of kinetic data that the rate-determining step in 21-hydroxylation of progesterone in the steady state was the dissociation of product from P450 C21, whereas the conversion to deoxycortisol was the rate-determining step in the reaction of 17alpha-hydroxyprogesterone. The difference in the rate-determining steps in the reactions for the two substrates was clearly demonstrated in the pre-steady-state kinetics.

Conversion of pregnenolone to DHEA by human 17alpha-hydroxylase/17, 20-lyase (P450c17). Evidence that DHEA is produced from the released intermediate, 17alpha-hydroxypregnenolone

Most previous studies using reconstituted systems and fast kinetics suggest that the conversion of pregnenolone to dehydroepiandrosterone (DHEA; the precursor of androgen and estrogen biosynthesis) by P450c17 does not require the release of the intermediate 17alpha-OHPreg (a precursor of cortisol biosynthesis). With such a mechanism, it is difficult to conceive how high amounts of DHEA may be produced in some cells or tissues, such as the testis and cells from the adrenal reticularis, while in other tissues such as the fasciculata zone, high levels of 17alpha-OHPreg are synthesized. In this report, we address this matter using intact transfected cells, which better reflect the actual cellular conditions. Furthermore, by using transfected cells, we can conveniently analyze human enzymes, as we are not restricted by the availability of human tissues as in the case of methods using purified or partially purified enzymes. Using intact HEK-293 cells transfected with human P450c17 in culture, we showed, in a time course study of the transformation of pregnenolone, that there is an accumulation of 17alpha-OHPreg, and that, subsequently, the accumulated 17alpha-OHPreg decreases with a concomitant increase in DHEA production. The DHEA/17alpha-OHPreg ratio changes from 0.1 :1 after 1 h incubation to 50 : 1 after 20 h. This result strongly suggests that the transformation of Preg to DHEA proceeds through two steps in which DHEA is produced from the released intermediate 17alpha-OHPreg. We also show that high levels of substrate vs. enzyme concentration will lead to high hydroxylase activity whereas the reverse will increase the lyase activity. The result is in good agreement with recent observations suggesting that surrounding enzymes and steroids could modulate the lyase activity. Cotransfection of vectors expressing cytochrome b5 and NADPH cytochrome P450 reductase indicates that both are required for an optimum production of DHEA.

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency

More than 90% of cases of congenital adrenal hyperplasia (CAH, the inherited inability to synthesize cortisol) are caused by 21-hydroxylase deficiency. Females with severe, classic 21-hydroxylase deficiency are exposed to excess androgens prenatally and are born with virilized external genitalia. Most patients cannot synthesize sufficient aldosterone to maintain sodium balance and may develop potentially fatal "salt wasting" crises if not treated. The disease is caused by mutations in the CYP21 gene encoding the steroid 21-hydroxylase enzyme. More than 90% of these mutations result from intergenic recombinations between CYP21 and the closely linked CYP21P pseudogene. Approximately 20% are gene deletions due to unequal crossing over during meiosis, whereas the remainder are gene conversions--transfers to CYP21 of deleterious mutations normally present in CYP21P. The degree to which each mutation compromises enzymatic activity is strongly correlated with the clinical severity of the disease in patients carrying it. Prenatal diagnosis by direct mutation detection permits prenatal treatment of affected females to minimize genital virilization. Neonatal screening by hormonal methods identifies affected children before salt wasting crises develop, reducing mortality from this condition. Glucocorticoid and mineralocorticoid replacement are the mainstays of treatment, but more rational dosing and additional therapies are being developed.

YM116, 2-(1H-imidazol-4-ylmethyl)-9H-carbazole, decreases adrenal androgen synthesis by inhibiting C17-20 lyase activity in NCI-H295 human adrenocortical carcinoma cells

The concentrations of androstenedione and dehydroepiandrosterone, products of C17-20 lyase, in the medium after a 6-hr incubation of NCI-H295 cells were decreased by YM116 (2-(1H-imidazol-4-ylmethyl)-9H-carbazole) (IC50: 3.6 and 2.1 nM) and ketoconazole (IC50: 54.9 and 54.2 nM). 17Alpha-hydroxyprogesterone, a product of 17alpha-hydroxylase, was increased by YM116 (1-30 nM) and by ketoconazole (10-300 nM) and then was decreased at higher concentrations of both agents (IC50: 180 nM for YM116, 906 nM for ketoconazole), indicating that YM116 and ketoconazole were 50- and 16.5-fold more specific inhibitors of C17-20 lyase, respectively, than 17alpha-hydroxylase. Compatible with these findings, progesterone, a substrate of 17alpha-hydroxylase, was increased by these agents. Cortisol production was inhibited by YM116 and ketoconazole (IC50: 50.4 and 80.9 nM, respectively). YM116 was a 14-fold more potent inhibitor of androstenedione production than cortisol production, whereas ketoconazole was a nonselective inhibitor of the production of both steroids. YM116 and ketoconazole inhibited the C17-20 lyase activity in human testicular microsomes (IC50: 4.2 and 17 nM, respectively). These results demonstrate that YM116 reduces the synthesis of adrenal androgens by preferentially inhibiting C17-20 lyase activity.

Molecular engineering study on electron transfer from NADPH-P450 reductase to rat mitochondrial P450c27 in yeast microsomes

We have reported the localization on yeast microsomes for a modified P450c27 (mic-P450c27) that contains the microsomal targeting signal of bovine P450c17 in front of the mature form of rat mitochondrial P450c27 (Sakaki, T., Akiyoshi-Shibata, M., Yabusaki, Y., and Ohkawa, H. (1992) J. Biol. Chem. 267, 16497-16502). In this study, we found that mic-P450c27 could be reduced by NADPH in the yeast microsomes without supplement of its physiological redox partners, adrenodoxin and NADPH-adrenodoxin reductase. In order to elucidate the direct electron transfer from NADPH-P450 reductase to mic-P450c27, we carried out simultaneous expression of mic-P450c27 and yeast P450 reductase. The reduction rate of mic-P450c27 was increased by overproduction of yeast P450 reductase, roughly in proportion to the reductase content in the microsomes. In addition, we constructed a fused enzyme between mic-P450c27 and yeast P450 reductase. The reduction rate of heme iron in the fused enzyme was too rapid to be measured. These recombinant yeast microsomes showed a notable 27-hydroxylation activity toward 5beta-cholestane-3alpha,7alpha, 12alpha-triol in the absence of adrenodoxin and adrenodoxin reductase. Finally, we purified mic-P450c27 from the recombinant yeast microsomes and reconstituted the hydroxylation system in liposomal membranes using the purified mic-P450c27 and yeast NADPH-P450 reductase. Mic-P450c27 was reduced by NADPH and showed its monooxygenase activity on the reconstituted system. Therefore, yeast NADPH-P450 reductase alone was found to transfer two electrons from NADPH to mic-P450c27. These results clearly show that mic-P450c27 not only localizes on the microsomes but also functions as a microsomal cytochrome P450 that accepts electrons from NADPH-P450 reductase.

Reconstitution of the steroidogenic pathway from cholesterol to aldosterone in liposome membranes

A steroidogenic pathway from cholesterol to aldosterone was reconstituted in liposome membranes using cytochromes P-450scc, P-450C21 and P-450(11) beta, and 3 beta-hydroxysteroid dehydrogenase/ delta 5-delta 4 isomerase (3 beta HSD/I) with their electron transfer systems. All of the enzymes were purified from bovine adrenocortical mitochondria and microsomes. The cholesterol metabolism in the liposomal reconstituted system was compared with that in the combined organella system composed of bovine adrenocortical mitochondria and microsomes, where the activity of P-450(17) alpha,lyase was inhibited by bifonazole. The metabolic activities in these two systems were similar except for aldosterone production. Aldosterone was produced in the liposomal system but not in the combined organella system. 4-fold increase in the amount of P-450scc in the liposomal system enhanced the activity of 3 beta HSD/I, P-450C21 and 11 beta-hydroxylase of P-450(11) beta but decreased 18-hydroxycorticosterone and aldosterone production by P-450(11) beta, supporting our previous findings describing the regulation mechanism of aldosterone synthesis (Kominami, S., Harada, D. and Takemori, S. (1994) Biochim. Biophys. Acta 1192, 234). It was demonstrated using the liposomal reconstituted system that the increase in the amount of one enzyme did not only increase the metabolizing activity of that enzyme but also affect other enzyme in various ways.

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.

Inhibition of androgen synthesis by 22-hydroximino-23,24-bisnor-4-cholen-3-one

The 22-hydroximino-23,24-bisnor-4-cholen-3-one (22-oxime) was synthesized and evaluated as an inhibitor of 17 alpha-hydroxylase/C17,20-lyase in rat testicular microsomes and the 5 alpha-reducatase of human prostatic microsomes from patients with benign prostatic hypertrophy. The 22-oxime demonstrated moderate inhibition for the 17 alpha-hydroxylase (Ki 74 nM vs. Km 29 nM) with progesterone as substrate and potent inhibition (Ki 18 nM vs. Km 76 nM) for the C17,20-lyase activity with 17 alpha-hydroxyprogesterone as substrate. Further investigation of this enzyme with progesterone as substrate demonstrated the inhibition occurred mainly at the 17 alpha-hydroxylation step of the progesterone substrate. The 22-oxime also demonstrated potent and competitive inhibition of 5 alpha-reductase in human prostatic microsomes (Ki 1.4 nM vs. Km 14 nM). When adult male rats were injected subcutaneously (sc) daily with 22-oxime (50 mg/kg/day) for 21 days, the concentrations of serum and testicular testosterone were significantly reduced by 65% and 59%, respectively, in comparison to vehicle-treated controls. Furthermore, both testosterone and DHT concentrations in rat prostatic tissue were significantly decreased by 60% and 44% compared to control tissue. Serum LH concentrations were unchanged in the 22-oxime-treated group compared to the control group. This indicates that the reduction in androgen concentrations in animals treated with this compound is not due to its influence on pituitary feedback mechanisms which result in reduced LH secretion. These results suggest that 22-oxime is effective in reducing androgen synthesis through the inhibition of 17 alpha-hydroxylase, C17,20-lyase, and 5 alpha-reductase both in vitro and in vivo.

Significance of steroidogenic enzymes in the pathogenesis of hyperfunctioning and non-hyperfunctioning adrenal tumor

To elucidate the mechanisms of abnormal steroid production in hyperfunctioning and non-hyperfunctioning adrenal tumors, we examined both the activities and amounts of steroidogenic cytochromes P450 in the tumor and non-tumor portions of these adrenals at the posttranslational (protein) level. Adrenals from 5 patients with primary aldosteronism, 5 with Cushing's syndrome, 1 with deoxycorticosterone (DOC)-producing adenoma, 10 with non-hyperfunctioning adrenal adenoma, and 5 subjects with normal control adrenals (obtained from patients with renal cell carcinoma) were used in our studies. Activities of P450scc, P45011 beta and P450aldo, and P450C21 and P-45017 alpha were assayed in a reconstituted enzyme system using 20 alpha-hydroxycholesterol, DOC, and progesterone, respectively, and the substrate and the extracted products were analyzed by HPLC. Enzyme amounts were determined by immunoblot analysis with anti-bovine P450scc, P45011 beta, and P450C21 IgG, and anti-porcine P45017 alpha IgG. Human P450aldo was only detected in the tumor portion of primary aldosteronism adrenals, with both activities and amounts of other P-450s similar to those in the non-tumor portion of primary aldosteronism and normal controls. In Cushing's syndrome, both activities and amounts of P45017 alpha and P450C21 were significantly increased in the tumor compared with those in the non-tumor portion of Cushing's syndrome and normal controls. In DOC-producing adenoma, both activities and amounts of P45017 alpha and P45011 beta in the tumor portion of the adenoma decreased compared with normal control, while those of other P450s were similar to normal controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning and expression of guinea pig cytochrome P450c17 cDNA (steroid 17 alpha-hydroxylase/17,20 lyase): tissue distribution, regulation, and substrate specificity of the expressed enzyme

In mammalian and fish species, P450c17 mediates both 17 alpha-hydroxylase and 17,20-lyase activities in the synthesis of steroid hormones. Previous results have shown that among the adrenal steroid hydroxylase enzymes involved in adrenal C19 steroid and glucocorticoid synthesis, regulation of cytochrome P450c17 is of primary importance because it is localized at the key branch between glucocorticoid and C19 steroid synthesis. A cDNA library from guinea pig adrenal was constructed, and the complete 17 alpha-hydroxylase cytochrome P450 cDNA was isolated. The guinea pig P450c17 cDNA includes the full-length coding region (1,524 nucleotide), the complete 3' untranslated region (169 nucleotide), and 39 bases of the 5' untranslated region. Our clone shares most of the features of the other P450c17 cDNAs; however, in addition, we identified a novel conserved region of 18 amino acids located in exon I between residues 80 and 97. This region presents the highest percentage of identity among the other P450c17 enzymes and is positioned one helixturn upstream of the important Ser106 on the corresponding human form. On Northern blot, the cDNA hybridizes with a major 1.8-kb mRNA and with two other related P450c17 mRNA of about 3 and 4 kb. P450c17 mRNA is equally distributed in male and female gonads and adrenals. Characterization of the enzymatic activity shows that 17 alpha-hydroxylase and 17,20-lyase are carried by a single protein, but in homogenates 17,20-lyase activity is barely detectable. Moreover, we demonstrate in vitro and in vivo that the guinea pig enzyme preferentially has very high levels of 17 alpha-hydroxylase and 17,20-lyase activities only toward delta 4 steroids. Second-messenger cyclic adenosine monophosphate and adrenocorticotropin specifically increased the abundance of P450c17 mRNA levels in guinea pig adrenal cells.

Regulation mechanism of the catalytic activity of bovine adrenal cytochrome P-450(11)beta

In our previous paper (Ikushiro et al. (1992) J. Biol. Chem. 267, 1464), two catalytic states were proposed for bovine adrenocortical P-450(11)beta at 37 degrees C: one in liposome membranes and the other in liposome membranes containing P-450scc. Similar reaction characteristics were observed at 5 degrees C and all the experiments in this study were performed at 5 degrees C. P-450(11)beta-proteoliposomes had relatively low 11 beta-hydroxylase activity and could catalyze aldosterone formation from 11-deoxycorticosterone. Relatively high 11 beta-hydroxylase activity was observed in P450(11)beta-proteoliposomes containing P-450scc and in Tween-20 solubilized P-450(11)beta, in which no aldosterone formation could be detected. Optical titration indicated binding of corticosterone to P-450(11)beta to be much weaker in the Tween-20 solubilized state than in proteoliposomes. Corticosterone competitively inhibited 11 beta-hydroxylation reaction of P-450(11)beta-proteoliposomes, but neither in P-450(11)beta-proteoliposomes containing P-450scc nor in the Tween-20 solubilized system. The binding of corticosterone to P-450(11)beta was concluded quite weak in proteoliposomes in the presence of P-450scc and in the Tween-20 solubilized state. Aldosterone formation thus was not possible in these systems. Inability of the bovine adrenocortical zonae fasciculata and reticularis to produce aldosterone may be due to the weak binding of corticosterone to P-450(11)beta in these zones.

Cytochrome P-450(17 alpha,lyase)-mediating pathway of androgen synthesis in bovine adrenocortical cultured cells

Cytochrome P-450(17 alpha,lyase) mediating pathway of dehydroepiandrosterone (DHA) formation from pregnenolone was investigated in primary cultures of bovine adrenocortical fasciculata-reticularis cells. To determine whether DHA formation proceeds predominantly by successive monooxygenase reactions without 17 alpha-hydroxypregnenolone leaving P-450(17 alpha,lyase) the cells were incubated with [14C]pregnenolone and 17 alpha-[3H]hydroxypregnenolone in the presence of Trilostane. Results of the double-substrate double-label experiments indicate that in the presence of high concentration of pregnenolone most of DHA was formed, directly from pregnenolone by the successive reactions. Since the concentration of pregnenolone usually exceeds that of 17 alpha-hydroxypregnenolone in the adrenal glands, DHA is concluded to be formed predominantly by successive reactions from pregnenolone without 17 alpha-hydroxypregnenolone leaving P-450(17 alpha,lyase) in vivo. By chronic ACTH treatment, the activities of 17 alpha-hydroxylation and DHA formation in adrenocortical cultured cells became higher concomitantly with the increase of P-450(17 alpha,lyase) content. Most of DHA was found to be formed by successive reactions from pregnenolone even under such conditions.

The role of cytochrome b5 in adrenal microsomal steroidogenesis

The role of cytochrome b5 in adrenal microsomal steroidogenesis was studied in guinea pig adrenal microsomes and also in the liposomal system containing purified cytochrome P-450s and NADPH-cytochrome P-450 reductase. Preincubation of the microsomes with anti-cytochrome b5 immunoglobulin decreased both 17 alpha- and 21-hydroxylase activity in the microsomes. In liposomes containing NADPH-cytochrome P-450 reductase and P-450C21 or P-450(17) alpha,lyase, addition of a small amount of cytochrome b5 stimulated the hydroxylase activity while a large amount of cytochrome b5 suppressed the hydroxylase activity. The effect of cytochrome b5 on the rates of the first electron transfer to P-450C21 in liposome membranes was determined from stopped flow measurements and that of the second electron transfer was estimated from the oxygenated difference spectra in the steady state. It was indicated that a small amount of cytochrome b5 activated the hydroxylase activity by supplying additional second electrons to oxygenated P-450C21 in the liposomes while a large amount of cytochrome b5 might suppress the activity through the interferences in the interaction between the reductase and P-450C21.

Kinetic control of steroidogenesis by steroid concentration in guinea pig adrenal microsomes

For clarification of the effects of steroid concentration on steroidogenesis of adrenal microsomes, the kinetic parameters, Km and kcat, were determined in the steady-state for progesterone and 17 alpha-hydroxyprogesterone metabolism catalyzed by P-450C21 and P-450(17 alpha lyase) in guinea pig adrenal microsomes. At a high concentration of progesterone, it was equally metabolized by P-450C21 and P-450(17 alpha lyase), while at a low concentration, it was hydroxylated at 17 alpha-position with twice higher rate than at 21-position. 17 alpha-Hydroxyprogesterone is apparently metabolized preferentially by P-450C21 at any concentration. Although the productions of deoxycortisol and androstenedione from 17 alpha-hydroxyprogesterone were strongly inhibited by progesterone, androstenedione formation from progesterone was not inhibited by a high concentration of progesterone. The addition of liposomal P-450C21 to the reaction medium containing adrenal microsomes caused a decrease in the concentration of 17 alpha-hydroxyprogesterone released into the medium in the steady state reaction, but this had no effect on the activity of androstenedione formation from high concentrations of progesterone. It thus follows that androstenedione is produced by successive monooxygenase reactions without the release of 17 alpha-hydroxyprogesterone from P-450(17 alpha lyase) at a high concentration of progesterone, which is the condition of the adrenal microsomes in vivo.

Cytochrome P-450scc-catalyzed production of progesterone from 22R-hydroxycholest-4-en-3-one by way of 20,22-dihydroxycholest-4-en-3-one

Transient accumulation of a dihydroxylated steroid was found when 22R-hydroxycholest-4-en-3-one was used as the substrate for a reconstituted cholesterol side-chain cleavage system derived from bovine adrenocortical mitochondria. The indications were that the accumulated steroid was an intermediate in the cytochrome P-450scc-catalyzed reaction. The retention time of the accumulated intermediate was identical with that of authentic 20,22-dihydroxycholest-4-en-3-one on HPLC. When 22R-hydroxycholesterol and 22R-hydroxycholest-4-en-3-one were incubated simultaneously, the total amount of reaction products was essentially the same as that observed with 22R-hydroxycholest-4-en-3-one alone. Under the conditions employed, the apparent turnover number of cytochrome P-450scc for 22R-hydroxycholesterol was calculated to be 77 nmol/min/nmol P-450 from the amount of pregnenolone formed, whereas the apparent turnover number for 22R-hydroxycholest-4-en-3-one was 64 nmol/min/nmol P-450 with respect to the intermediate formation and 77 nmol/min/nmol P-450 with respect to the progesterone formation. The apparent turnover number for 20,22-dihydroxycholest-4-en-3-one was about 125 nmol/min/nmol P-450, which was not significantly different from that of 20,22-dihydroxycholesterol. The apparent Km for 22R-hydroxycholesterol was about 20 microM and those for 22R-hydroxycholest-4-en-3-one and 20,22-dihydroxycholest-4-en-3-one were 50 and 40 microM, respectively. Thus, 22R-hydroxycholest-4-en-3-one was efficiently metabolized to progesterone by way of 20,22-dihydroxycholest-4-en-3-one by cytochrome P-450scc.