Regulation of cholesterol metabolism in rat adrenal mitochondria

Diethylumbelliferyl phosphate inhibits steroidogenesis by interfering with a long-lived factor acting between protein kinase A activation and induction of the steroidogenic acute regulatory protein (StAR)

Diethylumbelliferyl phosphate (DEUP) is an organophosphate cholesteryl ester hydrolase inhibitor that blocks steroidogenesis mainly by preventing cholesterol transport into the mitochondria of steroidogenic cells. In the present study, we show that DEUP blocks the cAMP-stimulated mitochondrial accumulation of the 30-kDa mitochondrial proteins (recently named steroidogenic acute regulatory StAR proteins) that are believed to be the cycloheximide-sensitive factors induced by trophic hormones and cAMP. Inhibition of mitochondrial StAR accumulation by DEUP is dose dependent and closely parallels inhibition of progesterone synthesis. Stimulated lactate production, another cAMP-dependent process in MA-10 cells, is also inhibited by DEUP. Inhibition of protein kinase A (PKA) action would explain the inhibition of these two unrelated processes. However, the cytosolic PKA activity of DEUP-treated MA-10 10 cells was normal. Moreover, the activity of purified PKA was unaffected by DEUP. The inhibition of StAR synthesis was not caused by a direct effect of DEUP on the labile proteins since DEUP-treated cells required more than 24 h to recover steroidogenic capacity after DEUP treatment. Further evidence that the synthesis of StAR was not directly affected was obtained using the constitutively active R2C cells. Progesterone synthesis by these cells also involves StAR, but neither StAR synthesis or steroid synthesis is sensitive to DEUP. Lactate formation in dibutyryl-cAMP-stimulated R2C cells is, however, sensitive to inhibition by DEUP. These data can be best explained by DEUP acting on a long-lived factor involved in the cAMP/PKA response pathway, but not involved in constitutive steroidogenesis.

Modulation of adrenal cell functions by cadmium salts: 2. Sites affected by CdCl2 during unstimulated steroid synthesis

In previous studies cadmium chloride (CdCl2) nonlethally inhibited Y-1 adrenal mouse adrenal tumour cell 20-dihydroxyprogesterone (20DHP) secretion, affecting unstimulated and stimulated steroidogenic pathway sites differently. We studied CdCl2 effects on unstimulated steroidogenesis using Y-1 cells incubated 0.5 h in medium with or without cadmium (using the concentration that inhibited ACTH-stimulated steroid secretion by 50%). Exogenously added 20-hydroxycholesterol (20OHC), 22(R)-hydroxycholesterol (22OHC), 25-hydroxycholesterol (25OHC), pregnenolone (PREG), or progesterone (PROG) were used to bypass any rate-limited steroidogenic pathway sites that CdCl2 might inhibit. 25OHC is a biologically active nonpathway steroid, while 20OHC, 22OHC, PREG, and PROG are pathway steroids; each increased unstimulated 20DHP secretion nearly 10-fold. Although CdCl2 could not reduce dibutyryl cyclic AMP- (dbcAMP)-stimulated 20DHP secretion significantly, it did significantly reduce basal and 25OHC-induced 20DHP secretion 25% below untreated levels. When 20OHC, 22OHC, PREG, or PROG were incubated with unstimulated Y-1 cells, their synthesis into 20DHP was unaffected by cadmium. dbcAMP bypasses the plasma membrane enzyme complex that synthesizes intracellular cAMP during exogenous ACTH stimulation; dbcAMP was not inhibited by CdCl2. The rate-limited step accelerated by cAMP involves plasma membrane and/or cytoplasmic cholesterol transport to and through outer and inner mitochondrial membranes before the cholesterol is synthesized into pregnenolone by side-chain cleavage enzymes on the inner membrane matrix face. Little is known regarding the mechanisms controlling unstimulated steroidogenesis. Under unstimulated conditions the 25-, 20- and 22(R)-monohydroxyls of cholesterol facilitate plasma membrane, cytoplasm and inner and outer mitochondrial solubility, diffusion and/or transport to bypass rate-limited steps and augment unstimulated steroid synthesis. Since conversion of endogenous mitochondrial cholesterol and 25OHC, but not dbcAMP-mobilized cytoplasmic cholesterol, 20OHC or 22OHC conversion, to 20DHP is inhibited by CdCl2, this suggests that (a) control of mitochondrial cholesterol supplies is independent of the cAMP-regulated mitochondrial steps in the 20DHP steroid synthetic pathway, (b) CdCl2 specifically inhibited endogenous mitochondrial cholesterol and 25OHC utilization, (c) CdCl2 toxicity may affect adrenal, testicular, ovarian, and placental basal steroidogenic functions, and (d) 25OHC may be a useful compound to examine unstimulated steroid synthesis.

Evidence that angiotensin II decreases mitochondrial calcium in the glomerulosa cell

The present studies were performed using primary monolayer cultures of bovine glomerulosa cells to determine whether the elevation in cytosolic calcium concentration produced by angiotensin II was accompanied by an elevation in mitochondrial calcium. Exchangeable mitochondria calcium content was assessed indirectly by measuring the changes in cytosolic calcium concentration and calcium efflux produced by the mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Total mitochondrial calcium content was also assessed directly by atomic absorption spectroscopy. CCCP had a direct effect to promote calcium release from an oligomycin/antimycin-sensitive (mitochondrial) calcium pool in permeabilized cells. In intact cells, CCCP caused rapid reductions in cellular ATP content and the ratio of ATP to ADP. Still, its effects on calcium dynamics were exerted primarily at the mitochondrial level as evidenced by inhibition with ruthenium red, but not dantrolene. As expected, angiotensin II produced a rapid increase in calcium efflux and an equally rapid and sustained increase in cytosolic calcium concentration. Nonetheless, CCCP-stimulated elevations in cytosolic calcium concentration and calcium efflux were reduced by angiotensin II in a concentration-dependent manner. Total mitochondrial calcium content was also lower in angiotensin-treated than in control cells. These results indicate that angiotensin II causes a net decrease in mitochondrial calcium stores. On the basis of these data, it is proposed that alterations in calcium metabolism initiated by angiotensin II are exerted not only at the membrane and cytosolic levels but also at the level of the mitochondria. Changes in mitochondrial calcium dynamics may directly contribute to the regulation of mitochondrial steroidogenic enzymes by angiotensin II.

Synergistic stimulation of pregnenolone synthesis in rat adrenal mitochondria by n-hexane and cardiolipin

n-Hexane and cardiolipin each stimulate pregnenolone production by isolated rat adrenal mitochondria. Following corticotropin (ACTH) stimulation, mitochondrial cholesterol metabolism exhibits a fast phase lasting 2 min, followed by a 10-fold slower metabolism. ACTH suppression by dexamethazone or cycloheximide (CX) treatment removes this fast phase. n-Hexane, at concentrations approaching 80% of the aqueous solubility limit (approximately 0.08 mM), selectively stimulates the slow phase of metabolism, while cardiolipin (100 microM) stimulates only the fast phase. Other alkanes and ethers are effective. The effect of n-hexane is dependent on mitochondrial integrity, as evidenced by decreased effects in hypoosmotically shocked mitochondria (outer membrane disrupted) and ineffectiveness in sonicated mitochondria (both membranes disrupted). n-Hexane apparently enhances the transfer of outer membrane cholesterol to inner membrane P-450scc. Stimulation by cardiolipin is retained by disrupted mitochondria and may involve enhanced availability of P-450scc to inner membrane cholesterol. When added together, these agents produce more than additive effects on cholesterol metabolism. Preincubation with n-hexane did not increase reactive cholesterol, suggesting that enhanced cholesterol transport occurs only in concert with metabolism of inner membrane cholesterol. Uptake of alkanes into mitochondrial membranes may effect structural changes that facilitate outer to inner membrane cholesterol transfer, but major changes are excluded by the effectiveness of isocitrate as a reductant for P-450scc. In combination, n-hexane and cardiolipin reproduce the effect of the ACTH-sensitive sterol regulatory peptide on mitochondria [R. C. Pedersen and A. C. Brownie (1983) Proc. Natl. Acad. Sci. USA 80, 1882-1886], suggesting that peptide action on adrenal mitochondria may resolve into two analogous components.

Transduction of ACTH signal from plasma membrane to mitochondria in adrenocortical steroidogenesis. Effects of peptide, phospholipid, and calcium

The conversion of cholesterol to pregnenolone by adrenocortical mitochondria is the rate-limiting step in steroidogenesis. This process is stimulated dramatically by the action of ACTH through the sequential reactions, in which adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase and ribosomal protein synthesis are all involved. The de novo synthesized protein, the so-called labile protein with a half-life of approx 10 min, is believed to stimulate the cholesterol side chain cleavage reaction by an unknown mechanism. Available evidence indicates that the electron on transfer reaction from NADPH to P-450scc is mediated rapidly by adrenodoxin reductase and p-450 scc. In addition, these redox components are inactivated slowly with a half-life of 3.5 days after hypophysectomy. It is known that the corticoid output from adrenocortical cells starts within 5 min and reaches the maximum after 10-15 min of ACTH administration to animals. One can assume that under normal physiological conditions, both O2 and NADPH are not limiting. Additionally, mitochondrial inner membranes are poor in cholesterol. In this context, the availability of substrate cholesterol to P450scc is the most likely candidate for the regulatory mechanism.

Importance of the unsaturated fatty acyl group of phospholipids in their stimulatory role on rat adrenal mitochondrial steroidogenesis

We have investigated the relationship between chemical properties of various phospholipids and their steroidogenic activity for adrenal mitochondria prepared from dexamethasone/cycloheximide-treated quiescent rats. Phospholipids studied include those purified from bovine and rat adrenal mitochondria, obtained from commercial sources, and reduced by catalytic hydrogenation. All phospholipids were subjected to analysis of their fatty acyl groups and examined for their steroidogenic activities. From these experiments, we came to the following conclusions: The degree of unsaturation in the fatty acyl moiety correlates with their steroidogenic activities regardless of head groups. Namely, polyunsaturation appears to be more important than monounsaturation with a relative insensitivity toward their head groups. Saturated phospholipids exhibit an inhibition for steroidogenic activity. Cardiolipins, which are steroidogenic, appear exceptional. Their head groups may partially participate in the activity in addition to their high content of unsaturated fatty acids. The importance of the adrenoyl (C22:4) group in phospholipids is suggested.

Sex differences in adrenocortical structure and function. XX. The effects of gonadectomy and testosterone or estradiol replacement on cholesterol content and distribution in the gland

Sex differences in the distribution of free and esterified cholesterol in the mitochondria and lipid droplets of the rat adrenal glands, as well as their dependence on gonadal hormones were studied. For these purposes intact, gonadectomized and gonadectomized-testosterone or estradiol replaced rats were employed. The concentration of free cholesterol [FC] as well as esterified cholesterol [EC] in full homogenates of decapsulated glands was higher in male than in female rats. Neither orchiectomy nor testosterone replacement had an effect on FC and EC concentration. Ovariectomy increased FC and EC concentration in full adrenal homogenates, an effect reversed by estradiol replacement. Similar changes were found in the lipid droplets. The concentration of FC and EC in the adrenal mitochondria was higher in male than in female rats. Orchiectomy increased the concentration of FC and decreased the concentration of EC. Testosterone administration resulted in further increase in FC concentration and restored EC concentration to the level of the control group. On the other hand ovariectomy increased FC and EC concentration, an effect reversed by estradiol replacement. Results obtained clearly showed that sex differences in FC and EC concentration in rat adrenal gland depend mainly on estradiol which lowers FC and EC concentration in the adrenal lipid droplets and mitochondria.

Imidazole antimycotics: inhibitors of steroid aromatase

Miconazole and clotrimazole, members of a class of imidazole agents which have broad spectrum antimycotic activity, were shown to be potent inhibitors of steroid aromatase activity of human placental microsomes. The I50 values for the inhibition of aromatase activity by miconazole, clotrimazole, ketoconazole, and aminoglutethimide were 0.6, 1.8, 60 and 44 microM respectively. The most effective compound, miconazole, exhibited competitive kinetics with respect to androstenedione, the aromatase substrate. The apparent inhibitory constant (Ki) was 55 nM, under assay conditions where the apparent Km for androstenedione was 220 nM. The inhibition of aromatase activity by miconazole was shown to be reversible by dilution. Miconazole was a relatively poor inhibitor of the cholesterol side chain cleavage activity of a placental mitochondria-enriched fraction, while both clotrimazole and ketoconazole markedly inhibited this mitochondrial monooxygenase activity. Spectrophotometric studies revealed that miconazole bound to the cytochrome P-450 component of the placental microsomal aromatase complex and had negligible effect on NADPH-cytochrome c (P-450) reductase activity. These results strongly support direct interaction of miconazole with microsomal cytochrome P-450 in human placental microsomes with high affinity resulting in the inhibition of aromatase activity.

The effect of calcium on cholesterol side-chain-cleavage enzyme in superovulated rat ovaries

Cholesterol side-chain-cleavage enzyme activity in mitochondria isolated from heavily luteinized rat ovaries was determined using isocitrate as an electron donor and [4-14C]cholesterol to start the reaction. The activity of the enzyme was reduced when more than 10 microM calcium was added to the mitochondrial preparations. When 100 microM EGTA or 2 mM ATP was added to the reaction an increase in enzyme activity was observed and ATP was able to partially overcome the calcium-induced inhibition.

Inhibition of adrenal steroidogenesis by the anesthetic etomidate

The use of the intravenous anesthetic etomidate for prolonged sedation has been associated with low levels of plasma cortisol and increased mortality. We measured the cortisol and aldosterone responses to ACTH stimulation in five patients receiving etomidate, and we also studied the direct effects of etomidate on enzymes in the rat steroidogenic pathway. One patient who was receiving a 20-hour infusion of etomidate (1.3 to 1.5 mg per kilogram of body weight per hour) had marked adrenocortical suppression that was still evident four days after etomidate was discontinued. Four surgical patients receiving etomidate during their operations were all found to have adrenal suppression four hours after the operation; mean (+/- S.D.) increases in cortisol and aldosterone after ACTH stimulation were only 1.8 +/- 0.5 micrograms per deciliter and 0.5 +/- 1.1 ng per deciliter, respectively. In rat adrenal cells, etomidate produced a concentration-dependent blockade of the two mitochondrial cytochrome P-450-dependent enzymes, cholesterol-side-chain cleavage enzyme, and 11 beta-hydroxylase, without evident inhibition of the microsomal enzymes in the glucocorticoid pathway. Physicians should be aware that etomidate inhibits adrenal steroidogenesis, and they should consider treating selected patients with corticosteroids if etomidate is used.

Effect of cholesterol and protein content on membrane fluidity and 3 beta-hydroxysteroid dehydrogenase activity in mitochondrial inner membranes of bovine adrenal cortex

The steroid biosynthetic enzymes in the adrenal cortex are localised in endoplasmic reticulum and mitochondrial membranes. For some of the enzymes in endoplasmic reticulum the activity appears to be modulated by lipid fluidity, (21-hydroxysteroid hydroxylase and 3 beta-hydroxysteroid dehydrogenase). A mechanism for the regulation of corticosteroid biosynthesis mediated by the membrane fluidity has been suggested. Therefore a study of the mitochondrial inner membrane of the bovine adrenal cortex has been undertaken in comparison with a previous study of the endoplasmic reticulum. The kinetic parameters of the 3 beta-hydroxysteroid dehydrogenase were studied as a function of pH and temperature. No thermal transition can be observed in the Arrhenius plot for this enzyme in contrast with the results obtained for the microsomal enzyme. Membrane fluidity using, as fluorescent probes, diphenylhexatriene and a set of n-(9-anthroyloxy)fatty acids has been also studied as a function of temperature with or without addition of cholesterol. No thermal transition in the lipid phase can be observed. The addition of cholesterol to total mitochondrial membrane as to a lipid extract of the membrane decreases fluidity to the same extent as it does with microsomes. The presence of a large amount of protein in mitochondria has an effect which is additive to that of the cholesterol.

The catalytic cycle of cytochrome P-450scc and intermediates in the conversion of cholesterol to pregnenolone

Cytochrome P-450scc as isolated is a cholesterol-depleted low-spin haemoprotein; addition of cholesterol results in formation of a high-spin complex. Cytochrome P-450scc--cholesterol is a one-electron acceptor on titration with NADPH. Cytochrome P-450scc--cholesterol can be anaerobically reduced to the ferrous state which, on oxygenation, forms an oxygenated cytochrome P-450scc--cholesterol complex. This oxygenated complex in the absence of adrenodoxin autoxidises to ferric cytochrome P-450scc--cholesterol without oxidation of cholesterol. The decay of the oxygenated complex is first-order, k = 9.3 X 10(-3) S-1 at 4 degrees C. The rate of autoxidation is influenced by pH, ionic strength and the chemical nature of bound sterol. The activation energy of autoxidation is 75 kJ mol-1. Addition of equimolar amounts of adrenodoxin to cytochrome P-450scc--cholesterol followed by stoichiometric reduction under anaerobic conditions and subsequent oxygenation, allows single catalytic turnover cycles of cytochrome P-450scc to be observed. This has led to detection of intermediates in the conversion of cholesterol to pregnenolone and a precursor/product sequence of cholesterol----22-hydroxycholesterol----20,22-dihydroxy-cholesterol ----pregnenolone has been established. Addition of oxidised adrenodoxin to oxygenated cytochrome P-450scc--cholesterol results in formation of 22-hydroxycholesterol.

The role of lysine-rich proteins in the acute steroidogenic response of rat adrenal cells to ACTH

The possibility that a lysine-rich protein was involved in the acute stimulation of steroidogenesis by ACTH was investigated using [3H] labelled lysine and isolated adrenal cells. The results demonstrated that cycloheximide inhibited steroidogenesis in a dose-dependent, rapid fashion and inhibited the incorporation of radioactive lysine into protein. However cells incubated in a lysine-free medium showed the same response to ACTH as cells incubated in a lysine-containing medium. It was also demonstrated that ACTH had no effect on the incorporation of tritiated lysine into the protein or small peptide fractions. These observations suggest that a rapidly synthesised, lysine-rich protein is not involved in the acute response to ACTH.

Sterol-phospholipid interactions in model membranes. Effect of polar group substitutions in the cholesterol side-chain at C20 and C22

The interactions of phospholipids with four different cholesterol derivatives substituted with one OH or one keto group at position C20 or C22 of the side-chain were studied. The derivatives were the 22,R-hydroxy; 22,S-hydroxy; 22-keto- and 20,S-hydroxycholesterol. Two aspects of the interactions were investigated: (1) the effect of the cholesterol derivatives on the gel leads to liquid crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) and of dielaidoylphosphatidylethanolamine (DEPE) monitored by differential scanning calorimetry and (2) The effect on the lamellar leads to hexagonal HII phase transition of DEPE monitored by DSC and by 31P-NMR to determine structural changes. The gel leads to liquid crystalline phase transition was affected by the cholesterol derivatives to a much larger extent in the case of DPPC than of DEPE. In both cases, there was a differential effect of the four derivatives, the 22,R-hydroxycholesterol being the less effective. In DPPC-sterol 1:1 systems, 22,R-hydroxycholesterol does not suppress the melting transition, the delta H values becomes 7.1 kcal X mol-1 as compared to 8.2 kcal X mol-1 for the pure lipid. 22,S-OH cholesterol has a much stronger effect (delta H = 3.1 kcal X mol-1) and 22-ketocholesterol suppresses the transition completely. In DEPE mixtures of all these compounds, the melting transition of the phospholipid is still observable. The transition temperature was shifted to lower values (-13.5 degrees C in the presence of 20,S-OH cholesterol). The delta H of the transition was lowered by these compounds except in DEPE-22,R-OH cholesterol mixtures and the cooperativity of the transition (reflected by the width at half peak height) was reduced. The lamellar leads to hexagonal HII phase transition was also affected by the presence of these cholesterol derivatives. The transition temperature value was depressed with all these compounds. 20,S-OH cholesterol was the most effective followed by 22,R-OH cholesterol. The delta H of the transition was not strongly affected. The molecular interfacial properties of these derivatives were studied by the monomolecular film technique. It is most likely that 22,R-OH cholesterol due to the hydroxyl groups at the 3 beta- and 22,R-positions orients with the sterol nucleus lying flat at the air/water interface, since the compression isotherm of either the pure sterol or the DOPC-sterol mixture (molar ratio, 1:1) monomolecular film exhibits a transition at approx. 103 A2.(ABSTRACT TRUNCATED AT 400 WORDS)

Polypeptide activators of cholesterol side-chain cleavage

A rate-determining step in the cAMP-dependent action of ACTH on adrenal steroid biosynthesis is the interaction of cholesterol substrate with the cholesterol side-chain cleavage cytochrome P-450 in the mitochondrion. This interaction is rapidly and reversibly sensitive to inhibitors of protein synthesis. For this reason a hormone-dependent, labile protein activator of cholesterol side-chain cleavage has long been postulated as an obligatory intermediate in the tropic regulation of this reaction. Applying recent advances in liquid chromatography, two-dimensional gel electrophoresis, and enzyme reconstitution into liposomes, several laboratories have now reported the isolation and partial characterization of polypeptide candidates for the status of "labile protein."

Ketoconazole blocks adrenal steroidogenesis by inhibiting cytochrome P450-dependent enzymes

Ketoconazole has recently been shown to interfere with steroidogenesis in patients and rat in vitro systems. In this study we attempted to elucidate the site of inhibition in the adrenal gland. Although ketoconazole impaired adrenocorticotropic hormone stimulated cyclic (c)AMP production, dibutyrl cAMP addition did not bypass the steroidogenic blockade indicating that the critical ketoconazole-inhibited step was distal to cAMP. Addition of radiolabeled substrates to isolated adrenal cells and analysis of products by high performance liquid chromatography demonstrated a ketoconazole block between deoxycorticosterone (DOC) and corticosterone. This 11-hydroxylase step is carried out by a P450-dependent mitochondrial enzyme. No restriction of progesterone or pregnenolone conversion to DOC was detected, steps carried out by non-P450-dependent microsomal enzymes. Inhibition of cholesterol conversion to pregnenolone by mitochondrial fractions indicated a second block at the side chain cleavage step, another mitochondrial P450-dependent enzyme. Adrenal malate dehydrogenase, a non-P450-dependent mitochondrial enzyme was not inhibited while renal 24-hydroxylase, a P450-dependent mitochondrial enzyme in another organ, was blocked by ketoconazole. We conclude that ketoconazole may be a general inhibitor of mitochondrial P450 enzymes. This finding suggests that patients receiving ketoconazole be monitored for side effects relevant to P450 enzyme inhibition. Further, we raise the possibility that this drug action may be beneficially exploited in situations where inhibition of steroidogenesis is a therapeutic goal.

Characterization of human cholesterol side chain cleavage enzyme (EC 1.14.15x) of human term placental mitochondria

The cholesterol side chain cleavage enzyme (EC 1.14.15x) in mitochondria of a human term placenta was partially characterized. Enzyme activity was determined by separation of [26-14C]-cholesterol and [5-14C]-isocaproic acid formed by side chain cleavage. Since the amounts of unlabeled cholesterol were too large, a KM of cholesterol could not be determined. The apparent KM value of NADPH is 6.25 x 10(-4) M. A pH optimum was found at pH 9.5 (Tris buffer) and a temperature optimum at 40 C. The metal ions Sr2+ and Ba2+ showed no inhibition at 1 and 10 mM and a moderate inhibition at 100 mM. In low concentrations (1 mM), Mg2+ and Ca2+ slightly stimulated the enzyme whereas in higher concentrations (100 mM) an inhibitory effect was observed. A strong inhibition was achieved with 1 mM Zn2+, Cd2+, Cu2+ and by 10 and 100 mM Fe2+, Mn2+, Co2+ and Ni2+. During preincubation of the enzyme without radioactive substrate, a rapid loss in enzyme activity in relation to enzyme concentration was observed (initial activity = 100%) (preincubation time in hours): 0.5 h (97%), 1 h (55%) and 1.5 h (34%). A dose-dependent inhibition of the enzyme by the following proteins was achieved: bovine serum protein, human serum protein, human immunoglobulin G and ovalbumin. Furthermore, a dose-dependent inhibition was found with the membrane lipids lecithin and sphingosine.

Partial characterization of placental 3 beta-hydroxysteroid dehydrogenase (EC 1.1.1.145), delta 4-5isomerase (EC 5.3.3.1) in human term placental mitochondria

A partial characterization of human term placental 3 beta-HSDH in mitochondria is reported. Apparent KM of pregnenolone: 70 nM. A dose-dependent stimulation of 3 beta-HSDH by NAD+ or NADP+ was observed in the range from 10(-6) to 10(-3) M (KM value of NAD+: 20 microM). At equimolar concentrations NAD+ is more than 10-fold as effective a cofactor of the 3 beta-HSDH than NADP+. pH optimum: 9.5 (glycine-NaOH buffer). Temperature optimum 40-45 degrees C. A rapid loss of 3 beta-HSDH activity was found after preincubation of the enzyme at 37 degrees C after 30 min; less than 50% of initial enzyme activity is present. No inhibition was obtained by Mg2+, Ca2+ Sr2+ and Ba2+ (1-100 mM). A strong inhibition was achieved with 1 mM Zn2+, Cd2+, Cu2+ and 10 mM and 100 mM Fe2+, Mn2+, Co2+ and Ni2+.

The influence of the side chain on sterol side-chain cleavage in rat adrenal glands

The cholesterol side-chain cleavage enzyme system of rat adrenal cortex, the enzyme catalyzing a rate-limiting step of adrenal steroidogenesis, was shown to metabolize a series of cholesterol analogues to pregnenolone. In the presence of Ca2+, rat adrenocortical mitochondria converted the analogue with two less methylene groups (C25) than cholesterol into pregnenolone at a faster rate than cholesterol. The analogues with one or three less methylene groups (C26 or C24) were metabolized at a similar rate to cholesterol. Lengthening the non-polar side chain produced analogues that did not appear to be metabolized. Studies of the metabolism of these analogues in isolated rat adrenocortical carcinoma cells showed that the C24 and C25 analogues were converted into pregnenolone much more efficiently than was cholesterol or the C26 sterol. The experimental findings are explained in terms of the differing ability of each exogenously added sterol to gain access to the active site of the sterol side-chain cleavage enzyme by passage through the membranes of the adrenal cell.

Regulation of cytochrome P450 activities in adrenocortical mitochondria from normal rats and human neoplastic tissues

From our immunocytochemical studies, cytochrome P450scc and P45011 beta systems were localized on the matrix side of inner membrane of the mitochondria in the parenchymal cells of adrenal cortex. However, the degree of immunocytochemical staining varied from one mitochondrion to another within a single cell; some stained intensely along the entire inner membrane, some stained only along segments of the inner membrane, and some did not stain at all. The ratio of stained to unstained mitochondria was approximately unity in untreated rats, while stained mitochondria greatly increased upon ACTH administration suggesting that the population of cytochrome P450 system-containing mitochondria increased upon long-term ACTH action. By the combined use of flash photolysis and substrate difference spectroscopy, quantitative determination of substrate-bound and free forms of P450scc and P45011 beta in mitochondria became possible. Increases in total amounts of P450scc and cholesterol resulted from the long-term ACTH administration to rats. The ratio of cholesterol-bound P450scc to its free form was essentially unchanged under these conditions. On the other hand, amounts of P45011 beta were not increased significantly by long-term ACTH administration. The ratio of DOC-bound P45011 beta to the free form increased significantly, however, by long-term ACTH administration and also in the mitochondria from patients suffering from Cushing's syndrome.

Stimulation by calcium and other cations of the cholesterol binding to steroid-free cytochrome P-450scc purified from bovine adrenocortical mitochondria: the implication of ACTH-mediated calcium homeostasis on the cholesterol availability

By utilizing purified cytochrome P-450scc from bovine adrenocortical mitochondria and cholesterol-containing dioleoylglycerophosphocholine liposomes, we have demonstrated a dramatic stimulation (2-3 fold) of cholesterol binding to the steroid-free cytochrome by Ca++. We theorize that ACTH modulates the increase of intracellular Ca++ concentration resulting in the increase of the cholesterol availability to the mitochondrial cytochrome.

Characterization of cytochrome P-450scc-containing liposomes

Purified cytochrome P450scc from bovine adrenocortical mitochondria was incorporated into liposomes by the cholate-dilution method utilizing either dialysis or Sephadex gel filtration. Among synthetic phospholipids tested, dioleoylglycerophosphocholine showed the best stability during the incorporation of P450scc into liposomes. A maximum amount of heme was incorporated into liposomes at a molar ratio of phospholipid to the cytochrome of approx. 200. When P450scc was incorporated into the dioleoylglycerophosphocholine liposomes by the cholate-filtration method, the P450scc-containing liposomes showed two major populations on the elution pattern of the Sepharose 4B gel filtration, and were seen at a diameter of 200-600 A and its aggregated forms. When the cytochrome was incorporated into dioleoylglycerophosphocholine liposomes or cholesterol-free adrenocortical mitochondrial liposomes, P450scc was less stable than P450scc in aqueous solution. Cholesterol or adrenodoxin markedly stabilized the liposomal P450scc. Liposomal P450scc required cholesterol for its optimum reduction with adrenodoxin, adrenodoxin reductase, and NADPH in the presence of CO. About 70% of the total heme in the dioleoylglycerophosphocholine liposomes was reduced by the enzymatic reduction in the presence of cholesterol, indicating that 70% of the total molecules are exposed to the surface of the outer monolayer. In order to see the location of the heme in membrane, the dioleoylglycerophosphocholine-liposomal P450scc was subjected to p-chloromercuriphenyl sulfonic acid treatment. This reagent destroyed the liposomal P450scc. These results suggest that the heme is located in the proximity of the p-chloromercuriphenyl sulfonic acid reacting sites which are exposed to the surface, or located on the vincinity of polar heads of the membrane.

The effect of azastene, cyanoketone and trilostane upon respiration and cleavage of the cholesterol side chain in mitochondria from bovine adrenal cortex

Mitochondria prepared from bovine adrenal cortex and incubated with ADP and phosphate respired at about 45% of the rat observed in the presence of an uncoupler of oxidative phosphorylation; there was, however, little inefficiency in the reactions involved in the phosphorylation of ADP. Three inhibitors of the 3 beta-hydroxysteroid dehydrogenase (azastene, cyanoketone and trilostane) were employed with a view to preventing pregnenolone metabolism and thus aiding the assay of cholesterol side-chain cleavage. Freshly made solutions of these inhibitors did not modify mitochondrial respiratory rates, at concentrations of 10 microM. In contrast, solutions maintained at 0-4 degrees C for one week subsequently inhibited the respiratory rate of uncoupled mitochondria. When fresh solutions of the inhibitors were used in the assays of cholesterol side-chain cleavage, 10 microM azastene did not significantly inhibit pregnenolone metabolism. Cyanoketone and trilostane were both significant inhibitors of pregnenolone metabolism, but 10 microM cyanoketone reduced the initial rate of cholesterol side-chain cleavage by 50% in the presence of 10 mM malate, although this inhibition did not occur in the presence of 10 mM DL-isocitrate. Thus trilostane may be the preferred inhibitor of the 3 beta-hydroxysteroid dehydrogenase during studies of cholesterol side-chain cleavage in vitro.

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

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

On the synergistic action of androgen and FSH on progestin secretion of cultured rat granulosa cells. Cellular and mitochondrial cholesterol metabolism

The effect of FSH and androgen on the conversion of cholesterol into progesterone by cultured rat granulosa cells (GC) was studied in intact cells or mitochondrial preparations. Culture of GC for immature hypophysectomized diethylstilbestrol-treated rats for 48 h in the presence of ovine FSH (5 microgram/ml) alone, or FSH + testosterone (Te; 0.5 microgram/ml) caused a slight increase in the activity of the mitochondrial marker enzyme succinic dehydrogenase, while Te had no effect. Culture with the hormones for 48 h had no significant effect on the levels of free and esterified cellular cholesterol. GC monolayers after 48 h with or without FSH and Te converted [3H]cholesterol into 4 major metabolites, 3 of which were secreted into the medium and, in thin-layer chromatographic behavior, resembled pregnenolone, progesterone and 20 alpha-dihydroprogesterone. The total amount of the 3 C-21 steroids was higher (p less than 0.01) in FSH- or Te-treated than in control cells, and combined treatment had a synergistic effect. The uptake of labeled cholesterol (4--10%) was significantly higher (p less than 0.01) in cells pretreated with FSH or Te, whereas a combined FSH and Te treatment had an additive effect. Mitochondria isolated from GC monolayers took up cholesterol in a temperature-dependent fashion, but this uptake was not affected by hormonal pretreatment. In the presence of cyanoketone, the mitochondrial fractions activity converted cholesterol into pregnenolone. This activity was enhanced by FSH or Te (p less than 0.01), and further enhancement was observed with FSH + Te; the combined effect appeared to be more than additive (p = 0.05). The results suggest that both FSH and Te enhance the activity of cholesterol side-chain cleavage, but do not affect the transport of cholesterol into the mitochondria. A possible hormonal effect on a pre-mitochondrial step is discussed.

Presence of a low molecular weight inhibitor of succinate-supported cholesterol side chain cleavage in rat ovaries

1. Low molecular weight fractions (mol. wt. 3500-10 000) prepared from cytosols of luteinized rat ovaries inhibited succinate-supported cholesterol side chain cleavage by intact ovarian mitochondria utilizing endogenous or exogenous sterol as substrate. 2. The low molecular weight fractions inhibited steroid secretion by collagenase-dispersed ovarian cells stimulated with lutropin or dibutyryl cyclic AMP. 3. Steroidogenesis by intact mitochondria incubated with NADPH was enhanced by the low molecular weight ovarian fraction, but cholesterol side chain cleavage carried out by sonicated mitochondria incubated with NADPH was unaffected. 4. Succinate-supported mitochondrial respiration was stimulated by the low molecular weight factor, apparently by uncoupling of oxidative phosphorylation. The uncoupling seems to be the mechanism by which steroid synthesis is inhibited. 5. The low molecular weight factor was heat-labile and not extracted by activated charcoal. Similar heat-labile material capable of inhibiting succinate-supported mitochondrial steroid synthesis was not found in low molecular weight fractions prepared from rat kidney, liver, spleen, brain, plasma and bovine corpus luteum. 6. Treatment of rats with cycloheximide 1 h before killing resulted in a reduction of inhibitory activity in ovarian low molecular weight cytosolic fractions. 7. We conclude that ovarian cytosols contain a low molecular weight factor, presumably a protein, which inhibits mitochondrial cholesterol side chain cleavage by uncoupling oxidative phosphorylation. The physiological function of this factor remains to be determined.

Effects of cholesterol, hydroxycholesterols and calcium on pregnenolone production rates in mitochondrial fractions from rat testes

The in vitro regulation of the mitochondrial conversion of cholesterol to pregnenolone in rat testis tissue has been further investigated. Pregnenolone production rates by isolated mitochondrial fractions could be stimulated by the addition of cholesterol. The stimulation was always highest in mitochondria isolated from lutropin-treated testes relative to control and cycloheximide treated testes. Addition 20- or 25-hydroxycholesterol resulted in a greater stimulation of pregnenolone production rates and these rates were unaffected by prior treatment with cycloheximide. When both cholesterol and 20- or 25-hydroxycholesterol were present in the incubation medium, pregnenolone production rates were mainly influenced by the hydroxycholesterol, even in the presence of a ten-fold excess of cholesterol. Ca2+ in vitro stimulated pregnenolone production rates from endogenous cholesterol as well as from added cholesterol. However, pregnenolone production rates in the presence of hydroxycholesterol were not influence by the addition of Ca2+ in vitro.

Pregnenolone biosynthesis in isolated cells of Snell rat adrenocortical carcinoma 494

Adrenocorticotrophin (ACTH) produced an insignificant stimulation of pregnenolone biosynthesis from endogenous precursors in isolated cells prepared from the rat Snell adrenal carcinoma 494. On the addition of 25-hydroxycholesterol, the rate of pregnenolone synthesis increased 10-fold. These results, noting also the very low cholesterol content of the tumor cells, suggested that lack of cholesterol was responsible for the poor steroidogenic response of the cells to ACTH. Endogenous pregnenolone production was sensitive to cytochalasin B as well as cycloheximide. However, pregnenolone synthesis after the addition of 25-hydroxycholesterol was not affected by these inhibitors. Removal of cycloheximide from the cells resulted in the immediate restoration of the initial rate of pregnenolone synthesis from endogenous precursors. This suggested that cycloheximide was interfering with the action of a stable activated intracellular messenger.