Cytochrome P-450 of bovine adrenal mitochondria. Ligand binding to two forms resolved by EPR spectroscopy

Rodent StAR mRNA is substantially regulated by control of mRNA stability through sites in the 3'-untranslated region and through coupling to ongoing transcription

The steroidogenic acute regulator (StAR) gene is transcribed to 1.6 kb and 3.5 kb mRNAs that differ only through the length of the 3'-untranslated region (3'-UTR). These forms result from alternative polyadenylation sites in exon 7. These sites are utilized similarly in unstimulated adrenal cells whereas Br-cAMP selectively stimulates 3.5 kb mRNA. After removal of Br-cAMP, 3.5 kb mRNA declines rapidly (t(1/2) = 2 h) while 1.6 kb mRNA responds more slowly. This selective degradation is more evident in testis MA10 cells and is seen even in the presence of Br-cAMP. Transfection of Y-1 cells with CMV promoted StAR vectors confirmed that the 3.5 kb form is less stable and that Br-cAMP slowly increases this instability. Basal instability resides solely in the extended 3'-UTR which contains AU-rich elements. Br-cAMP enhances this degradation of 3.5 kb mRNA but additionally requires translated and 5'-UTR sequences. Degradation of both forms is arrested by inhibitors of transcription or translation, indicating that mRNA stability is coupled to these processes independent of the extended 3'-UTR. Br-cAMP stimulates substantial selective synthesis of 3.5 kb StAR mRNA despite this instability. The preferential generation of the unstable form may facilitate rapid increases and decreases of StAR activity in response to external changes.

Characterization of the rat Star gene that encodes the predominant 3.5-kilobase pair mRNA. ACTH stimulation of adrenal steroids in vivo precedes elevation of Star mRNA and protein

The steroidogenic acute regulatory protein (STAR) participates in steroidogenesis through the mitochondrial transfer of cholesterol to cytochrome P450scc. The rat adrenal Star gene is transcribed as a 3. 5-kilobase pair (kb) and 1.6-kb mRNA with the larger mRNA predominating ( approximately 85% of total) in vivo. Hypophysectomy (HPX) produced a 3-5-fold decrease in Star mRNA along with a loss of adrenal steroids, whereas P450scc mRNA decreased by less than 2-fold. Adrenocorticotropic hormone (ACTH) treatment of HPX rats maximally stimulated steroidogenesis rates within 5 min with over 10-fold elevation of steady state blood levels occurring within 10 min. For intact rats there was a 5-10-fold larger increase, paralleling previously observed elevations of cholesterol-cytochrome P450scc association and metabolism in subsequently isolated adrenal mitochondria. ACTH did not increase either total STAR protein or a group of modified forms until at least 30 min after completion of acute stimulation, indicating that elevated translation of STAR protein cannot alone mediate this acute stimulation. Parallel slow changes in STAR protein and corticosterone formation after ACTH treatment are consistent with participation of STAR forms as co-regulators of these hormonal responses. ACTH stimulation of HPX rats increased Star mRNA by 2.5-fold within 20 min and by 4.5-fold after 1 h, thus preceding the rise in the STAR protein. A 3.5-kb Star cDNA clone isolated from a rat adrenal cDNA library exhibited a 0.9-kb open reading frame and a 2.5-kb 3'-untranslated region (3'-UTR). The open reading frame sequence differed at only 12 amino acids from that of the mouse Star. The rat Star gene seven exons with exon 7 encoding the entire 2.5 kb of 3'-UTR of the 3.5-kb mRNA. The 3'-UTR sequence suggests that 1.6- and 3.5-kb mRNA are formed by an alternative usage of different polyadenylation signals. Multiple UUAUUUA(U/A)(U/A) motifs also suggest additional regulation through this extended 3'-UTR. Although elevation of STAR protein by ACTH does not cause the acute increase in adrenal cholesterol metabolism, changes in the turnover or distribution of an active STAR subfraction cannot be excluded.

Control of cholesterol access to cytochrome P450scc in rat adrenal cells mediated by regulation of the steroidogenic acute regulatory protein

Cholesterol conversion to pregnenolone by cytochrome P450scc in steroidogenic cells, including those of the adrenal cortex, is determined by hormonal control of cholesterol availability. Intramitochondrial cholesterol movement to P450scc, which retains hormonal activation in isolated mitochondria, is apparently dependent on peripheral benzodiazepine receptor and the recently cloned steroidogenic acute regulatory (StAR) protein. In rat adrenal cells, StAR is formed as a 37-kDa precursor that is transferred to the mitochondrial inner membrane following phosphorylation by hormonally activated protein kinase A, and processed to multiple forms, some of which turn over very rapidly. In bovine cells, StAR undergoes three modifications forming a set of eight proteins seen in both glomerulosa and fasciculata cells. In the former, cyclic AMP and angiotensin II each decrease two forms and elevate six forms. Significantly, the major change seen after activation may not involve phosphorylation of StAR. Cholesterol transfer across mitochondrial membranes is also activated in isolated mitochondria by GTP and low concentrations of Ca2+, apparently prior to activation by StAR. Depletion of StAR by cycloheximide inhibits cholesterol transfer but is overcome by uptake of Ca2+ into the matrix. This activation of cellular cholesterol transport is sustained in adrenal cells permeabilized by Streptolysin O. In rat adrenal cells cAMP elevates 3.5- and 1.6-kb mRNA, hybridized by a 1.0-kb StAR cDNA. A 3.5-kb rat adrenal cDNA that encodes all except the 5' end of the longest StAR mRNA has been characterized. The corresponding gene sequence is distributed across seven exons. The shorter mRNA may arise from polyadenylation signals early in exon 7. However, the 3.5-kb mRNA comprises 80-90% of untreated rat adrenal StAR mRNA and may therefore provide the prime source for in vivo translation of StAR protein.

Inhibitors of P450-dependent steroid biosynthesis: from research to medical treatment

A number of cytochrome P450-dependent enzymes are major targets for both steroidal and nonsteroidal compounds that may be of use in the treatment of a number of androgen-independent, androgen-, estrogen- and other steroid-dependent diseases. Compounds of interest are for example aminoglutethimide and derivatives; esters of 4-pyridineacetic acid; imidazole derivatives, such as ketoconazole, liarozole, fadrozole, CGS 18320 B; bis-chlorophenyl-pyrimidine analogues; triazole derivatives vorozole and CGS 20267, and steroidal aromatase inhibitors such as 4-hydroxyandrostenedione. Some of them (e.g. ketoconazole) triggered studies to find new possibilities in medical treatment. Others are of use clinically or under clinical evaluation to provide a palliative treatment and/or cure to patients with for example prostatic carcinoma, breast cancer, hypercortisolism and benign prostatic hyperplasia.

Magnetic resonance study of the structure and functions of cytochrome P450

Cytochrome P450 is a membrane-bound enzyme providing oxidation of numerous organic compounds in organisms. The objective of this review is to show the wide possibilities that are provided by Electron Spin Resonance (ESR) and Nuclear Magnetic Resonance (NMR) techniques to the study of the structure and functions of this unique enzyme. High sensitivity of ESR spectra of cytochrome P450 to its functional state and interaction with substrates and inhibitors is illustrated. NMR and proton relaxation make it possible to obtain unique information about the structure of the active center of cytochrome P450 under physiological conditions. ESR and NMR methods allow one to obtain structural data on location of substrates, inhibitors, and their spin-labeled analogs with respect to Fe3+ ions in the enzyme-active center. Of special interest seems to be coupling of ESR with the affinity modification method. For this purpose, the spin-labeled analogs of cytochrome P450 substrates containing alkylating groups were used. As a result, an important datum has been obtained on the structure of active centers of cytochrome P450 in microsomes and in a highly purified state. In conclusion, the problems of the structure and functions of cytochrome P450, which can be most efficiently resolved with the use of magnetic resonance methods, are discussed.

The influence of oxygen donor ligation on the spectroscopic properties of ferric cytochrome P-450: ester, ether and ketone co-ordination to the haem iron

Homogeneous low-spin complexes of cytochrome P-450-CAM with esters, ethers and ketones have been prepared and characterized by u.v.-visible absorption, circular dichroism (CD), magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopy. Direct haem iron ligation has been verified by competition for binding with well-known haem ligands. The u.v.-visible spectra of the oxygen-donor complexes exhibit near-u.v. (delta) transitions near 356 nm, Soret maxima at 417 +/- 1 nm, beta bands near 536 nm and alpha peaks near 569 nm, with alpha greater than beta in intensity. Negative delta and Soret CD troughs are seen. The MCD spectra have minima at approximately 356 nm, intense derivative-shaped Soret features centred at approximately 416 nm and four characteristic features beyond 450 nm. The EPR spectra of these complexes, while similar to that of the native enzyme, exhibit slight variances. Anomalous spectral and substrate binding properties have been reported in the study of cytochrome P-450 under conditions employing solvents and non-phosphate buffers containing oxygen functionalities, and have been attributed to 'solvent effects'. The present work, in combination with our previous report of alcohol, amide and carboxylate oxygen donor complexes of cytochrome P-450, is evidence that a wide variety of oxygen-donor species are capable of direct ligation to the haem iron of cytochrome P-450. This leads us to suggest oxygen-donor ligation to cytochrome P-450 as the origin of spectral and substrate binding anomalies previously attributed to solvent effects.

Zonal distribution of cytochrome P-450s (P-450(11) beta and P-450scc) and their relation to steroidogenesis in bovine adrenal cortex

Zona glomerulosa and zonae fasciculata-reticularis of bovine adrenal cortex were separated by our improved method using a remodeled microtome. A considerable amount of fresh mitochondria and microsomes was quickly obtained. Specific content of cytochrome P-450 in the mitochondria was higher in zonae fasciculata-reticularis than in zona glomerulosa. When metyrapone was added to the mitochondria and the drug-induced difference spectra were measured, there appeared two phases in the titration curve; the first phase was saturated at a concentration of 3 microM and the second phase appeared at concentrations higher than 10 microM. The first phase seemed to be due to the substrate-free P-450(11) beta and the second phase to the cholesterol-bound P-450scc. From these spectral studies contents of cytochrome P-450scc and P-450(11) beta were estimated, and those were found to be equal for each zone. From similar studies on substrate-induced difference spectra, there seemed to be no difference between two zones in the affinity of cytochrome P-450(11) beta for deoxycorticosterone and 11-deoxycortisol. This was also the case with 11 beta-hydroxylase activity and substrate specificity for deoxycorticosterone and 11-deoxycortisol.

The effect of ether stress and cycloheximide treatment on cholesterol binding and enzyme turnover of adrenal cortical cytochrome P450scc

Relative rate constants for the formation of pregnenolone from cytochrome P450scc bound cholesterol in adrenal cortical mitochondria of stressed, stressed plus cycloheximide treated and dexamethasone treated rats were calculated from the ratios of initial rates of pregnenolone formation and the pregnenolone induced difference spectrum. In mitochondria from adrenals removed under aerobic conditions in vivo, the rate constant for the enzyme in stressed rats is twice a high as the rate constant for the enzyme from the stressed plus cycloheximide group, and four times as high as that for the enzyme from dexamethasone treated rats. Anoxia for 5 min in the intact gland increases the rate constant in all groups. Pregnenolone difference spectra are higher in mitochondria from stressed plus cycloheximide treated rats than in mitochondria from stressed rats, when adrenals are removed aerobically. It is concluded that ACTH increases cholesterol binding to cytochrome P450scc, by increasing either the enzymes affinity for its substrate or the availability of cholesterol and in addition promotes turnover of the enzyme. Both of these effects of ACTH are inhibited by cycloheximide.

The transfer of cholesterol and hydroxycholesterol derivatives from liposome to soluble cytochrome P-450 scc

We have studied the cholesterol-binding reaction with purified steroid-free cytochrome P-450scc. By mixing an aqueous solution of cholesterol-, pregnenolone- and progesterone-free cytochrome with cholesterol-containing liposomes, the low to high spin conversion of the hemoprotein was observed spectrophotometrically and by electron spin resonance spectroscopy. When the binding rates were compared at a fixed molar heme: cholesterol ratio of 1 : 1, 20 mol% cholesterol-dioleoylglycerophosphocholine liposomes react with the cytochrome at a faster rate than 50 mol% cholesterol-dimyristoylglycerophosphocholine liposomes, indicating that the availability of cholesterol molecules in an unsaturated membrane is better than in a saturated membrane. When the number of cholesterol-dioleoylglycerophosphocholine liposomes was increased, the cholesterol-binding rates increased markedly. These results imply that the collision of the liposomes with the soluble hemoprotein molecules plays an important role in the binding reaction under our experimental conditions. The binding reaction was found to be temperature-dependent with a refractive temperature at near 20 degrees C. From the comparison of the binding abilities among cholesterol derivatives tested, wer conclude that the alpha-face of the A-B transfused rings and the portion of the hydrocarbon side-chain of cholesterol are important for the binding. Additionally, polar derivatives had faster rates than non-polar steroids in structurally homologous series.

Magnetic and natural circular dichroism spectra of cytochgromes P-450(11) beta and P-450scc purified from bovine adrenal cortex

Magnetic (MCD) and natural circular dichroism (CD) spectra various complexes of cytochrome P-450(11) beta (P-450(11) beta) and cytochrome P-450scc (P-450scc) from bovine adrenal cortex were measured from 250 nm to 700 nm. MCD and CD spectral contours of cytochromes P-450(11) beta and P-450scc in the Soret and visible regions were, as a whole, analogous to those of cytochromes P-450 from rabbit liver microsomes and also from Pseudomonas putida in their high-spin ferric, high-spin ferrous and ferrous-CO complexes. MCD spectrum of the low-spin ferric P-450scc free from the substrate, cholesterol, was very similar to that caused by addition of 20 alpha-hydroxycholesterol, a reaction intermediate. However, it was distinct from those of the low-spin ferric P-450(11) beta and P-450scc complexes caused by addition of external nitrogenous ligands. The electronic states of the heme in the low-spin ferric P-450 free from substrates seemed to be subtly different from those of low-spin complexes coordinated with external nitrogenous ligands. Soret CD spectra of ferric low-spin complexes were not so different from each other. Upon reduction of high-spin ferric P-450(11) beta or P-450scc, the Soret CD magnitudes increased significantly in contrast with those of other P-450s, the Soret CD magnitudes of which decrease upon reduction. This may reflect an increased proximity of the neighbouring aromatic groups upon reduction of high-spin P-450(11) beta or P-450scc. High substrate specificity of adrenal P-450s compared with liver P-450s can be explained in view of the above findings. THe CD spectra in the near ultraviolet region (250-350 nm) were found to be quite sensitive to the spin change for ferric P-450scc, while the MCD spectra in this region did not reflect substantially the spin state of the enzyme. MCD parameters of cytochrome P-450s were compared to those of other hemoproteins in diagrams describing selected MCD spectral values of hemoproteins so far available and were discussed in connection with the structures of the heme environment of P-450.

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.

Inhibition of bovine adrenocortical cytochrome P-450scc by 3,3'-dimethoxybenzidine

The effect of 3,3'=dimethoxybenzidine (o-dianisidine) on the conversion of cholesterol to pregnenolone was investigated in a reconstituted side chain cleavage system using enzymes purified from bovine adrenal cortex; d-p-aminoglutethimide was also assayed under similar conditions for comparison. 3,3'-Dimethoxybenzidine was found to be a potent inhibitor of pregnenolone formation, causing 50% inhibition at a concentration of 1.5 microM when using 70 microM cholesterol - this dose is approximately one fourth that required of 3-methoxybenzidine and one twentieth that required of benzidine for equal inhibition. In the same system, d-p-aminoglutethimide exhibited an I50 value of about 55 microM. No effects of 3,3'-dimethoxybenzidine on adrenodoxin reductase or adrenodoxin activities could be detected, and inhibition of side chain cleavage could be relieved by dilution suggesting that the inhibitor acts by reversibly binding to cytochrome P-450scc.

3-Methoxybenzidine: a potent inhibitor of cholesterol side chain cleavage cytochrome P-450

The effect of 3-methoxybenzidine on the conversion of cholesterol to pregnenolone was investigated using a reconstituted enzyme system comprised of adrenodoxin, adrenodoxin reductase and cytochrome P-450scc purified from bovine adrenal cortex. Under conditions where the cytochrome P-450scc concentration was rate-limiting, 3-methoxybenzidine was found to be a potent inhibitor, causing 50% inhibition at 7 microM when using a cholesterol concentration of 70 microM. The parent compound, benzidine, was much less effective, exhibiting an I50 value of approximately 40 microM. No effect of 3-methoxybenzidine was observed on the adrenodoxin reductase and adrenodoxin-catalyzed reduction of cytochrome c by NADPH, and it is concluded that 3-methoxybenzidine acts on cytochrome P-450scc in inhibiting cholesterol side chain cleavage.

Electron paramagnetic resonance studies of the purified cytochrome P-450scc and P-45011beta from bovine adrenocortical mitochondria

Electron paramagnetic resonance studies have been carried out on two species of cytochrome P-450 (P-450scc and P-45011beta) purified from bovine adrenocortical mitochondria. The g values of the steroid-bound cytochromes in the high spin form were determined at 4.2 degrees K to be 8.07, 3.60 and 1.70 for P-450scc and 8.00, 3.65 and 1.71 for P-45011beta. The E/D values were estimated to be 0.103 for P-450scc and 0.099 for P-45011beta. Either high spin P-450 was converted into the low spin form by the treatment with an NADPH dependent electron donating system and subsequent gel filtration in order to remove the steroid. The g values of the low spin ferric cytochromes were 2.423, 2.247 and 1.914 for P-450scc and 2.430, 2.251 and 1.919 for P-45011beta at 77 degrees K. The values for magnitude of delta/gamma, magnitude of V/gamma and k were 5.69, 5.21 and 1.11 for P-450scc and 5.94, 5.38 and 1.16 for P-45011beta. These studies indicate that there are some differences in the ferric heme environment between P-450scc and P-45011beta.