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

Underestimated reactions and regulation patterns of adrenal cytochromes P450

Although cytochrome P450 (CYP) systems including the adrenal ones are being investigated since many years, there are still reactions and regulation patterns that have been underestimated ever since. This review discusses neglected ones to bring them into the focus of investigators working in the field. Novel substrates and reactions described for adrenal CYPs recently point to the fact that different from what has been believed for many years, adrenal CYPs are less selective than previously thought. The conversion of steroid sulfates, intermediates of steroid biosynthesis as well as of exogenous compounds are being discussed here in more detail and consequences for further studies are drawn. Furthermore, it was shown that protein-protein interactions may have an important effect not only on the activity of adrenal CYPs, but also on the product pattern of the reactions. It was found that, as expected, the stoichiometry of CYP:redox partner plays an important role for tuning the activity. In addition, competition between different CYPs for the redox partner and for electrons and possible alterations by mutants in the efficiency of electron transfer play an important role for the activity and product pattern. Moreover, the influence of phosphorylation and small charged molecules like natural polyamines on the activity of adrenal systems has been demonstrated in-vitro indicating a possible regulation of adrenal CYP reactions by affecting redox partner recognition and binding affinity. Finally, an effect of the genetic background on the consequences of mutations in adrenal CYPs found in patients was suggested from corresponding in-vitro studies indicating that a different genetic background might be able to significantly affect the activity of a CYP mutant.

Effect of sulfonated steroids on steroidogenic cytochrome P450-dependent steroid hydroxylases

In the last decades, sulfonated steroids evolved from inactive metabolites intended for excretion to highly relevant compounds involved in many physiological processes. Investigations of the impact of sulfonated steroids on the steroid hormone biosynthesis revealed that, on the one hand, these can serve as substrate for steroidogenic cytochromes P450 and, on the other hand, these are able to influence the catalytic properties of these enzymes. In this review the relevance of sulfonated steroids for the steroid hormone biosynthesis will be discussed.

New assumptions about oxidative processes involved in steroid hormone biosynthesis: is the role of cytochrome P-450-activated dioxygen limited to hydroxylation reactions or are dioxygen insertion reactions also possible?

The traditional conception of the chemical pathways leading to the formation of the steroid hormones is derived by piecing together the results of several independent in vitro incubation experiments. The results of these experiments have led to the assumption that some relevant cytochrome P-450's (P-450scc, P-450arom, P-450aldo, etc.) are "polyfunctional" and catalyze several successive hydroxylation reactions, which lead to the formation of the hormonal products. This essay offers an alternative view. It advances the suggestion that the oxygenated intermediates in the relevant biosynthetic conversions are reactive species that are formed by addition of both atoms of dioxygen onto two neighboring carbon atoms of steroidal precursors. Space-filled Stuart molecular models, generated by a computer program, suggest that the oxidized intermediates resemble hydroperoxides or cyclic peroxides (1,2-dioxanes). For the aromatization process required for estrogen biosynthesis, the atoms of dioxygen are bonded to C-2 and C-19 of the C19-precursor. For aldosterone formation, dioxygen is bonded to C-11 and C-18 of an appropriate precursor. Moreover, the results obtained from a computer program that provides information about "molecular mechanics" (bond angles and bond distances as well as total potential energies for each conformation of a molecule) suggest that consideration be given to the possibility that cortisol also can be biosynthesized by P-450-activated dioxygen addition to C-11 and C-17 of an appropriate precursor. Neither the traditional view of steroidogenic pathways nor the suggestions advanced here have been established by compelling experimental findings. Both hypotheses are saddled with untested assumptions, which are necessary because the dynamic processes can only be discerned by indirect means. The origins of some naturally occurring steroids hydroxylated at C-17, C-18 and C-19 are examined in the light of the suggestions made in this essay.

Reflections on sterol sidechain cleavage process catalyzed by cytochrome P450(scc)

The essay examines the evidence upon which the presently accepted version of the mechanism of the cytochrome P450(scc)-catalyzed-cleavage of the sidechain of cholesterol is based. This analysis indicates that the generally held view of the process (two consecutive hydroxylations, followed by cleavage of the resulting glycol) most likely does not describe the true mechanism. The available evidence can not be used to support this traditional notion. Two alternative hypotheses are proposed.

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

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

Comparison of the multiple molecular forms of bovine adrenocortical P450scc with those of corpus luteum P450scc

1. Bovine adrenocortical P450scc was resolved into several fractions by chromatography on AH-Sepharose 4B followed by gel filtration on Toyopearl HW55S. All fractions contained P450scc of the same molecular size and the P450scc could be resolved into 3-4 major and more than 10 minor isoelectric point forms by isoelectric focusing on polyacrylamide gel in the presence of Emulgen 913. 2. Both the AH-Sepharose chromatography profile and the isoelectric focusing pattern of the adrenocortical P450scc were more complex than those of the corpus luteum P450scc. The corpus luteum P450scc was practically devoid of the neutral to acidic isoelectric point forms. 3. Three to four P450scc subfractions with different isoelectric focusing pattern were obtained from a purified preparation of adrenocortical P450scc by ion-exchange chromatography on DEAE-Toyopearl 650S or DEAE-Sephadex A25. These P450scc subfractions showed essentially the same spectral properties, catalytic activity, molecular weight and N-terminal amino acid sequence. 4. The most acidic (the latest eluting) subfraction was composed mostly of the neutral to acidic isoelectric point forms. The sedimentation characteristics of this subfraction was also studied. 5. The structural basis of the multiple molecular forms was discussed.

Multiple molecular forms of cytochrome P-450SCC purified from bovine corpus luteum mitochondria

Cytochrome P-450 related to side-chain cleavage of cholesterol (P-450SCC) was isolated from bovine corpus luteum mitochondria in the form of its stable cholesterol complex. The isolation procedure included ammonium sulfate fractionation and chromatography on omega-aminohexyl-Sepharose (AH-Sepharose). Corpus luteum P-450SCC was resolved into one minor (AH-I) and two major (AH-II and AH-III) fractions by the chromatography. Results of re-chromatography suggested the possibility that AH-III Fraction was originally complexed with lipidic material. The two major fractions purified by the re-chromatography (AH-IIR and AH-IIIR Fractions) showed essentially a single band on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and their absorption spectra were indistinguishable from each other. Both fractions were further resolved into two major and some minor bands of P-450SCC by isoelectric focusing on polyacrylamide gel in the presence of a non-ionic detergent, as detected by protein staining, heme staining and immunoblot analysis with anti-bovine P-450SCC monoclonal antibody. Both AH-IIR and AH-IIIR Fractions were further resolved by high-performance liquid chromatography (HPLC) on SP-TSK gel column into two fractions, SP-I and SP-II. These fractions had the same N-terminal amino acid sequence, showed similar catalytic activity and resolved into one major and a few minor bands on isoelectric focusing on polyacrylamide gel. Much more heterogeneity was observed in purified P-450SCC preparations from bovine adrenal cortex mitochondria. These results indicated the presence of multiple molecular forms of corpus luteum P-450SCC as well as adrenal cortex P-450SCC. Computer simulation studies were carried out in order to analyze the mechanism of formation of multiple bands on isoelectric focusing. The multiple bands of corpus luteum P-450SCC could be explained by postulating the presence of two isozymes (or molecular forms) having a pair of sites each with or without a charged group.

Existence of multiple forms of cytochrome P-450scc purified from bovine adrenocortical mitochondria

Three fractions of cytochrome P-450scc (denoted as fractions a, b, and c) were purified by a new procedure from bovine adrenocortical mitochondria. The amino-acid content analyses of these three fractions showed no difference. NH2-terminal amino-acid sequences of cytochrome P-450scc fractions, a and b agreed completely with the sequence deduced by nucleotide sequence of cDNA of cytochrome P-450scc mRNA (Morohashi, K., Fujii-Kuriyama, Y., Okada, Y., Sogawa, K., Hirose, T., Inayama, S. and Omura, T. (1984) Proc. Natl. Acad. Sci. USA 81, 4647-4651), whereas the sequence of fraction c showed a missing of isoleucine at the NH2-terminal. COOH-terminal ámino-acid sequences of fractions a, b and c were -Gln-Ala-COOH, identical with the deduced sequence from the cDNA. Measurements of the enzymatic activities of cholesterol side-chain cleavage reaction revealed no distinct difference among these three fractions. Although each of these fractions appeared as a single protein staining band upon SDS-polyacrylamide gel electrophoresis, these fractions showed heterogeneities upon two-dimensional electrophoresis and chromatofocusing. Fraction a contained the major form of cytochrome P-450scc, and its isoelectric point was estimated to be pH 7.8 by isoelectric focusing under both native and denatured conditions, and this value was confirmed by chromatofocusing. Neither of the carbohydrate-specific stainings (such as periodic acid-Schiff staining and lectin-peroxidase stainings using concanavalin A, wheat-germ agglutinin, and soybean agglutinin) of purified cytochrome P-450scc fractions after the electrophoretic resolution on SDS-polyacrylamide gel could show cytochrome P-450scc fractions as glycoproteins, suggesting that the heterogeneities were not due to the glycosylation state.

Studies of the active site of cytochrome P-450scc with a high-affinity spin-labeled inhibitor

The intramolecular site of P-450scc for conversion of cholesterol to pregnenolone involves a substrate site, an active site, and a site for transmission of electrons. The substrate site was studied with a high-affinity, high-potency nitroxide spin-labeled inhibitor of cholesterol side-chain cleavage. This substance, 17 alpha-hydroxy-11-deoxycorticosterone nitroxide (SL-V), has an affinity comparable to that of the most active substrate inhibitors ever reported and 2-50 times greater than that of the natural substrate cholesterol. Competition experiments with cholesterol and its analogues confirmed that SL-V binds reversibly to the substrate site. Titration experiments showed a single binding site on the P-450 molecule. The substrate site is on the apoprotein and has little or no direct interaction with the heme. Spin-spin interactions between the Fe3+ and side-chain or A-ring spin-labeled groups could not be demonstrated, which is consistent with carbons 22 and 20 being closest to the heme iron. We postulate that substrate disrupts a histidine nitrogen coordination with the heme iron and induces conformational changes in the apoprotein. These changes lead to increased affinity for iron-sulfur protein.

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.

The side-chain cleavage of cholesterol sulfate--III. The effect of adrenodoxin, membrane phospholipids and Tween 80 on the kinetics of oxidation of the sterol sulfate by a reconstituted cholesterol desmolase system

This paper reports the Km values of a reconstituted cholesterol side-chain cleavage system for cholesterol sulfate, cholesterol, and adrenodoxin, determined under several experimental conditions. The Km values for adrenodoxin change depending on whether cholesterol or its sulfate is used as the substrate. Moreover, the Km values for both of the substrates and for adrenodoxin are greatly modulated by both membrane phospholipids, isolated from adrenal mitochondria, and Tween 80, 0.002%. In the absence of detergents or phospholipids, the enzyme system shows a high affinity for cholesterol sulfate, but is inhibited when high concentrations of the sterol sulfate are added to the incubation mixture. Raising the concentration of adrenodoxin in the assay mixture prevents the substrate inhibition. When cholesterol sulfate is incorporated into micelles containing the phospholipids, the enzyme system does not display substrate inhibition, and the kinetics of cleavage of the sterol sulfate are relatively independent of the concentration of adrenodoxin in the assay mixture. In the absence of phospholipids, the apparent kinetics of cleavage of cholesterol and its sulfate are quite different from each other, but when incorporated into micelles containing phospholipids, the kinetics of cleavage of the two substrates are similar to each other.

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

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

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

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

The side-chain cleavage of cholesterol sulfate--I. The effect of adrenodoxin on the binding of cholesterol sulfate to cytochrome P-450scc

Difference spectroscopy was used to measure the binding of cholesterol sulfate (CS) to cytochrome P-450scc. The uncomplexed cytochrome and the complex of the cytochrome with adrenodoxin (ADX) were both titrated with CS in order to test whether ADX increased the affinity of the cytochrome for the sterol sulfate. The addition of ADX to the cytochrome had different effects on the binding of the sterol sulfate depending on several factors including: (1) The method of preparation of the cytochrome P-450scc, (2) The concentration of cytochrome P-450scc, (3) The method by which CS was suspended in aqueous solution, and (4) Whether or not the solutions of cytochrome contained non-ionic detergents. The results of this study suggest that the method of isolation of cytochrome P-450scc, and non-ionic detergents, greatly modulate the apparent affinity of cytochrome P-450scc for CS. In the absence of detergents the addition of adrenodoxin to dilute solutions of cytochrome P-450scc appears to enhance only slightly (1- to 2-fold) the affinity of the cytochrome for the sterol sulfate.

Utilization of intramitochondrial membrane cholesterol by cytochrome P-450-dependent cholesterol side-chain cleavage reaction in bovine adrenocortical mitochondria: steroidogenic and non-steroidogenic pools of cholesterol in the mitochondrial inner membranes

Inner and outer submitochondrial membranes were prepared after disruption of malate-treated bovine adrenocortical mitochondria. It was found that a part of the endogenous cholesterol in the inner membrane (approx. 50%) was rapidly utilized by the cholesterol side-chain cleavage reaction. The utilization of cholesterol in the outer membrane, on the other hand, was inefficient and slow in spite of the fact that cholesterol concentration is higher in the outer than in the inner membrane. When the inner membrane prepared from untreated mitochondria was incubated for 20 min in the presence of a reconstituted cytochrome P-450-reducing system, the inner membrane cholesterol was depleted by approximately 70%. The half-life of the depletion reaction was 2-3 min. In addition, when the outer membrane plus the soluble fraction from the untreated mitochondria were added as a source of cholesterol to the inner membrane fraction, a marginal increase in the production of steroids was observed. From these results it is concluded that a portion of the inner membrane cholesterol can be steroidogenic, whereas the rest of the cholesterol is non-steroidogenic.

Adrenal cortical 11 beta-hydroxylase and side-chain cleavage enzymes. Requirement for the A- or B-pyridyl ring in metyrapone for inhibition

The adrenal cortical enzyme systems, 11 beta-hydroxylase, P-450 11 beta, and the side-chain cleavage complex, P-450 scc, differ only in their cytochrome P-450s. Structural modifications of metyrapone, an inhibitor of cytochrome P-450 enzyme systems, have been made to determine the requirement for the A- or B-pyridyl ring for inhibition of P-45011 beta and P-450 scc activities. Three new analogs of metyrapone (A-phenylmetyrapone, B-phenylmetyrapone and diphenylmetyrapone) were synthesized and evaluated as inhibitors using a crude, defatted bovine adrenal cortical mitochondrial preparation. Characterization of the mitochondrial preparation demonstrated: enhancement of both activities by the addition of 15.0 microM adrenodoxin, the addition of 1% ethanol decreased both activities less than 10%, and the apparent Km of deoxycorticosterone for P-45011 beta was 6.8 microM and the apparent Km of cholesterol for P-450 scc was 21.6 microM. Inhibition of P-45011 beta and P-450 scc activities with these compounds demonstrated: the B-pyridyl ring of metyrapone is required for inhibition of both activities whereas requirement for the A-ring is less stringent, and the four metyrapone analogs were more selective inhibitors of P-45011 beta activity. These studies suggest that the A-phenyl metyrapone analog is a good candidate for further development of a selective adrenocortical radiopharmaceutical.

The domain structure of the cholesterol side-chain cleavage cytochrome P-450 from bovine adrenocortical mitochondria

A homogeneous cytochrome P-450scc preparation with a specific enzyme content of 18 nmol/1 mg protein has been obtained using affinity chromatography on adrenodoxin-Sepharose under optimal conditions of the protein adsorption onto and desorption from the affinity sorbent. The data on the N-terminal amino acid sequence of the enzyme, along with the results of electrophoretic and spectrophotometric analyses favoured the multistage cholesterol transformation to pregnenolone to be catalyzed by single species of cytochrome P-450scc consisting of one polypeptide chain. Limited proteolysis of cytochrome P-450scc with trypsin resulted, at the initial stages, in the formation (in an equimolar ratio) of two large polypeptide fragments, I and II, with Mr 27000 and 22000, respectively. Prolonged action of trypsin led to the digestion of fragment II and the formation of a stoichiometric amount of fragment III, Mr of about 14000. Cytochrome P-450scc converted by trypsin into equimolar mixtures of fragments I and II or I and III retained the major spectral and functional properties of the native protein. The aspartyl-prolyl linkages, sulphhydryl groups, and surface tyrosine residues are distributed nonuniformly among fragments I and II. These data, as well as a different resistance of the fragments to the action of trypsin, suggest that cytochrome P-450scc consists of two independently folded domains linked with a short loop of the polypeptide chain, the domains being rigidly associated under neutral conditions.

The effects of activating peptides and proteins on the activity of reconstituted cholesterol desmolase systems

The kinetics of cleavage of both cholesterol and cholesterol sulfate by reconstituted cholesterol side-chain cleavage systems are profoundly modified by non-ionic detergents and phospholipids. The Km value for adrenodoxin is changed depending on whether cholesterol or cholesterol sulfate is used as the substrate and by the presence of detergents and phospholipids in the assay mixture. The activity of the enzyme system is also affected by reducing agents such as dithiothreitol. All these factors must be considered when the effects of protein and peptide activators of the cholesterol side-chain cleavage system are interpreted.

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.